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Which WANs are implemented using dedicated leased line connections from the corporate edge point to the provider networks?

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Local Area Network (LAN) is where we connect the endpoint devices like servers, desktops, telephones, and access points. To connect the device within the LAN to a different device within the different LAN, we are using a Wide Area Network (WAN). In WAN, two or more local area networks can relate to different layer 3 devices like routers or firewalls. The common type of WAN is leased line connection.

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Understanding Leased Lines

A leased line, sometimes called a dedicated line, is a dedicated point-to-point link and fixed-bandwidth data connection. A leased line is not a dedicated cable. It is a reserved dedicated leased line circuit (either a copper or a fiber optic cable) between two points.

The leased line transfers data in both directions using a full-duplex transmission. It uses two pairs of wires (full-duplex cable), that each wire is used in a unidirectional transmission of data network. A leased line is not a long physical cable extended to two or more locations as others perceived. It uses a specialized switching device that acts as a signal booster to make the connection a point-to-point link and reach a remote destination.

Organizations are not building their infrastructure to create a dedicated connection to their other branches as it is expensive and difficult to implement. They use the infrastructure of an Internet Service Provider on a fixed monthly fee, which is why it is called a leased line.

The below diagram shows how the leased line connects two branches:

leased line

ISP as a Leased Line

A leased line can be of any medium as long as it connects two branches together regardless if it has network circuitry in between. It can be an MPLS, Fiber Optic, DSL, or Satellite. The local Internet Service Provider is the best way to acquire a leased line as they have a huge geographical network infrastructure. It can either be a monthly or yearly subscription, depending on the terms with the ISP.

From the Optical Network Terminal (ONT) located on the customer’s branch, the traffic will go to the Optical Line Termination (OLT) located in the ISP premise, where it multiplexed and processed all the optical signals coming from the customers.

From OLT, it will then go to the edge routers where it uses VRF and adds labels if it is using MPLS. From edge routers, it will then go to the core router using BGP as the overlay protocol and IGP like OSPF as the underlay protocol. From core routers, it will transmit the data to the other edge router, go to OLT, and finally to the ONT, which is located to the other branch of the customer. The transmission equipment used in between the core routers, edge routers, OLT, and ONT is either Synchronous Digital Hierarchy (SDH) or Dense Wavelength Division Multiplexing (DWDM).

Lease Line Advantages and Disadvantages

Most leased lines have a Service Level of Agreement (SLA) to the ISP, which guarantees a reliable and stable internet connection. Because the leased line is a dedicated communication channel, your network will have reliable internet access, continuous data flow, higher bandwidth can be achieved and controlled. You can implement a leased line when you want a completely secured and superior quality of service for your network.

On the other hand, leased lines can be expensive because they need a dedicated cable and switching circuitry. Not only that, the leased line is not scalable as it is a permanent physical connection.

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Objectives
Upon completion of this chapter, you will be able to answer the following questions:
■ What is the purpose of a WAN?
■ How do WANs operate?
■ What WAN services are available?
■ What are the differences between private
WAN technologies?
■ What are the differences between public
WAN technologies?
■ What is the appropriate WAN protocol and
service for a specific network requirement?
Key Terms
This chapter uses the following key terms. You can find the definitions in the Glossary.
2 Connecting Networks v6 Companion Guide
last-mile Page 15
central office (CO) Page 15
toll network Page 16
dialup modem Page 16
modulates Page 16
demodulates Page 16
Access server Page 16
broadband modem Page 16
channel service unit/data service unit
(CSU/DSU) Page 17
WAN switch Page 17
circuit-switched network Page 17
public switched telephone network
(PSTN) Page 18
Integrated Services Digital Network
(ISDN) Page 18
packet-switched network (PSN) Page 18
virtual circuit (VC) Page 19
data-link connection identifiers
(DLCIs) Page 19
latency Page 19
jitter Page 19
private WAN infrastructure Page 21
public WAN infrastructure Page 21
broadband connections Page 21
Synchronous Optical Networking
(SONET) Page 22
Synchronous Digital Hierarchy (SDH) Page 22
light-emitting diodes (LEDs) Page 22
dense wavelength division multiplexing
(DWDM) Page 22
multiplex Page 22
E3 Page 24
optical carrier (OC) Page 24
time-division multiplexed (TDM) Page 26
Basic Rate Interface (BRI) Page 26
Primary Rate Interface (PRI) Page 27
nonbroadcast multi-access (NBMA) Page 28
permanent virtual circuits (PVCs) Page 28
Metropolitan Ethernet (MetroE) Page 30
Ethernet over MPLS (EoMPLS) Page 31
Virtual Private LAN Service (VPLS) Page 31
Multiprotocol Label Switching (MPLS) Page 32
very small aperture terminal (VSAT) Page 33
DSL modem Page 34
DSL access multiplexer (DSLAM) Page 34
point of presence (POP) Page 34
cable modems (CM) Page 35
headend Page 35
cable modem termination system
(CMTS) Page 35
municipal Wi-Fi Page 36
WiMAX Page 36
satellite Internet Page 36
3G/4G wireless Page 38
Long-Term Evolution (LTE) Page 38
teleworker Page 38
site-to-site VPNs Page 38
remote-access VPNs Page 39
Chapter 1: WAN Concepts 3
Introduction (1.0)
Businesses must connect LANs to provide communications between them, even
when these LANs are far apart. Wide-area networks (WANs) are used to connect
remote LANs. A WAN may cover a city, country, or global region. A WAN is owned
by a service provider, and a business pays a fee to use the provider’s WAN network
services.
Different technologies are used for WANs than for LANs. This chapter introduces
WAN standards, technologies, and purposes. It covers selecting the appropriate WAN
technologies, services, and devices to meet the changing business requirements of an
evolving enterprise.
Class Activity 1.0.1.2: Branching Out
Your medium-sized company is opening a new branch office to serve a wider, clientbased
network. This branch will focus on regular, day-to-day network operations but
will also provide TelePresence, web conferencing, IP telephony, video on demand, and
wireless services.
Although you know that an ISP can provide WAN routers and switches to accommodate
the branch office connectivity for the network, you prefer to use your own
customer premises equipment (CPE). To ensure interoperability, Cisco devices have
been used in all other branch-office WANs.
As the branch-office network administrator, you are responsible for researching possible
network devices for purchase and use over the WAN.
WAN Technologies Overview (1.1)
In this section, you learn about WAN access technologies available to small- to
medium-sized business networks.
Purpose of WANs (1.1.1)
In this topic, you learn the purpose of the WAN.
Why a WAN? (1.1.1.1)
A WAN operates beyond the geographic scope of a LAN. As shown Figure 1-1,
WANs are used to interconnect the enterprise LAN to remote LANs in branch sites
and telecommuter sites.
4 Connecting Networks v6 Companion Guide
Figure 1-1 WANs Interconnect Users and LANs
A WAN is owned by a service provider. A user must pay a fee to use the provider’s
network services to connect remote sites. WAN service providers include carriers, such
as a telephone network, cable company, or satellite service. Service providers provide
links to interconnect remote sites for the purpose of transporting data, voice, and video.
In contrast, LANs are typically owned by an organization. They are used to connect
local computers, peripherals, and other devices within a single building or other small
geographic area.
Are WANs Necessary? (1.1.1.2)
Without WANs, LANs would be a series of isolated networks. LANs provide both
speed and cost-efficiency for transmitting data over relatively small geographic areas.
However, as organizations expand, businesses require communication among geographically
separated sites. The following are some examples:
■ Regional or branch offices of an organization need to be able to communicate
and share data with the central site.
■ Organizations need to share information with other customer organizations.
For example, software manufacturers routinely communicate product and
promotional information to distributors that sell their products to end users.
■ Employees who travel on company business frequently need to access
information that resides on their corporate networks.
Chapter 1: WAN Concepts 5
Home computer users also need to send and receive data across increasingly larger
distances. Here are some examples:
■ Consumers now commonly communicate over the Internet with banks, stores,
and a variety of providers of goods and services.
■ Students do research for classes by accessing library indexes and publications
located in other parts of their country and in other parts of the world.
It is not feasible to connect computers across a country, or around the world, with
physical cables. Therefore, different technologies have evolved to support this communication
requirement. Increasingly, the Internet is being used as an inexpensive
alternative to enterprise WANs. New technologies are available to businesses to provide
security and privacy for their Internet communications and transactions. WANs
used by themselves, or in concert with the Internet, allow organizations and individuals
to meet their wide-area communication needs.
WAN Topologies (1.1.1.3)
Interconnecting multiple sites across WANs can involve a variety of service provider
technologies and WAN topologies. Common WAN topologies are
■ Point-to-point topology
■ Hub-and-spoke topology
■ Full mesh topology
■ Dual-homed topology
Point-to-Point
A point-to-point topology, as shown in Figure 1-2, employs a point-to-point circuit
between two endpoints. Typically involving dedicated leased-line connections like
a T1 or an E1 line, a point-to-point connection provides a Layer 2 transport service
through the service provider network. Packets sent from one site are delivered to the
other site and vice versa. A point-to-point connection is transparent to the customer
network, as if there was a direct physical link between two endpoints.
Figure 1-2 Point-to-Point Topology
6 Connecting Networks v6 Companion Guide
Hub-and-Spoke
If a private network connection between multiple sites is required, a point-to-point
topology with multiple point-to-point circuits is one option. Each point-to-point
circuit requires its own dedicated hardware interface that will require multiple
routers with multiple WAN interface cards. This interface can be expensive. A less
expensive option is a point-to-multipoint topology, also known as a hub-and-spoke
topology.
With a hub-and-spoke topology, all spoke circuits can share a single interface to the
hub. For example, spoke sites can be interconnected through the hub site using virtual
circuits and routed subinterfaces at the hub. A hub-and-spoke topology is also
an example of a single-homed topology. Figure 1-3 displays a sample hub-and-spoke
topology consisting of four routers with one router as a hub connected to the other
three spoke routers across a WAN cloud.
Figure 1-3 Hub-and-Spoke Topology
Full Mesh
One of the disadvantages of hub-and-spoke topologies is that all communication
has to go through the hub. With a full mesh topology using virtual circuits, any site
can communicate directly with any other site. The disadvantage here is the large
number of virtual circuits that need to be configured and maintained. Figure 1-4
displays a sample full mesh topology consisting of four routers connected to each
other across a WAN cloud.
Dual-homed Topology
A dual-homed topology provides redundancy. As shown in Figure 1-5, the spoke
routers are dual-homed and redundantly attached to two hub routers across a WAN
Chapter 1: WAN Concepts 7
cloud. The disadvantage to dual-homed topologies is that they are more expensive
to implement than a single-homed topology. The reason is that they require
additional networking hardware, like additional routers and switches. Dual-homed
topologies are also more difficult to implement because they require additional,
and more complex, configurations. However, the advantage of dual-homed topologies
is that they offer enhanced network redundancy, load balancing, distributed
computing or processing, and the ability to implement backup service provider
connections.
Figure 1-4 Full Mesh Topology
Figure 1-5 Dual-Homed Topology
8 Connecting Networks v6 Companion Guide
Evolving Networks (1.1.1.4)
Every business is unique, so how an organization grows depends on many factors.
These factors include the types of products or services the business sells, the management
philosophy of the owners, and the economic climate of the country in
which the business operates.
In slow economic times, many businesses focus on increasing their profitability by
improving the efficiency of their existing operations, increasing employee productivity,
and lowering operating costs. Establishing and managing networks can represent
significant installation and operating expenses. To justify such a large expense,
companies expect their networks to perform optimally and to be able to deliver
an ever-increasing array of services and applications to support productivity and
profitability.
The example used in this chapter and shown in Figure 1-6 is of a fictitious company
called SPAN Engineering. This topic will illustrate how SPAN’s network requirements
change as the company grows from a small, local business into a global enterprise.
Figure 1-6 SPAN Engineering
Small Office (1.1.1.5)
SPAN Engineering, an environmental consulting firm, has developed a special process
for converting household waste into electricity and is developing a small pilot project
for a municipal government in its local area. The company, which has been in business
for four years, is a small office consisting of 15 employees: six engineers, four
computer-aided drawing (CAD) designers, a receptionist, two senior partners, and
two office assistants.
Chapter 1: WAN Concepts 9
SPAN Engineering’s management is working to win full-scale contracts after the pilot
project successfully demonstrates the feasibility of the company’s process. Until
then, the company must manage its costs carefully.
As shown in Figure 1-7, SPAN Engineering uses a single LAN to share information
between computers and to share peripherals, such as a printer, a large-scale plotter
(to print engineering drawings), and fax equipment.
Figure 1-7 Connecting a Small Office
The company has recently upgraded its LAN to provide inexpensive voice over IP
(VoIP) service to save on the costs of separate phone lines for employees.
Internet connectivity is provided using a common broadband service called digital
subscriber line (DSL), which is supplied by the local telephone service provider.
Because SPAN has so few employees, bandwidth is not a significant problem.
The company cannot afford in-house IT support staff, so it uses support services purchased
from the DSL provider. The company also uses a hosting service rather than
purchasing and operating its own FTP and email servers.
Campus Network (1.1.1.6)
Five years later, SPAN Engineering has grown rapidly. The company was contracted
to design and implement a full-size waste conversion facility soon after the successful
implementation of its first pilot plant. Since then, SPAN has won other projects in
neighboring municipalities and in other parts of the country.
10 Connecting Networks v6 Companion Guide
To handle the additional workload, the business has hired more staff and leased
more office space. It is now a small- to medium-sized business with several hundred
employees. Many projects are being developed at the same time, and each requires a
project manager and support staff. The company has organized itself into functional
departments, with each department having its own organizational team. To meet its
growing needs, the company has moved into several floors of a larger office building.
As the business has expanded, the network has also grown. Instead of a single small
LAN, the network now consists of several subnetworks, each devoted to a different
department. For example, all the engineering staff is on one LAN, while the marketing
staff is on another LAN. These multiple LANs are joined to create a companywide
network, or campus, which spans several floors of the building.
Figure 1-8 shows an example of SPAN’s campus network.
Figure 1-8 Connecting a Campus Network
The business now has in-house IT staff to support and maintain the network. The
network includes dedicated servers for email, data transfer, and file storage, and webbased
productivity tools and applications. In addition, a company intranet provides
in-house documents and information to employees. An extranet provides project
information to designated customers.
Branch Networks (1.1.1.7)
Another six years later, SPAN Engineering has been so successful with its patented
process that demand for its services has skyrocketed. New projects are underway
Chapter 1: WAN Concepts 11
in multiple cities. To manage those projects, the company has opened small branch
offices closer to the project sites.
This situation presents new challenges to the IT team. To manage the delivery of
information and services throughout the company, SPAN Engineering now has a data
center, which houses the various databases and servers of the company. To ensure
that all parts of the business are able to access the same services and applications
regardless of where the offices are located, the company must now implement
a WAN.
For its branch offices that are in nearby cities, the company decides to use private
dedicated lines through a local service provider, as shown in Figure 1-9. However,
for those offices that are located in other countries, the Internet is an attractive WAN
connection option. Although connecting offices through the Internet is economical,
this approach introduces security and privacy issues that the IT team must address.
Figure 1-9 Connecting Branch Networks
Distributed Network (1.1.1.8)
SPAN Engineering has now been in business for 20 years and has grown to thousands
of employees distributed in offices worldwide, as shown in Figure 1-10.
The cost of the enterprise network and its related services is a significant
expense. The company is looking to provide its employees with the best network services
at the lowest cost. Optimized network services would allow each employee to
work at a high rate of efficiency.
12 Connecting Networks v6 Companion Guide
Figure 1-10 SPAN Engineering
To increase profitability, SPAN Engineering must reduce its operating expenses. It
has relocated some of its office facilities to less expensive areas. The company is
also encouraging teleworking and virtual teams. Web-based applications, including
web conferencing, e-learning, and online collaboration tools, are being used
to increase productivity and reduce costs. Site-to-site and remote-access virtual
private networks (VPNs) enable the company to use the Internet to connect
easily and securely with employees and facilities around the world. To meet these
requirements, the network must provide the necessary converged services and
secure Internet WAN connectivity to remote offices and individuals, as shown
in Figure 1-11.
As seen in this example, network requirements of a company can change dramatically
as the company grows over time. Distributing employees saves costs in many ways,
but it puts increased demands on the network.
A network not only must meet the day-to-day operational needs of the business
but also must be able to adapt and grow as the company changes. Network
designers and administrators meet these challenges by carefully choosing network
technologies, protocols, and service providers. They must also optimize their
networks by using many of the network design techniques and architectures
described in this course.
Activity 1.1.1.9: Identify WAN Topologies
Refer to the online course to complete this activity.
Interactive
Graphic
Chapter 1: WAN Concepts 13
Figure 1-11 Connecting a Global Enterprise Network
WAN Operations (1.1.2)
In this topic, you learn how WANs operate.
WANs in the OSI Model (1.1.2.1)
WAN operations focus primarily on the physical layer (OSI Layer 1) and the data
link layer (OSI Layer 2), as illustrated in Figure 1-12. WAN access standards typically
describe both physical layer delivery methods and data link layer requirements.
The data link layer requirements include physical addressing, flow control, and
encapsulation.
WAN access standards are defined and managed by a number of recognized
authorities:
■ Telecommunications Industry Association (TIA)
■ Electronic Industries Alliance (EIA)
■ International Organization for Standardization (ISO)
■ Institute of Electrical and Electronics Engineers (IEEE)
Layer 1 protocols describe how to provide electrical, mechanical, operational, and
functional connections to the services of a communications service provider.
14 Connecting Networks v6 Companion Guide
Figure 1-12 WANs Operate in Layers 1 and 2
Layer 2 protocols define how data is encapsulated for transmission toward a remote
location and the mechanisms for transferring the resulting frames. A variety of different
technologies are used, such as the Point-to-Point Protocol (PPP), Frame Relay,
and Asynchronous Transfer Mode (ATM). Some of these protocols use the same
basic framing or a subset of the High-Level Data Link Control (HDLC) mechanism.
Most WAN links are point-to-point. For this reason, the address field in the Layer 2
frame is usually not used.
Common WAN Terminology (1.1.2.2)
One primary difference between a WAN and a LAN is that a company or organization
must subscribe to an outside WAN service provider to use WAN carrier network
services. A WAN uses data links provided by carrier services to access the Internet
and connect different locations of an organization to each other. These data links also
connect to locations of other organizations, to external services, and to remote users.
The physical layer of a WAN describes the physical connections between the company
network and the service provider network. Figure 1-13 illustrates the terminology
commonly used to describe WAN connections:
■ Customer premises equipment (CPE): The CPE consists of the devices and
inside wiring located on the enterprise edge connecting to a carrier link. The subscriber
(that is, customer) either owns the CPE or leases the CPE from the service
provider. A subscriber, in this context, is a company that arranges for WAN
services from a service provider.
Chapter 1: WAN Concepts 15
Figure 1-13 WAN Terminology
■ Data communications equipment (DCE): This is an EIA term. Also called data
circuit-terminating equipment by the ITU. The DCE consists of devices that put
data on the local loop. The DCE primarily provides an interface to connect subscribers
to a communication link on the WAN cloud.
■ Data terminal equipment (DTE): These customer devices pass the data from a
customer network or host computer for transmission over the WAN. The DTE
connects to the local loop through the DCE.
■ Demarcation point: This point is established in a building or complex to separate
customer equipment from service provider equipment. Physically, the demarcation
point is the cabling junction box, located on the customer premises, that
connects the CPE wiring to the local loop. It is usually placed so that a technician
can access it easily. The demarcation point is the place where the responsibility
for the connection changes from the user to the service provider. When
problems arise, it is necessary to determine whether the user or the service provider
is responsible for troubleshooting or repair.
■ Local loop: This loop is the actual copper or fiber cable that connects the CPE to
the CO of the service provider. The local loop is also sometimes called the last-mile.
■ Central office (CO): The CO is the local service provider facility or building
that connects the CPE to the provider network.
16 Connecting Networks v6 Companion Guide
■ Toll network: This network consists of the long-haul, all-digital, fiber-optic
communications lines, switches, routers, and other equipment inside the WAN
provider network.
WAN Devices (1.1.2.3)
Many types of devices are specific to WAN environments, as shown in Figure 1-14,
and are described in the list that follows.
Figure 1-14 Common WAN Devices
■ Dialup modem: Voiceband modems are considered to be a legacy WAN technology.
A voiceband modem modulates (that is, converts) the digital signals
produced by a computer into voice frequencies. These frequencies are then transmitted
over the analog lines of the public telephone network. On the other side
of the connection, another modem demodulates the sounds back into a digital
signal for input to a computer or network connection.
■ Access server: This server controls and coordinates dialup modem, dial-in, and
dial-out user communications. Considered to be a legacy technology, an access
server may have a mixture of analog and digital interfaces and support hundreds
of simultaneous users.
■ Broadband modem: This type of digital modem is used with high-speed DSL
or cable Internet service. Both operate in a similar manner to the voiceband
modem but use higher broadband frequencies to achieve higher transmission
speeds.
Chapter 1: WAN Concepts 17
■ Channel service unit/data service unit (CSU/DSU): Digital leased lines require
a CSU and a DSU. A CSU/DSU can be a separate device like a modem, or it can
be an interface on a router. The CSU provides termination for the digital signal and
ensures connection integrity through error correction and line monitoring. The DSU
converts the line frames into frames that the LAN can interpret and vice versa.
■ WAN switch: This multiport internetworking device is used in service provider
networks. These devices typically switch traffic, such as Frame Relay or ATM,
and operate at Layer 2.
■ Router: This device provides internetworking and WAN access interface ports
that are used to connect to the service provider network. These interfaces may be
serial connections, Ethernet, or other WAN interfaces. With some types of WAN
interfaces, an external device, such as a DSU/CSU or modem (analog, cable, or
DSL), is required to connect the router to the local service provider.
■ Core router/Multilayer switch: This router or multilayer switch resides within
the middle or backbone of the WAN, rather than at its periphery. To fulfill this
role, a router or multilayer switch must be able to support multiple telecommunications
interfaces of the highest speed used in the WAN core. It must also be
able to forward IP packets at full speed on all of those interfaces. The router or
multilayer switch must also support the routing protocols being used in the core.
Note
The preceding list is not exhaustive, and other devices may be required, depending on the
WAN access technology chosen.
WAN technologies are either circuit-switched or packet-switched. The type of device
used depends on the WAN technology implemented.
Circuit Switching (1.1.2.4)
A circuit-switched network is one that establishes a dedicated circuit (or channel)
between nodes and terminals before the users may communicate. Specifically, circuit
switching dynamically establishes a dedicated virtual connection for voice or data
between a sender and a receiver. Before communication can start, it is necessary to establish
the connection through the network of the service provider, as shown in Figure 1-15.
As an example, when a subscriber makes a telephone call, the dialed number is used
to set switches in the exchanges along the route of the call so that there is a continuous
circuit from the caller to the called party. Because of the switching operation
used to establish the circuit, the telephone system is called a circuit-switched
network. If the telephones are replaced with modems, the switched circuit is able to
carry computer data.
18 Connecting Networks v6 Companion Guide
Figure 1-15 Circuit-Switched Network
If the circuit carries computer data, the usage of this fixed capacity may not
be efficient. For example, if the circuit is used to access the Internet, a burst of
activity occurs on the circuit while a web page is transferred. This burst could be
followed by no activity while the user reads the page and then another burst of
activity while the next page is transferred. This variation in usage between none and
maximum is typical of computer network traffic. Because the subscriber has sole
use of the fixed capacity allocation, switched circuits are generally an inefficient
way of moving data.
The two most common types of circuit-switched WAN technologies are the
public switched telephone network (PSTN) and the Integrated Services Digital
Network (ISDN).
Packet Switching (1.1.2.5)
In contrast to circuit switching, a packet-switched network (PSN) splits traffic data
into packets that are routed over a shared network. Packet-switching networks do not
require a circuit to be established, and they allow many pairs of nodes to communicate
over the same channel.
The switches in a PSN determine the links that packets must be sent over based on
the addressing information in each packet. The following are two approaches to this
link determination:
■ Connectionless systems: Full addressing information must be carried in each
packet. Each switch must evaluate the address to determine where to send the
packet. An example of a connectionless system is the Internet.
■ Connection-oriented systems: The network predetermines the route for a
packet, and each packet only has to carry an identifier. The switch determines
Chapter 1: WAN Concepts 19
the onward route by looking up the identifier in tables held in memory. The
set of entries in the tables identifies a particular route or circuit through the
system. When the circuit is established temporarily while a packet is traveling
through it and then breaks down again, it is called a virtual circuit (VC).
An example of a connection-oriented system is Frame Relay. In the case
of Frame Relay, the identifiers used are called data-link connection
identifiers (DLCIs).
Note
Frame Relay systems are commonly being replaced by Ethernet WANs.
Because the internal links between the switches are shared between many users, the
cost of packet switching is lower than that of circuit switching. However, latency
(delays) and jitter (variability of delay) are greater in packet-switched networks than
in circuit-switched networks. The reason is that the links are shared, and packets must
be entirely received at one switch before moving to the next. Despite the latency and
jitter inherent in shared networks, modern technology allows satisfactory transport
of voice and video communications on these networks.
In Figure 1-16, SRV1 is sending data to SRV2. As packets traverse the provider network,
they arrive at the first provider switch. Packets are added to the queue and forwarded
after other packets in the queue have been forwarded. Eventually, the packets
reach SRV2.
Figure 1-16 Packet-Switched Network
20 Connecting Networks v6 Companion Guide
Activity 1.1.2.6: Identify WAN Terminology
Refer to the online course to complete this activity.
Selecting a WAN Technology (1.2)
In this section, you learn how to select WAN access technologies to satisfy business
requirements.
WAN Services (1.2.1)
In this topic, you learn about different WAN services available.
WAN Link Connection Options (1.2.1.1)
ISPs can use are several WAN access connection options to connect the local loop to
the enterprise edge. These WAN access options differ in technology, speed, and cost.
Each has distinct advantages and disadvantages. Familiarity with these technologies
is an important part of network design.
As shown in Figure 1-17 and described in the list that follows, an enterprise can get
WAN access in two ways.
Figure 1-17 WAN Access Options
Interactive
Graphic
Chapter 1: WAN Concepts 21
■ Private WAN infrastructure: Service providers may offer dedicated
point-to-point leased lines, circuit-switched links, such as PSTN or ISDN,
and packet-switched links, such as Ethernet WAN, ATM, or Frame Relay.
■ Public WAN infrastructure: Service providers provide Internet access using
broadband services such as DSL, cable, and satellite access. Broadband
connections are typically used to connect small offices and telecommuting
employees to a corporate site over the Internet. Data traveling between corporate
sites over the public WAN infrastructure should be protected using VPNs.
Note
Frame Relay systems are commonly being replaced by Ethernet WANs.
The topology in Figure 1-18 illustrates some of these WAN access technologies.
Figure 1-18 WAN Access Technologies
Service Provider Network Infrastructure (1.2.1.2)
When a WAN service provider receives data from a client at a site, it must forward
the data to the remote site for final delivery to the recipient. In some cases, the
remote site may be connected to the same service provider as the originating site. In
other cases, the remote site may be connected to a different ISP, and the originating
ISP must pass the data to the connecting ISP.
22 Connecting Networks v6 Companion Guide
Long-range communications are usually those connections between ISPs, or between
branch offices in very large companies.
Service provider networks are complex. They consist mostly of high-bandwidth
fiber-optic media, using either the Synchronous Optical Networking (SONET)
or Synchronous Digital Hierarchy (SDH) standard. These standards define how
to transfer multiple data, voice, and video traffic over optical fiber using lasers or
light-emitting diodes (LEDs) over great distances.
Note
SONET is an American-based ANSI standard, while SDH is a European-based ETSI and ITU
standard. Both are essentially the same and, therefore, often listed as SONET/SDH.
A newer fiber-optic media development for long-range communications is called
dense wavelength division multiplexing (DWDM). DWDM multiplies the
amount of bandwidth that a single strand of fiber can support, as illustrated
in Figure 1-19.
Figure 1-19 DWDM
DWDM enables long-range communication in several ways:
■ DWDM enables bidirectional (for example, two-way) communications over one
strand of fiber.
■ It can multiplex more than 80 different channels of data (that is, wavelengths)
onto a single fiber.
Chapter 1: WAN Concepts 23
■ Each channel is capable of carrying a 10 Gb/s multiplexed signal.
■ It assigns incoming optical signals to specific wavelengths of light (that is,
frequencies).
■ It can amplify these wavelengths to boost the signal strength.
■ It supports SONET and SDH standards.
DWDM circuits are used in all modern submarine communications cable systems and
other long-haul circuits, as illustrated in Figure 1-20.
Figure 1-20 Service Provider Networks Use DWDM
Activity 1.2.1.3: Classify WAN Access Options
Refer to the online course to complete this activity.
Private WAN Infrastructures (1.2.2)
In this topic, you compare private WAN technologies.
Leased Lines (1.2.2.1)
When permanent dedicated connections are required, a point-to-point link is used to
provide a pre-established WAN communications path from the customer premises to
Interactive
Graphic
24 Connecting Networks v6 Companion Guide
the provider network. Point-to-point lines are usually leased from a service provider
and are called leased lines.
Leased lines have existed since the early 1950s; for this reason, they are referred to
by different names such as leased circuits, serial link, serial line, point-to-point link,
and T1/E1 or T3/E3 lines.
The term leased line refers to the fact that the organization pays a monthly lease fee
to a service provider to use the line. Leased lines are available in different capacities
and are generally priced based on the bandwidth required and the distance between
the two connected points.
In North America, service providers use the T-carrier system to define the digital
transmission capability of a serial copper media link, while Europe uses the E-carrier
system, as shown in Figure 1-21. For instance, a T1 link supports 1.544 Mb/s, an
E1 supports 2.048 Mb/s, a T3 supports 43.7 Mb/s, and an E3 connection supports
34.368 Mb/s. Optical carrier (OC) transmission rates are used to define the digital
transmitting capacity of a fiber-optic network.
Figure 1-21 Sample Leased-Line Topology
Table 1-1 describes the advantages and disadvantages of using leased lines.
Table 1-1 Advantages/Disadvantages of Leased Lines
Advantages Disadvantages
Simplicity: Point-to-point communication
links require minimal expertise to install
and maintain.
Cost: Point-to-point links are generally the
most expensive type of WAN access. The
cost of leased-line solutions can become
significant when they are used to connect
many sites over increasing distances.
In addition, each endpoint requires an
interface on the router, which increases
equipment costs.
Chapter 1: WAN Concepts 25
Advantages Disadvantages
Quality: Point-to-point communication links
usually offer high service quality, if they
have adequate bandwidth. The dedicated
capacity removes latency or jitter between
the endpoints.
Limited flexibility: WAN traffic is often
variable, and leased lines have a fixed
capacity, so the bandwidth of the line seldom
matches the need exactly. Any change to the
leased line generally requires a site visit by
ISP personnel to adjust capacity.
Availability: Constant availability is essential
for some applications, such as e-commerce.
Point-to-point communication links provide
permanent, dedicated capacity, which is
required for VoIP or Video over IP.
The Layer 2 protocol is usually HDLC or PPP.
Dialup (1.2.2.2)
Dialup WAN access may be required when no other WAN technology is available. For
example, a remote location could use modems and analog dialed telephone lines to provide
low capacity and dedicated switched connections, as shown in Figure 1-22. Dialup
access is suitable when intermittent, low-volume data transfers are needed.
Figure 1-22 Sample Dialup Topology
Traditional telephony uses a copper cable, called the local loop, to connect the telephone
handset in the subscriber premises to the CO. The signal on the local loop
26 Connecting Networks v6 Companion Guide
during a call is a continuously varying electronic signal that is a translation of the
subscriber voice into an analog signal.
Traditional local loops can transport binary computer data through the voice telephone
network using a dialup modem. The modem modulates the binary data into an
analog signal at the source and demodulates the analog signal to binary data at the
destination. The physical characteristics of the local loop and its connection to the
PSTN limit the rate of the signal to less than 56 kb/s.
For small businesses, these relatively low-speed dialup connections are adequate
for the exchange of sales figures, prices, routine reports, and email. Using automatic
dialup at night or on weekends for large file transfers and data backup can take
advantage of lower off-peak rates. These rates, often referred to as tariffs or toll
charges, are based on the distance between the endpoints, time of day, and the duration
of the call.
The advantages of modem and analog lines are simplicity, availability, and low
implementation cost. The disadvantages are the low data rates and a relatively
long connection time. The dedicated circuit has little delay or jitter for
point-to-point traffic, but voice or video traffic does not operate adequately at
these low bit rates.
Note
Although very few enterprises support dialup access, it is still a viable solution for remote
areas with limited WAN access options.
ISDN (1.2.2.3)
Integrated Services Digital Network (ISDN) is a circuit-switching technology that
enables the local loop of a PSTN to carry digital signals, resulting in higher capacity
switched connections.
ISDN changes the internal connections of the PSTN from carrying analog signals to
time-division multiplexed (TDM) digital signals. TDM allows two or more signals,
or bit streams, to be transferred as subchannels in one communication channel. The
signals appear to transfer simultaneously; but physically, the signals are taking turns
on the channel.
Figure 1-23 displays a sample ISDN topology. The ISDN connection may require a
terminal adapter (TA), which is a device used to connect ISDN Basic Rate Interface
(BRI) connections to a router.
Chapter 1: WAN Concepts 27
Figure 1-23 Sample ISDN Topology
The two types of ISDN interfaces are as follows:
■ Basic Rate Interface (BRI): ISDN BRI is intended for the home and small enterprise
and provides two 64 kb/s bearer channels (B) for carrying voice and data
and a 16 kb/s delta channel (D) for signaling, call setup, and other purposes. The
BRI D channel is often underused because it has only two B channels to control
(see Figure 1-24).
Figure 1-24 ISDN BRI
■ Primary Rate Interface (PRI): ISDN is also available for larger installations.
In North America, PRI delivers 23 B channels with 64 kb/s and one D channel
with 64 kb/s for a total bit rate of up to 1.544 Mb/s. This includes some
additional overhead for synchronization. In Europe, Australia, and other
parts of the world, ISDN PRI provides 30 B channels and one D channel,
for a total bit rate of up to 2.048 Mb/s, including synchronization overhead
(see Figure 1-25).
28 Connecting Networks v6 Companion Guide
Figure 1-25 ISDN PRI
BRI has a call setup time that is less than a second, and the 64 kb/s B channel provides
greater capacity than an analog modem link. In comparison, the call setup time
of a dialup modem is approximately 30 or more seconds with a theoretical maximum
of 56 kb/s. With ISDN, if greater capacity is required, a second B channel can be
activated to provide a total of 128 kb/s. This permits several simultaneous voice conversations,
a voice conversation and data transfer, or a video conference using one
channel for voice and the other for video.
Another common application of ISDN is to provide additional capacity as needed on
a leased-line connection. The leased line is sized to carry average traffic loads while
ISDN is added during peak demand periods. ISDN is also used as a backup if the
leased line fails. ISDN tariffs are based on a per-B channel basis and are similar to
those of analog voice connections.
With PRI ISDN, multiple B channels can be connected between two endpoints. This
allows for videoconferencing and high-bandwidth data connections with no latency
or jitter. However, multiple connections can be very expensive over long distances.
Note
Although ISDN is still an important technology for telephone service provider networks, it has
declined in popularity as an Internet connection option with the introduction of high-speed
DSL and other broadband services.
Frame Relay (1.2.2.4)
Frame Relay is a simple Layer 2 nonbroadcast multi-access (NBMA) WAN technology
used to interconnect enterprise LANs. A single router interface can be used to
connect to multiple sites using permanent virtual circuits (PVCs). PVCs are used to
carry both voice and data traffic between a source and destination, and support data
rates up to 4 Mb/s, with some providers offering even higher rates.
An edge router requires only a single interface, even when multiple VCs are used.
The leased line to the Frame Relay network edge allows cost-effective connections
between widely scattered LANs.
Chapter 1: WAN Concepts 29
Frame Relay creates PVCs, which are uniquely identified by a data-link connection
identifier (DLCI). The PVCs and DLCIs ensure bidirectional communication from one
DTE device to another.
For instance, in Figure 1-26, R1 will use DLCI 102 to reach R2 while R2 will use
DLCI 201 to reach R1.
Figure 1-26 Sample Frame Relay Topology
ATM (1.2.2.5)
Asynchronous Transfer Mode (ATM) technology is capable of transferring voice,
video, and data through private and public networks. It is built on a cell-based
architecture rather than on a frame-based architecture. ATM cells are always a
fixed length of 53 bytes. The ATM cell contains a 5-byte ATM header followed
by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying
voice and video traffic because this traffic is intolerant of delay. Video and voice
traffic do not have to wait for larger data packets to be transmitted, as shown in
Figure 1-27.
The 53-byte ATM cell is less efficient than the bigger frames and packets of Frame
Relay. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte
payload. When the cell is carrying segmented network layer packets, the overhead
is higher because the ATM switch must be able to reassemble the packets at the
30 Connecting Networks v6 Companion Guide
destination. A typical ATM line needs almost 20 percent greater bandwidth than
Frame Relay to carry the same volume of network layer data.
Figure 1-27 Sample ATM Topology
ATM was designed to be extremely scalable and to support link speeds of T1/E1 to
OC-12 (622 Mb/s) and faster.
As with other shared technologies, ATM allows multiple VCs on a single leased-line
connection to the network edge.
Note
ATM networks are now considered to be a a legacy technology.
Ethernet WAN (1.2.2.6)
Ethernet was originally developed to be a LAN access technology. Originally, Ethernet
was not suitable as a WAN access technology because at that time, the maximum
cable length was one kilometer. However, newer Ethernet standards using fiber-optic
cables have made Ethernet a reasonable WAN access option. For instance, the IEEE
1000BASE-LX standard supports fiber-optic cable lengths of 5 km, while the IEEE
1000BASE-ZX standard supports cable lengths up to 70 km.
Service providers now offer Ethernet WAN service using fiber-optic cabling. The
Ethernet WAN service can go by many names, including Metropolitan Ethernet
Chapter 1: WAN Concepts 31
(MetroE), Ethernet over MPLS (EoMPLS), and Virtual Private LAN Service
(VPLS). A sample Ethernet WAN topology is shown in Figure 1-28.
Figure 1-28 Sample Ethernet WAN Topology
An Ethernet WAN offers several benefits:
■ Reduced expenses and administration: Ethernet WAN provides a switched,
high-bandwidth Layer 2 network capable of managing data, voice, and video all
on the same infrastructure. This characteristic increases bandwidth and eliminates
expensive conversions to other WAN technologies. The technology enables businesses
to inexpensively connect numerous sites in a metropolitan area, to each
other, and to the Internet.
■ Easy integration with existing networks: Ethernet WAN connects easily to
existing Ethernet LANs, reducing installation costs and time.
■ Enhanced business productivity: Ethernet WAN enables businesses to take
advantage of productivity-enhancing IP applications that are difficult to implement
on TDM or Frame Relay networks, such as hosted IP communications,
VoIP, and streaming and broadcast video.
Note
Ethernet WANs have gained in popularity and are now commonly being used to replace the
traditional Frame Relay and ATM WAN links.
32 Connecting Networks v6 Companion Guide
MPLS (1.2.2.7)
Multiprotocol Label Switching (MPLS) is a multiprotocol high-performance WAN
technology that directs data from one router to the next. MPLS is based on short
path labels rather than IP network addresses.
MPLS has several defining characteristics. It is multiprotocol, meaning it has the ability
to carry any payload including IPv4, IPv6, Ethernet, ATM, DSL, and Frame Relay
traffic. It uses labels that tell a router what to do with a packet. The labels identify
paths between distant routers rather than endpoints, and while MPLS actually routes
IPv4 and IPv6 packets, everything else is switched.
MPLS is a service provider technology. Leased lines deliver bits between sites, and
Frame Relay and Ethernet WAN deliver frames between sites. However, MPLS can
deliver any type of packet between sites. MPLS can encapsulate packets of various
network protocols. It supports a wide range of WAN technologies including
T-carrier/E-carrier links, Carrier Ethernet, ATM, Frame Relay, and DSL.
The sample topology in Figure 1-29 illustrates how MPLS is used. Notice that the
different sites can connect to the MPLS cloud using different access technologies.
Figure 1-29 Sample MPLS Topology
In the Figure 1-29, CE refers to the customer edge; PE is the provider edge router,
which adds and removes labels; and P is an internal provider router, which switches
MPLS labeled packets.
VSAT (1.2.2.8)
All private WAN technologies discussed so far used either copper or fiber-optic
media. What if an organization needed connectivity in a remote location where no
service providers offer WAN service?
Chapter 1: WAN Concepts 33
Very small aperture terminal (VSAT) is a solution that creates a private WAN
using satellite communications. A VSAT is a small satellite dish similar to those used
for home Internet and TV. VSATs create a private WAN while providing connectivity
to remote locations.
Specifically, a router connects to a satellite dish that is pointed to a service provider’s
satellite. This satellite is in geosynchronous orbit in space. The signals must travel
approximately 35,786 kilometers (22,236 miles) to the satellite and back.
The example in Figure 1-30 displays a VSAT dish on the roofs of the buildings communicating
with a satellite thousands of kilometers away in space.
Figure 1-30 Sample VSAT Topology
Activity 1.2.2.9: Identify Private WAN Infrastructure Terminology
Refer to the online course to complete this activity.
Public WAN Infrastructure (1.2.3)
In this topic, you compare public WAN technologies.
Interactive
Graphic
34 Connecting Networks v6 Companion Guide
DSL (1.2.3.1)
DSL technology is an always-on connection technology that uses existing twistedpair
telephone lines to transport high-bandwidth data, and provides IP services to
subscribers. A DSL modem converts an Ethernet signal from the user device to a
DSL signal, which is transmitted to the central office.
Multiple DSL subscriber lines are multiplexed into a single, high-capacity link using
a DSL access multiplexer (DSLAM) at the provider location referred to as the point
of presence (POP). DSLAMs incorporate TDM technology to aggregate many subscriber
lines into a single medium, generally a T3 connection. Current DSL technologies
use sophisticated coding and modulation techniques to achieve fast data rates.
There is a wide variety of DSL types, standards, and emerging standards. DSL is now
a popular choice for enterprise IT departments to support home workers. Generally,
a subscriber cannot choose to connect to an enterprise network directly but must
first connect to an ISP, and then an IP connection is made through the Internet to the
enterprise. Security risks are incurred in this process but can be mediated with security
measures.
The topology in Figure 1-31 displays a sample DSL WAN connection.
Figure 1-31 Sample DSL Topology
Chapter 1: WAN Concepts 35
Cable (1.2.3.2)
Coaxial cable is widely used in urban areas to distribute television signals. Network
access is available from many cable television providers. This access allows for greater
bandwidth than the conventional telephone local loop.
Cable modems (CMs) provide an always-on connection and a simple installation. A
subscriber connects a computer or LAN router to the cable modem, which translates
the digital signals into the broadband frequencies used for transmitting on a cable
television network. The local cable TV office, which is called the cable headend,
contains the computer system and databases needed to provide Internet access. The
most important component located at the headend is the cable modem termination
system (CMTS), which sends and receives digital cable modem signals on a cable
network and is necessary for providing Internet services to cable subscribers.
Cable modem subscribers must use the ISP associated with the service provider. All
the local subscribers share the same cable bandwidth. As more users join the service,
available bandwidth may drop below the expected rate.
The topology in Figure 1-32 displays a sample cable WAN connection.
Figure 1-32 Sample Cable Topology
36 Connecting Networks v6 Companion Guide
Wireless (1.2.3.3)
Wireless technology uses the unlicensed radio spectrum to send and receive data.
The unlicensed spectrum is accessible to anyone who has a wireless router and wireless
technology in the device he or she is using.
Until recently, one limitation of wireless access has been the need to be within the
local transmission range (typically less than 100 feet) of a wireless router or a wireless
modem that has a wired connection to the Internet. The following new developments
in broadband wireless technology are changing this situation:
■ Municipal Wi-Fi: Many cities have begun setting up municipal wireless networks.
Some of these networks provide high-speed Internet access for free or
for substantially less than the price of other broadband services. Others are for
city use only, allowing police and fire departments and other city employees to
do certain aspects of their jobs remotely. To connect to a municipal Wi-Fi, a subscriber
typically needs a wireless modem, which provides a stronger radio and
directional antenna than conventional wireless adapters. Most service providers
provide the necessary equipment for free or for a fee, much like they do with
DSL or cable modems.
■ WiMAX: Worldwide Interoperability for Microwave Access (WiMAX) is a
new technology that is just beginning to come into use. It is described in the
IEEE standard 802.16. WiMAX provides high-speed broadband service with
wireless access and provides broad coverage like a cell phone network rather
than through small Wi-Fi hotspots. WiMAX operates in a similar way to
Wi-Fi, but at higher speeds, over greater distances, and for a greater number of
users. It uses a network of WiMAX towers that are similar to cell phone towers.
To access a WiMAX network, subscribers must subscribe to an ISP with
a WiMAX tower within 30 miles of their location. They also need some type
of WiMAX receiver and a special encryption code to get access to the base
station.
■ Satellite Internet: Typically, rural users use this type of technology where
cable and DSL are not available. A VSAT provides two-way (upload and
download) data communications. The upload speed is about one-tenth of the
500 kb/s download speed. Cable and DSL have higher download speeds, but
satellite systems are about 10 times faster than an analog modem. To access
satellite Internet services, subscribers need a satellite dish, two modems
(uplink and downlink), and coaxial cables between the dish and the modem.
Figure 1-33 displays an example of a WiMAX network.
Chapter 1: WAN Concepts 37
Figure 1-33 Sample Wireless Topology
3G/4G Cellular (1.2.3.4)
Increasingly, cellular service is another wireless WAN technology being used to connect
users and remote locations where no other WAN access technology is available,
as shown in Figure 1-34. Many users with smartphones and tablets can use cellular
data to email, surf the web, download apps, and watch videos.
Figure 1-34 Sample Cellular Topology
Phones, tablet computers, laptops, and even some routers can communicate through
to the Internet using cellular technology. These devices use radio waves to communicate
through a nearby mobile phone tower. The device has a small radio antenna, and
the provider has a much larger antenna sitting at the top of a tower somewhere within
miles of the phone.
38 Connecting Networks v6 Companion Guide
These are two common cellular industry terms:
■ 3G/4G Wireless: Abbreviation for third-generation and fourth-generation
cellular access. These technologies support wireless Internet access.
■ Long-Term Evolution (LTE): Refers to a newer and faster technology and is
considered to be part of fourth-generation (4G) technology.
VPN Technology (1.2.3.5)
Security risks are incurred when a teleworker or a remote office worker uses a
broadband service to access the corporate WAN over the Internet. To address security
concerns, broadband services provide capabilities for using VPN connections to
a network device that accepts VPN connections, which are typically located at the
corporate site.
A VPN is an encrypted connection between private networks over a public network,
such as the Internet. Instead of using a dedicated Layer 2 connection, such as a leased
line, a VPN uses virtual connections called VPN tunnels, which are routed through
the Internet from the private network of the company to the remote site or
employee host.
Using VPN offers several benefits:
■ Cost savings: VPNs enable organizations to use the global Internet to connect
remote offices, and to connect remote users to the main corporate site. This
eliminates expensive, dedicated WAN links and modem banks.
■ Security: VPNs provide the highest level of security by using advanced encryption
and authentication protocols that protect data from unauthorized access.
■ Scalability: Because VPNs use the Internet infrastructure within ISPs and
devices, it is easy to add new users. Corporations are able to add large amounts
of capacity without adding significant infrastructure.
■ Compatibility with broadband technology: VPN technology is supported by
broadband service providers such as DSL and cable. VPNs allow mobile workers
and telecommuters to take advantage of their home high-speed Internet service
to access their corporate networks. Business-grade, high-speed broadband connections
can also provide a cost-effective solution for connecting remote offices.
There are two types of VPN access:
■ Site-to-site VPNs: Site-to-site VPNs connect entire networks to each other; for
example, they can connect a branch office network to a company headquarters
network, as shown in Figure 1-35. Each site is equipped with a VPN gateway,
Chapter 1: WAN Concepts 39
such as a router, firewall, VPN concentrator, or security appliance. In the
Figure 1-35, a remote branch office uses a site-to-site-VPN to connect with the
corporate head office.
Figure 1-35 Sample Site-to-Site VPN Topology
■ Remote-access VPNs: Remote-access VPNs enable individual hosts, such as
telecommuters, mobile users, and extranet consumers, to access a company
network securely over the Internet. Each host (Teleworker 1 and Teleworker 2)
typically has VPN client software loaded or uses a web-based client, as shown
in Figure 1-36.
Activity 1.2.3.6: Identify Public WAN Infrastructure Terminology
Refer to the online course to complete this activity.
Interactive
Graphic
40 Connecting Networks v6 Companion Guide
Figure 1-36 Sample Remote-Access VPN Topology
Selecting WAN Services (1.2.4)
In this topic, you learn how to select the appropriate WAN protocol and service for a
specific network requirement.
Choosing a WAN Link Connection (1.2.4.1)
There are many important factors to consider when choosing an appropriate WAN
connection. For a network administrator to decide which WAN technology best
meets the requirements of a specific business, he or she must answer the following
questions:
What is the purpose of the WAN?
There are a few issues to consider:
■ Will the enterprise connect local branches in the same city area, connect remote
branches, or connect to a single branch?
■ Will the WAN be used to connect internal employees, or external business partners
and customers, or all three?
■ Will the enterprise connect to customers, connect to business partners, connect
to employees, or some combination of these?
■ Will the WAN provide authorized users limited or full access to the company
intranet?
Chapter 1: WAN Concepts 41
What is the geographic scope?
There are a few issues to consider:
■ Is the WAN local, regional, or global?
■ Is the WAN one-to-one (single branch), one-to-many branches, or many-to-many
(distributed)?
What are the traffic requirements?
There are a few issues to consider:
■ What type of traffic must be supported (data only, VoIP, video, large files,
streaming files)? This determines the quality and performance requirements.
■ What volume of traffic type (voice, video, or data) must be supported for each
destination? This determines the bandwidth capacity required for the WAN connection
to the ISP.
■ What Quality of Service is required? This may limit the choices. If the traffic is
highly sensitive to latency and jitter, eliminate any WAN connection options that
cannot provide the required quality.
■ What are the security requirements (data integrity, confidentiality, and security)?
These are important factors if the traffic is of a highly confidential nature, or if it
provides essential services, such as emergency response.
Choosing a WAN Link Connection (Cont.) (1.2.4.2)
In addition to gathering information about the scope of the WAN, the administrator
must also determine the following:
■ Should the WAN use a private or public infrastructure? A private infrastructure
offers the best security and confidentiality, whereas the public Internet
infrastructure offers the most flexibility and lowest ongoing expense. The choice
depends on the purpose of the WAN, the types of traffic it carries, and available
operating budget. For example, if the purpose is to provide a nearby branch with
high-speed secure services, a private dedicated or switched connection may be
best. If the purpose is to connect many remote offices, a public WAN using the
Internet may be the best choice. For distributed operations, a combination of
options may be the solution.
■ For a private WAN, should it be dedicated or switched? Real-time, highvolume
transactions have special requirements that could favor a dedicated line,
such as traffic flowing between the data center and the corporate head office. If
the enterprise is connecting to a local single branch, a dedicated leased line could
42 Connecting Networks v6 Companion Guide
be used. However, that option would become very expensive for a WAN connecting
multiple offices. In that case, a switched connection might be better.
■ For a public WAN, what type of VPN access is required? If the purpose of the
WAN is to connect a remote office, a site-to-site VPN may be the best choice. To
connect teleworkers or customers, remote-access VPNs are a better option. If the
WAN is serving a mixture of remote offices, teleworkers, and authorized customers,
such as a global company with distributed operations, a combination of VPN
options may be required.
■ Which connection options are available locally? In some areas, not all WAN
connection options are available. In this case, the selection process is simplified,
although the resulting WAN may provide less than optimal performance.
For example, in a rural or remote area, the only option may be VSAT or
cellular access.
■ What is the cost of the available connection options? Depending on the option
chosen, the WAN can be a significant ongoing expense. The cost of a particular
option must be weighed against how well it meets the other requirements. For
example, a dedicated leased line is the most expensive option, but the expense
may be justified if it is critical to ensure secure transmission of high volumes
of real-time data. For less demanding applications, a less expensive switched or
Internet connection option may be more suitable.
Using the preceding guidelines, as well as those described by the Cisco Enterprise
Architecture, a network administrator should be able to choose an appropriate WAN
connection to meet the requirements of different business scenarios.
Lab 1.2.4.3: Researching WAN Technologies
In this lab, you will complete the following objectives:
Part 1: Investigate Dedicated WAN Technologies and Providers
Part 2: Investigate a Dedicated Leased-Line Service Provider in Your Area
Chapter 1: WAN Concepts 43
Summary (1.3)
Class Activity 1.3.1.1: WAN Device Modules
Your medium-sized company is upgrading its network. To make the most of the
equipment currently in use, you decide to purchase WAN modules instead of
new equipment.
All branch offices use either Cisco 1900 or 2911 series ISRs. You will be updating
these routers in several locations. Each branch has its own ISP requirements to
consider.
To update the devices, focus on the following WAN module access types:
■ Ethernet
■ Broadband
■ T1/E1 and ISDN PRI
■ BRI
■ Serial
■ T1 and E1 Trunk Voice and WAN
■ Wireless LANs and WANs
A business can use private lines or the public network infrastructure for WAN connections.
A public infrastructure connection can be a cost-effective alternative to a
private connection between LANs, as long as security is also planned.
WAN access standards operate at Layers 1 and 2 of the OSI model, and are defined
and managed by the TIA/EIA, ISO, and IEEE. A WAN may be circuit-switched or
packet-switched.
There is common terminology used to identify the physical components of WAN
connections and who, the service provider or the customer, is responsible for which
components.
Service provider networks are complex, and the service provider’s backbone networks
consist primarily of high-bandwidth fiber-optic media. The device used for interconnection
to a customer is specific to the WAN technology that is implemented.
Permanent, dedicated point-to-point connections are provided by using leased
lines. Dialup access, although slow, is still viable for remote areas with limited WAN
options. Other private connection options include ISDN, Frame Relay, ATM, Ethernet
WAN, MPLS, and VSAT.
44 Connecting Networks v6 Companion Guide
Public infrastructure connections include DSL, cable, wireless, and 3G/4G cellular.
Security over public infrastructure connections can be provided by using remoteaccess
or site-to-site VPNs.


Objectives
Upon completion of this chapter, you will be able to answer the following questions:
■ What is the purpose of a WAN?
■ How do WANs operate?
■ What WAN services are available?
■ What are the differences between private
WAN technologies?
■ What are the differences between public
WAN technologies?
■ What is the appropriate WAN protocol and
service for a specific network requirement?
Key Terms
This chapter uses the following key terms. You can find the definitions in the Glossary.
2 Connecting Networks v6 Companion Guide
last-mile Page 15
central office (CO) Page 15
toll network Page 16
dialup modem Page 16
modulates Page 16
demodulates Page 16
Access server Page 16
broadband modem Page 16
channel service unit/data service unit
(CSU/DSU) Page 17
WAN switch Page 17
circuit-switched network Page 17
public switched telephone network
(PSTN) Page 18
Integrated Services Digital Network
(ISDN) Page 18
packet-switched network (PSN) Page 18
virtual circuit (VC) Page 19
data-link connection identifiers
(DLCIs) Page 19
latency Page 19
jitter Page 19
private WAN infrastructure Page 21
public WAN infrastructure Page 21
broadband connections Page 21
Synchronous Optical Networking
(SONET) Page 22
Synchronous Digital Hierarchy (SDH) Page 22
light-emitting diodes (LEDs) Page 22
dense wavelength division multiplexing
(DWDM) Page 22
multiplex Page 22
E3 Page 24
optical carrier (OC) Page 24
time-division multiplexed (TDM) Page 26
Basic Rate Interface (BRI) Page 26
Primary Rate Interface (PRI) Page 27
nonbroadcast multi-access (NBMA) Page 28
permanent virtual circuits (PVCs) Page 28
Metropolitan Ethernet (MetroE) Page 30
Ethernet over MPLS (EoMPLS) Page 31
Virtual Private LAN Service (VPLS) Page 31
Multiprotocol Label Switching (MPLS) Page 32
very small aperture terminal (VSAT) Page 33
DSL modem Page 34
DSL access multiplexer (DSLAM) Page 34
point of presence (POP) Page 34
cable modems (CM) Page 35
headend Page 35
cable modem termination system
(CMTS) Page 35
municipal Wi-Fi Page 36
WiMAX Page 36
satellite Internet Page 36
3G/4G wireless Page 38
Long-Term Evolution (LTE) Page 38
teleworker Page 38
site-to-site VPNs Page 38
remote-access VPNs Page 39
Chapter 1: WAN Concepts 3
Introduction (1.0)
Businesses must connect LANs to provide communications between them, even
when these LANs are far apart. Wide-area networks (WANs) are used to connect
remote LANs. A WAN may cover a city, country, or global region. A WAN is owned
by a service provider, and a business pays a fee to use the provider’s WAN network
services.
Different technologies are used for WANs than for LANs. This chapter introduces
WAN standards, technologies, and purposes. It covers selecting the appropriate WAN
technologies, services, and devices to meet the changing business requirements of an
evolving enterprise.
Class Activity 1.0.1.2: Branching Out
Your medium-sized company is opening a new branch office to serve a wider, clientbased
network. This branch will focus on regular, day-to-day network operations but
will also provide TelePresence, web conferencing, IP telephony, video on demand, and
wireless services.
Although you know that an ISP can provide WAN routers and switches to accommodate
the branch office connectivity for the network, you prefer to use your own
customer premises equipment (CPE). To ensure interoperability, Cisco devices have
been used in all other branch-office WANs.
As the branch-office network administrator, you are responsible for researching possible
network devices for purchase and use over the WAN.
WAN Technologies Overview (1.1)
In this section, you learn about WAN access technologies available to small- to
medium-sized business networks.
Purpose of WANs (1.1.1)
In this topic, you learn the purpose of the WAN.
Why a WAN? (1.1.1.1)
A WAN operates beyond the geographic scope of a LAN. As shown Figure 1-1,
WANs are used to interconnect the enterprise LAN to remote LANs in branch sites
and telecommuter sites.
4 Connecting Networks v6 Companion Guide
Figure 1-1 WANs Interconnect Users and LANs
A WAN is owned by a service provider. A user must pay a fee to use the provider’s
network services to connect remote sites. WAN service providers include carriers, such
as a telephone network, cable company, or satellite service. Service providers provide
links to interconnect remote sites for the purpose of transporting data, voice, and video.
In contrast, LANs are typically owned by an organization. They are used to connect
local computers, peripherals, and other devices within a single building or other small
geographic area.
Are WANs Necessary? (1.1.1.2)
Without WANs, LANs would be a series of isolated networks. LANs provide both
speed and cost-efficiency for transmitting data over relatively small geographic areas.
However, as organizations expand, businesses require communication among geographically
separated sites. The following are some examples:
■ Regional or branch offices of an organization need to be able to communicate
and share data with the central site.
■ Organizations need to share information with other customer organizations.
For example, software manufacturers routinely communicate product and
promotional information to distributors that sell their products to end users.
■ Employees who travel on company business frequently need to access
information that resides on their corporate networks.
Chapter 1: WAN Concepts 5
Home computer users also need to send and receive data across increasingly larger
distances. Here are some examples:
■ Consumers now commonly communicate over the Internet with banks, stores,
and a variety of providers of goods and services.
■ Students do research for classes by accessing library indexes and publications
located in other parts of their country and in other parts of the world.
It is not feasible to connect computers across a country, or around the world, with
physical cables. Therefore, different technologies have evolved to support this communication
requirement. Increasingly, the Internet is being used as an inexpensive
alternative to enterprise WANs. New technologies are available to businesses to provide
security and privacy for their Internet communications and transactions. WANs
used by themselves, or in concert with the Internet, allow organizations and individuals
to meet their wide-area communication needs.
WAN Topologies (1.1.1.3)
Interconnecting multiple sites across WANs can involve a variety of service provider
technologies and WAN topologies. Common WAN topologies are
■ Point-to-point topology
■ Hub-and-spoke topology
■ Full mesh topology
■ Dual-homed topology
Point-to-Point
A point-to-point topology, as shown in Figure 1-2, employs a point-to-point circuit
between two endpoints. Typically involving dedicated leased-line connections like
a T1 or an E1 line, a point-to-point connection provides a Layer 2 transport service
through the service provider network. Packets sent from one site are delivered to the
other site and vice versa. A point-to-point connection is transparent to the customer
network, as if there was a direct physical link between two endpoints.
Figure 1-2 Point-to-Point Topology
6 Connecting Networks v6 Companion Guide
Hub-and-Spoke
If a private network connection between multiple sites is required, a point-to-point
topology with multiple point-to-point circuits is one option. Each point-to-point
circuit requires its own dedicated hardware interface that will require multiple
routers with multiple WAN interface cards. This interface can be expensive. A less
expensive option is a point-to-multipoint topology, also known as a hub-and-spoke
topology.
With a hub-and-spoke topology, all spoke circuits can share a single interface to the
hub. For example, spoke sites can be interconnected through the hub site using virtual
circuits and routed subinterfaces at the hub. A hub-and-spoke topology is also
an example of a single-homed topology. Figure 1-3 displays a sample hub-and-spoke
topology consisting of four routers with one router as a hub connected to the other
three spoke routers across a WAN cloud.
Figure 1-3 Hub-and-Spoke Topology
Full Mesh
One of the disadvantages of hub-and-spoke topologies is that all communication
has to go through the hub. With a full mesh topology using virtual circuits, any site
can communicate directly with any other site. The disadvantage here is the large
number of virtual circuits that need to be configured and maintained. Figure 1-4
displays a sample full mesh topology consisting of four routers connected to each
other across a WAN cloud.
Dual-homed Topology
A dual-homed topology provides redundancy. As shown in Figure 1-5, the spoke
routers are dual-homed and redundantly attached to two hub routers across a WAN
Chapter 1: WAN Concepts 7
cloud. The disadvantage to dual-homed topologies is that they are more expensive
to implement than a single-homed topology. The reason is that they require
additional networking hardware, like additional routers and switches. Dual-homed
topologies are also more difficult to implement because they require additional,
and more complex, configurations. However, the advantage of dual-homed topologies
is that they offer enhanced network redundancy, load balancing, distributed
computing or processing, and the ability to implement backup service provider
connections.
Figure 1-4 Full Mesh Topology
Figure 1-5 Dual-Homed Topology
8 Connecting Networks v6 Companion Guide
Evolving Networks (1.1.1.4)
Every business is unique, so how an organization grows depends on many factors.
These factors include the types of products or services the business sells, the management
philosophy of the owners, and the economic climate of the country in
which the business operates.
In slow economic times, many businesses focus on increasing their profitability by
improving the efficiency of their existing operations, increasing employee productivity,
and lowering operating costs. Establishing and managing networks can represent
significant installation and operating expenses. To justify such a large expense,
companies expect their networks to perform optimally and to be able to deliver
an ever-increasing array of services and applications to support productivity and
profitability.
The example used in this chapter and shown in Figure 1-6 is of a fictitious company
called SPAN Engineering. This topic will illustrate how SPAN’s network requirements
change as the company grows from a small, local business into a global enterprise.
Figure 1-6 SPAN Engineering
Small Office (1.1.1.5)
SPAN Engineering, an environmental consulting firm, has developed a special process
for converting household waste into electricity and is developing a small pilot project
for a municipal government in its local area. The company, which has been in business
for four years, is a small office consisting of 15 employees: six engineers, four
computer-aided drawing (CAD) designers, a receptionist, two senior partners, and
two office assistants.
Chapter 1: WAN Concepts 9
SPAN Engineering’s management is working to win full-scale contracts after the pilot
project successfully demonstrates the feasibility of the company’s process. Until
then, the company must manage its costs carefully.
As shown in Figure 1-7, SPAN Engineering uses a single LAN to share information
between computers and to share peripherals, such as a printer, a large-scale plotter
(to print engineering drawings), and fax equipment.
Figure 1-7 Connecting a Small Office
The company has recently upgraded its LAN to provide inexpensive voice over IP
(VoIP) service to save on the costs of separate phone lines for employees.
Internet connectivity is provided using a common broadband service called digital
subscriber line (DSL), which is supplied by the local telephone service provider.
Because SPAN has so few employees, bandwidth is not a significant problem.
The company cannot afford in-house IT support staff, so it uses support services purchased
from the DSL provider. The company also uses a hosting service rather than
purchasing and operating its own FTP and email servers.
Campus Network (1.1.1.6)
Five years later, SPAN Engineering has grown rapidly. The company was contracted
to design and implement a full-size waste conversion facility soon after the successful
implementation of its first pilot plant. Since then, SPAN has won other projects in
neighboring municipalities and in other parts of the country.
10 Connecting Networks v6 Companion Guide
To handle the additional workload, the business has hired more staff and leased
more office space. It is now a small- to medium-sized business with several hundred
employees. Many projects are being developed at the same time, and each requires a
project manager and support staff. The company has organized itself into functional
departments, with each department having its own organizational team. To meet its
growing needs, the company has moved into several floors of a larger office building.
As the business has expanded, the network has also grown. Instead of a single small
LAN, the network now consists of several subnetworks, each devoted to a different
department. For example, all the engineering staff is on one LAN, while the marketing
staff is on another LAN. These multiple LANs are joined to create a companywide
network, or campus, which spans several floors of the building.
Figure 1-8 shows an example of SPAN’s campus network.
Figure 1-8 Connecting a Campus Network
The business now has in-house IT staff to support and maintain the network. The
network includes dedicated servers for email, data transfer, and file storage, and webbased
productivity tools and applications. In addition, a company intranet provides
in-house documents and information to employees. An extranet provides project
information to designated customers.
Branch Networks (1.1.1.7)
Another six years later, SPAN Engineering has been so successful with its patented
process that demand for its services has skyrocketed. New projects are underway
Chapter 1: WAN Concepts 11
in multiple cities. To manage those projects, the company has opened small branch
offices closer to the project sites.
This situation presents new challenges to the IT team. To manage the delivery of
information and services throughout the company, SPAN Engineering now has a data
center, which houses the various databases and servers of the company. To ensure
that all parts of the business are able to access the same services and applications
regardless of where the offices are located, the company must now implement
a WAN.
For its branch offices that are in nearby cities, the company decides to use private
dedicated lines through a local service provider, as shown in Figure 1-9. However,
for those offices that are located in other countries, the Internet is an attractive WAN
connection option. Although connecting offices through the Internet is economical,
this approach introduces security and privacy issues that the IT team must address.
Figure 1-9 Connecting Branch Networks
Distributed Network (1.1.1.8)
SPAN Engineering has now been in business for 20 years and has grown to thousands
of employees distributed in offices worldwide, as shown in Figure 1-10.
The cost of the enterprise network and its related services is a significant
expense. The company is looking to provide its employees with the best network services
at the lowest cost. Optimized network services would allow each employee to
work at a high rate of efficiency.
12 Connecting Networks v6 Companion Guide
Figure 1-10 SPAN Engineering
To increase profitability, SPAN Engineering must reduce its operating expenses. It
has relocated some of its office facilities to less expensive areas. The company is
also encouraging teleworking and virtual teams. Web-based applications, including
web conferencing, e-learning, and online collaboration tools, are being used
to increase productivity and reduce costs. Site-to-site and remote-access virtual
private networks (VPNs) enable the company to use the Internet to connect
easily and securely with employees and facilities around the world. To meet these
requirements, the network must provide the necessary converged services and
secure Internet WAN connectivity to remote offices and individuals, as shown
in Figure 1-11.
As seen in this example, network requirements of a company can change dramatically
as the company grows over time. Distributing employees saves costs in many ways,
but it puts increased demands on the network.
A network not only must meet the day-to-day operational needs of the business
but also must be able to adapt and grow as the company changes. Network
designers and administrators meet these challenges by carefully choosing network
technologies, protocols, and service providers. They must also optimize their
networks by using many of the network design techniques and architectures
described in this course.
Activity 1.1.1.9: Identify WAN Topologies
Refer to the online course to complete this activity.
Interactive
Graphic
Chapter 1: WAN Concepts 13
Figure 1-11 Connecting a Global Enterprise Network
WAN Operations (1.1.2)
In this topic, you learn how WANs operate.
WANs in the OSI Model (1.1.2.1)
WAN operations focus primarily on the physical layer (OSI Layer 1) and the data
link layer (OSI Layer 2), as illustrated in Figure 1-12. WAN access standards typically
describe both physical layer delivery methods and data link layer requirements.
The data link layer requirements include physical addressing, flow control, and
encapsulation.
WAN access standards are defined and managed by a number of recognized
authorities:
■ Telecommunications Industry Association (TIA)
■ Electronic Industries Alliance (EIA)
■ International Organization for Standardization (ISO)
■ Institute of Electrical and Electronics Engineers (IEEE)
Layer 1 protocols describe how to provide electrical, mechanical, operational, and
functional connections to the services of a communications service provider.
14 Connecting Networks v6 Companion Guide
Figure 1-12 WANs Operate in Layers 1 and 2
Layer 2 protocols define how data is encapsulated for transmission toward a remote
location and the mechanisms for transferring the resulting frames. A variety of different
technologies are used, such as the Point-to-Point Protocol (PPP), Frame Relay,
and Asynchronous Transfer Mode (ATM). Some of these protocols use the same
basic framing or a subset of the High-Level Data Link Control (HDLC) mechanism.
Most WAN links are point-to-point. For this reason, the address field in the Layer 2
frame is usually not used.
Common WAN Terminology (1.1.2.2)
One primary difference between a WAN and a LAN is that a company or organization
must subscribe to an outside WAN service provider to use WAN carrier network
services. A WAN uses data links provided by carrier services to access the Internet
and connect different locations of an organization to each other. These data links also
connect to locations of other organizations, to external services, and to remote users.
The physical layer of a WAN describes the physical connections between the company
network and the service provider network. Figure 1-13 illustrates the terminology
commonly used to describe WAN connections:
■ Customer premises equipment (CPE): The CPE consists of the devices and
inside wiring located on the enterprise edge connecting to a carrier link. The subscriber
(that is, customer) either owns the CPE or leases the CPE from the service
provider. A subscriber, in this context, is a company that arranges for WAN
services from a service provider.
Chapter 1: WAN Concepts 15
Figure 1-13 WAN Terminology
■ Data communications equipment (DCE): This is an EIA term. Also called data
circuit-terminating equipment by the ITU. The DCE consists of devices that put
data on the local loop. The DCE primarily provides an interface to connect subscribers
to a communication link on the WAN cloud.
■ Data terminal equipment (DTE): These customer devices pass the data from a
customer network or host computer for transmission over the WAN. The DTE
connects to the local loop through the DCE.
■ Demarcation point: This point is established in a building or complex to separate
customer equipment from service provider equipment. Physically, the demarcation
point is the cabling junction box, located on the customer premises, that
connects the CPE wiring to the local loop. It is usually placed so that a technician
can access it easily. The demarcation point is the place where the responsibility
for the connection changes from the user to the service provider. When
problems arise, it is necessary to determine whether the user or the service provider
is responsible for troubleshooting or repair.
■ Local loop: This loop is the actual copper or fiber cable that connects the CPE to
the CO of the service provider. The local loop is also sometimes called the last-mile.
■ Central office (CO): The CO is the local service provider facility or building
that connects the CPE to the provider network.
16 Connecting Networks v6 Companion Guide
■ Toll network: This network consists of the long-haul, all-digital, fiber-optic
communications lines, switches, routers, and other equipment inside the WAN
provider network.
WAN Devices (1.1.2.3)
Many types of devices are specific to WAN environments, as shown in Figure 1-14,
and are described in the list that follows.
Figure 1-14 Common WAN Devices
■ Dialup modem: Voiceband modems are considered to be a legacy WAN technology.
A voiceband modem modulates (that is, converts) the digital signals
produced by a computer into voice frequencies. These frequencies are then transmitted
over the analog lines of the public telephone network. On the other side
of the connection, another modem demodulates the sounds back into a digital
signal for input to a computer or network connection.
■ Access server: This server controls and coordinates dialup modem, dial-in, and
dial-out user communications. Considered to be a legacy technology, an access
server may have a mixture of analog and digital interfaces and support hundreds
of simultaneous users.
■ Broadband modem: This type of digital modem is used with high-speed DSL
or cable Internet service. Both operate in a similar manner to the voiceband
modem but use higher broadband frequencies to achieve higher transmission
speeds.
Chapter 1: WAN Concepts 17
■ Channel service unit/data service unit (CSU/DSU): Digital leased lines require
a CSU and a DSU. A CSU/DSU can be a separate device like a modem, or it can
be an interface on a router. The CSU provides termination for the digital signal and
ensures connection integrity through error correction and line monitoring. The DSU
converts the line frames into frames that the LAN can interpret and vice versa.
■ WAN switch: This multiport internetworking device is used in service provider
networks. These devices typically switch traffic, such as Frame Relay or ATM,
and operate at Layer 2.
■ Router: This device provides internetworking and WAN access interface ports
that are used to connect to the service provider network. These interfaces may be
serial connections, Ethernet, or other WAN interfaces. With some types of WAN
interfaces, an external device, such as a DSU/CSU or modem (analog, cable, or
DSL), is required to connect the router to the local service provider.
■ Core router/Multilayer switch: This router or multilayer switch resides within
the middle or backbone of the WAN, rather than at its periphery. To fulfill this
role, a router or multilayer switch must be able to support multiple telecommunications
interfaces of the highest speed used in the WAN core. It must also be
able to forward IP packets at full speed on all of those interfaces. The router or
multilayer switch must also support the routing protocols being used in the core.
Note
The preceding list is not exhaustive, and other devices may be required, depending on the
WAN access technology chosen.
WAN technologies are either circuit-switched or packet-switched. The type of device
used depends on the WAN technology implemented.
Circuit Switching (1.1.2.4)
A circuit-switched network is one that establishes a dedicated circuit (or channel)
between nodes and terminals before the users may communicate. Specifically, circuit
switching dynamically establishes a dedicated virtual connection for voice or data
between a sender and a receiver. Before communication can start, it is necessary to establish
the connection through the network of the service provider, as shown in Figure 1-15.
As an example, when a subscriber makes a telephone call, the dialed number is used
to set switches in the exchanges along the route of the call so that there is a continuous
circuit from the caller to the called party. Because of the switching operation
used to establish the circuit, the telephone system is called a circuit-switched
network. If the telephones are replaced with modems, the switched circuit is able to
carry computer data.
18 Connecting Networks v6 Companion Guide
Figure 1-15 Circuit-Switched Network
If the circuit carries computer data, the usage of this fixed capacity may not
be efficient. For example, if the circuit is used to access the Internet, a burst of
activity occurs on the circuit while a web page is transferred. This burst could be
followed by no activity while the user reads the page and then another burst of
activity while the next page is transferred. This variation in usage between none and
maximum is typical of computer network traffic. Because the subscriber has sole
use of the fixed capacity allocation, switched circuits are generally an inefficient
way of moving data.
The two most common types of circuit-switched WAN technologies are the
public switched telephone network (PSTN) and the Integrated Services Digital
Network (ISDN).
Packet Switching (1.1.2.5)
In contrast to circuit switching, a packet-switched network (PSN) splits traffic data
into packets that are routed over a shared network. Packet-switching networks do not
require a circuit to be established, and they allow many pairs of nodes to communicate
over the same channel.
The switches in a PSN determine the links that packets must be sent over based on
the addressing information in each packet. The following are two approaches to this
link determination:
■ Connectionless systems: Full addressing information must be carried in each
packet. Each switch must evaluate the address to determine where to send the
packet. An example of a connectionless system is the Internet.
■ Connection-oriented systems: The network predetermines the route for a
packet, and each packet only has to carry an identifier. The switch determines
Chapter 1: WAN Concepts 19
the onward route by looking up the identifier in tables held in memory. The
set of entries in the tables identifies a particular route or circuit through the
system. When the circuit is established temporarily while a packet is traveling
through it and then breaks down again, it is called a virtual circuit (VC).
An example of a connection-oriented system is Frame Relay. In the case
of Frame Relay, the identifiers used are called data-link connection
identifiers (DLCIs).
Note
Frame Relay systems are commonly being replaced by Ethernet WANs.
Because the internal links between the switches are shared between many users, the
cost of packet switching is lower than that of circuit switching. However, latency
(delays) and jitter (variability of delay) are greater in packet-switched networks than
in circuit-switched networks. The reason is that the links are shared, and packets must
be entirely received at one switch before moving to the next. Despite the latency and
jitter inherent in shared networks, modern technology allows satisfactory transport
of voice and video communications on these networks.
In Figure 1-16, SRV1 is sending data to SRV2. As packets traverse the provider network,
they arrive at the first provider switch. Packets are added to the queue and forwarded
after other packets in the queue have been forwarded. Eventually, the packets
reach SRV2.
Figure 1-16 Packet-Switched Network
20 Connecting Networks v6 Companion Guide
Activity 1.1.2.6: Identify WAN Terminology
Refer to the online course to complete this activity.
Selecting a WAN Technology (1.2)
In this section, you learn how to select WAN access technologies to satisfy business
requirements.
WAN Services (1.2.1)
In this topic, you learn about different WAN services available.
WAN Link Connection Options (1.2.1.1)
ISPs can use are several WAN access connection options to connect the local loop to
the enterprise edge. These WAN access options differ in technology, speed, and cost.
Each has distinct advantages and disadvantages. Familiarity with these technologies
is an important part of network design.
As shown in Figure 1-17 and described in the list that follows, an enterprise can get
WAN access in two ways.
Figure 1-17 WAN Access Options
Interactive
Graphic
Chapter 1: WAN Concepts 21
■ Private WAN infrastructure: Service providers may offer dedicated
point-to-point leased lines, circuit-switched links, such as PSTN or ISDN,
and packet-switched links, such as Ethernet WAN, ATM, or Frame Relay.
■ Public WAN infrastructure: Service providers provide Internet access using
broadband services such as DSL, cable, and satellite access. Broadband
connections are typically used to connect small offices and telecommuting
employees to a corporate site over the Internet. Data traveling between corporate
sites over the public WAN infrastructure should be protected using VPNs.
Note
Frame Relay systems are commonly being replaced by Ethernet WANs.
The topology in Figure 1-18 illustrates some of these WAN access technologies.
Figure 1-18 WAN Access Technologies
Service Provider Network Infrastructure (1.2.1.2)
When a WAN service provider receives data from a client at a site, it must forward
the data to the remote site for final delivery to the recipient. In some cases, the
remote site may be connected to the same service provider as the originating site. In
other cases, the remote site may be connected to a different ISP, and the originating
ISP must pass the data to the connecting ISP.
22 Connecting Networks v6 Companion Guide
Long-range communications are usually those connections between ISPs, or between
branch offices in very large companies.
Service provider networks are complex. They consist mostly of high-bandwidth
fiber-optic media, using either the Synchronous Optical Networking (SONET)
or Synchronous Digital Hierarchy (SDH) standard. These standards define how
to transfer multiple data, voice, and video traffic over optical fiber using lasers or
light-emitting diodes (LEDs) over great distances.
Note
SONET is an American-based ANSI standard, while SDH is a European-based ETSI and ITU
standard. Both are essentially the same and, therefore, often listed as SONET/SDH.
A newer fiber-optic media development for long-range communications is called
dense wavelength division multiplexing (DWDM). DWDM multiplies the
amount of bandwidth that a single strand of fiber can support, as illustrated
in Figure 1-19.
Figure 1-19 DWDM
DWDM enables long-range communication in several ways:
■ DWDM enables bidirectional (for example, two-way) communications over one
strand of fiber.
■ It can multiplex more than 80 different channels of data (that is, wavelengths)
onto a single fiber.
Chapter 1: WAN Concepts 23
■ Each channel is capable of carrying a 10 Gb/s multiplexed signal.
■ It assigns incoming optical signals to specific wavelengths of light (that is,
frequencies).
■ It can amplify these wavelengths to boost the signal strength.
■ It supports SONET and SDH standards.
DWDM circuits are used in all modern submarine communications cable systems and
other long-haul circuits, as illustrated in Figure 1-20.
Figure 1-20 Service Provider Networks Use DWDM
Activity 1.2.1.3: Classify WAN Access Options
Refer to the online course to complete this activity.
Private WAN Infrastructures (1.2.2)
In this topic, you compare private WAN technologies.
Leased Lines (1.2.2.1)
When permanent dedicated connections are required, a point-to-point link is used to
provide a pre-established WAN communications path from the customer premises to
Interactive
Graphic
24 Connecting Networks v6 Companion Guide
the provider network. Point-to-point lines are usually leased from a service provider
and are called leased lines.
Leased lines have existed since the early 1950s; for this reason, they are referred to
by different names such as leased circuits, serial link, serial line, point-to-point link,
and T1/E1 or T3/E3 lines.
The term leased line refers to the fact that the organization pays a monthly lease fee
to a service provider to use the line. Leased lines are available in different capacities
and are generally priced based on the bandwidth required and the distance between
the two connected points.
In North America, service providers use the T-carrier system to define the digital
transmission capability of a serial copper media link, while Europe uses the E-carrier
system, as shown in Figure 1-21. For instance, a T1 link supports 1.544 Mb/s, an
E1 supports 2.048 Mb/s, a T3 supports 43.7 Mb/s, and an E3 connection supports
34.368 Mb/s. Optical carrier (OC) transmission rates are used to define the digital
transmitting capacity of a fiber-optic network.
Figure 1-21 Sample Leased-Line Topology
Table 1-1 describes the advantages and disadvantages of using leased lines.
Table 1-1 Advantages/Disadvantages of Leased Lines
Advantages Disadvantages
Simplicity: Point-to-point communication
links require minimal expertise to install
and maintain.
Cost: Point-to-point links are generally the
most expensive type of WAN access. The
cost of leased-line solutions can become
significant when they are used to connect
many sites over increasing distances.
In addition, each endpoint requires an
interface on the router, which increases
equipment costs.
Chapter 1: WAN Concepts 25
Advantages Disadvantages
Quality: Point-to-point communication links
usually offer high service quality, if they
have adequate bandwidth. The dedicated
capacity removes latency or jitter between
the endpoints.
Limited flexibility: WAN traffic is often
variable, and leased lines have a fixed
capacity, so the bandwidth of the line seldom
matches the need exactly. Any change to the
leased line generally requires a site visit by
ISP personnel to adjust capacity.
Availability: Constant availability is essential
for some applications, such as e-commerce.
Point-to-point communication links provide
permanent, dedicated capacity, which is
required for VoIP or Video over IP.
The Layer 2 protocol is usually HDLC or PPP.
Dialup (1.2.2.2)
Dialup WAN access may be required when no other WAN technology is available. For
example, a remote location could use modems and analog dialed telephone lines to provide
low capacity and dedicated switched connections, as shown in Figure 1-22. Dialup
access is suitable when intermittent, low-volume data transfers are needed.
Figure 1-22 Sample Dialup Topology
Traditional telephony uses a copper cable, called the local loop, to connect the telephone
handset in the subscriber premises to the CO. The signal on the local loop
26 Connecting Networks v6 Companion Guide
during a call is a continuously varying electronic signal that is a translation of the
subscriber voice into an analog signal.
Traditional local loops can transport binary computer data through the voice telephone
network using a dialup modem. The modem modulates the binary data into an
analog signal at the source and demodulates the analog signal to binary data at the
destination. The physical characteristics of the local loop and its connection to the
PSTN limit the rate of the signal to less than 56 kb/s.
For small businesses, these relatively low-speed dialup connections are adequate
for the exchange of sales figures, prices, routine reports, and email. Using automatic
dialup at night or on weekends for large file transfers and data backup can take
advantage of lower off-peak rates. These rates, often referred to as tariffs or toll
charges, are based on the distance between the endpoints, time of day, and the duration
of the call.
The advantages of modem and analog lines are simplicity, availability, and low
implementation cost. The disadvantages are the low data rates and a relatively
long connection time. The dedicated circuit has little delay or jitter for
point-to-point traffic, but voice or video traffic does not operate adequately at
these low bit rates.
Note
Although very few enterprises support dialup access, it is still a viable solution for remote
areas with limited WAN access options.
ISDN (1.2.2.3)
Integrated Services Digital Network (ISDN) is a circuit-switching technology that
enables the local loop of a PSTN to carry digital signals, resulting in higher capacity
switched connections.
ISDN changes the internal connections of the PSTN from carrying analog signals to
time-division multiplexed (TDM) digital signals. TDM allows two or more signals,
or bit streams, to be transferred as subchannels in one communication channel. The
signals appear to transfer simultaneously; but physically, the signals are taking turns
on the channel.
Figure 1-23 displays a sample ISDN topology. The ISDN connection may require a
terminal adapter (TA), which is a device used to connect ISDN Basic Rate Interface
(BRI) connections to a router.
Chapter 1: WAN Concepts 27
Figure 1-23 Sample ISDN Topology
The two types of ISDN interfaces are as follows:
■ Basic Rate Interface (BRI): ISDN BRI is intended for the home and small enterprise
and provides two 64 kb/s bearer channels (B) for carrying voice and data
and a 16 kb/s delta channel (D) for signaling, call setup, and other purposes. The
BRI D channel is often underused because it has only two B channels to control
(see Figure 1-24).
Figure 1-24 ISDN BRI
■ Primary Rate Interface (PRI): ISDN is also available for larger installations.
In North America, PRI delivers 23 B channels with 64 kb/s and one D channel
with 64 kb/s for a total bit rate of up to 1.544 Mb/s. This includes some
additional overhead for synchronization. In Europe, Australia, and other
parts of the world, ISDN PRI provides 30 B channels and one D channel,
for a total bit rate of up to 2.048 Mb/s, including synchronization overhead
(see Figure 1-25).
28 Connecting Networks v6 Companion Guide
Figure 1-25 ISDN PRI
BRI has a call setup time that is less than a second, and the 64 kb/s B channel provides
greater capacity than an analog modem link. In comparison, the call setup time
of a dialup modem is approximately 30 or more seconds with a theoretical maximum
of 56 kb/s. With ISDN, if greater capacity is required, a second B channel can be
activated to provide a total of 128 kb/s. This permits several simultaneous voice conversations,
a voice conversation and data transfer, or a video conference using one
channel for voice and the other for video.
Another common application of ISDN is to provide additional capacity as needed on
a leased-line connection. The leased line is sized to carry average traffic loads while
ISDN is added during peak demand periods. ISDN is also used as a backup if the
leased line fails. ISDN tariffs are based on a per-B channel basis and are similar to
those of analog voice connections.
With PRI ISDN, multiple B channels can be connected between two endpoints. This
allows for videoconferencing and high-bandwidth data connections with no latency
or jitter. However, multiple connections can be very expensive over long distances.
Note
Although ISDN is still an important technology for telephone service provider networks, it has
declined in popularity as an Internet connection option with the introduction of high-speed
DSL and other broadband services.
Frame Relay (1.2.2.4)
Frame Relay is a simple Layer 2 nonbroadcast multi-access (NBMA) WAN technology
used to interconnect enterprise LANs. A single router interface can be used to
connect to multiple sites using permanent virtual circuits (PVCs). PVCs are used to
carry both voice and data traffic between a source and destination, and support data
rates up to 4 Mb/s, with some providers offering even higher rates.
An edge router requires only a single interface, even when multiple VCs are used.
The leased line to the Frame Relay network edge allows cost-effective connections
between widely scattered LANs.
Chapter 1: WAN Concepts 29
Frame Relay creates PVCs, which are uniquely identified by a data-link connection
identifier (DLCI). The PVCs and DLCIs ensure bidirectional communication from one
DTE device to another.
For instance, in Figure 1-26, R1 will use DLCI 102 to reach R2 while R2 will use
DLCI 201 to reach R1.
Figure 1-26 Sample Frame Relay Topology
ATM (1.2.2.5)
Asynchronous Transfer Mode (ATM) technology is capable of transferring voice,
video, and data through private and public networks. It is built on a cell-based
architecture rather than on a frame-based architecture. ATM cells are always a
fixed length of 53 bytes. The ATM cell contains a 5-byte ATM header followed
by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying
voice and video traffic because this traffic is intolerant of delay. Video and voice
traffic do not have to wait for larger data packets to be transmitted, as shown in
Figure 1-27.
The 53-byte ATM cell is less efficient than the bigger frames and packets of Frame
Relay. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte
payload. When the cell is carrying segmented network layer packets, the overhead
is higher because the ATM switch must be able to reassemble the packets at the
30 Connecting Networks v6 Companion Guide
destination. A typical ATM line needs almost 20 percent greater bandwidth than
Frame Relay to carry the same volume of network layer data.
Figure 1-27 Sample ATM Topology
ATM was designed to be extremely scalable and to support link speeds of T1/E1 to
OC-12 (622 Mb/s) and faster.
As with other shared technologies, ATM allows multiple VCs on a single leased-line
connection to the network edge.
Note
ATM networks are now considered to be a a legacy technology.
Ethernet WAN (1.2.2.6)
Ethernet was originally developed to be a LAN access technology. Originally, Ethernet
was not suitable as a WAN access technology because at that time, the maximum
cable length was one kilometer. However, newer Ethernet standards using fiber-optic
cables have made Ethernet a reasonable WAN access option. For instance, the IEEE
1000BASE-LX standard supports fiber-optic cable lengths of 5 km, while the IEEE
1000BASE-ZX standard supports cable lengths up to 70 km.
Service providers now offer Ethernet WAN service using fiber-optic cabling. The
Ethernet WAN service can go by many names, including Metropolitan Ethernet
Chapter 1: WAN Concepts 31
(MetroE), Ethernet over MPLS (EoMPLS), and Virtual Private LAN Service
(VPLS). A sample Ethernet WAN topology is shown in Figure 1-28.
Figure 1-28 Sample Ethernet WAN Topology
An Ethernet WAN offers several benefits:
■ Reduced expenses and administration: Ethernet WAN provides a switched,
high-bandwidth Layer 2 network capable of managing data, voice, and video all
on the same infrastructure. This characteristic increases bandwidth and eliminates
expensive conversions to other WAN technologies. The technology enables businesses
to inexpensively connect numerous sites in a metropolitan area, to each
other, and to the Internet.
■ Easy integration with existing networks: Ethernet WAN connects easily to
existing Ethernet LANs, reducing installation costs and time.
■ Enhanced business productivity: Ethernet WAN enables businesses to take
advantage of productivity-enhancing IP applications that are difficult to implement
on TDM or Frame Relay networks, such as hosted IP communications,
VoIP, and streaming and broadcast video.
Note
Ethernet WANs have gained in popularity and are now commonly being used to replace the
traditional Frame Relay and ATM WAN links.
32 Connecting Networks v6 Companion Guide
MPLS (1.2.2.7)
Multiprotocol Label Switching (MPLS) is a multiprotocol high-performance WAN
technology that directs data from one router to the next. MPLS is based on short
path labels rather than IP network addresses.
MPLS has several defining characteristics. It is multiprotocol, meaning it has the ability
to carry any payload including IPv4, IPv6, Ethernet, ATM, DSL, and Frame Relay
traffic. It uses labels that tell a router what to do with a packet. The labels identify
paths between distant routers rather than endpoints, and while MPLS actually routes
IPv4 and IPv6 packets, everything else is switched.
MPLS is a service provider technology. Leased lines deliver bits between sites, and
Frame Relay and Ethernet WAN deliver frames between sites. However, MPLS can
deliver any type of packet between sites. MPLS can encapsulate packets of various
network protocols. It supports a wide range of WAN technologies including
T-carrier/E-carrier links, Carrier Ethernet, ATM, Frame Relay, and DSL.
The sample topology in Figure 1-29 illustrates how MPLS is used. Notice that the
different sites can connect to the MPLS cloud using different access technologies.
Figure 1-29 Sample MPLS Topology
In the Figure 1-29, CE refers to the customer edge; PE is the provider edge router,
which adds and removes labels; and P is an internal provider router, which switches
MPLS labeled packets.
VSAT (1.2.2.8)
All private WAN technologies discussed so far used either copper or fiber-optic
media. What if an organization needed connectivity in a remote location where no
service providers offer WAN service?
Chapter 1: WAN Concepts 33
Very small aperture terminal (VSAT) is a solution that creates a private WAN
using satellite communications. A VSAT is a small satellite dish similar to those used
for home Internet and TV. VSATs create a private WAN while providing connectivity
to remote locations.
Specifically, a router connects to a satellite dish that is pointed to a service provider’s
satellite. This satellite is in geosynchronous orbit in space. The signals must travel
approximately 35,786 kilometers (22,236 miles) to the satellite and back.
The example in Figure 1-30 displays a VSAT dish on the roofs of the buildings communicating
with a satellite thousands of kilometers away in space.
Figure 1-30 Sample VSAT Topology
Activity 1.2.2.9: Identify Private WAN Infrastructure Terminology
Refer to the online course to complete this activity.
Public WAN Infrastructure (1.2.3)
In this topic, you compare public WAN technologies.
Interactive
Graphic
34 Connecting Networks v6 Companion Guide
DSL (1.2.3.1)
DSL technology is an always-on connection technology that uses existing twistedpair
telephone lines to transport high-bandwidth data, and provides IP services to
subscribers. A DSL modem converts an Ethernet signal from the user device to a
DSL signal, which is transmitted to the central office.
Multiple DSL subscriber lines are multiplexed into a single, high-capacity link using
a DSL access multiplexer (DSLAM) at the provider location referred to as the point
of presence (POP). DSLAMs incorporate TDM technology to aggregate many subscriber
lines into a single medium, generally a T3 connection. Current DSL technologies
use sophisticated coding and modulation techniques to achieve fast data rates.
There is a wide variety of DSL types, standards, and emerging standards. DSL is now
a popular choice for enterprise IT departments to support home workers. Generally,
a subscriber cannot choose to connect to an enterprise network directly but must
first connect to an ISP, and then an IP connection is made through the Internet to the
enterprise. Security risks are incurred in this process but can be mediated with security
measures.
The topology in Figure 1-31 displays a sample DSL WAN connection.
Figure 1-31 Sample DSL Topology
Chapter 1: WAN Concepts 35
Cable (1.2.3.2)
Coaxial cable is widely used in urban areas to distribute television signals. Network
access is available from many cable television providers. This access allows for greater
bandwidth than the conventional telephone local loop.
Cable modems (CMs) provide an always-on connection and a simple installation. A
subscriber connects a computer or LAN router to the cable modem, which translates
the digital signals into the broadband frequencies used for transmitting on a cable
television network. The local cable TV office, which is called the cable headend,
contains the computer system and databases needed to provide Internet access. The
most important component located at the headend is the cable modem termination
system (CMTS), which sends and receives digital cable modem signals on a cable
network and is necessary for providing Internet services to cable subscribers.
Cable modem subscribers must use the ISP associated with the service provider. All
the local subscribers share the same cable bandwidth. As more users join the service,
available bandwidth may drop below the expected rate.
The topology in Figure 1-32 displays a sample cable WAN connection.
Figure 1-32 Sample Cable Topology
36 Connecting Networks v6 Companion Guide
Wireless (1.2.3.3)
Wireless technology uses the unlicensed radio spectrum to send and receive data.
The unlicensed spectrum is accessible to anyone who has a wireless router and wireless
technology in the device he or she is using.
Until recently, one limitation of wireless access has been the need to be within the
local transmission range (typically less than 100 feet) of a wireless router or a wireless
modem that has a wired connection to the Internet. The following new developments
in broadband wireless technology are changing this situation:
■ Municipal Wi-Fi: Many cities have begun setting up municipal wireless networks.
Some of these networks provide high-speed Internet access for free or
for substantially less than the price of other broadband services. Others are for
city use only, allowing police and fire departments and other city employees to
do certain aspects of their jobs remotely. To connect to a municipal Wi-Fi, a subscriber
typically needs a wireless modem, which provides a stronger radio and
directional antenna than conventional wireless adapters. Most service providers
provide the necessary equipment for free or for a fee, much like they do with
DSL or cable modems.
■ WiMAX: Worldwide Interoperability for Microwave Access (WiMAX) is a
new technology that is just beginning to come into use. It is described in the
IEEE standard 802.16. WiMAX provides high-speed broadband service with
wireless access and provides broad coverage like a cell phone network rather
than through small Wi-Fi hotspots. WiMAX operates in a similar way to
Wi-Fi, but at higher speeds, over greater distances, and for a greater number of
users. It uses a network of WiMAX towers that are similar to cell phone towers.
To access a WiMAX network, subscribers must subscribe to an ISP with
a WiMAX tower within 30 miles of their location. They also need some type
of WiMAX receiver and a special encryption code to get access to the base
station.
■ Satellite Internet: Typically, rural users use this type of technology where
cable and DSL are not available. A VSAT provides two-way (upload and
download) data communications. The upload speed is about one-tenth of the
500 kb/s download speed. Cable and DSL have higher download speeds, but
satellite systems are about 10 times faster than an analog modem. To access
satellite Internet services, subscribers need a satellite dish, two modems
(uplink and downlink), and coaxial cables between the dish and the modem.
Figure 1-33 displays an example of a WiMAX network.
Chapter 1: WAN Concepts 37
Figure 1-33 Sample Wireless Topology
3G/4G Cellular (1.2.3.4)
Increasingly, cellular service is another wireless WAN technology being used to connect
users and remote locations where no other WAN access technology is available,
as shown in Figure 1-34. Many users with smartphones and tablets can use cellular
data to email, surf the web, download apps, and watch videos.
Figure 1-34 Sample Cellular Topology
Phones, tablet computers, laptops, and even some routers can communicate through
to the Internet using cellular technology. These devices use radio waves to communicate
through a nearby mobile phone tower. The device has a small radio antenna, and
the provider has a much larger antenna sitting at the top of a tower somewhere within
miles of the phone.
38 Connecting Networks v6 Companion Guide
These are two common cellular industry terms:
■ 3G/4G Wireless: Abbreviation for third-generation and fourth-generation
cellular access. These technologies support wireless Internet access.
■ Long-Term Evolution (LTE): Refers to a newer and faster technology and is
considered to be part of fourth-generation (4G) technology.
VPN Technology (1.2.3.5)
Security risks are incurred when a teleworker or a remote office worker uses a
broadband service to access the corporate WAN over the Internet. To address security
concerns, broadband services provide capabilities for using VPN connections to
a network device that accepts VPN connections, which are typically located at the
corporate site.
A VPN is an encrypted connection between private networks over a public network,
such as the Internet. Instead of using a dedicated Layer 2 connection, such as a leased
line, a VPN uses virtual connections called VPN tunnels, which are routed through
the Internet from the private network of the company to the remote site or
employee host.
Using VPN offers several benefits:
■ Cost savings: VPNs enable organizations to use the global Internet to connect
remote offices, and to connect remote users to the main corporate site. This
eliminates expensive, dedicated WAN links and modem banks.
■ Security: VPNs provide the highest level of security by using advanced encryption
and authentication protocols that protect data from unauthorized access.
■ Scalability: Because VPNs use the Internet infrastructure within ISPs and
devices, it is easy to add new users. Corporations are able to add large amounts
of capacity without adding significant infrastructure.
■ Compatibility with broadband technology: VPN technology is supported by
broadband service providers such as DSL and cable. VPNs allow mobile workers
and telecommuters to take advantage of their home high-speed Internet service
to access their corporate networks. Business-grade, high-speed broadband connections
can also provide a cost-effective solution for connecting remote offices.
There are two types of VPN access:
■ Site-to-site VPNs: Site-to-site VPNs connect entire networks to each other; for
example, they can connect a branch office network to a company headquarters
network, as shown in Figure 1-35. Each site is equipped with a VPN gateway,
Chapter 1: WAN Concepts 39
such as a router, firewall, VPN concentrator, or security appliance. In the
Figure 1-35, a remote branch office uses a site-to-site-VPN to connect with the
corporate head office.
Figure 1-35 Sample Site-to-Site VPN Topology
■ Remote-access VPNs: Remote-access VPNs enable individual hosts, such as
telecommuters, mobile users, and extranet consumers, to access a company
network securely over the Internet. Each host (Teleworker 1 and Teleworker 2)
typically has VPN client software loaded or uses a web-based client, as shown
in Figure 1-36.
Activity 1.2.3.6: Identify Public WAN Infrastructure Terminology
Refer to the online course to complete this activity.
Interactive
Graphic
40 Connecting Networks v6 Companion Guide
Figure 1-36 Sample Remote-Access VPN Topology
Selecting WAN Services (1.2.4)
In this topic, you learn how to select the appropriate WAN protocol and service for a
specific network requirement.
Choosing a WAN Link Connection (1.2.4.1)
There are many important factors to consider when choosing an appropriate WAN
connection. For a network administrator to decide which WAN technology best
meets the requirements of a specific business, he or she must answer the following
questions:
What is the purpose of the WAN?
There are a few issues to consider:
■ Will the enterprise connect local branches in the same city area, connect remote
branches, or connect to a single branch?
■ Will the WAN be used to connect internal employees, or external business partners
and customers, or all three?
■ Will the enterprise connect to customers, connect to business partners, connect
to employees, or some combination of these?
■ Will the WAN provide authorized users limited or full access to the company
intranet?
Chapter 1: WAN Concepts 41
What is the geographic scope?
There are a few issues to consider:
■ Is the WAN local, regional, or global?
■ Is the WAN one-to-one (single branch), one-to-many branches, or many-to-many
(distributed)?
What are the traffic requirements?
There are a few issues to consider:
■ What type of traffic must be supported (data only, VoIP, video, large files,
streaming files)? This determines the quality and performance requirements.
■ What volume of traffic type (voice, video, or data) must be supported for each
destination? This determines the bandwidth capacity required for the WAN connection
to the ISP.
■ What Quality of Service is required? This may limit the choices. If the traffic is
highly sensitive to latency and jitter, eliminate any WAN connection options that
cannot provide the required quality.
■ What are the security requirements (data integrity, confidentiality, and security)?
These are important factors if the traffic is of a highly confidential nature, or if it
provides essential services, such as emergency response.
Choosing a WAN Link Connection (Cont.) (1.2.4.2)
In addition to gathering information about the scope of the WAN, the administrator
must also determine the following:
■ Should the WAN use a private or public infrastructure? A private infrastructure
offers the best security and confidentiality, whereas the public Internet
infrastructure offers the most flexibility and lowest ongoing expense. The choice
depends on the purpose of the WAN, the types of traffic it carries, and available
operating budget. For example, if the purpose is to provide a nearby branch with
high-speed secure services, a private dedicated or switched connection may be
best. If the purpose is to connect many remote offices, a public WAN using the
Internet may be the best choice. For distributed operations, a combination of
options may be the solution.
■ For a private WAN, should it be dedicated or switched? Real-time, highvolume
transactions have special requirements that could favor a dedicated line,
such as traffic flowing between the data center and the corporate head office. If
the enterprise is connecting to a local single branch, a dedicated leased line could
42 Connecting Networks v6 Companion Guide
be used. However, that option would become very expensive for a WAN connecting
multiple offices. In that case, a switched connection might be better.
■ For a public WAN, what type of VPN access is required? If the purpose of the
WAN is to connect a remote office, a site-to-site VPN may be the best choice. To
connect teleworkers or customers, remote-access VPNs are a better option. If the
WAN is serving a mixture of remote offices, teleworkers, and authorized customers,
such as a global company with distributed operations, a combination of VPN
options may be required.
■ Which connection options are available locally? In some areas, not all WAN
connection options are available. In this case, the selection process is simplified,
although the resulting WAN may provide less than optimal performance.
For example, in a rural or remote area, the only option may be VSAT or
cellular access.
■ What is the cost of the available connection options? Depending on the option
chosen, the WAN can be a significant ongoing expense. The cost of a particular
option must be weighed against how well it meets the other requirements. For
example, a dedicated leased line is the most expensive option, but the expense
may be justified if it is critical to ensure secure transmission of high volumes
of real-time data. For less demanding applications, a less expensive switched or
Internet connection option may be more suitable.
Using the preceding guidelines, as well as those described by the Cisco Enterprise
Architecture, a network administrator should be able to choose an appropriate WAN
connection to meet the requirements of different business scenarios.
Lab 1.2.4.3: Researching WAN Technologies
In this lab, you will complete the following objectives:
Part 1: Investigate Dedicated WAN Technologies and Providers
Part 2: Investigate a Dedicated Leased-Line Service Provider in Your Area
Chapter 1: WAN Concepts 43
Summary (1.3)
Class Activity 1.3.1.1: WAN Device Modules
Your medium-sized company is upgrading its network. To make the most of the
equipment currently in use, you decide to purchase WAN modules instead of
new equipment.
All branch offices use either Cisco 1900 or 2911 series ISRs. You will be updating
these routers in several locations. Each branch has its own ISP requirements to
consider.
To update the devices, focus on the following WAN module access types:
■ Ethernet
■ Broadband
■ T1/E1 and ISDN PRI
■ BRI
■ Serial
■ T1 and E1 Trunk Voice and WAN
■ Wireless LANs and WANs
A business can use private lines or the public network infrastructure for WAN connections.
A public infrastructure connection can be a cost-effective alternative to a
private connection between LANs, as long as security is also planned.
WAN access standards operate at Layers 1 and 2 of the OSI model, and are defined
and managed by the TIA/EIA, ISO, and IEEE. A WAN may be circuit-switched or
packet-switched.
There is common terminology used to identify the physical components of WAN
connections and who, the service provider or the customer, is responsible for which
components.
Service provider networks are complex, and the service provider’s backbone networks
consist primarily of high-bandwidth fiber-optic media. The device used for interconnection
to a customer is specific to the WAN technology that is implemented.
Permanent, dedicated point-to-point connections are provided by using leased
lines. Dialup access, although slow, is still viable for remote areas with limited WAN
options. Other private connection options include ISDN, Frame Relay, ATM, Ethernet
WAN, MPLS, and VSAT.
44 Connecting Networks v6 Companion Guide
Public infrastructure connections include DSL, cable, wireless, and 3G/4G cellular.
Security over public infrastructure connections can be provided by using remoteaccess
or site-to-site VPNs.


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What type of WAN technology provides a dedicated connection from the service provider?

Private WAN infrastructure: Service providers may offer dedicated point-to-point leased lines, circuit-switched links, such as PSTN or ISDN, and packet-switched links, such as Ethernet WAN, ATM, or Frame Relay.

What are the 3 WAN connection types?

List of WAN connection Types: Automatic IP. Static IP. PPPoE.

Which technologies are used to implement WANs?

The following communication and networking technologies have been used to implement WANs..
Asynchronous Transfer Mode..
Cable modem..
Dial-up internet..
Digital subscriber line..
Fiber-optic communication..
Frame Relay..
Leased line..

Which communication method is used in WAN connections?

The communication subnet of WAN mainly uses the packet-switched network. It can use the public packet-switched network, satellite communication network and wireless packet-switched network, to connect those different regions of the LAN or computer systems, in order to achieve the purpose of resource sharing.

wans implemented dedicated leased line connections corporate edge point provider networks Private WAN examples WAN connectivity options Dedicated WAN connection

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