Objectives
Upon completion of this chapter, you will be able to

■
Identify different types of WAN connections, encapsulations, and protocols

■
Know the difference between a WAN and LAN and which type of addresses
each uses

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Chapter 10
WANs and Routers
In this chapter, you learn about WAN devices, technologies, and standards. In addition,
you learn about the function of a router in a WAN.
Please be sure to look at this chapter’s e-Labs, Videos, and PhotoZooms that you will find
on the CD-ROM accompanying this book. These CD elements are designed to supple-
ment the material and reinforce the concepts introduced in this chapter.
WAN Characteristics
A wide-area network (WAN) is a data communications network that extends across a
large geographic area. WANs often use transmission facilities provided by common carriers,
for example, telephone companies.
A WAN differs from a local-area network (LAN) in several ways. For example, unlike a
LAN, which connects workstations, peripherals, terminals, and other devices in a single
building or other small geographic area, a WAN makes data connections across a broad
geographic area. Companies use a WAN to connect various company sites so that infor-
mation can be exchanged between distant offices.
A WAN operates at the physical layer (Layer 1) and the data link layer (Layer 2) of the
OSI reference model. It interconnects LANs that are usually separated by large geographic
areas. WANs provide for the exchange of data packets/frames between

routers/switches
and the LANs they support.
Table 10-1 lists some CPU information indicative of specific examples of data networks.

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512 Chapter 10: WANs and Routers
As shown in Figure 10-1, the major characteristics of WANs are as follows:

■ They connect devices that are separated by wide geographical areas.

■
They use the services of carriers such as the regional Bell operating companies
(RBOCs), Sprint, and MCI.

■
They use serial connections of various types to access bandwidth over large
geographic areas.

■
They connect devices that are separated by wide geographical areas.
Table 10-1 Data Networks
Name Location of Hosts Distances Between Devices
LAN
Classroom
Room 10 m
LAN
School
Building 100 m
LAN
University

Campus 1000 m = 1 km
WAN
Cisco Systems, Inc.
Country 100,000 m = 100 km
WAN
Africa
Continent 1,000,000 m = 1000 km
WAN
Internet
Planet 10,000,000 m = 10,000 km
WAN
Earth and Artificial
Satellites
Earth-Moon Systems 100,000,000 m = 100,000 km

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WAN Characteristics 513
Figure 10-1 WAN Connection
More Information: WAN Performance Criteria
Component Uptime
Each physical component of the WAN can be monitored and measured for its availability using
uptime. Uptime is the opposite of downtime: It is the amount of time that the device is func-
tional and in service relative to the users’ requirements for its availability. It is common for
uptime to be statistically overstated by measuring it on a 7-day-24-hour basis, even though the
users' requirements might be for only 5 days-a-week 12 hours a day. Remember to tailor this
metric and every other metric as closely as possible to your users’ stated requirements for net-
work performance.
Although electronic devices are highly reliable, they eventually fail. Most manufacturers pro-
vide a mean time between failure (MTBF) rating for their equipment as a reassurance of how
reliable their products really are. Typically, MTBF ratings are in the tens of thousands of hours.

These hours could conceivably translate into years of trouble-free service. Unfortunately, these
ratings are statistically derived. The actual time between failures of any given device depends
greatly on a number of factors, including the following:

■ Ambient temperature ranges of the operating environment

■
Cleanliness of the commercial electric power

■
Quality of the handling of the devices before and during operation
Monitoring and tracking uptime of individual components enables you to demonstrate to your
user community how well you are satisfying their requirements for the network’s availability.
Component uptime data can also be charted over time to identify potentially problematic com-
ponents in your network infrastructure. Such trends can provide information about the general
reliability of a given type or brand of hardware, which then can be used to identify individual
components that might be at risk of failure.
continues

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514 Chapter 10: WANs and Routers
More Information: WAN Performance Criteria (Continued)
Note that the term availability is sometimes used to generically describe aggregate network
uptime. Unfortunately, it is not a good metric. In theory, network availability provides a quanti-
fied synopsis of the network’s readiness. In practice, availability is so nebulous that it is virtu-
ally meaningless.
To illustrate this point, if a router at a premise location fails, the entire network is unavailable to
the users at that location. The network, however, is available to users at every other location.
They will not be able to access hosts at the affected location, but they will also not be impeded
from accessing every other host in the network. The extent to which the network is available

varies greatly by location and by usage requirements. Therefore, quantifying network availabil-
ity can be more onerous than it is valuable.
Traffic Volumes
One of the more important metrics for any WAN is the volume of traffic that it is expected to
support. Volume is almost always volatile; it varies with time, business cycles, seasons, and so
on. In other words, you can count on traffic volumes being anything but constant. Given this
volatility, it is important to measure volumes in two different ways, maximum volumes and
average volumes:

■ The maximum volume that you expect the network to support is known as the peak
volume. As its name implies, this peak volume is the greatest amount of traffic that
you expect the network to have to support.

■
Average volumes are the traffic loads that you can reasonably expect during the course of
a business day from any given work location.
Establishing these two traffic volumes is critical to sizing the WAN’s transmission facilities, as
well as the facilities of its routers. If you expect any given location to generate a traffic load of
100 kbps during the course of a business day, for example, a 56-kbps transmission facility is
clearly inadequate.
Delay
Delay is one of the more common metrics that can be used to measure network performance.
Delay is the time that elapses between two events. In data communications, these two events
are typically the transmission and reception of data. Therefore, delay is the total amount of
time that is required by the network to transport a packet from its point of origin to its destina-
tion. Given this definition, delay is an aggregate phenomenon with many potential causes.
Three of the more common types of delay are the following:

■
Propagation delay—Propagation delay is the cumulative amount of time required to trans-

mit, or propagate, the data across an end-to-end transmission path. The network infra-
structure within each transmission facility in the network path directly contributes to the
aggregate forwarding delay of any given transmission. An additional factor in propagation
delay is traffic volume. The more traffic that is flowing across a given facility, the less
bandwidth that is available for new transmissions. Propagation delay is inherent in terres-
trial circuits, regardless of whether they traverse glass or copper media or are transmitted
through the air using microwave radio frequencies.

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WAN Characteristics 515
WAN Devices
The following devices are used in WANs, as represented in Figure 10-2:

■
Routers—Offer many services, including internetworking and WAN interface ports.

■ Switches—Connect to WAN bandwidth for voice, data, and video communication.

■
Modems—Include interfaces to voice-grade services (analog dialup modems);
channel service units/digital service units (CSU/DSUs) that interface T1/E1 ser-
vices; and terminal adapters/Network Termination 1s (TAs/NT1s) that interface
Integrated Services Digital Network (ISDN) services.

■
Communication servers—Concentrate dial-in and dial-out user communication.
Figure 10-2 Common WAN Devices
WAN Standards
WAN data-link protocols describe how frames are carried between systems on a single
data link. They include protocols designed to operate over dedicated point-to-point,

multipoint, and multiaccess switched services, such as Frame Relay. WAN standards
are defined and managed by a number of recognized authorities, including the follow-
ing agencies:

■
International Telecommunication Union-Telecommunication Standardization
Sector (ITU-T), formerly the Consultative Committee for International Tele-
graph and Telephone (CCITT)
More Information: WAN Performance Criteria (Continued)

■
Satellite uplink/downlink delays—Some transmission facilities are satellite-based, requir-
ing the signal to be transmitted up to the satellite and then transmitted back down from
the satellite. Because of the great distances between the terrestrial transmission facilities
and the satellite, these delays can be quite noticeable.

■ Forwarding delay—Forwarding delay in a network is the amount of time that a physical
device needs to receive, buffer, process, and forward data. The actual forwarding delay of
any given device can vary over time. Individual devices operating at or near capacity will
likely experience a greater forwarding delay than comparable devices that are less utilized.
Additionally, forwarding delay can be exacerbated by heavy traffic or error conditions in
the network. Forwarding delay can be described as latency within individual components.

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516 Chapter 10: WANs and Routers

■ International Organization for Standardization (ISO)

■
Internet Engineering Task Force (IETF)


■
Electronic Industries Association (EIA)

■ Institute of Electrical and Electronics Engineers (IEEE)
The WAN physical layer describes the interface between the data terminal equipment
(DTE) and the data circuit-terminating equipment (DCE). Typically, a DCE device
is service provider equipment that provides clocking and switching services within a
network, and a DTE device is an attached customer device. In this model, the services
offered to the DTE are made available through a modem or a CSU/DSU (see Figure 10-3).
Figure 10-3 WAN Services
To ensure that the correct protocol is used, you must configure the appropriate Layer 2
encapsulation on the router. The choice of the protocol depends on the WAN technol-
ogy and communicating equipment being utilized.
WAN physical layer protocols describe how to provide electrical, mechanical, opera-
tional, and functional connections for WAN services. These services are usually
obtained from WAN service providers, such as RBOCs; alternate carriers; and Post,
Telephone, and Telegraph (PTT) agencies.
The following physical layer standards are specified:

■
EIA/TIA-232—Common physical layer interface standard that supports unbal-
anced circuits at signal speeds of up to 64 kbps. Closely resembles the V.24
specification.

■
EIA/TIA-449—A faster (up to 2 Mbps) version of EIA/TIA-232 capable of
longer cable runs.

■

V.24—Physical layer interface between DTE and DCE. V.24 is essentially the
same as the EIA/TIA-232.

■
V.35—A synchronous, physical layer protocol for communications between a
network access device and a packet network. V.35 is most commonly used in the
United States and in Europe and is recommended for speeds up to 48 kbps.
(Router)
DTE
CSU/DSU
(Modem)
DCE
EIA/TIA–232, V.35,
X.21,HSSI, and others

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WAN Characteristics 517

■ X.21—Protocol for serial communications over synchronous digital lines. The
X.21 protocol is used primarily in Europe and Japan.

■ G.703—ITU-T electrical and mechanical specifications for connections between
telephone company equipment and DTE using BNC connectors and operating at
E1 data rates.

■
EIA-530—Refers to two electrical implementations of EIA/TIA-449: RS-422 (for
balanced transmission) and RS-423 (for unbalanced transmission).
Synchronous serial lines have several data link encapsulations associated with them,
the characteristics for which are listed in Table 10-2.

Table 10-2 Synchronous Serial Line Data Link Encapsulations
Encapsulation Characteristics
High-Level Data Link
Control (HDLC)
An IEEE standard.
Is the default encapsulation on point-to-point dedicated
links, and circuit-switch connections.
Cisco HDLC is a proprietary version, bit-oriented, syn-
chronous data link layer protocol typically used when
communicating between two Cisco devices.
Cisco HDLC might be incompatible with other ven-
dors’ versions due to the method of implementation
those vendors have chosen.
Supports both point-to-point and multipoint configura-
tions with minimal overhead.
Frame Relay Uses high-quality digital facilities.
Uses simplified framing with no error correction mech-
anisms, which means it can send Layer 2 information
much more rapidly than other WAN protocols.
Industry-standard switched data link layer protocol
that handles multiple virtual circuits.
Next generation to X.25, Frame Relay is streamlined to
eliminate some of the time-consuming processes that
were employed in X.25, such as error correction and
flow control.
continues

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518 Chapter 10: WANs and Routers
Point-to-Point Protocol

(PPP)
Described by RFC 1661.
Two standards developed by the IETF.
Contains a Protocol field to identify the network layer
protocol.
Provides router-to-router and host-to-network connec-
tions over synchronous and asynchronous circuits.
Designed to work with several network layer protocols,
such as Internet Protocol (IP) and Internetwork Packet
Exchange (IPX).
Built-in security mechanisms, such as Password
Authentication Protocol (PAP) and Challenge Hand-
shake Authentication Protocol (CHAP).
Synchronous Data Link
Control (SDLC) Protocol
IBM-designed WAN data link protocol for System Net-
work Architecture (SNA) environments.
Largely being replaced by the more versatile HDLC.
Serial Line Internet Pro-
tocol (SLIP)
Extremely popular WAN data link protocol for carry-
ing IP packets.
Replaced in many applications by the more versatile PPP.
Standard protocol for point-to-point serial connections
using Transmission Control Protocol/Internet Protocol
(TCP/IP).
Link Access Procedure
Balanced (LAPB)
Used by X.25.
Has extensive error-checking capabilities.

Link Access Procedure on
the D channel (LAPD)
Protocol used for signaling and call setup on an Inte-
grated Services Digital Network (ISDN) D channel.
Link Access Procedure
Frame (LAPF)
Used for Frame-Mode Bearer Services.
Similar to LAPD, used with Frame Relay technologies.
X.25/Link Access Proce-
dure Balanced (LAPB)
An ITU-T standard that defines how connections
between DTE and DCE are maintained for remote ter-
minal access and computer communications in public
data networks.
Specifies LAPB, a data link layer protocol.
Predecessor to Frame Relay.
Table 10-2 Synchronous Serial Line Data Link Encapsulations (Continued)
Encapsulation Characteristics

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