CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 55 pps

The network infra-structure within each transmission facility in the network path directly contributes to the aggregate forwarding delay of any given transmission.. WAN Characteristics 5

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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

Awide-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

LAN Classroom

LAN School

LAN University

WAN Cisco Systems, Inc

WAN Africa

WAN Internet

WAN Earth and Artiﬁcial Satellites

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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

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:

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.

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Telecommuter

Mobile Worker

SanJose Headquarters

Denver Branch Office Service Provider

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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:

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 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.

Router

WAN Bandwidth Switch Modem

Comm.

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■ 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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■ 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

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

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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)

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