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X.25 and Frame Relay X.25 is an analog, packet-switched technology designed for long-distance data transmission and standardized by the ITU in the mid-1970s.. max-The X.25 standard speci

must navigate between the sender and receiver all reduce the actual throughput The number

of central offices, switches, and modems through which your phone call travels also affectthroughput Each time the signal passes through a switch or is converted from analog to dig-ital or digital to analog, it loses a little throughput If you’re surfing the Web, for example, bythe time a Web page returns to you, the connection may have lost from 5 to 30 Kbps, and youreffective throughput might have been reduced to 30 Kbps or less In addition, the FCC (Fed-eral Communications Commission), the regulatory agency that sets standards and policy fortelecommunications transmission and equipment in the United States, limits the use of PSTNlines to 53 Kbps to reduce the effects of crosstalk Thus, you will never actually achieve full 56-Kbps throughput using a modem over the PSTN

Nor can the PSTN provide the quality required by many network applications The quality of

a WAN connection is largely determined by how many data packets that it loses or that becomecorrupt during transmission, how quickly it can transmit and receive data, and whether itdrops the connection altogether To improve this quality, most protocols employ error check-ing techniques For example, TCP/IP depends on acknowledgments of the data it receives Inaddition, many (though not all) PSTN links are now digital, and digital lines are more reliablethan the older analog lines Such digital lines reduce the quality problems that once plaguedpurely analog PSTN connections

Although nearly all central offices in the PSTN handle digitized data, most still use circuitswitching rather than the more efficient packet switching Recall that in circuit switching, datatravels over a point-to-point connection that is reserved by a transmission until all of its datahas been transferred You might think that circuit switching makes the PSTN more secure thanother types of WAN connections; in fact, the PSTN offers only marginal security Because it

is a public network, PSTN presents many points at which communications can be interceptedand interpreted on their way from sender to receiver For example, an eavesdropper could eas-ily tap into the connection where your local telephone company’s line enters your house.The PSTN is not limited to servicing workstation dial-up WAN connections Following sec-tions describe other, more sophisticated WAN technologies that also rely on the public tele-phone network

X.25 and Frame Relay

X.25 is an analog, packet-switched technology designed for long-distance data transmission

and standardized by the ITU in the mid-1970s The original standard for X.25 specified a imum of 64-Kbps throughput, but by 1992 the standard was updated to include maximumthroughput of 2.048 Mbps It was originally developed as a more reliable alternative to the voicetelephone system for connecting mainframe computers and remote terminals Later it wasadopted as a method of connecting clients and servers over WANs

max-The X.25 standard specifies protocols at the Physical, Data Link, and Network layers of theOSI Model It provides excellent flow control and ensures data reliability over long distances

by verifying the transmission at every node Unfortunately, this verification also renders X.25

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comparatively slow and unsuitable for time-sensitive applications, such as audio or video Onthe other hand, X.25 benefits from being a long-established, well-known, and low-cost tech-nology X.25 was never widely adopted in the United States, but was accepted by other coun-tries and was for a long time the dominant packet-switching technology used on WANs aroundthe world.

Chapter 7 303

X.25 AND FRAME RELAY

Recall that, in packet switching, packets belonging to the same data stream may low different, optimal paths to their destination As a result, packet switching usesbandwidth more efficiently and allows for faster transmission than if each packet inthe data stream had to follow the same path, as in circuit switching Packet switching

fol-is also more flexible than circuit switching, because packet sizes may vary

NOTE

Frame Relay is an updated, digital version of X.25 that also relies on packet switching ITU

and ANSI standardized Frame Relay in 1984 However, because of a lack of compatibilitywith other WAN technologies at the time, Frame Relay did not become popular in the UnitedStates and Canada until the late 1980s Frame Relay protocols operate at the Data Link layer

of the OSI Model and can support multiple different Network and Transport layer protocols(for example, TCP/IP and IPX/SPX) The name is derived from the fact that data is sepa-rated into frames, which are then relayed from one node to another without any verification

or processing

An important difference between Frame Relay and X.25 is that Frame Relay does not antee reliable delivery of data X.25 checks for errors and, in the case of an error, either cor-rects the damaged data or retransmits the original data Frame Relay, on the other hand,simply checks for errors It leaves the error correction up to higher-layer protocols Partlybecause it doesn’t perform the same level of error correction that X.25 performs (and thus hasless overhead), Frame Relay supports higher throughput than X.25 It offers throughputsbetween 64 Kbps and 45 Mbps A Frame Relay customer chooses the amount of bandwidth

guar-he requires and pays for only that amount

Both X.25 and Frame Relay may be configured as SVCs (switched virtual circuits) or PVCs

(permanent virtual circuits) SVCs are connections that are established when parties need to transmit, then terminated after the transmission is complete PVCs are connections that are

established before data needs to be transmitted and maintained after the transmission is plete Note that in a PVC, the connection is established only between the two points (the senderand receiver); the connection does not specify the exact route the data will travel Thus, in aPVC, data may follow any number of paths from point A to point B For example, a trans-mission traveling over a PVC from Baltimore to Phoenix might go from Baltimore to Wash-ington, D.C., to Chicago, then to Phoenix; the next transmission over that PVC, however,might go from Baltimore to Boston to St Louis to Denver to Phoenix

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PVCs are not dedicated, individual links When you lease an X.25 or Frame Relay circuit from

your local carrier, your contract reflects the endpoints you specify and the amount of bandwidthyou require between those endpoints The service provider guarantees a minimum amount of

bandwidth, called the CIR (committed information rate) Provisions usually account for bursts

of traffic that occasionally exceed the CIR When you lease a PVC, you share bandwidth withthe other X.25 and Frame Relay users on the backbone PVC links are best suited to frequentand consistent data transmission

On networking diagrams, packet-switched networks such as X.25 and Frame Relay aredepicted as clouds, as shown in Figure 7-9, because of the indeterminate nature of their traf-fic patterns

FIGURE 7-9 A WAN using frame relay

You may have seen the Internet depicted as a cloud on networking diagrams, similar

to the Frame Relay cloud in Figure 7-9 In its early days, the Internet relied largely onX.25 and Frame Relay transmission—hence the similar illustration

NOTE

The advantage to leasing a Frame Relay circuit over leasing a dedicated service is that you payfor only the amount of bandwidth required Another advantage is that Frame Relay is muchless expensive than some newer WAN technologies offered today Also, Frame Relay is a long-established worldwide standard

On the other hand, because Frame Relay and X.25 use shared lines, their throughput remains

at the mercy of variable traffic patterns In the middle of the night, data over your Frame Relay

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network may zip along at 1.544 Mbps; during midday, when everyone is surfing the Web, itmay slow down to less than your CIR In addition, Frame Relay circuits are not as private (andpotentially not as secure) as dedicated circuits Nevertheless, because they use the same con-nectivity equipment as T-carriers, they can easily be upgraded to T-carrier dedicated lines.

ISDN

ISDN (Integrated Services Digital Network) is an international standard, originally

estab-lished by the ITU in 1984, for transmitting digital data over the PSTN In North America, astandard ISDN implementation wasn’t finalized until 1992, because telephone switch manu-facturers couldn’t agree on compatible technology for supporting ISDN The technology’suncertain start initially made telephone companies reluctant to invest in it, and ISDN didn’tcatch on as quickly as predicted However, in the 1990s ISDN finally became a popular method

of connecting WAN locations to exchange both data and voice signals

ISDN specifies protocols at the Physical, Data Link, and Transport layers of the OSI Model.These protocols handle signaling, framing, connection setup and termination, routing, flowcontrol, and error detection and correction ISDN relies on the PSTN for its transmissionmedium Connections can be either dial-up or dedicated Dial-up ISDN is distinguished fromthe workstation dial-up connections discussed previously because it relies exclusively on digi-tal transmission In other words, it does not convert a computer’s digital signals to analog beforetransmitting them over the PSTN Also, ISDN is distinguished because it can simultaneouslycarry as many as two voice calls and one data connection on a single line Therefore, ISDN caneliminate the need to pay for separate phone lines to support faxes, modems, and voice calls atone location

All ISDN connections are based on two types of channels: B channels and D channels The B channel is the “bearer” channel, employing circuit-switching techniques to carry voice, video,

audio, and other types of data over the ISDN connection A single B channel has a maximumthroughput of 64 Kbps (although it is sometimes limited to 56 Kbps by the ISDN provider)

The number of B channels in a single ISDN connection may vary The D channel is the “data”

channel, employing packet-switching techniques to carry information about the call, such assession initiation and termination signals, caller identity, call forwarding, and conference call-ing signals A single D channel has a maximum throughput of 16 or 64 Kbps, depending on thetype of ISDN connection Each ISDN connection uses only one D channel

In North America, two types of ISDN connections are commonly used: BRI (Basic Rate

Inter-face) and PRI (Primary Rate InterInter-face) BRI (Basic Rate InterInter-face) uses two B channels and

one D channel, as indicated by the notation 2B+D The two B channels are treated as separateconnections by the network and can carry voice and data or two data streams simultaneously

and separate from each other In a process called bonding, these two 64-Kbps B channels can

be combined to achieve an effective throughput of 128 Kbps—the maximum amount of datatraffic that a BRI connection can accommodate Most consumers who subscribe to ISDNfrom home use BRI, which is the most economical type of ISDN connection

Figure 7-10 illustrates how a typical BRI link supplies a home consumer with an ISDN link.From the telephone company’s lines, the ISDN channels connect to a Network Termination 1

device at the customer’s site The NT1 (Network Termination 1) device connects the

twisted-pair wiring at the customer’s building with the ISDN terminal equipment via RJ-11

(stan-dard telephone) or RJ-45 data jacks The ISDN TE (terminal equipment) may include

cards or standalone devices used to connect computers to the ISDN line (similar to anetwork adapter used on Ethernet or Token Ring networks)

FIGURE 7-10 A BRI link

So that the ISDN line can connect to analog equipment, the signal must first pass through a

terminal adapter A TA (terminal adapter) converts digital signals into analog signals for use

with ISDN phones and other analog devices (Terminal adapters are sometimes called ISDNmodems, though they are not, technically, modems.) Typically, telecommuters who want morethroughput than their analog phone line can offer choose BRI as their ISDN connection For

a home user, the terminal adapter would most likely be an ISDN router, whereas the terminalequipment could be an Ethernet card in the user’s workstation plus, perhaps, a phone

The BRI configuration depicted in Figure 7-10 applies to installations in North ica only Because transmission standards differ in Europe and Asia, different numbers

Amer-of B channels are used in ISDN connections in those regions

NOTE

PRI (Primary Rate Interface) uses 23 B channels and one 64-Kbps D channel, as represented

by the notation 23B+D PRI is less commonly used by individual subscribers than BRI is, but

it may be selected by businesses and other organizations that need more throughput As withBRI, the separate B channels in a PRI link can carry voice and data, independently of each other

or bonded together The maximum potential throughput for a PRI connection is 1.544 Mbps.PRI and BRI connections may be interconnected on a single network PRI links use the samekind of equipment as BRI links, but require the services of an extra network termination device,

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called a NT2 (Network Termination 2), to handle the multiple ISDN lines Figure 7-11

depicts a typical PRI link as it would be installed in North America

Individual customers who need to transmit more data than a typical modem can handle orwho want to use a single line for both data and voice may use ISDN lines ISDN, althoughnot available in every location of the United States, can be purchased from most local telephonecompanies Costs vary depending on the customer’s location PRI and B-ISDN are signifi-cantly more expensive than BRI Dial-up ISDN service is less expensive than dedicated ISDNservice In some areas, ISDN providers charge customers additional usage fees based on thetotal length of time they remain connected One disadvantage of ISDN is that it can span adistance of only 18,000 linear feet before repeater equipment is needed to boost the signal Forthis reason, it is only feasible to use for the local loop portion of the WAN link

Chapter 7 307

T-CARRIERS

FIGURE 7-11 A PRI link

T-Carriers

Another WAN transmission method that grew from a need to transmit digital data at high

speeds over the PSTN is carrier technology, which includes T1s, fractional T1s, and T3s carrier standards specify a method of signaling, which means they belong to the Physical layer

T-of the OSI Model A T-carrier uses TDM (time division multiplexing) over two wire pairs (onefor transmitting and one for receiving) to divide a single channel into multiple channels Forexample, multiplexing enables a single T1 circuit to carry 24 channels, each capable of 64-Kbps throughput; thus a T1 has a maximum capacity of 24 ×64 Kbps, or 1.544 Mbps Eachchannel may carry data, voice, or video signals The medium used for T-carrier signaling can

be ordinary telephone wire, fiber-optic cable, or wireless links

AT&T developed T-carrier technology in 1957 in an effort to digitize voice signals and therebyenable such signals to travel longer distances over the PSTN Before that time, voice signals,which were purely analog, were expensive to transmit over long distances because of the num-ber of connectivity devices needed to keep the signal intelligible In the 1970s, many busi-nesses installed T1s to obtain more voice throughput per line In the 1990s, with increased

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data communication demands, such as Internet access and geographically dispersed offices, T1sbecame a popular way to connect WAN sites.

The next section describes the various types of T-carriers, then the chapter moves on to rier connectivity devices

T-car-Types of T-Carriers

A number of T-carrier varieties are available to businesses today, as shown in Table 7-1 The

most common T-carrier implementations are T1 and, for higher bandwidth needs, T3 A T1

circuit can carry the equivalent of 24 voice or data channels, giving a maximum data

through-put of 1.544 Mbps A T3 circuit can carry the equivalent of 672 voice or data channels, giving

a maximum data throughput of 44.736 Mbps (its throughput is typically rounded up to 45Mbps for the purposes of discussion)

Table 7-1 Carrier specifications

and most of Asia In Europe, the standard high-speed carrier connections are E1 and

E3 Like T1s and T3s, E1s and E3s use time division multiplexing However, an E1

allows for 30 channels and offers 2.048-Mbps throughput An E3 allows for 480 nels and offers 34.368-Mbps throughput In Japan, the equivalent carrier standards

chan-are J1 and J3 Like a T1, a J1 connection allows for 24 channels and offers

1.544-Mbps throughput A J3 connection allows for 480 channels and offers 32.064-1.544-Mbpsthroughput Using special hardware, T1s can interconnect with E1s or J1s and T3swith E3s or J3s for international communications

NOTE

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The speed of a T-carrier depends on its signal level The signal level refers to the T-carrier’s

Physical layer electrical signaling characteristics as defined by ANSI standards in the early

1980s DS0 (digital signal, level 0) is the equivalent of one data or voice channel All other

signal levels are multiples of DS0

As a networking professional, you are most likely to work with T1 or T3 lines In addition toknowing their capacity, you should be familiar with their costs and uses T1s are commonlyused by businesses to connect branch offices or to connect to a carrier, such as an ISP Tele-phone companies also use T1s to connect their smaller central offices ISPs may use one ormore T1s or T3s, depending on the provider’s size, to connect to their Internet carriers

Because a T3 provides 28 times more throughput than a T1, many organizations may find thatmultiple T1s—rather than a single T3—can accommodate their throughput needs For exam-ple, suppose a university research laboratory needs to transmit molecular images over theInternet to another university, and its peak throughput need (at any given time) is 10 Mbps.The laboratory would require seven T1s (10 Mbps divided by 1.544 Mbps equals 6.48 T1s).Leasing seven T1s would prove much less expensive for the university than leasing a single T3.The cost of T1s varies from region to region On average, leasing a full T1 might cost between

$500 and $1500 to install, plus an additional $300 to $1000 per month in access fees Thelonger the distance between the provider (such as an ISP or a telephone company) and thesubscriber, the higher a T1’s monthly charge For example, a T1 between Houston and NewYork will cost more than a T1 between Washington, D.C., and New York Similarly, a T1 from

a suburb of New York to the city center will cost more than a T1 from the city center to a ness three blocks away

busi-For organizations that do not need as much as 1.544-Mbps throughput, a fractional T1 might

be a better option A fractional T1 lease allows organizations to use only some of the channels

on a T1 line and be charged according to the number of channels they use Thus, fractional T1bandwidth can be leased in multiples of 64 Kbps A fractional T1 is best suited to businessesthat expect their traffic to grow and that may require a full T1 eventually, but can’t currentlyjustify leasing a full T1

T3s are very expensive and are used by the most data-intensive businesses—for example, puter consulting firms that provide online data backups and warehousing for a number ofother businesses or large long-distance carriers A T3 is much more expensive than even mul-tiple T1s It may cost as much as $3000 to install, plus monthly service fees based on usage If

com-a customer uses the full T3 bcom-andwidth of 45 Mbps, for excom-ample, the monthly chcom-arges might

be as high as $18,000 Of course, T3 costs will vary depending on the carrier, your location,and the distance covered by the T3 In any event, however, this type of connection is signifi-cantly more expensive than a T1 Therefore, only businesses with extraordinary bandwidthrequirements should consider using T3s

site and the local telecommunications provider’s switching facility Connectivity hardware may

be purchased or leased If your organization uses an ISP to establish and service your T-carrierline, you will most likely lease the connectivity equipment If you lease the line directly fromthe local carrier and you anticipate little change in your connectivity requirements over time,however, you may want to purchase the hardware

T-carrier lines require specialized connectivity hardware that cannot be used with other WANtransmission methods In addition, T-carrier lines require different media, depending on theirthroughput In the following sections, you will learn about the physical components of a T-carrier connection between a customer site and a local carrier

Wiring

As mentioned earlier, the T-carrier system is based on AT&T’s original attempt to digitizeexisting long-distance PSTN lines T1 technology can use UTP or STP (unshielded or shieldedtwisted-pair) copper wiring—in other words, plain telephone wire—coaxial cable, microwave,

or fiber-optic cable as its transmission media Because the digital signals require a clean nection (that is, one less susceptible to noise and attenuation), STP is preferable to UTP ForT1s using STP, repeaters must regenerate the signal approximately every 6000 feet Twisted-pair wiring cannot adequately carry the high throughput of multiple T1s or T3 transmissions.Thus, for multiple T1s, coaxial cable, microwave, or fiber-optic cabling may be used For T3s,microwave or fiber-optic cabling is necessary

con-CSU/DSU (Channel Service Unit/Data Service Unit)

Although CSUs (channel service units) and DSUs (data service units) are actually two rate devices, they are typically combined into a single standalone device or an interface card

sepa-called a CSU/DSU The CSU/DSU is the connection point for a T1 line at the customer’s site The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring The DSU converts the T-carrier frames into

frames the LAN can interpret and vice versa It also connects T-carrier lines with terminatingequipment Finally, a DSU usually incorporates a multiplexer (In some T-carrier installations,the multiplexer can be a separate device connected to the DSU.) For an incoming T-carrierline, the multiplexer separates its combined channels into individual signals that can be inter-preted on the LAN For an outgoing T-carrier line, the multiplexer combines multiple signalsfrom a LAN for transport over the T-carrier After being demultiplexed, an incoming T-car-rier signal passes on to devices collectively known as terminal equipment Examples of termi-nal equipment include switches, routers, or telephone exchange devices that accept only voicetransmissions (such as a telephone switch)

Figure 7-12 depicts a typical use of a CSU/DSU with a point-to-point T1-connected WAN

In the following sections, you will learn how routers and switches integrate with CSU/DSUsand multiplexers to connect T-carriers to a LAN

Terminal Equipment

On a typical T1-connected data network, the terminal equipment will consist of switches,routers, or bridges Usually, a router or Layer 3 or higher switch is the best option, because thesedevices can translate between different Layer 3 protocols that might be used on the WAN andLAN The router or switch accepts incoming signals from a CSU/DSU and, if necessary, trans-lates Network layer protocols, then directs data to its destination exactly as it does on any LAN

On some implementations, the CSU/DSU is not a separate device, but is integrated withthe router or switch as an expansion card Compared to a standalone CSU/DSU, which mustconnect to the terminal equipment via a cable, an integrated CSU/DSU offers faster signalprocessing and better network performance In most cases, it is also a less expensive andlower-maintenance solution than using a separate CSU/DSU device Figure 7-13 illustrates

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T-CARRIERS

FIGURE 7-12 A point-to-point T-carrier connection

FIGURE 7-13 A T-carrier connection to a LAN through a router

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one way a router with an integrated CSU/DSU can be used to connect a LAN with a T1WAN link.

DSL

DSL (digital subscriber line) is a WAN connection method introduced by researchers at Bell

Laboratories in the mid-1990s It operates over the PSTN and competes directly with ISDNand T1 services Like ISDN, DSL can span only limited distances without the help of repeatersand is therefore best suited to the local loop portion of a WAN link Also, like its competitors,DSL can support multiple data and voice channels over a single line

DSL uses advanced data modulation techniques (which are Physical layer functions) to achieveextraordinary throughput over regular telephone lines To understand how DSL and voice sig-nals can share the same line, it’s helpful to recall that telephone lines carry voice signals over avery small range of frequencies, between 300 and 3300 Hz This leaves higher, inaudible fre-quencies unused and available for carrying data Also recall that in data modulation, a data sig-nal alters the properties of a carrier signal Depending on its version, DSL connection may use

a modulation technique based on amplitude or phase modulation However, in DSL, tion follows more complex patterns than the modulation you learned about earlier in thisbook The details of DSL modulation techniques are beyond the scope of this book However,you should understand that the types of modulation used by a DSL version affect its through-put and the distance its signals can travel before requiring a repeater The following sectiondescribes the different versions of DSL

modula-Types of DSL

The term xDSL refers to all DSL varieties, of which at least eight currently exist The

bet-ter-known DSL varieties include ADSL (asymmetric DSL), G.Lite (a version of ADSL),HDSL (High Bit-Rate DSL), SDSL (Symmetric or Single-Line DSL), VDSL (Very HighBit-Rate DSL), and SHDSL (Single-Line High Bit-Rate DSL)—the “x” in “xDSL” isreplaced by the variety name DSL types can be divided into two categories: asymmetricaland symmetrical

To understand the difference between these two categories, you must understand the concepts

of downstream and upstream data transmission The term downstream refers to data traveling from the carrier’s switching facility to the customer Upstream refers to data traveling from the

customer to the carrier’s switching facility In some types of DSL, the throughput rates fordownstream and upstream traffic differ That is, if you were connected to the Internet via aDSL link, you would be able to download images from the Internet more rapidly than youcould send them because the downstream throughput would be greater A technology that

offers more throughput in one direction than in the other is considered asymmetrical In

asym-metrical communications, downstream throughput is higher than upstream throughput metrical communication is well suited to users who receive more information from the networkthan they send to it—for example, people watching videoconferences or people surfing the

Asym-Web ADSL and VDSL are examples of asymmetrical DSL.

Conversely, symmetrical technology provides equal capacity for data traveling both upstream

and downstream Symmetrical transmission is suited to users who both upload and downloadsignificant amounts of data—for example, a bank’s branch office, which sends large volumes ofaccount information to the central server at the bank’s headquarters and, in turn, receives largeamounts of account information from the central server at the bank’s headquarters HDSL,

SDSL, and SHDSL are examples of symmetrical DSL.

DSL versions also differ in the type of modulation they use Some, such as the popular rate ADSL and VDSL, create multiple narrow channels in the higher frequency range to carrymore data For these versions, a splitter must be installed at the carrier and at the customer’spremises to separate the data signal from the voice signal before it reaches the terminal equip-ment (for example, the phone or the computer) G.Lite, a slower and less expensive version ofADSL, eliminates the splitter but requires the use of a filter to prevent high frequency DSLsignals from reaching the telephone Other types of DSL, such as HDSL and SDSL, cannotuse the same wire pair that is used for voice signals Instead, these types of DSL use the extrapair of wires contained in a telephone cable (that are otherwise typically unused)

full-The types of DSL also vary in terms of their capacity and maximum line length A VDSL linethat carries as much as 52 Mbps in one direction and as much as 6.4 Mbps in the oppositedirection can extend only a maximum of 1000 feet between the customer’s premises and thecarrier’s switching facility This limitation might suit businesses located close to a telephonecompany’s central office (for example, in the middle of a metropolitan area), but it won’t workfor most individuals The most popular form of DSL, ADSL, provides a maximum of 8 Mbpsdownstream and a maximum of 1.544 Mbps upstream However, the distance between the cus-tomer and the central office affects the actual throughput a customer will experience Close tothe central office, DSL achieves its highest maximum throughput The farther away the cus-tomer’s premises, the lower the throughput In the case of ADSL, a customer 9000 feet fromthe central office can potentially experience ADSL’s maximum potential throughput of 8 Mbpsdownstream At 18,000 feet away, the farthest allowable distance, the customer will experience

as little as 1.544-Mbps throughput Still, this throughput and this distance (approximately 3.4miles) renders ADSL suitable for most telecommuters Table 7-2 compares current specifica-tions for six DSL types

Table 7-2 Comparison of DSL types

Maximum Upstream Maximum Downstream Distance

ADSL “full rate”) 1 8 18,000

G.Lite (a type of ADSL) 0.512 1.544 25,000

HDSL or HDSL-2 1.544 or 2.048 1.544 or 2.048 18,000 or 12,000

SHDSL 2.36 or 4.7 2.36 or 4.7 26,000 or 18,000 VDSL 1.6, 3.2, or 6.4 12.9, 25.9, or 51.8 1000–4500

Chapter 7 313

DSL

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In addition to their data modulation techniques, capacity, and distance limitations, DSL typesvary according to how they use the PSTN Next, you will learn about how DSL connects to abusiness or residence over the PSTN.

DSL Connectivity

This section follows the path of an ADSL connection from a home computer, through the localloop, and to the telecommunications carrier’s switching facility Although variations exist, thisdescribes the most common implementation of DSL

Suppose you have an ADSL connection at home One evening you open your Web browserand request the home page of your favorite sports team to find the last game’s score As youknow, the first step in this process is establishing a TCP connection with the team’s Web server.Your TCP request message leaves your computer’s NIC and travels over your home network

to a DSL modem A DSL modem is a device that modulates outgoing signals and

demodu-lates incoming DSL signals Thus, it contains receptacles to connect both to your incomingtelephone line and to your computer or network connectivity device Because you’re usingADSL, the DSL modem also contains a splitter to separate incoming voice and data signals.The DSL modem may be external to the computer and connect to a computer’s Ethernet NICvia an RJ-45, USB, or wireless interface If your home network contains more than one com-puter and you want all computers to share the DSL bandwidth, the DSL modem must con-nect to a device such as a hub, switch, or router, instead of just one computer In fact, ratherthan using two separate devices, you could buy a router that combines DSL modem function-alities with the ability to connect multiple computers and share DSL bandwidth A DSLmodem is shown in Figure 7-14

Published distance limitations and throughput can vary from one service provider toanother, depending on how far the provider is willing to guarantee a particular level ofservice

NOTE

FIGURE 7-14 A DSL modem

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When your request arrives at the DSL modem, it is modulated according to the ADSL ifications Then the DSL modem forwards the modulated signal to your local loop—the linesthat connect your home with the rest of the PSTN For the first stretch of the local loop, thesignal continues over four-pair UTP wire At some distance less than 18,000 feet, it is com-bined with other modulated signals in a telephone switch If this switch is not in a central office,

spec-it forwards your request—this time over fiber-optic cable or a high-speed wireless link—toanother switch at the central office (To accept DSL signals, your telecommunications carriermust have newer digital switching equipment In areas of the country where carriers have notupdated their switching equipment, DSL service is not available.)

Inside the carrier’s switching facility, a splitter separates your line’s data signal (the TCP request)from any voice signals that are also carried on the line Next, your request is sent to a device

called a DSLAM (DSL access multiplexer), which aggregates multiple DSL subscriber lines

and connects them to a larger carrier or to the Internet backbone, as pictured in Figure 7-15.The request travels over the Internet until it reaches your sports team’s Web server Barringline problems and Internet congestion, the entire journey happens in a fraction of a second.After your team’s Web server accepts the connection request, the data follows the same path,but in reverse

Currently, ADSL is the most common form of DSL, but standards continue to evolve.Telecommunications carriers and manufacturers have positioned DSL as a competitor for T1,ISDN, and broadband cable services The installation, hardware, and monthly access costs forDSL are slightly less than those for ISDN lines and significantly less than the cost for T1s.(At the time of this writing, ADSL costs approximately $30 per month in the United States.)Considering that DSL technology can provide faster throughput than T1s, it presents a for-midable challenge to T-carrier services for business customers.

One drawback to DSL is that it is not available in all areas of the United States, either becausecarriers have not upgraded their switching equipment or because customers do not reside withinthe service’s distance limitations In addition, in its early years DSL was more expensive thanbroadband cable, its main competition among residential customers For these reasons, two-thirds of consumers in the United States use cable for broadband Internet access service

Broadband Cable

While local and long-distance phone companies strive to make DSL the preferred method ofInternet access for consumers, cable companies are pushing their own connectivity option This

option, called broadband cable or cable modem access, is based on the coaxial cable wiring

used for TV signals Such wiring can theoretically transmit as much as 56 Mbps downstreamand as much as 10 Mbps upstream Thus, broadband cable is an asymmetrical technology Real-istically, however, broadband cable throughput is limited (or throttled) by the cable companies,

so that customers are allowed, at most, 3-Mbps downstream and 1-Mbps upstream put The asymmetry of broadband cable makes it a logical choice for users who want to surfthe Web or download data from a network Some companies are also delivering music, video-conferencing, and Internet services over cable infrastructure

through-Broadband cable connections require that the customer use a special cable modem, a device

that modulates and demodulates signals for transmission and reception via cable wiring Cablemodems operate at the Physical and Data Link layer of the OSI Model, and therefore do notmanipulate higher-layer protocols such as IP or IPX The cable modem then connects to acustomer’s PC via an RJ-45, USB, or wireless interface to a NIC Alternately, the cable modemcould connect to a connectivity device, such as a hub, switch, or router, thereby supplying band-width to a LAN rather than to just one computer It’s also possible to use a device that com-bines cable modem functionality with a router; this single device can then provide both thebroadband cable connection and the capability of sharing the bandwidth between multiplenodes Figure 7-16 provides an example of a cable modem

Before customers can subscribe to broadband cable, however, their local cable company musthave the necessary infrastructure Traditional cable TV networks supply the infrastructure fordownstream communication (the TV programming), but not for upstream communication Toprovide Internet access through its network, the cable company must upgrade its existing equip-ment to support bidirectional, digital communications For starters, the cable company’s net-

work wiring must be replaced with HFC (hybrid fiber-coax), an expensive fiber-optic link that

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