ETWORKI G

users who want to interconnect multipleL or L s with particularly high-vol­ume traffic. The routing function in id ofthe device decides which line to use forany given packet.

Cisco Systems, Inco's AGS+ routersupports more than 20 T1lines. The linescan go to 20 different locations, or multi­ple lines can go to one location, providingincreased bandwidth and some protectionagainst line failures.

Dedicated T1lines are not always ec0­

nomically feasible, however, and trafficdoes not always travel in a teady stream,

pecially be een LAN . With T1 linescosting vera! per mile permonthin a national net , it .sense to allocate the T1 bandwidth amongseveral different applications, including

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channel, as well as the distance being cov­ered. In some cases, you won't need anyof these new options.

Such would be the case when a net­work manager wants to integrate severaldifferent work groups into a single, logicalnetwork. Most bridges and routers nowsupport 1.5M bit/sec. Tl links, hichshould provide sufficient bandwidth fortwo 10 bit/sec. regularly ex­changing data within a metropolitan area.Some LAN interconnectivity deviceseven support multiple T1links for those

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BY CARL MALAMUDSPECIAL10cw

ologies fo I- king LAetwor se erge in r

Fast packet, frame relay and cell relay:You've heard of them in relation to linkinglocal-area networks across a T1 circuit(see story page 77). ow that the technol­ogies are starting to show up in the formof real and announced products, you maybe wondering which capabilities to choosefor your LAN interconnection scheme.

The decision depends on the amount,type and speed of traffic crossing the T1

Tee1

ten ively train d by v ry one

of the t p manufactur r .

That why w kn w xactly

which n tworking produc can

and can t work for you.

Wh ther th y adhere to

indu try tandard. And wh ther

th y have the functionality and

compatibility to make your inv t-

LAN interconnection, voice and video.One way to do thi is through a T1 mul­

tiplexer, which rves as an entry point tothe T1 network for both LAN intercon­nectivity devices and other cu tomer pre­mi equipment such as private branchexchange. The multiplexer allocatessubchannels within the T1 link to LANs,voice, video and other applicationthrough circuit-switching technology.

While circuit switching is an inexpen­sive technique, it also requires dedicatedbandwidth allocation, which can be ineffi­cient if the traffic on each circuit is notcontinuous. The technologies that aretarting to address more flexible band-. th allocation are frame relay and its

counterpart, cell relay. The CCITT stan­dard- version of cell-relay technol-

,...--------- ---. ogy. called ynchronous Transfer

Mode. Frame relay and cell relay are bothfast-packet technologies.

Frame relay· both an interface and atransport service that handles packets ofvariable length. By contrast, cell relay in­terfaces and transports packets of fixedlength. Frame- and cell-relay interfacesand transport services can be mixed andmatched. For example, Stratacom's

12,500 packet assembler/disassembler(pAD) for its IPX T1 multiplexer usesframe relay on the application side of itsinterface and a proprietary cell-relaytechnique on the wide-area network side.

When both nodal processors - on thesending and receiving end - supportpacket-switching technologies, such asframe and cell relay, they can dynamicallyallocate bandwidth for applications asneeded instead of fixing a maximum foreach one. For instance, if data and voice

a Tl pipe, the nodal processor can8QlI~~ a little more data into the gapsbe een words, giving more rapidthroughput for the data application.

u h- rtraFrame relay parallels X.25 packet switch­ing in that it allows a LAN interconnec­tion device to tell a nodal processor itwishes to set up a virtual circuit in the pri­vate network. It is also specifically de­signed to handle traffic nt in bursts. It isfaster than traditional packet-switching inthat it eliminates error-detection and cor­rection on each network link, as does cellrelay. Error-ehecking at only the originand destination nodes is made possible bytoday's highly reliable communicationsmedia, such as fiber-optic cabling.

Frame-relay products are just startingto hit the market. Cisco and Stratacomare shipping products, and Codex Corp.,which has a 2096 investment in Stratacomand resells the IPX as its 6290 Tl multi­plexer, said it will also support frame relayon its 6525 X.25 packet switch and 6507PAD in first-quarter 1991.

While frame relay has shown up only inequipment for private networks, it can beused as an interconnection to a public datanetwork. In this case, the user needs topurchase only the bridge, router or gate­way that supports frame relay and let thecarrier provide the nodal processors andtransport service. U.S. Sprint Communi­cations Co. has announced frame-relayservice for late 1991, and orthem Tele­com, Inc. has announced equipment thatwill allow other carriers to offer publicframe-relay-based services in the future.

Senio,. write,. joanu M. Wezle,. con­tributed to tltis report.

DEC 61