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Page 1: Frame Relay - Understanding it

In simple terms, Frame Relay is apacket-switched network. A"frame", in the context of Frame

Relay, is a packet of data.When a data stream enters a Frame

Relay network it is broken into framesand each frame is sent across the net-work to the destination point. Theframes contain header informationwhich tells the intervening networknodes where to route them, and whichoutput port to use.

At any one time an individual linkin a Frame Relay network is carryingframes from many different sources enroute to many different destinations.This frame mixing or multiplexing isone of the keys to Frame Relay’s ad-vantages - it offers benefits over leasedlines in terms of performance, cost-sav-ing, manageability and resilience.

ComparisonsIt’s worth comparing traditional

point-to-point links with Frame Relaylinks in order to discuss the differencesbetween the two methods.

A traditional leased line is a point-to-point link. You pay for the lineirrespective of whether there is trafficon it. Consequently the line cost effi-

ciency, its cost per unit of data sent, canvary. When an organisation hasmultiple locations then connectingthem together using leased lines canbecome very expensive.

With leased lines, each message hasto complete before the next messagecan travel along the line. This can causeresponse time problems at sending/re-ceiving terminals and PCs. Forexample, a 3270 terminal may be slowto respond to a transaction request sentvia SNA over a leased line. With FrameRelay the frames from one data sourcecan be intermingled with those fromanother.

Because a Frame Relay line cancarry more data it is often more cost-ef-fective than leased lines. Users havereported substantial cost savings, 20 to30%, for example, as a result of substi-tuting a Frame Relay network for aleased line network.

ReliabilityWith leased lines there is significant

management overhead on the cus-tomer. Also Frame Relay networks canroute frames around failed links ornodes. Leased lines cannot, and usersof them often have to have dial-up lines

available as backups. Frame Relay isreliable enough that these backups canbe dispensed with, saving both costand management overhead. FrameRelay networks can be reconfiguredquite quickly so that, for example, abackup data centre can be used. Thiskind of reconfiguration would be im-possible with leased lines.

Network DescriptionA user will typically have a private

line to a node on a Frame Relay net-work. Line speed is fixed, and will besomewhere in the range of 56 Kbps to2 Mbps depending on the service thathas been purchased.

The network itself is composed oflines connecting nodes (also known asswitches). The receiving location alsohas a private line to a Frame Relaynode.

A permanent virtual circuit, or PVC,is defined to link the sending and re-ceiving end points. The circuit isbi-directional. Frames are routedacross the network from sender to re-ceiver using header information whichis added to the incoming data stream.

Note that it may be possible to haveswitched access to Frame Relay by, forexample, dialling up an access point onthe Frame Relay network over an ISDNinterface. Data then flows from the useracross an ISDN network and then intothe Frame Relay network. Each logicalconnection from a site via ISDN uses asingle ISDN channel; they cannot bemultiplexed into one ISDN channel.This may be a cost-effective way ofconnecting remote sites with low datatraffic rates to a Frame Relay network.

All the nodes have entry and exitports and a particular route throughthe network involves each node know-

UnderstandingFrame Relay

Who’s using Frame Relay technology, and why? Can it help ease your own comms bottlenecks? Chris Mellor reports.

Issue 74 Page 3 File: C0480.1

"Because a Frame Relay line can carrymore data it is often more cost-effective

than leased lines. Users have reported substantial cost savings,

20 to 30%, for example."


PC Network Advisor

Page 2: Frame Relay - Understanding it

ing which exit ports to use for framesin a message. Each frame has a datalink connection identifier - DLCI -which is used by the nodes to choosethe right exit port. A DLCI is not con-stant across the network. It is of onlylocal significance to a Frame Relaynode. The routing tables in each nodefor a PVC take care of alternately read-ing and assigning DLCI values inframe headers before they send theframe on to the next node.

When PVCs are first defined mis-in-stallation of DLCI numbers can be acommon error that prevents propermessage transmission and receipt.

The FRADData enters a Frame Relay network

by passing through a FRAD, which iseither a Frame Relay Access Device orFrame Relay Assembler/Disassemblerdepending on who you speak to. Eitherway, a FRAD is typically a router.

The FRAD breaks the data streamdown into sections (frames), adds theheader information and a check digit atthe end, and sends the frames across thenetwork. At the exit point of the net-work the frames are reassembled intoa continuous data stream once more.

Each frame contains 5 bytes ofheader information. This header size isconstant irrespective of how large theframe is. The larger the frame, thelower the overhead and the more effi-cient Frame Relay is at turningtheoretical bandwidth into availablebandwidth.

A Frame Relay node takes no inter-est in user data within a frame at all. Itlooks for a flag that starts a frame, theheader, and then for a check digit thatmarks the end of the user data.

Frames can be of variable size. Eachone is transmitted as a stand-alone en-

tity. If it gets corrupted en route throughthe network it is dropped. There is noframe error checking and recoverywithin the network. That is the respon-sibility of software that uses thenetwork. Thus Frame Relay requires avirtually error-free transmission sys-tem for this approach to be tenable.

Renting SpaceA telcomms company (telco to those

in the industry) will take a leased line,a T1, for example, and sell you FrameRelay bandwidth on it. So you are, ineffect, using only part of the T1 line.There may be 40 or more customerseach with their own Frame Relay band-width on the line. Frames arestatistically multiplexed on it.

The telco is sharing lines and nodesbetween many customers. This is onereason why a 56 Kbps Frame Relaynetwork is cheaper than a 56 Kbpsleased line.

CIRA user commits to deliver data to a

PVC at a Committed Information Rate(CIR), which is expressed as bits persecond. There could be two 28 KbpsCIR PVCs defined over a single 56Kbps interface to the network. Sincedata has to be submitted at the linespeed, 56 Kbps for example, the CIR isan average over a period of a few sec-onds. If you under-submit it doesn’tmatter. If you over-submit (called "bur-sting over your CIR"), excess framesmay be discarded if the network getscongested.

When you’re renting Frame Relaylines, you need to discuss CIR figureswith the company to ensure that theline can cope with the amount of trafficyou intend to throw at it.

Congestion occurs when more datais attempting to cross the network thanit can handle. When this is detected acongestion bit is added to a frameheader to tell the sending FRAD itought to slow down. It will then keepframes in its buffers until it stops re-ceiving congestion bits.

If buffer space fills up, or there isnone, then frames are discarded. Thenetwork knows the CIR for a senderand discards frames that have had aDiscard-Enable (DE) bit set in theirheader meaning that they represent aframe in excess of the sender’s CIR.

Some FRADs can set the DE bit tosignal that the frame is low priority.Thus senders can divide messages intonormal and low priority groups.

UNIThe UNI is the User Network Inter-

face and defines what user devicesneed to do to initiate, operate and re-ceive Frame Relay transmissions.

The PVCWhen a location or device is given

access to an existing Frame Relay net-work it means wiring in the access portand then configuring a permanent vir-tual circuit - PVC. A unique PVCconnects each of the user’s sendingnodes with each of their receivingnodes. Each PVC is defined by DLCIswith routing tables that designate theentry and exit ports on a node in thenetwork.

The PVC is ready to use wheneverdata needs to be sent. This keeps calllatency low.

Automatic Re-routingIf a node or line in a Frame Relay

network goes down, the network isautomatically reconfigured to routearound the failed component. This is amatter of re-setting routing tables ineach node on the network so that PVCsare redefined.

SVCThe Frame Relay specification

defines Switched Virtual Circuits -SVCs - as well as PVCs. A calling de-

File: C0480.2 Issue 74 Page 4

"Frames can be of variable size. Each oneis transmitted as a stand-alone entity. If it gets corrupted en route through

the network it is dropped."


PC Network Advisor

Page 3: Frame Relay - Understanding it

vice would request a connection to adestination device using internation-ally-recognized X.121 or E.164numbering plans. SVC services may becheaper than PVC services in circum-stances where there is a low traffic rateor low connection rate. Also SVC ser-vices mean that users don’t have topre-configure and manage PVCs andcan get additional bandwidth on de-mand. This is useful where networksare in a state of flux.

When an SVC is set up, any encap-sulation procedures needed are agreedduring the setup. The CIR is alsoagreed at that time. Note that Multi-casting is only available over PVCs, notSVCs.

LMIWhen there is no data passing

across from the network to a user thenetwork is polled every 10 seconds orso and should return a "keep alive"message signifying that the link isoperational. This polling is part of theLine Management Interface - LMI. Thelink will be assumed to be down if acertain number of keep alive failuresoccurs. This allows noise on the line tobe accommodated.

Frame Relay ForumIn 1991, 42 Frame Relay suppliers

formed the Frame Relay Forum. Thereare now over 300 members and it hasproved to be a very effective body inadvancing market take-up of FrameRelay technology.

The Forum has three subgroupswhich work to develop forum propo-sals. The Market Development andEducation Committee aims to stimu-late interest in the technology and itsbenefits as well as serving as a usergroup. Multivendor issues are the con-cern of the Interoperability and TestingCommittee.

The Technical Committee addressestechnical issues to encourage inter-operability and developments ofFrame Relay technology. There arefour main areas of activity: User Net-work Interfaces (UNI), Network-to-Network Interface (NNI), MulticastService and Multiprotocol Encapsula-tion Procedures.

The committee develops UNI stand-ards and conformance tests forvendors to check their products andservices. Items in the standard may bemandatory, highly desirable or notcritical.

Transferring data between networknodes belonging to different FrameRelay vendors is where the NNI issuessurface. A user may use Frame Relayservices from two or more carriers witheach carrier providing a segment ofthat user’s Frame Relay network.Whole PVCs are broken down intoPVC segments provided by each car-rier. The sum of the segments makes upa complete PVC. The committee de-cides what each network has to do tosupport such interoperability. A peer-to-peer interface operates between thecarriers providing each network seg-ment.

When a network detects that a UserNetwork Interface or NNI is inopera-tive, each network notifies the adjacentnetwork via the NNI that the PVC isinactive. The PVC status change is pro-pagated through the adjacentnetworks to the remote users. The NNIalso covers congestion managementprinciples and CIR coordination be-tween the network providers. Ideally auser should see and receive the sameservice from a multi-carrier FrameRelay network as from a single carriernetwork. This is what the committee isworking towards.

Multicasting, a supplementaryFrame Relay service, is the facility toaccept a frame at a UNI and broadcastthe frame to multiple destinations. InOne-Way multicasting, nodes in theFrame Relay network have an extraPVC. Frames transmitted on this PVCwill be delivered to all the neighbour-ing nodes. This is helpful formanagement traffic like routing tableupdates.

Two-Way multicasting allows asingle point or "root" to multicast dataunits to a specified group of users.Frames transmitted by the root are seenby all group members. Frames trans-mitted by group members aredelivered only to the root. This is usefulfor remote learning applications.

With N-Way multicasting framestransmitted by any group member areseen by all group members. This is use-

ful for conferencing situations. Multi-casting destinations may be on onenetwork or multiple networks.

LAN ConnectionsWhen Frame Relay is used to inter-

connect LANs then the LAN traffic(Ethernet or Token Ring, for example)is encapsulated in the frames so thatthere is no logical break in the LANstructure; the interconnected LANsseem to be a single one. MultiprotocolEncapsulation Procedures describe themethods used to do this. They coverother protocols as well and also includebridging and routing between LANs.

SNA And Frame RelaySNA is another packet switching

protocol developed from broadbandISDN. SNA links may well have usedleased 9.6 Kbps lines, either point-to-point or multidropped. Frame Relaywill provide much more speed thanthis. It will also support multiple proto-cols which the SNA links cannot. AsIBM sites have added PCs on LANsand Unix systems with TCP/IP to itsSNA 3270, LU6.2 etc networks, FrameRelay is an attractive option forcarrying these multiple protocols. With-out it, multiple SNA lines have to beinstalled.

IBM has ensured that its SNA pro-ducts support Frame Relay andsupports all SNA topologies acrossFrame Relay networks. For example, in1994 IBM released a new version of itsNetwork Control Protocol, NCP 7.1,that allowed SNA networks to fullyutilise Frame Relay. Also PS/2s run-ning OS/2 can have RoutExpander/2software and Wide Area Concentrator- WAC - hardware which enables themto access a Frame Relay network andfunction as a gateway for a small site.AIX RS/6000s can be connected to amainframe host via a Token Ring LANand Frame Relay rather than by SNAlinks between the mainframe and theUnix box. The same goes for terminalsand mainframes.

ATM And Frame RelayBoth Frame Relay and ATM

evolved from the broadband ISDN

Issue 74 Page 5 File: C0480.3

Frame Relay


PC Network Advisor

Page 4: Frame Relay - Understanding it

standards developed in the 1980s. Theessential difference is ATM’s fixedlength cell size versus Frame Relay’svariable length frames.

ATM has a fixed 53-byte cell size.That means the header overhead isconstant. If cells are not filled with datathen the header overhead as a percent-age of network traffic grows. ThusATM may be less efficient than FrameRelay when the data load factor in cellsis low.

Many analysts think that FrameRelay and ATM will co-exist with aneventual trend for ATM to capture thebackbone traffic and Frame Relay be-coming a submitting route to an ATMbackbone. ATM may spread to desktopdevices where its ability to cope wellwith delay-sensitive traffic likemultimedia make it suitable. In general,it is thought that Frame Relay transfersdata more efficiently up to 56 Kbps withATM being better at higher rates.

The Frame Relay Forum, togetherwith the ATM Forum, has defined aFrame Relay/ATM Network Inter-working Implementation Agreementto cover this area. They have also beenworking on Frame Relay-to-ATMprotocol conversion.

Voice And Frame RelayVoice traffic is very sensitive to

delay. If a voice input stream is brokenup into frames which are sent one afterthe other across a network and reas-sembled at the other end, the listenershould hear normal telephone speech.However, if there are intervals be-tween the frames then the speechbecomes disjointed with pauses be-tween sections. The sections may notbe at word or sentence boundarieswhich lowers the perceived quality stillfurther.

Frame Relay’s inability to prioritiseframes effectively doesn’t help andneither does the use of DE bit setting onframes above the CIR. You just cannotdiscard voice frames. Thus FrameRelay has been considered unsuitablefor voice traffic.

However, there are moves to enablevoice traffic to be sent over FrameRelay. The forum’s technical commit-tee is working on a framework for aVoice Over Frame Relay implementa-tion. Some vendors, like Scitec, havevoice solutions ready today. The com-mittee is also working on datacompression which should help fur-ther and make Frame Relay moresuitable for multimedia traffic.

Voice-capable FRADS chop bigframes up into small ones using ATMsegmentation and re-assembly (SAR)techniques. This stops large frames de-laying voice frames and hencelowering voice transmission quality.Integrating voice and data on a FrameRelay network can be effective for com-panies with international links. Voicecalls between countries are expensiveand carrying voice over Frame Relaycan save millions of dollars annually.

X.25 And Frame RelayIn Europe, X.25 lines have been

popular for LAN-to-LAN and SNAconnections. Frame Relay offers lowercall latency, faster performance andcheaper line cost. It also has a lowerdata overhead per message.

In X.25 the network handles errordetection and retransmission. FrameRelay networks let the user do this. Forexample, with the TCP/IP protocol,TCP establishes robust transport-levelconnections across a network and IP, alower level protocol in the ISO schemeof things, carries data packets across

the network. This combination handlesretransmission of data if errors occur.It is a perfect fit for Frame Relay.

The FutureThe boom in Internet usage is

prompting a growth in Frame Relaybecause Internet Access Providers useit to connect their systems to the Inter-net. In general Frame Relay is stillgrowing. The US market for it shouldreach in excess of $1.2 billion this year.It is expected by the Frame RelayForum that ATM backbones willspring up inside Frame Relay net-works but, overall, Frame Relaynetworking should increase in popu-larity.

File: C0480.4 Issue 74 Page 6


The AuthorChris Mellor is a freelance writerand consultant and can be con-tacted as [email protected].

For further information, contact theFrame Relay Forum at:

The Frame Relay Forum,303 Vintage Park Drive,Foster City,CA 94404,USA.Phone: +1 415 525 6916Fax: +1 415 578 6980Email: [email protected]

AT&T’s Web site at www.att.com/in-terspan/overview/att00171.html alsocontains useful information.

"The Frame Relay Forum, together withthe ATM Forum, has defined a Frame

Relay/ATM Network InterworkingImplementation Agreement."


PC Network Advisor