atm local area networks

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TMLocal Area Networks

LAN emulation offers a best-effort, connectionless, packet transfer service at theMAC sublayer, implemented on top of a connection-orientedATM network.

Peter Newman

uch interest has been expressed

in asynchronous transfer mode( A T M ) t e c h n o l og y , d u e t o i tsflexibility and its supp ort of m ulti-media traffic. Interest first camefrom the carr iers and the manu-

facturersofwideareanetworkingequipment, ndnowinterest is growing in the applica tion of A TM tech-nology to the local and cam pus area networkingenvironments.ATMoffersmuchgreatercapacitythanexisting shared medium LANs. It is scalable in thatthe capacity of a n ATh4 system isnot fundamentallylimited by the technolog y itself. Also it is designedto supp or t mul t imedia t ra f fic and i s capab le o fofferingseamless ntegrationwithwide rea ATM net-works, both public and p rivate.

Much of th e cur re n t d i scuss ion jus t i f i es theintroduction of AT M technology into the local area

on the basis of its ability to handle multim edia traf-fic. Howev er, the move toward A TM is equally like-ly to be driven by ATM’s more m unda ne benefits ofincreased bandwidth, and much gre ater manage-ability, for the continually increasing volume of reg-ular da ta commu nications traffic.

LAN Service Requirements

rATM technology o be successfully ntroducedF“ nto cus tom er p remises, i t mus t o f fe r LAN -l ik e s e r v ic e f o r d a t a t r a f f i c a n d b e c o m p a t i b l ewith the exist ing data commun ications protocols,app l ica tions , and equ ip ment . P resen t ly , LANsof fe r connec t ion less , “ bes t e f fo r t” ( i .e . , lost o rcorrup ted packets are not retransmitted) service orthe t rans fe r o f var iab le s ize da ta packe ts . They

o f f e r p o i n t - t o - p o i n t , m u l t i c a s t , a n d b r o a d c a s tt r a n s f e r . M a n y c u r r e n t p r o t o c o l s r e l y on t h ebroadcast capability. Users are not required to estab-l i sh a c o n n e c t i o n b e f o r e s u b m i t t i n g d a t a f o rtransmission,nor ar e hey required to define the traf-f i c c h a r a c t e r i s t ic s o f t h e i r d a t a i n a d v a n c e o ftransmission.Users simply submit traffic o the LANwhenever theywish,asfast as possible, and the L AN

PETER NEWMAN s

responsiblefor the system

architecture OfA TMswi tch

products at N.E .T Adaptive.

d y n a m i ca l ly s h a r e s t h e a v a i l ab l e b a n d w i d t h

between all active users.Most LANequipment conforms to the IE EE 802family of protocols (Fig. 1) . In this architecture,the data link layer is split into the logical link con-t r o l ( L L C ) a n d m e d i u m a c c e s s c o n t r o l ( M A C )sublayers.Th e LLC sublayer offers a comm on inter-f a c e t o t h e n e t w o r k l a y e r , w h il e e a c h d i f f e r e n tMA C pro toco l i s spec i f ic to a par t i cu la r LA N,e.g., ca me r sensemultiple accesswithcollisiondetec-t ion (CSM NC D) , Token Ring , Token Bus, e tc .All stationson an IEE E802 LAN are a ddressed usingagloballyunique48-bit individualaddresswith afla taddress space . Group addresses may be def inedfor multicast groups and a well-known broadcastaddress is also defined.

LANs are frequently nterconn ected with bridgesand rou ters to form larger networks. Bridges oper-

ateat theMACsublayer,andarepopularbecause heyrequire very l i t t le man ual configurat ion and aretransparent to the user. B ridges interconnect multi-ple LAN segments yet give the appearance to theuser of a single LAN. Route rs operate at the net-worklayerbutsupportonlyaffitesetofnetworklayer

protoco ls (not all protocols in commercial use a reroutable) . They offer greater control , bet ter man-ageme nt faci l i t ies , and may be used t o constructmuch large r networks than bridges.

Transparen t bridging is defined in two forms:loca l and remo te [l] Fig. 2). A local bridge con-nects LAN s directly attached to its ports. A remoteb r i d g e c o n n e c t s L A N s a c r o s s a n o n - I E E E 802interconnecting medium , typically a wide area net-work (e.g., X.25, frame relay, or T1private lines). Aremote bridge encapsulates each packet from the

source LAN within a protocol specific to th e inter-connecting medium. T he original packet is removedfrom the encapsulation at the remote bridge of thedest ination LAN. Comm unication between a sta-tion connected directly o the interconnecting medi-u m a n d a s t a t i o n o n a L A N i s n o t p e r m i t t e dunless the former run s the encapsulation bridgingprotocol.

86 0163-6804/94/ 04.00 1994IEEE IEEE Communications Magazine March 1994

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W Figure 1 The IEEE 802 family of LANprotocols.

It is possible to interfac e ATM directly to thetransport layer or the netwo rklayer of the OS1model.This offers efficiency by avoiding the un necessarycomplexity of the data link layer. How ever, her e aremany network layer protocols, and each on e wouldhave to be interfaced to ATM separately. To offergenera l compatibi l i ty with the instal led base ofnetworks and protocols, regardless of the networklayer and upper layer protocol stack, and to supporttransparent MA C bridging, an interface at the MA Csublayer is requ i r ed . This wil l pe rmi t the hugelegacy of existing LAN applications o m igrate to theATM environment without major upheaval.

Thu s a MAC sublayer should be deve lopedfor ATM LANs that offers the sam e connection-l e s s M A C s e r vi c e a s t h e I E E E 802 a n d F D D IMA C sublayers . Also we requ i re th e ab i l i ty toper form MA C layer b r idg ing be tween s ta t ionsa t tached d i rec t ly to an A TM LAN and s ta t ionsconnected o an IEE E 802LAN. Suchbridges shouldoffer t ransparent local bridging. T he ATM net-workis itself aLAN , and not simplyanetwork acrosswhich LANsmaybe interconnected,soremote bridg-i n g w o u l d b e i n a d e q u a t e . ( R e m o t e b r i d g in gwould require a station on the ATM network o knoww h e t h e r t h e d e s t i n a t i o n w a s a ls o o n t h e A T Mnetwork , o r on a n I E E E 802 network connectedvia a bridge, in order to select the required encap-

sulation protocol.)

Connectionless ServiceImplementation

TM switches are connection-oriented and d oA ot naturally support a connectionless service.S ince we have dec ided tha t an ATM LAN m ustoffer a connectionless service at the M AC sublay er,it is reasonable to consider implementing hat servicewith a connectionles s erver (a CLSF in ITU-speak)[2,3]. This is the proposed implementation for th eswitched multimegabit data service (SMD S) l] ndalso the B-ISDN connectionless data service (speci-fied in the ITU -T 1.364 recomm endation).

In itssimplestformaconnectionlessserversapack-

et switch at tached to an AT M switch. All vir tualchannelscarrying raffic hat requiresa connectionlessswitching service is directed by th e ATM switch tothe connec t ion less se rver . The connec t ion lessserversareconnected ogetherw ithvirtual aths throughthe ATM switches o form a “virtualoverlaynetwork,”(Fig. 3) . This is basical ly the same architec tural

solution as narrowband ISDN ntegrated accessto se para te switching facilities.

Implement ing the connec t ion less se rver as apacket switch separately from the ATM switch hasa number of disadvantages.Primarily, it substantial-ly restricts the bandwidth available for switchingconnectionless traffic. Thu s the ATM promise ofhigh bandw idth and scalable capacity is lost. Also itn e g a t e s t h e p o s s i b il i ty o f a s i n g l e i n t e g r a t e dswitching mechan ism for multim edia services. It is

an appro ach based upon th e existing solution todata netw orking. This approach is popular with thepublic carr iers because i t confines the statis t icalmultiplexing to the connectionless server and avoidsthe requirem ent to sup port statistical traffic directlyin the AT M ne twork . I t may be a use fu l pub l icservice for sending the occ asional datagram , but inthe local a rea, statistical switching directly at theATM layer will offer far greater performa nce.

On the Fly ConnectionlessImplementation

I n a n a l t e r n at i v e i m p l e m e n t a t i o n , h e c o n n e c -tionless server is integrated into th e port c ards of theATM swi tch in a d i s t r ibu ted ma nner [4 ]. Eachport card dete cts vir tual channels carrying con-nectionless traffic and inspects them for cells thatcontain the beginningof apac ket (BOMcells). Then

a routing operat ion is performed upon the dest i-nation ad dress in the payload of the BOM cell .Th e forwarding table is updated to transmit the cellsof this packet on the next hop toward itsdestination.The entry is remov ed from the fonvardin g able whent h e e n d o f t h e p a c k e t ( E O M c e l l ) is d e t e c t e d .Thism ethod perm itsconnectionless switching at the

W Figure 2. Local and remote bridging

IEEE Commu nications Magaz ine Mdrch 1994 87




o implement

a connec-

tionless

service using

switched

virtual

connections

a connection

must be

established

for each data

conversa tion.

W Figure 3.B-ISDN connectionless data service implementation.

packet lev el without re assemb ly of cells into pack-ets prior t o switching. However, it requires th e useof AAL 3/4 whereas the da ta community has shownmuch grea ter interest in th e use of AAL 5 for datacommunication. This method m ay be used to imple-ment a cen t ra l i zed connec t ion less se rver [ 5 6 ] .To achieve a distr ibuted implementat ion requiresfast route resolution and the n fast allocation of a fre emul t ip lex ing iden t i f i e r (M ID) by the requ i redoutputport,for every BOM cell [4]. dditional hard-ware, an d hence additional cost, is implied but lit-t l e i n c r e a s e d f u n c t i o n a l i t y i s o f f e re d b e y o n ddirectATM. Whyadd hardware toperform fastrout-ing and connec t ion se tu p on a per packet basis?Th e same result can be achieved by performing rout-ing and conn ection setup on a per call basis using

permanent or switched virtual connections.

Permanent Virtual Connections

Th e s implest approach t o implement ing a con-nectionless service on to p of a connection -orient-ed ne twork i s to use a mesh of semi-permanentconnections. Each en d-station has a virtual chan-n e l t o e v e r y o t h e r e n d - s t a t i o n i n t h e n e t w or k .T h i s m a y b e a c c e p t a b l e f o r a s m a l l n u m b e r o fnodes bu t the ma in tenance of the mesh of con-n e c t i o n s r a p i d l y b e c o m e s u n a c c e p t a b l e a s t h enumber of end-points grows.

A m ore practical approach is to interconnectcus tom er p remises ATM swi tches wi th a semi-perma nent mesh of v i r tua l pa ths (F ig . 4). I t is asimilar approach to the “virtual overlay netw or kof connectionless servers xcept the connec-

t ionless servers are removed. T he stat is tical mul-t iplexing tha t th e connectionless server providedi s n o w i m p l e m e n t e d w i t h in t h e c u s t o m e rpremises ATM switches.

This appr oac h is well-suited to the intercon-nection of ATM LANs across the public networkto formaprivatew ide area ATM network [7,8] .Theconnectionless service is supp ortedvia switchedvir-

tual channels. Across he wide area, thevirtual chan-nels are carr ied within the semi-permanent vir tu-al paths so that th e public network is not involvedin the signaling,does not have to offer statisticalmul-tiplexing,and is not involved in reserving reso urceso n a p e r v i r t u a l c h a n n e l b a s i s. B a n d w i d t h f o reach v i r tua l pa th i s rese rved ac ross the publ icnetwo rkand thecus tomer premises ATMsw itchsta-tistically multiplexes traffic for the wide a rea ontothe virtual paths. If th e public network only offersc o n s t a n t b i t - r a t e v i rt u a l p a t h s , r e s e r v i n g t h eb a n d w i d t h s t a t i ca l l y f o r e a c h v i r t u a l p a t h c a nresu l t in under -u t i l i za t ion of bandwid th due totheverybu rsty na ture of data traffic. Two techniqueshave been p roposed for dynamically varying thebandwid th o f each v i r tua l pa th accord ing to the

load: bandw idth renegotiation [9-111, and bandwidthadvertising [12]. A more efficient solution may beachieved if the public network offers variable bit-rate virtual paths w ith statistical multiplexing [131.

Switched Virtual Connections

To mplementa con nectionless serviceusing switchedvirtual connection s,a connection must be establishedf o r e a c h d a t a c o n v e r s a t i o n . E s t a b l is h i n g a n dreleasing a connection to transmit eac h individualpacket is ineff icient because i t places too high aload on t he signaling service a nd imposes a co n-nection set up delay before the tran sfer of every pack-et . However, the majori ty of dat a conversat ionsconsist of th e exchange of mult iple packets, so ifa conne ction is established for the d uration of theconversat ion the overhead of connection setup is

minimized. In the local area, we are not requiredto reserve network resources for idle vir tual con-nections; a virtual connection with no traffic needconsume nonetwork resources other than entries nthe connection tables. Thus many virtual connec-tions can be cached at each station, on the assump-tion that f uture conversations to thos e destinationsare l ikely to arise. Even in a pure connectionless

88 IEEECommunications Magazine March 1994




Figure4 Private wide area network conne ctionlessdata service implementation.

i m p l e m e n t a t i o n s u c h a s I P , e a c h s t a t i o n m u s tmain ta in a l imi ted cache of mappings be tweennetwork addresse s and physical addresses. Fur-thermore, each mapping s obtained from an addressresolution protocol , which is an operat ion c om-parable to connection setup.

T h e a b o v e a r g u m e n t s u g g e s t s t h a t , fo r a nATM network offering a local area d ata service,the dist inction between connectionless and con-nection-oriented implementation is being eroded.This s due to a fundamentalprinciple of ATM switch-ing: separat io n of the connection from the net-work resources (bandwidth and buffers) consumedb y t h e c o n n e c t i o n . A n A T M c o n n e c t i o n i s alightweight connection, i.e., it consum es no band-widthwhen inactive and avery large number of inac-t ive vir tual conn ections can be suppo rted by thenetwork. In previous connection-oriented packetnetworks such as X.25, resources (packet buffers)

were dedicated to eachvirtual circuit by the networkin order to offer rel iable service. ATM gives usthe oppor tun i ty to s epara t e the v i rtua l connec-t ion f rom any gua ran tee o f se rv ice ac ross tha tconnection . A best-effort connectionlessservicecanbe em ula ted us ing AT M vi r tua l connec t ions iftheresourcesofthenetw orkarenot eservedbyindi-v idua l connec t ions bu t a re dynamica l ly sharedbetween all active connections.

LAN Emulation

n IEE E 802 LAN offers a connectionless MACA ervice, supporting arbitration am ong stationsfor access o a sharedphysical transmissionmedium(e.g., the coax cable or the hub back plane). In con-trast, ATM offers a connection-oriented comm u-

nication service based on switched point-to-pointphysical media. For connectionless M AC service ontop of ATM , a protocol layer emulating the con-nectionless service of a LANmust be designed ontop ofthe ATM adaptation layer AAL).ewillcallthis the ATM MAC sublayer (Fig. 5 . T h e A T MMA C sublayer emulates he LAN service by creatin g

the appearance of avirtual shared medium out of anactual switched point-to-point network.

I n a n e x i s t i ng I E E E 8 0 2 L A N s e g m e n t , a llcommunica t ion (un icas t , mul t i cas t, and broad-c a s t) i s b r o a d c a s t t o a l l s t a t i o n s o n t h e s h a r e dphysical medium , and each station filters out thepacketsitwants to receive.Aphysica1LAN egmentcan be emula te d by connec t ing a g roup of endstat ions on the AT M network to an ATM mult i-c a s t v i r t u a l c o n n e c t i o n , w h ic h e m u l a t e s t h eb r o a d c a s t p h y s i ca l m e d i u m o f t h e IEEE 8 0 2

H Figure 5. The A T M M A C sublayer structure or an end statio n.

IEEE Communications Magazine March 1994 89




n A TM

network

uses a

hierarchical

addressformat to

identi8 each

ATM port on

the network.

W Figure6 ATM address format specifiedby the AT M Forum.

LAN. It becomes the broadcast channel of the ATMLANsegm ent. Any stat ionmaybroadcast toal loth-e rs on the ATM LAN segment by transmitting o nthe sh ared ATM multicast virtual connection.

I n c u r r en t I E E E 802 LANs, the membersh ipof an individual LAN segm ent is defined by phys-ical connec tion o he physical shared medium. Mem-bership of an ATM LANsegme nt s defined by logicalconnection to th e multicast ATMvirtualconnectiont h a t e m u l a t e s t h e b r o a d c a s t c h a n n e l f o r t h a tATM LAN segment . So membership of an ATMLANsegment is defined logically (stored in somemanagem ent database) ra ther than physically (as

in an IEEE 802LAN . This offers terminal mobil-ity and increased flexibility n network ma nagemen tand has led to the use of the term “vir tual LAN”to describe an ATM LANsegment.

A LA N segment cou ld be emula ted by t rans -mitt ingallofthe traff icfor thesegmen t onitsbro ad-cast channel, but mo st LAN traffic is unicast an dit is more efficient to su pport unicast communica-t ion us ing po in t - to -po in t ATM vi r tua l connec -tions. Greater security results because the unicastt ra f fic appea rs on ly a t the two comm unica t ingstat ions and is not broadcast to al l s tat ions on th eLANsegment.An ATML ANsegment canthenoffermuch higher ag gregate bandwidth than if all traf-ficwere transmitted on the sam e broadcast channel.Also, the use of individual virtual connectio ns forun icas t t r a f fic permi t s much grea te r con t ro l o f

the quali ty of service ( through put, delay, proba-bility of cell loss, etc.).

T o e s t a b l i s h a p o i n t - t o - p o i n t A T M v i r t u a lconnection for each instance of unicast commu ni-cation, the current location of the destination e nds t a t i o n m u s t b e d i s c o v e r e d a n d e x p r e ss e d a s ades t ina tion address tha t the A TM s ignal ing se r -vice can understand. This operatio n s called addressresolution. Th e ATM signalingservicemust then beinvoked to establish a point-to-point A TM virtualconnection to the dest ination with the appropri-ate quality of service. Within th e en d station, thisoperatio n should be implemented in the software oft h e A T M M A C s u b l a y e r, so a s t o o f f e r a t r a n s -parent service to the L LC sublayer (Fig. 5).

Addressing

To emulate the service of an I E E E 802LAN wemust sup por t address ing us ing the 48-b it MA Caddress. Thisaddressh asa flat addresssp aceandiden-tifies a network interface in the end station whetherit connects to Ethern et, Token R ing, or FD DI, etc.EachATMMACentitymusttherefore alsobeassigneda 4 8 -b i t M A C a d d r e s s , f r o m t h e s a m e a d d r e s sspace, to identify it. The M AC ad dress is assigned

by the manufac turer of the network interface and isguaranteed to be globally unique. Thu s i t can beused to identlfy an end station (or a particular net-work interface on an nd station). It allows the endstation to be relatively mobile (they get disconn ect-ed and reappe ar at a different location fairly often)sincetheMACaddresscontainsnohintofthelocation

of the end station.The d irect use of a M AC address to communi-

catewith an end station is acceptable in a single LANsegment or across a l imited nu mbe r of LAN seg-ments interc onnec ted via bridges. However, largebridged networks becom e very diff icult to ma n-

age and introduceexcessivebroadcas t raffic attempt-ing to locate end stat ions. The address space of alarge netw ork is usually organized hierarchically(e.g., the telephone network is organized by areacode, central off ice number, a nd customer’s l inenumber) . This makes i t much easier to locate anypart icular point on the network but such a struc-ture greatly restricts the mobility of the addr essedobjects. Thisrestriction is perfectlyreaso nable if theaddress ed object is relatively static, such a s a pa r-ticular pho ne jack in th e office wall.

An ATM network uses a hierarchical addres sfor-mat t o iden ti fy each ATM por t on the ne twork.The address resolution operation binds the relativelymobile, end-station, MACaddress to he fixed phys-ica l address o f the A TM por t t o which i t i s cur -rently connected. When a n end station is attached

t o a n A T M s w i tc h p o r t a r e g i s t r a t io n p r o t o c o le x c h an g e s t h e M A C a d d r e s s a n d A T M a d d r e s sbetween the ATM network and the end station. Botht h e M A C a d d r e s s a n d t h e A T M a d d r e s s al s on e e d t o s u p p o r t g r o u p a d d r e s s e s f o r m u l t ic a s tconnections.

The signalingprotocol being developed or broad-band-ISDN permits an ATM address to be divid-ed into an address and a sub-address [141.T h e A T MForum recommends that the address describe thepoint of attachm ent to the public network (if con-nected to the publicnetwork) and that the sub-addressidentify a part icular en d stat ion within a privatenetwork [15].

Th e ATM Forum spec i f ica t ion permi t s twoaddress ormats o be used to specify an ATM add ress.On e is the hierarchical ISDN telephone num ber-

i n g s c h e m e E.164, a n d t h e o t h e r i s a 2 0 - b y t eaddress defined by the ATM Forum and modeledafter the address format of an OS1network serviceaccess point (Fig. 6). This address structure con-tains an initial string of seven bytes, allocated bynational and international authorities to identify aparticularorganization,e.g., an AT M service provider,a p r i v a te A T M n e t w o r k , a n A T M v e n d o r , e t c .

90 IEEE Communications Magazine March 1994




Thi s is followed by the fo ur bytes of the routingd o m a i n a n d a r e a , w h i ch t h e o r g a n i z a t i o n c anallocate in some hierarchical mann er of its ownchoosing. Next comes a field, named the end sys-tem identifier,which contains avalid IEE E 802 MACaddress. The S EL field is not used. An alternativeformat al lows the eight bytes after the A FI f ieldto contain an E.164 address. This option permitsboth public address and private sub-address to becombined into a single ATM address.

Address ResolutionAddress reso lu t ion i s implemented by e i ther abroadcast mechanism similar to IP ARP or a dis-tributed database mechanism. In both mechanisms,the source sends an address reso lu t ion reques tcontaining the dest ination MAC ad dress and i tsown MAC and ATM addresses.

In a broadcast mechanism, for unicast addressresolution, the source broadcasts the request toal l s tat ions on the local ATM L AN seg ment andto all ATM LAN segments connected via bridges.Al l s ta t ions check th e reques ted MA C addressa n d t h e s t a t i o n t h a t o w n s t h e r e q u e s t e d M A Caddre ss rep l ies wi th i t s cur ren t ATM ad dress .The reply may be sent on the broadcast channelfor the ATM LA N segment , o r the des t ina t ionmay set up an A TM connection to the source and

deliver the reply. Fo r m ulticast address resolutionan algorithm may be defined to convert from ag roupM A C a d d r e s s t o a g r o u p A T M a d d r e s s . ( T h egroup MA C address is hashed onto a set of con-t iguous g roup A TM addresses p re -a l located t oeach ATM LAN segment.) Alternatively, a sim-ple server mechanism may be implemented formulticast addresses.

In a database address resolution mechanism,r e q u e s t s a r e r e c e iv e d b y a n a d d r e s s s e r v e r i nthe ne twork . The se rver main ta ins a t ab le con-ta in ing MA C to ATM address mappings whichis updated as part of the registration protocol everytime an end stat ion joins or leaves the network.Both unicast and multicast address resolution maybe offered by the server. The server must b e imple-m e n t e d a s a d i s t r i b u t e d d a t a b a s e to g u a r dagainst hardware failure, so it is likely to requir e

a more complex implementa t ion than a b road-cast implementation.

Acom plication ariseswhen the destination is notdirectlyattached to the ATM network but is attachedto an I EE E 802 LAN connected to the ATM net-work via a bridge. In the broadcast implementa-tion, the bridge can reply to the address resolutionrequestwithitsown ATMaddress, asaproxyfor hedest ination , if i t contains the dest in ation MA Caddress in i ts forwarding table. In the da tabaseapproach , the address se rver o n t h e A T M n e t -work must contain en tries not only for the direct-ly a t t a c h e d s t a t i o n s b u t a l s o f o r a l l s t a t i o n sattached to IE EE 802 LANs accessiblevia bridges.To achieve his each bridge must continually updatethe addressserverw ith the contentsof its forwardingtable. In large bridged networks, avery large add ress

table would result and substantial t raff ic wouldbe required to keep it up to date.

The twoapproaches o address resolution can becombined transparently to the user. End stationsshould assume that a broadcast mechanism is inuse. If ATM multicast connections are implementedusing a mult icast server (see sect ion on mult i-

cast), then the multicast server can also act as anaddress se rver . The mul t icas t se rver can in te r -cep t address reso lu t ion reques t s submi t ted fo rbroadcast to an AT M LAN segment and respondwith the requested address from its database. Thea d d r e s s r e s o l u t i o n d a t a b a s e c a n b e c o m p i l e dfrom the exchange of addresses in the registra-t ion protoc ol , an d also by a learning algori thmsimilar to those used by transpa rent bridges. If noentry is found in the databas e for the requestedaddress, then th e server should use th e broadcast

address resolution mechanism.

Virtual LANs

Th e entire network could be configured as a sin-gle ATM LA N segment but there are advantagesin par t i tion ing i t in to mul t ip le ATM LAN seg-m e n t s [16]. A T M L A N s e g m e n t s c a n b e o r g a -nized along administrative bound aries providingincreased security across boundaries. Partitioningalso increases he manageabilityof the network, andit limits the amo unt and extent of broadcast traf-f ic . A l s o , p a r t i t i o n i n g i n t o m u l t i p l e s e g m e n t sfacilitates interworking between ATM LANs andthe existingbase of installed LANs, bridges, routers,and pro toco ls . The broad cas t channe l o f eachATM LAN segm ent is limited to the members ofthat segment. Broadcast t raff ic on an A TM LANsegment will not escape the boundary of that seg-ment unless segments are connected together viaa bridge.

Figure 7 shows a network with three ATM LANsegments. A n ATM LAN segment is not confinedtoendstationsconnected oasingleATMswitch.Mem-bers can be accep ted f rom any po in t o f a t t ach-ment in the ATM network. Th e network managercan determine the assignment of end stations o ATMLAN seg ments using a graphical network man-agement tool. Thereafter the networkwill recognizee a c h e n d s t a t i o n b y i ts M A C a d d r e s s , a n d t h eATM LAN seg ments to which it belongs can beobtained from a management database. UnknownM A C a d d r e s s e s c a n b e a s s i g ne d t o a d e f a u l tATM LAN segm ent. Alternatively, membershipof ATM LAN segments can be determined by thephysical port to which the end station is connect-

ed, with idle ports assigned a default LA N mem-bership.

When t he end station is plugged into an ATMswitch port, the registration protocol inform s theATM network of the end station’s MA C address.The network looks up the M AC address in itsvirtualLAN database to determ ine to which ATM LANsegment (o r segments ) the end s ta t ion be longs .The ne twork then in forms the ATM MA C sub-layer in the end station of the various param etersof its ATM LAN segm ents. On e paramete r is theATM g roup address of the broadcast channel foreach ATM LAN segm ent of which the end s ta -tion is a member. T he end station can then invokethe signaling mechanism to establish a connec-tion to th e ATM multicast virtual connection thatemulates the broadcast channel for each of the ATM

LAN segm ents to which i t belongs. So each endstation can be attached to any physical ATM portand still remain con nected to its assigned virtualA T M L A N s e g m e n t . T h u s m o v e s a n d c h a n g e scan be m ade within the corpora te campus with-out requir ing a change of network layer addressor any other action by the network man ager.

ddress

resolution is

implemented

by either a

broadcastmechanism

similar to IP

ARP or a

distributed

database

mechanism.

IEEE Communications Magazine March 1994 91




W Figure 7 An eramplenetwork showingATMLANsegments.

I n e r n etwo r k i n g

I f a L A N e m u l a t i o n s e r v i c e i s o f f e r e d a t t h eMA C sublayer then hos t s , b r idges , and rou te rscan be interconnected transparently across ATMLANsegm entsin the same manner asexist ing IEE E802LANsegments. Comm unication between IEE E802 a n d A T M L A N s e g m e n ts c a n a l s o b e s u p -ported by bridging or routing. A bridge, router, o rend station (e.g., a server) can be connecte d to mul-tiple ATM LAN segments. Eachinterface to anATMLANsegment has a separate (software) ATM M ACe n t i t y w it h i t s o w n M A C a d d r e s s b u t r e q u i r e s

only a single physical connection to the ATM net-work. Figure 8givesanexample of the protocol struc-ture of a bridged connection between stat ions onan ATM LAN segment and an IEEE 802 LAN.

ATM LAN segmen ts can be select ively inter-c o n n e c t e d u s i ng a r o u t e r , w h i c h c a n b e p r o -g r a m m e d to p e r m i t o n l y c e r t ai n m a c h i n e s o rprotocols o communicatebetweenspecificsegments.

In this way, the network adm inistrator can imple-me nt certain security measures. However, once twoendstationson differentMM segments have exchangedpacke ts ac ross one o r more rou te rs us ing a com-mon ne twork layer p ro toco l , they can d i scovereach other’s ATM address and establish adirectATMconnection to transfer data. Adirect A TM connectionoffers greater performance than is available at thenetwork layer across one o r more routers. Hence,inside an ATM network, the role of the multipro-tocol router is likely to evolve to that of an addressresolution service at the network layer becausethe forwarding of datawill m igrate to the specialized

hardware of the AT M switch.Th e func t ion tha t the ro u te r per forms i s very

similar to the function tha t s ignaling in the ATMnetwork performs. Both employ an underlying data-gram transfer service. Both require a routing proto-col to rou te traffic across the network between twoendpo in t s spec i fied by a par t i cu la r address . AnA T M s i g na l in g s e r v ic e o n ly u n d e r s t a n d s A T M

92 IEEECommunications Magazin e March 1994




W Figure 8. Local bridging of L A N a n d ATM LA N segments.

addres ses where as a mul t ip ro toco l rou te r canunderstand the network layer addresses of eachprotocol t supports. It is possible o comb ine both func-tions within the ATM switch such that conn ectionrequests to the signaling service may specify any ofthe supported network layer addresses or an A TMaddress.However, hiswill significantlycomplicate thesignaling service. A b etter approach may be to com-bine both fun ctions in the AT M switch but to main-ta in the separa t io n of func t iona l ity fo r ease o fmaintenance , upgrade, and testability.

The Fast Track

A major advantage of LAN emulation at the

MAC sublayer is that i t is common to all higherlayer protocols. By implem enting a single ATMMAC su blayer we are able to sup port al l higherlayer protocols. With a single common interface wehave com patibility with the existing installed baseof “legacy networks.” However, compatibi l i tywith the legacy of protocol develo pmen t is notwitho ut cost. Figure 9 gives an artist’s impressionof the maze of demul t ip lex ing pa ths tha t havedeveloped due to the rapid growth in local area net-work ing . Encapsu la t io n , demul t ip lex ing , andaddress resolution in the data link layer reduce thepacke t p rocessing per formance a t the ne tworkinterface. So it may be worth interfacing popularnetwork layer protocols directly rom the netwo rk layerto the ATM adaptation layer, cutting out the con-ven t iona l da ta l ink layer a l toge ther . However ,

any protocol that we interface directly o ATM fromt h e n e t w o r k l a y e r c a n n o l o n g e r b e b r i d g e dbecause it has no M AC sublayer. Such a protocolwould require a router to interconnect an ATM LANand an IEEE 802 LAN.

A ne twork layer p ro toco l can be con nec tedd i rec t ly to ATM, us ing an ap proach s imi la r to

LAN emulation at the MA C sublayer. The majordifference is that the address resolution mecha-n ism mus t t r ans la te d i rec t ly from th e ne tworkl a y er a d d r e s s t o t h e A T M a d d r e s s [171. Sinceeach network layer protocol must be interfacedto ATM individually, this approach m akes sensefor the most popular protocols; others may use LANemulation at the M AC sublayer. Both approach-es may happily coexist , just as curren t co mm er-cial networks choose to route some protocols andbridge others.

Now that we have begun to prune the protocold e m u l t i p l e x i n g t r e e , w h y s t o p a t t h e n e t w o r klayer? I t has long been argued t hat offering mul-

tiplexing at every layer in the pr otoco l stack sig-nif icantly reduces throughput [18]. Multiplexingis only need ed at a single layer in the proto colstack. For an ATM end stat ion this layer shouldbe the ATM layer. Ult imately the ATM VCI canbe used as the dem ult iplexing identif ier all theway u p to the application process, but this impliesa radical departu re from the existingprotocol stack.Comp atibility s retained using the full protocol stackfor connection setup but , onc e the conn ection isestablished, the full protocol stack can be switchedout For th e data transfer phase, the VCI can be usedto demul t ip lex d i rec t ly from the AAL throughthe transport layer o the applicationprocess (assum-ing an all ATM end-to-end connection).

Switch Requirements forATM LANs

e design of an ATM switch to support an ATMr AN is likely to differ in a number of importantrespects from a switch designed to offer B-ISDNservice in th e public network.

IEEE Communicat ions Magazine March 1994 93




wo

fundamental

classes of

service

are beingconsidered

forATM

networks:

guaranteed

and best-

effort.

Figure 9 Artist’s impressionof he current protocol demultiplexing tree.

Best -Ef for t Service

On e of the most fundamental concepts in packetswitching s that users contend dynam ically or accessto apo olof s hared bandwidth.Thisis statisticalmul-tiplexing ut it implies he probability hat at timesmore t ra f fic wil l a r r ive than can be se rv iced bythe available bandwidth. If the overload is shortlived it is sufficien t to buf fer th e excess traffic. If it ispossible for the excess traffic to exceed th e availablebuffer capacity a congestion control m echanism isrequired t o share the available network resource s( b a n d w i d t h a n d b u f f e r m e m o r y ) d y n a m i ca l lybetwee n all contending users. This class of serv ice is

generally referred to as “best-effort” n that the net-work offers no s e c erformance guarantee to theuser.The term “availablebit-rate”(AE3R) s also used

effort. For the guarante ed service, the traffic char-acteristics of the sou rce are specified at call setupand the ne twork e i ther guaran tees a par t i cu la rquali ty of service for the dura t ion of the cal l ( interms of delay and cell loss probab ility) or rejectsthe call.

For a constant b i t-rate (CBR ) guaranteed con-nection, only the required pe ak cell rate is specified(and possibly the cell jitter). Th e network ens uresthat bandwidth for this conn ection is always avail-able. This is similar to circu it switching but usingcellsinsteadofo ctets. t ismore flexiblethan raditionalcircuit switching in that any reques ted b it-ra te may

be supported up to the link capacity.Foravariab le bit-rate (V BR) gu aranteed servicethe reauired traffic characteristics a re sDecified at

to describe this service suggesting that th e networkattemptstosharethebandwidthcurrentlyavailableforthis class of s ewice betw een active users.

Two fund amental classes of service are beingconsideredforATM networks: guarante ed and best-

call setup using so me statistical definition’(e.g.,peakrate, sustainable rate, and m aximum burst length).Again the netw ork guarantees th e quali ty of ser-viceorrejectsthecall[19-22].ThisseMcepermitssta-tisticalmultiplexing ut the statistical characteristics

94 IEEE Commun ications Magazine March 1994




of the source must be known in advance.For th e best-effort ABR) ervice only the peak

rate of the source is specified at cal l setup, andusers are expected to adjust their ra te in responseto feedback received from the network. T he best-effort service makes use of the bandw idth rema in-ing after serving the guaran teed traffic.

Ano ther app roach for bursty traffic is to reservebandwidth for each b urst using a fast reservationprotocol [23,24]. Operatio n at the burst rate require simplem entat ion in hardware in each switch and

requires specific raffic manag emen t cells to requestand release bandw idth for each burst. This methodbecomes m ore inef f icien t as the peak t ransmis -sion rate of each burst appro aches the link capac-ity [25,26], which is exactly the m ode of op erat iona LAN would prefer.

InaLAN eachendstationwishesto ransmit atthefull line rate of its network interfa ce to achieve aslow a latency as possible. So the allocation of con-stant bit-rate gua ranteed bandwidth, for each con-nection,will severelyrestrict th e amoun t ofbandw idthava i lab le to each co nnec t ion . I t wil l r esu l t inincreas ed latency and inefficient utilization o f band-width since da ta traffic is very bursty. Bandw idthcould be al located on a stat is tical basis, for eac hconnection, if the traff ic characterist ics of eachsource were known. However, data applicat ionscannot predict their bandwidth requirem ents or sourcetraffic characteristics in advance of transmission.This s because their access to he network iscontrolledby the operating system and the opera ting systemconcurrently chedulesa numberof applications, eachwith differing raffic characteristics.Clearly a best-effortservice is the most natural fit for bursty data trafficin the local area.

Mul ti pl e Traffic Classes

It is most likely that a n ATM local area netwo rk willbe required ooffer aguaranteedservice o handlereal-time traffic (e.g. voice, video), co mp one nts of multi-media se rv ices, o r c i rcu i t em ula t ion t ra f f ic , inadditiontoabest-effortdataservice.Suchtrafficisab1eto specify its traffic characteristics and may the re-fore request aqualityofserviceguaranteefromhenet-work. The switch hardware nee ds to ensure that a t

no tim e will the quality of service of th e guaran teedtraffic be adversely affected by best-effort traffic.

Th e simplest approach involves separatin g thecell buffering in the A TM switch into at least twotraffic classes, implem ented in sepa rate physicalor logical queues. Gu arante ed traffic is placed in on eq u e u e a n d b e s t -e f f o r t i n t h e o t h e r . T h e q u e u eservice algorithm alwaysserves the g uarantee d traf-fic in prefe rence to th e best-effort traffic [27 28].M o r e c o m p l e x q u e u i n g s t r u c t u r e s a n d s e rv i c ea lgor i thm s have been inves t iga ted bu t i t i s no tclear that the enhanced performance justifies theincreased complexity [29,30].

Burst Buffering

Th e dimensioning of the cell buffers for the guaran-teed, con stant bit-rate trafficin an ATM switch s relat-

ed t o the j i t ter in cell arr ival for t raff ic of known(and enfo rced) characteristics. T his may be achievedwith relativelysmall buffers ypicallysome hundredsof cells, either per port o r shared across a numberofports. T ooff er a best-effort service for statistical traf-fic, buffering needs t o be provided in relation to theburst (orpack et) arrival statisticsofthe traffic.Thiswill

require much larger buffers everal megabytes ofbuffering is comm on in current bridges and routers.Thu s burst buf fe r ing cann ot be implem ented instatic RAM w ithin th e switching elements that formthe switch fabric of m any cur rent switch designs.Burst buffering may require DRAM or video RAMand is more l ikely to be im plemented in the po rtc a r d s o r b e t w e e n g r o u p s o f p o r t c a r d s a n d t h eswitch fabric.

Congestion Control

A best-effort service must perm it stations to con-tend dynamically for access to a pool of sha red band-width. In an IEEE 802 LAN, the shared mediump r o v id e s t h e s h a r e d b a n d w i d th a n d t h e M A Csublayer provides arbitration . In an A TM switch,each ou tp u t por t i s a poo l o f shared bandwid th ,regardless of sw itch capacity.

If the best-effort service is carrying data pro to-co ls tha t use a window f low cont ro l mechanismther e will be a limit on the am ount of data that anyconnection can inject into the network. This willpermit the burst buffers to be dimensioned accord-ing to the num ber of active connections that may be

supported [31]. As l ink bandwidths increase, thesize of the window must also incre ase to maintainhigh throughputw hichwill reduce he numb er of activec o n n e c t i o n s t h a t m a y b e s u p p o r t e d f o r a g i v e nbuffer size. Also it would be unwise for the networkto rely on corr ect s izing of the user’s window tomaintain a n acceptable quali ty of service. Som eform of congestion control scheme must be imple-mented in a n ATM local area network to supportthe statistical sharing of bandwidth between c om-peting stations without prior bandwidth reservation.

Three fundam ental approachesare available forconge stion control: over-provisioning in term s of

bandwidth o r buffers; loss mechanisms; and delaymechanisms.

o offer a

best-effort

service,

buffering

needs to be

provided in

relation to the

burst arrival

statistics of

the trafic.

Over Provisioning hen E thern e t was f ir s tintroduced, 10 Mb/s shared betwee n al l s tat ionsseem ed like an infinite amou nt of bandwidth , yetcurrently a bandwidth of 10 Mb/s per end stat ionis a popular goal . Clearly bandwidth over-provi-s i o n i n g h a s i t s l i m i t s a s a c o n g e s t i o n c o n t r o l

mechanism. Over-provisioning in term s of buffer-i n g a l s o h a s f i n i t e li m i t a t i o n s d e t e r m i n e d b ydelay. If th e delay through th e buffer exceeds theretransmission tim eout of the higher layer proto-cols, additional retransmissio n trafficwill be insert-ed into th e network during a period of congestion[32, 331.

LossMechanisms Loss mechanisms discard traf-fic during periods of congestion. On e approa ch usesthe cel l loss priori ty bit in the cel l header to dis-card low priority traffic in preference to high priorityt r a f f i c w h e n e v e r t h e b u f f e r l e n g t h e x c e e d s athreshold [34-361. This can be useful for sou rces hatcan code their in formation in to multiple priority lev-e l s , such as the h igh an d low def in it ion comp o-nen ts o f a v ideo s igna l , bu t i t i s d i ff i cul t to see

how dat a traffic could be coded in to two loss pri-oritiesat theMACsublaye r tomakeuseofthism ech-a n i s m . If t h e loss m e c h a n i s m s i m p ly d i s c a r d strafficwhen he buffer overflows, each discarded cellis likely to belong to a different packet. Therefo rem a n y p a c k e t s w i ll r e q u i r e r e t r a n s m i s s io n a n dbandwidth is wasted by the onward transmission





W Figure 1O.Accessflow control.

of the remain ing ce l ls f rom cor ru p ted packe ts .Simulation studies of TCP over ATM with a sim-p l e c e l l d i s c a r d c o n g e s t i o n m e c h a n i s m h a v eshown s ign if ican t th rough put degrad a t ion andhigh levels of packet retransm ission even thoughTC P has an in te rn a l congest ion con t ro l mecha-nism [37,38].

An a l te rnat ive loss mechanism requ i res tha tthe peak transmission rate of each virtual connec-tion be declared when it is established and that theend of each burs t be marked in the ce l l header .Each ou tpu t por t main ta ins an ind ica tion of theinstantaneous sum of the peak traffic rates arrivingat the ou tput port . If an arriving burst causes thesum to exceed the l ink capac i ty the burs t i s d i s-c a r d e d [21]. T h i s s c h e m e h a s t h e a d v a n t a g e ofdiscarding entir e packets but requ ires each buffer

to maintain a count of the number of cells it con-tains for each virtual connection.A similar scheme is to randomly select act ive

vir tual connections for discard when the bufferexceeds a threshold , e.g., [39]. Such a schem e willalso discard complete packets rather than ra ndomcel ls and may have a s imple r implem enta t ion .Loss schemes that drop entire packets have a muchbetter performance than random cell loss schemesas fewer packets require retransmission a nd theremainingcells from corrupted packets are not trans-mitted beyon d the point of congestion [47].

Delay Mechanisms e lay mechanism s usenegative feedback from th e point of congestionb a c k t o w a r d s t h e s o u r c e t o r e d u c e t h e t r a f fi cen te r ing the ne twork. Forward exp l ic i t conges-

t i o n n o t i f i c a t i o n ( F E C N ) s e n d s a c o n g e s t i o ni n d ic a t io n a l o n g t h e f o r w a r d d a t a p a t h t o t h edest ination [40 411. Th e des t ina tion then takess o m e a c t i o n t o c a u s e t h e s o u r c e t o r e d u c e i t stransmission ra te such as closing a window in ahigher layer protocol or sen ding an explici t s ig-na l . Backward exp l ici t conges t ion no t i f i ca t ion(BECN) sends the congestion indication direct ly

back to the source a long a re tu rn pa th [42,48].On receipt of this indicat ion the source reduc esits transmission rate directly. BEC N can respond toc o n g es t io n m u c h m o r e r a p id l y t h a n F E C N b u trequires congestion notification cells to be in sert-ed into thenetworkwhereasFECNcan simplymarka b i t in the ce ll header as i t passes th rough th ep o i n t o f c o n g e s t i o n . O t h e r d e l a y m e c h a n i s m shave been proposed that use credit orbackpressureo n e a c h v i r t u a l c o n n e c t i o n , o n a l i n k - b y - l in kbas i s be tween swi tches and u l t imate ly back to

the sou rce [e .g . , 43-45]. T h i s a p p r o a c h o f f e r sm u c h t i g h t e r f l o w c o n t r o l b u t i s c o n s i d e r a b l ymore complex to implement.

Access Flow ControlA T M i s on l y o n e o f a n u m b e r o f t e c h n o l o g i e scapable of offering high performanc e local areanetworking. To achieve general acceptan ce in thecommerc ia l wor ld , AT M must a t t a in a cos t pe rpor t comp arab le wi th com pet ing technolog ies .On e simple technique to reduce the cost per porti s to mul tip lex mul t ip le s ta t ion s on to the sam eAT M switchport(Fig. 10).Thisisno t unreasonable.A n FD DI ring, for example, multiplexes all of thestat ions into a shared medium of 100Mb/s capac-ity. Offering the capability to multiplex up to 8 o r16stat ionsontoeach 155Mb/sswitchport allowsthe

user a f lexible degree of concen trat ion to adjustthe cost of each ATM interface to the performancerequired. D ata traffic is extremely bursty. The pro-portion of time each user requires th e full 155Mb/sis likely to be very small. Thus, for th e m ajority ofcommercialdata an d interactiveapplications,a mod-erate deg ree of concentrat ion a t the access portsis unlikely to be noticed by the user.

An access f low cont ro l mechanism su ppor t st h e m u l t ip l e x i n g o f m u l t i p l e 155 M b / s A T Maccess por t s on to a s ing le 155Mb/s ATM switchp o r t . A s i m p l e s t a r t / s t o p m e c h a n i s m f o r t h eentire b est-effort traffic class on e ach access linki s a l l t h a t i s r e q u i r e d . T h e g u a r a n t e e d t r a f fi cdoes n o t req u i re access f low cont ro l s ince con-cen t ra t ion i s t aken in t o account dur ing the ca l la c c e p t a n c e p r o c e s s a n d t h e e n t i r e c a ll w i l l

be re jec ted i f insuf f ic ien t ban dwid th i s ava i l -a b l e . ( A c c es s f l o w c o n t r o l f o r t h e g u a r a n t e e dclass of traffic may be useful, however, to coo rdi-nate traffic arrival and t o prevent buffer overflowin small input buffers.) The gen eric f low control(GF C) function in B-ISDN is designed to offer flowcont ro lac rossanaccessl ink . At p resen t , the s tan-d a r d s b o d i e s s e e m t o b e c o n v e r g i n g o n aminimum functionali ty th at wil l permit flowcon-t ro l o f th e bes t -e f fo r t t r a f fic c lass us ing two ofthe G FC bits in the cel l header.

Access flow cont ro l and conges t ion con t ro lare differe nt functions. Both ar e required. Accessflow control operates on each traff ic class on thea c c e s s l in k a s a s i n g l e u n i t or a t l e a s t o n t h ebest-effort traffic class). It is not se lective betweendifferent virtual conn ections within a traffic class

because a l l connec t ions pass th rough the sa mebott leneck he mult iplexed switch port . Con-gestion control, however, attempts to share resource sw i t h i n t h e n e t w o r k b e t w e e n c o m p e t i n g u s e r sand, to achieve this , i t must op erate on a per vir-tua l connec t ion bas i s because on ly the v i r tua lconne ctions passing through th e point of conges-tion require flow control.

96 IEEECommu nications Magazine March 1994




Multicast

The re a re two c lasses o f mul t icas t connec t ion :one-to-many and many-to-many. (Many-to-one con-nections may be regard ed as a subset of many-to-m a n y c o n n e c t io n s . ) L A N e m u l a t i o n r e q u i r e smany-to-many ATM connections. A many-to-manyconnec t ion i s a mul t ipo in t co nnec t ion , wi th as ing le g roup address , on which any memb er o fthe group may transmit and all mem bers receive.Som e implementations require a m ulticast server

an d o th ers use t he mul t icas t capab i li t i e s o f anATM switch directly.

I n t h e s e r v e r i m p l e m e n t a t i o n , e a c h g r o u pmem ber establishes a unicast connection to th e serv-er and a one-to-many connection s established fromthe server to the group. Th e server implementa-t ion mus t be employed if AA L 5 i s used on themul t icas t connec t ion , because AAL 5 can on lyperform reassembly on virtual connectio ns origi-nating from a single source. If m ultiple sources trans-mit simultaneously on the sam e multicast virtualconne ction, their cel ls will becom e interleave da n d A A L 5 provides no mechanism to reassem-ble such a ce l l s t ream. Th e mul t icas t se rver isused to resequ ence the cel l s tream, so that cellsfrom d ifferent sources are no t interleaved on anymulticast connection [46].

If AAL314 s used for multicast conn ections henthe m ultiplexing identifier (MI D) field can iden -tify the source; thus no server is required to act asa relay. The use of AAL 14 for multicast connec-t ions limi t s the members o f an A TM L AN seg-ment to less than 1024 and requires th e networkto assign MID values. Another approach,whichper-mits the use of AA L 5 without requiring a server,i s to es tab l i sh a o n e - t o - m a n y c o n n e c t io n f r o meach member of the group o all others. This approachrapidly con sume s VC Is and is much mo re diff i-cu l t to manage because a l l o f the one- to -manyconnections must be updated whenever a mem -ber joins or leaves the group.

Conclus ionTM technology is currently being applied to local

A nd cam pus area networking, where it offersgreatly increased bandwidth and su pports broad-band services. However, i t must interw ork withthe existing installed base of LANs, bridges, routers ,and protocols. While a n interface to AT M couldbe of fe red f rom the t ranspor t l ayer o r the ne t -work layer o f the OS1 m o d e l , s u c h i n t e r f a c e swould be protocol specific. To o ffer general com-pa t ib i l ity regard less o f the ne twork and u ppe rlayer protocol stack and to suppo rt ransparent MACb r i d gi n g , a n i n t e r f a c e a t t h e M A C s u b l a y e r isrequired. To avoid the requirement to modify theprotocol stack in every end station, this ATM M ACsublayer should em ulate th e service offered by anIEEE 802 L A N . T h u s t h e A T M M A C s u b l a ye rshould offer a best-effort, connectionless, datagra mtransfer service.

Al though a connec t ion less se rv ice cou ld beoffered by a connectionless server, this approachdenies the full benefits of ATM and requires sub-stantial hardware in addition to the AT M switch.A so lu t ion based on a fu l l mesh of semiperma-nen t v i r tual connec t ions i s adeq ua te fo r a verysmall network but becomes difficult to manage as

the num ber of end stations increases. A n approachbased on switchedvirtual connections offers he mostflexible solution .

LAN emulation using switched virtual con nec-t ions requ i res address reso lu t ion to loca te thedestination end station followed by connection estab-lishment to the resulting ATM address. Add ress res-o l u t i o n m a y b e i m p l e m e n t e d b y a b r o a d c a s ttechnique or by an address server. Th e broadcastmeth od is the simplest for small networks but a com-bina t ion of bo th i s p robably be t te r . T he use o f

the MA C address to identify an en d stat ion, withdynamic binding to i ts current physical locationin the ATM network, allows he ATM LAN segmentto be viewedas avirtual LAN . Thevirtual LANmodelpermits end stations to move and c hange physicall o c a t i o n w h il e m a i n t a i n in g c o n n e c t i o n t o t h esame A TM LA N segmen t. This greatly simplifiesthe management of large data networks.

The suppor t o f LAN emula t ion imposes ce r -t a i n r e q u i r e m e n t s o n t h e d e s i g n o f a n A T Mswitch not necessarily fou nd in switches designed forthe public B-ISDN service. The bandw idth instan-taneously available to the best-effort service on eachoutput port must be dynamically shared betweenall contending users, which requires substantial burstbuffering to absorb short bursts of traffic arrivingin excessof the available output p ort capacity. Longer

burs t s requ i re a feedback con t ro l mechanism.Also , a swi tch of fe r ing LAN emu la t ion may berequired to support concentration for data traffic,to offer th e user f lexibil i ty in matching the per-f o r m a n c e r e q u ir e m e n t s t o t h e A T M p o r t c o s t .This wil l require an access f low control m echa-nism.

Many technical and adm inistrative issues rema into be solved and agreed up on in the applicat ionofAT M technology to the publicnetwork (e.g., raf-f i c manage ment , bes t -e f for t se rv ice, m ul t i cas t ,tar iff ing). However, the re is a growing demandfor high bandwidth networking in the local and cam -p u s a r e a t h a t c a n b e w e ll s a t i s f ie d u s i n g A T Mtechnology for L AN em ulation. Solutions exist tothe technical problems introduced by LAN emu-lation and the issues of standardizatio n, interworking,and vendor interoperabil ity are curren tly being

a d d r e s s e d i n t h e s t a n d a r d s b o d i e s a n d by t h eATM Forum.

Acknowledgement

Th e architecture of N.E.T. Adaptive s LAN em u-lation servicewas designed by John B urnett tow hom

am g rateful for many patient and detailed expla-nation s of the design issues.

TM

technology is

being applied

to local and

cam pus area

networking,

where it

oflers greatly

increased

bandwidth

and supports

broadband

services.

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Biography

PETERNEWMANs responsible or the system architecture of ATM switch

products for N.E.T Adaptive, Redwood City, Calif ornia. He is currently

working on congestioncontrolforATM inth eTraffic Management Group

of t he ATM Forum. He was a research fellow at the Computer Labora-

tow of the University of Cambridge where in 1988 he rece ived a Ph.D.

for research in fast packet switching. He began working on fast pack-

et switching in 1981 for t he Telecommunicat ions and Computer Sys-

tems Research Laboratories of the G.E.C. (UK).

98 IEEECommunications Magazine March 1994

atm local area networks

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