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One of the important effects caused by queueing in the network is theintroduction of cell delay variation in that not all cells associated with aparticular connection will suffer the sam

Asynchronous Transfer

Mode (ATM)

All this buttoning and unbuttoning

18th century suicide note

The history of telecommunications is basically a history of technology.Advances in technology have led to new networks, each new networkoffering a range of new services to the user The result is that we now have awide range of networks supporting different services We have the telexnetwork, the telephone network, the ISDN, packet-switched data networks,circuit-switched data networks, mobile telephone networks, the leased linenetwork, local area networks, metropolitan area networks, and so on Morerecently we have seen the introduction of networks to support Frame Relayand SMDS services The problem is that the increasing pace of developments

in applications threatens to make new networks obsolete before they canproduce a financial return on the investment

To avoid this problem it has long been the telecommunications engineer’sdream to develop a universal network capable of supporting the completerange of services, including of course those that have not yet been thought of.The key to this is a switching fabric flexible enough to cater for virtually anyservice requirements ATM is considered by many to be as close to this as weare likely to get in the foreseeable future This chapter explains the basic ideas

of ATM, how it can carry different services in a unified way, and how it willprovide seamless networking over both the local and wide areas, i.e TotalArea Networking Section 5.1 gives a general overview of the key features ofATM with an explanation of the underlying principles Section 5.2 puts a bitmore flesh on the skeleton Section 5.3 looks at how SMDS and Frame Relayare carried over an ATM network, and section 5.4 looks briefly at ATM in localarea networks

Copyright © 1995, 1999 John Wiley & Sons Ltd Print ISBN 0-471-98464-7 Online ISBN 0-470-84153-2

Figure 5.1 ATM cells: the universal currency of exchange

Cell switching

The variety of networks has arisen because the different services have theirown distinct requirements But despite this variety, services can be categorisedbroadly as continuous bit-stream oriented, in that the user wants the remoteend to receive the same continuous bit-stream that is sent; or as bursty, in thatthe information generated by the user’s application arises in distinct burstsrather than as a continuous bit-stream Generally speaking, continuousbit-stream oriented services map naturally on to a circuit-switched network,whereas bursty services tend to be better served by packet-switched networks.Any ‘universal’ switching fabric therefore needs to combine the best features

of circuit-switching and packet-switching, while avoiding the worst.There is also great diversity in the bit rates that different services need.Interactive screen-based data applications might typically need a few kilobitsper second Telephony needs 64 kbits/s High-quality moving pictures mayneed tens of megabits per second Future services (such as holographic 3Dtelevision or interactive virtual reality) might need many tens of megabits persecond So the universal network has to be able to accommodate a very widerange of bit rates

The technique that seems best able to satisfy this diversity of needs is whathas come to be called cell-switching which lies at the heart of ATM Incell-switching the user’s information is carried in short fixed-length packetsknown as cells As standardised for ATM, each cell contains a 5-octet headerand a 48-octet information field, as shown in Figure 5.1 On transmissionlinks, both between the user and the network and between switches withinthe network, cells are transmitted as continuous streams with no intervening

Figure 5.2 Cell switching

spaces So if there is no information to be carried, empty cells are transmitted

to maintain the flow

User information is carried in the information field, though for reasons thatwill become clear the payload that a cell carries is sometimes not quite48-octets The cell header contains information that the switches use to routethe cell through the network to the remote terminal Because it is only 5-octetslong the cell header is too short to contain a full address identifying theremote terminal and it actually contains a label that identifies a connection Socell-switching, and therefore ATM, is intrinsically connection-oriented (wewill see later how connectionless services can be supported by ATM) Byusing different labels for each connection a terminal can support a largenumber of simultaneous connections to different remote terminals Differentconnections can support different services Those requiring high bit-rates(such as video) will naturally generate more cells per second than thoseneeding more modest bit rates In this way ATM can accommodate very widediversity in bit rate

The basic idea of ATM is that the user’s information, after digital encoding

if not already in digital form, is accumulated by the sending terminal until acomplete cell payload has been collected, a cell header is then added and thecomplete cell is passed to the serving local switch for routing through thenetwork to the remote terminal The network does not know what type ofinformation is being carried in a cell; it could be text, it could be speech, itcould be video, it might even be telex! Cell switching provides the universalswitching fabric because it treats all traffic the same (more or less—read onfor more detail), whatever service is being carried

Figure 5.2 illustrates the principle of cell switching A number of transmissionlinks terminate on the cell switch, each carrying a continuous stream of cells

All cells belonging to a particular connection arrive on the same transmissionlink and are routed by the switch to the desired outgoing link where they areinterleaved for transmission on a first-come-first-served basis with cellsbelonging to other connections For simplicity only one direction of transmission

is shown The other direction of transmission is treated in the same way,though logically the two directions of transmission for a connection are quiteseparate Indeed, as we shall see, one of the features of ATM is that the nature

of the two channels forming a connection (one for each direction oftransmission) can be configured independently

Following usual packet-switching parlance, ATM connections are morecorrectly known as ‘virtual’ connections to indicate that, in contrast with realconnections, a continuous end-to-end connection is not provided between theusers But to make for easier reading in what follows, the term ‘virtual’ isgenerally omitted; for connection read virtual connection

A connection is created through the network by making appropriate entries

in routing look-up tables at every switch en route This would be at

subscription time for a permanent virtual circuit (PVC) or call set-up time for

a switched virtual circuit (SVC) (for simplicity here, aspects of signalling areomitted) Each (horizontal) entry in the routing look-up table relates to aspecific connection and associates an incoming link and the label used on thatlink to identify the connection with the desired outgoing link and the labelused on that link to identify the connection Note that different labels are used

on incoming and outgoing transmission links to identify the same connection(if they happen to be the same it is pure coincidence)

Figure 5.2 shows successive cells arriving on incoming link m, each

associated with a different connection, i.e they have different labels on that

link The routing table shows that the cell with incoming label x should be routed out on link o It also shows that the label to be used for this connection

on outgoing link o is y Similarly, the incoming cell with label w should be routed out on link p, with the new label z It is clear therefore that different

connections may use the same labels, but not if they are carried on the sametransmission link

Because of the statistical nature of traffic, no matter how carefully designed

an ATM network is, there will be occasions (hopefully rare) when resources(usually buffers) become locally overloaded and congestion arises In thissituation there is really no choice but to throw cells away To increase theflexibility of ATM, bearing in mind that some services are more tolerant ofloss than others, a priority scheme has been added so that when congestionarises the network can discard cells more intelligently than would otherwise

be the case There is a single bit in the cell header (see Figure 5.10) known asthe Cell Loss Priority bit (CLP) that gives an indication of priority Cells withCLP set to 1 are discarded by the network before cells with CLP set to 0 Aswill be seen, different cells belonging to the same connection may havedifferent priority

Choice of cell length

If cells are going in the same direction, the switch may route themsimultaneously to the same outgoing link Since only one cell can actually betransmitted at a time, it is necessary to include buffer storage to holdcontending cells until they can be transmitted Contending cells queue fortransmission on the outgoing links By choosing a short cell length, the

queueing delay that is incurred by cells en route through the network can be

kept acceptably short

Another important consideration that favours a short cell length is the time

it takes for a terminal to accumulate enough information to fill a cell, usuallyreferred to as the packetisation delay For example, for a digital telephonewhich generates digitally encoded speech at 64 kbit/s it takes about 6 ms tofill a cell This is delay that is introduced between the speaker and the listener,additional to any queueing delays imposed by the network Speech isparticularly sensitive to delay because of the unpleasant effects of echo thatarise when end-to-end delays exceed about 20 ms

One of the important effects caused by queueing in the network is theintroduction of cell delay variation in that not all cells associated with aparticular connection will suffer the same delay in passing through thenetwork Although cells may be generated by a terminal at regular intervals(as for example for 64 kbit/s speech) they will not arrive at the remoteterminal with the same regularity Some will be delayed more than others Toreconstitute the 64 kbit/s speech at the remote terminal a reconstructionbuffer is needed to even out the variation in cell delay introduced by thenetwork This buffer introduces yet more delay, often referred to asdepacketisation delay Clearly, the shorter the cell the less cell delay variationthere will be and the shorter the depacketisation delay

So the shorter the cell the better But this has to be balanced against thehigher overhead which the header represents for a shorter cell length, and the53-octet cell has been standardised for ATM as a compromise The saga of thischoice is interesting and reflects something of the nature of internationalstandardisation Basically, Europe wanted very short cells with an informationfield of 16 to 32 octets so that speech could be carried without the need toinstall echo suppressors, which are expensive The USA on the other handwanted longer cells with a 64 to 128 octet information field to increase thetransmission efficiency; the transmission delays on long distance telephonecircuits in the USA meant that echo suppressors were commonly fittedanyway CCITT (now ITU-TS) went halfway and agreed an information field

of 48 octets, thought by many to combine the worst of both worlds!

Network impairments

The dynamic allocation of network resources inherent in cell-switchingbrings the flexibility and transmission efficiencies of packet switching,

whereas the short delays achieved by having short fixed-length cells tendtowards the more predictable performance of circuit switching Nevertheless,impairments do arise in the network, as we have seen, and they play a centralrole in service definition and network design, as we shall see The mainimpairments are as follows.

• Delay: especially packetisation delay, queueing delay, and depacketisationdelay, though additionally there will be switching delays and propagationdelay

• Cell delay variation: different cells belonging to a particular virtualconnection will generally suffer different delay in passing through thenetwork because of queueing

• Cell loss: may be caused by transmission errors that corrupt cell headers,congestion due to traffic peaks or equipment failure

• Cell misinsertion: corruption of the cell header may cause a cell to berouted to the wrong recipient Such cells would be lost to the intendedrecipient and inserted into the wrong connection

Control of these impairments in order to provide an appropriate quality ofservice over a potentially very wide range of services is one of the dominatingthemes of ATM

The traffic contract

From what we have seen of cell-switching so far it should be clear that newconnections would compete for the same network resources (transmissioncapacity, switch capacity and buffer storage) as existing connections It isimportant, therefore, to make sure that creating a new connection would notreduce the quality of existing connections below what is acceptable to theusers But what is acceptable to users? We have seen that one of the keyattractions of ATM is its ability to support a very wide range of services Thesewill generally have different requirements, and what would be an acceptablenetwork performance for one service may be totally unacceptable for another.Voice, for example, tends to be more tolerant to cell loss than data, but muchless tolerant to delay Furthermore, for the network to gauge whether it hasthe resources to handle a new connection it needs to know what the demands

of that connection would be

A key feature of ATM is that for each connection a traffic contract is agreedbetween the user and the network This contract specifies the characteristics

of the traffic the user will send into the network on that connection and itspecifies the quality of service (QoS) that the network must maintain Thecontract places an obligation on the network; the user knows exactly whatservice he is paying for and will doubtless complain to the service provider if

he does not get it And it places an obligation on the user; if he exceeds theagreed traffic profile the network can legitimately refuse to accept the excess

traffic on the grounds that doing so may compromise the quality of theexisting connections and thereby breach the contracts already agreed forthose connections But provided that the user stays within the agreed trafficprofile the network should support the quality of service requested Thecontract also provides the basis on which the network decides whether it hasthe resources available to support a new connection; if it does not then thenew connection request is refused.

Traffic characteristics are described in terms of parameters such as peak cellrate, together with an indication of the profile of the rate at which cells will besent into the network The quality of service is specified in terms ofparameters relating to accuracy (such as cell error ratio), dependability (such

as cell loss ratio), and speed (such as cell transfer delay and cell delayvariation) Some of these parameters are self-explanatory, some are not Theyare covered in more detail later, but serve here to give a flavour of what isinvolved

We may summarise this as follows:

• for each connection the user indicates his service requirements to thenetwork by means of the traffic contract;

• at connection set-up time the network uses the traffic contract to decide,before agreeing to accept the new connection, whether it has the resourcesavailable to support it while maintaining the contracted quality of service

of existing connections; the jargon for this is connection admission control(CAC);

• during the connection the network uses the traffic contract to check thatthe users stay within their contracted service; the jargon for this is usageparameter control (UPC)

How this is achieved is considered in more detail in section 5.2

Adaptation

ATM, then, offers a universal basis for a multiservice network by reducing allservices to sequences of cells and treating all cells the same (more or less) But,first we have to convert the user’s information into a stream of cells, and ofcourse back again at the other end of the connection This process is known asATM adaptation, and is easier said than done! The basic idea behindadaptation is that the user should not be aware of the underlying ATMnetwork infrastructure (we will look at exceptions to this later when weintroduce native-mode ATM services)

Circuit emulation—an example of ATM adaptation

For example, suppose that the user wants a leased-line service; this shouldappear as a direct circuit connecting him to the remote end, i.e the ATM

Figure 5.3 ATM adaptation for circuit emulation

network should emulate a real circuit The user transmits a continuousclocked bit stream, at say 256 kbit/s, and expects that bit stream to bedelivered at the remote end with very little delay and with very fewtransmission errors (and similarly in the other direction of transmission) Asshown in Figure 5.3, at the sending end the adaptation function would dividethe user’s bit-stream into a sequence of octets When 47 octets of informationhave been accumulated they are loaded into the information field of a celltogether with a one-octet sequence number The appropriate header is added,identifying the connection, and the cell is sent into the network for routing tothe remote user as described above This process of chopping the userinformation up so that it fits into ATM cells is known as segmentation

At the receiving end the adaptation function performs the inverse operation

of extracting the 47 octets of user information from the cell and clocking themout to the recipient as a continuous bit-stream, a process known as re-assembly.This is not trivial The network will inevitably have introduced cell delayvariation, which will have to be compensated for by the adaptation process.The clock will have to be recreated so that the bit stream can be clocked out tothe recipient at the same rate at which it was input by the sender Theone-octet sequence number sent in every cell allows the terminating equipment

to detect whether any cells have been lost in transit through the network (not

that anything can be done in this case to recover the lost information but theloss can be signalled to the application).

To overcome the cell delay variation the adaptation process will use are-assembly buffer (sometimes called the play-out buffer) The idea is that there-assembly buffer stores the payloads of all the cells received for thatconnection, for a period equal to the maximum time a cell is expected to take

to transit the network, which includes cell delay variation This means that ifthe information is clocked out of the re-assembly buffer at the same clock rate

as the original bit stream (256 kbit/s in this example) the re-assembly buffershould never empty and the original bit stream would be recreated (neglectingany loss of cells)

The re-assembly buffer would also be used to recreate the play-out clock.Typically a phase-locked loop would be used to generate the clock The filllevel of the buffer, i.e the number of cells stored, would be continouslycompared with the long-term mean fill level to produce an error signal for thephase-locked loop to maintain the correct clock signal

It is clear from this simple (and simplified!) example that the adaptationprocess must reflect the nature of the service to be carried and that a singleadaptation process such as that outlined above will not work for all services.But it is equally clear that having a different adaptation process for everypossible application is not practicable CCITT has defined a small number ofadaptation processes, four in all, each applicable to a broad class of serviceshaving features in common The example shown above (circuit emulation)could be used to support any continuous bit rate service, though the bit rateand quality of service needed would depend on the exact service required

The ATM protocol reference model

A layered reference model for ATM has been defined as a framework for thedetailed definition of standard protocols and procedures, as shown in Figure5.4 (I.321) There are essentially three layers relating to ATM: the physicallayer; the ATM layer; and the ATM adaptation layer (It should be noted thatthese layers do not generally correspond exactly with those of the OSI 7-layermodel.) Each of the layers is composed of distinct sublayers, as shown.Management protocols, not shown, are also included in the referencemodel, for both layer management and plane management For the sake ofbrevity these are not covered here

The physical layer

The physical layer is concerned with transporting cells from one interfacethrough a transmission channel to a remote interface The standards embrace

a number of types of transmission channel, both optical and electrical,including SDH (synchronous digital hierarchy) and PDH (plesiochronous

Figure 5.5 ATM bearer service

Figure 5.4 The ATM protocol reference model

digital hierarchy) The physical layer may itself generate and insert cells intothe transmission channel, either to fill the channel when there are no ATMcells to send or to convey physical layer operations and maintenanceinformation: these cells are not passed to the ATM Layer The physical layer isdivided into a physical medium (PM) sublayer, which is concerned only withmedium-dependent functions such as line coding, and a transmissionconvergence (TC) sublayer, which is concerned with all the other aspectsmentioned above of converting cells from the ATM layer into bits fortransmission, and vice versa for the other direction of transmission

The ATM Layer (I.361)

The ATM layer is the layer at which multiplexing and switching of cells takeplace It provides virtual connections between end-points and maintains thecontracted quality of service by applying a connection admission controlprocedure at connection set-up time and by policing the agreed trafficcontract while the connection is in progress The ATM layer provides tohigher layers a service known as the ATM bearer service, as shown in Figure 5.5

The ATM adaptation layer (AAL) (I.363)

The ATM adaptation layer, invariably referred to simply as the AAL,translates between the service required by the user (such as voice, video,Frame Relay, SMDS, X.25) and the ATM bearer service provided by the ATMlayer It is composed of the convergence sublayer (CS) and the segmentationand reassembly sublayer (SAR) The convergence sublayer performs a variety

of functions which depend on the actual service being supported, includingclock recovery, compensating for cell delay variation introduced by thenetwork, and dealing with other impairments introduced by the networksuch as cell loss The segmentation and reassembly sublayer segments theuser’s information, together with any supporting information added by theconvergence sublayer, into blocks that fit into the payload of successive ATMcells for transport through the network, and in the other direction oftransmission it reassembles cells received from the network to recreate theuser’s information as it was before segmentation at the sending end.Four types of AAL have been defined To further minimise the variety theAAL convergence sublayer is itself divided into a common part (CPCS) and aService Specific part (SSCS) For each type of AAL the common part dealswith those features that the supported services have in common, whereas theservice specific part deals with things that are different The AALs areconsidered in more detail in section 5.2

As Figure 5.4 shows, different AALs are used for signalling and for the datapath; that is the control plane and user plane in CCITT parlance In effectsignalling is viewed as a special type of service and a signalling AAL (SAAL)has been developed to support it

The ATM adaptation layer is not mandatory for the data path (i.e the userplane), and may be omitted Applications that can use the ATM bearer service

as provided by the ATM Layer may do so Indeed, it seems likely that in thefullness of time, when ATM is common and ubiquitous, applications will bedesigned to use the ATM bearer service directly rather than via an intermediateservice such as Frame Relay In the case of permanent virtual connectionsthere is no requirement for user signalling, so the signalling AAL may also bemissing

Virtual paths and virtual circuits

We now take the story of cell-switching a bit further So far we haveconsidered that a straightforward virtual connection is created between the

Figure 5.6 The ATM cell header

Figure 5.7 Virtual paths and virtual channels

end users, and that labels are used in the cell headers so that each cell can beassociated with the appropriate connection In fact for ATM two types ofvirtual connection have been defined the virtual path connection (VPC) andthe virtual channel connection (VCC), and the label actually consists of twodistinct parts, as shown in Figure 5.6: a virtual path identifier (VPI) and thevirtual channel identifier (VCI) (we will look at the other header fields insection 5.2)

A virtual path connection is a semi-permanent connection which carries agroup of virtual channels all with the same end-points On any physical linkthe VPIs identify the virtual paths, and the VCIs identify the virtual channels,

as shown in Figure 5.7 VPIs used on one physical link may be re-used onothers, but VPIs on the same physical link are all different A VCI relates to avirtual path; so different virtual paths may re-use VCIs, but VCIs on the samevirtual path are all different

Virtual paths and virtual circuits both have traffic contracts These arenotionally independent, and different virtual channels in the same virtualpath may have different qualities of service But the quality of service of avirtual channel cannot be better than that of the virtual path in which it is carried.Perhaps the easiest way to explain the idea of Virtual Paths is to look at afew examples of how they might be used

Figure 5.8 A virtual path connection between user sites

Virtual path example 1—flexible interconnection of user sites

Companies often want to interconnect geographically remote sites Ifconventional leased lines are used for this, several of them are usuallyneeded, typically involving a mix of bit rates and qualities Some may be used

to interconnect the company’s PABXs Others might be used to interconnectLAN routers, or to support videoconferencing, or whatever

With ATM the company could lease from the network operator a singlevirtual path connection between the two sites, as shown in Figure 5.8.Each site has a PABX for telephony, a local area network, and avideoconferencing facility The virtual path supports four virtual channelswith VCI = 1, 2, 3 and 4 At each site an ATM multiplexer performs the ATMadaptation function

The virtual channel with VCI = 1 carries a circuit-emulation service, ineffect interconnecting the PABXs by a 2048 kbit/s private circuit

The virtual channel with VCI = 2 carries a Frame Relay service at 512 kbit/sinterconnecting the two LANs via the routers

The virtual channel with VCI = 3 carries 64 kbit/s constant bit rate voice forthe videoconferencing facility, and the fourth, with VCI = 4, carries constantbit rate video at 256 kbit/s also for the videoconferencing facility

The virtual path connection is set up on a subscription basis by the networkoperator via virtual path cross-connection switches in the ATM network, onlyone of which is shown for simplicity The virtual path cross-connection

switches are simple cell-switches and operate as decribed in Figure 5.2 But, inthis case the switches look only at the VPIs: they do not look at the VCIs whichare transported through the virtual path connection unchanged and unseen

by the network The routing look-up table therefore has entries relating only

to VPIs In this case all cells incoming on port m with VPI = 1 are routed out onport p with new VPI = 5 The VCIs are unchanged by the VP cross-connect andare the same both ends

This use of a virtual path simplifies the network switching requirementssubstantially, since the virtual channels do not have to be individuallyswitched It gives the user great flexibility to use the capacity of the virtualpath in any way he wishes, since the virtual channel structure within thevirtual path is not seen by the network The user could, for example, set upother virtual channels between the ATM multiplexers to interconnect otherdevices such as surveillance cameras for remotely checking site security Orvirtual channels could be allocated different bit rates at different times of theday to exploit the daily variations in traffic But the user must make sure thatthe aggregate demand of the virtual channels does not exceed the trafficcontract agreed for the virtual path

Depending on tariffs and service requirements, it may of course be betterfor the company to lease several virtual paths between the two sites, eachconfigured to carry virtual channels needing a particular quality of service.The requirements of voice, as reflected in the circuit emulation serviceconnecting the two PABXs, are quite different from LAN interconnectionrequirements So it might be beneficial to use different virtual paths for these

Virtual path example 2—flexible network access

The above example is a bit unrealistic in that it only provides for trafficbetween the two sites In practice a great deal of the traffic, especially voicetraffic, would need to be routed to third parties Figure 5.9 shows how anadditional virtual path (VPI = 2) is used to provide PABX access to the PSTN.Figures 5.8 and 5.9 show only one direction of transmission The otherdirection of transmission is treated in an identical way It should be noted,however, that although connections generally involve two-way transmission,that is a channel in each direction, this is not mandatory: one-way connectionsare permitted

In a practical network virtual path cross-connection switching and virtualchannel switching are likely to be combined in one switching system Thiswould give even greater flexibility than shown above For example, the user’straffic could be segregated by the ATM multiplexer into voice and data, alldata being carried on one virtual path, and all voice traffic being carried on asecond, whether intended for the remote site or the PSTN The combinedVP/VC switch could then route the voice traffic as required

Figure 5.9 Flexible access using virtual paths

Virtual path example 3—Using virtual paths within the ATM network

The distinction between virtual paths and virtual channels can substantiallysimplify switching and multiplexing in the network, and adds an importantdegree of freedom to network designers and operators In effect virtual pathscan be used to create a logical network topology that is quite distinct from that

of the physical links A virtual path can interconnect two switches even ifthere is no direct link between them For routing purposes the virtual pathconstitutes a direct connection Virtual paths can also be used betweenswitches as a way of logically partitioning different types of traffic that needdifferent qualities of service This can significantly simplify connectionadmission control schemes

The above account is intended to provide an accessible picture of ATM(basically what it is, and perhaps what it is not) and the emphasis has been onclarity of explanation rather than detail For some readers this is enough; theycan pass on to other chapters Other readers will want more, and they shouldplough on

Figure 5.10 The ATM cell header

The ATM cell

We begin by revisiting the cell header So far we have looked at the function ofthe VPI, the VCI and the CLP bit Now we will look at the others

There are in fact two slightly different cell header formats, one used at theUser Network Interface (UNI), the other at the Network–Network Interface(NNI) as shown in Figure 5.10 They differ only in that the UNI formatincludes an additional field, the generic flow control field (GFC), which theNNI format does not The NNI format takes advantage of the available space

to have a longer VPI

The generic flow control field is intended to provide a multiple accesscapability (similar to the MAC Layer in LANs) whereby a number of ATMterminals and devices can be attached to a single network interface, eachgetting access to set-up and clear connections and transfer data on theseconnections in a standardised and controlled way At the time of writing thedetails of this multiple access scheme have not been formally agreed, but arelikely to be based on the Orwell protocol developed at BT Laboratories.Clearly there is no requirement for this feature at the NNI, and the NNIformat fills the first four bits of the cell header with an extention of the VPIfield, permitting more virtual paths to be supported

The 3-bit PT field indicates the type of payload being carried by the cell.There are basically two types of payload: those which carry user information

Figure 5.11 Service classes

and those which do not Cells carrying user information are identified byhaving 0 in the most significant bit of the PT field (Strictly speaking there aresix cases where this is not true: they are identified by virtue of having specificVCI values that are reserved for signalling or management purposes and notavailable to carry user information.) Cells with 1 in the most significant bit ofthe PT field carry OAM or resource management information We do notconsider them further here (the interested reader should consult the standards(I.610 and I.371))

In user information cells the middle bit of the PT field is a congestionindication bit: 0 signifies that congestion has not been encountered by the cell;

1 indicates that the cell has actually experienced congestion The leastsignificant bit of the PT field in a user information cell is the ATM-user-to-ATM-user indication, which is passed unchanged by the network and delivered tothe ATM Layer user; that is, the AAL, at the other end of the connection In thenext section we will see an example of the ATM-user-to-ATM-user indicationbeing used by one of the AALs

The last octet of the cell header contains the header error control (HEC).This is used to check whether the cell header has been corrupted duringtransmission (it is also used by the physical layer to detect cell boundaries).The error checking code used is capable of correcting single bit errors anddetecting multiple bit errors Cells that are received with uncorrectable errors

in the header are discarded

Services and adaptation (I.363)

To keep the number of adaptation algorithms to a minimum four distinctclasses of service have been defined, designated A, B, C and D (I.362) Asshown in Figure 5.11, they differ in terms of whether they require a strict timerelationship between the two ends, whether they are constant bit rate (CBR)

or variable bit rate (VBR), and whether they are connection-oriented orconnectionless (CLS)

Class A services are constant-bit-rate and connection-oriented, and involve

a strict timing relationship between the two ends of the connection Circuitemulation, as outlined in the previous section, is a good example of a Class Aservice PCM-encoded speech is another

Figure 5.12 AALs and service classes

Class B services are also connection-oriented with a strict time relationshipbetween the two ends, but have a variable bit rate The development ofvariable bit rate services is still in its infancy, but typically they involve coding

of voice or video using compression algorithms that try to maintain thequality associated with the peak bit rate of the information while exploitingthe fact that for much of the time the actual information rate is a lot less thanthe peak rate

Class C services too are connection-oriented, and have a variable bit rate.But they do not involve a strict time relationship between the two ends, andare generally more tolerant of delay than the real-time services of classes Aand B Information is transferred in variable-length blocks (that is, packets orframes from higher-layer protocols) Connection-oriented data services such

as Frame Relay and X.25 are examples of class C services Signalling is another.Class D services are variable-bit-rate, do not involve a strict time relationshipbetween the two ends, and are connectionless Again, information is transferred

in variable-length blocks SMDS is perhaps the best-known example.There are actually eight possible combinations of timing relationship,connection-mode and bit-rate The other four combinations not covered byclasses A to D do not produce viable services For example, the idea of aconnectionless-mode service with a strict timing relationship between thetwo ends does not really mean anything

Clearly, services that differ enough to belong to different classes as definedabove are likely to need significantly different things from the AAL, andseveral types of AAL have been defined to support service classes A to D, asshown in Figure 5.12

Note that it is not quite as straightforward as having a different type of AALfor each of the four service classes, though this was the original intention Whathappened was that AAL types 1, 2, 3 and 4 were defined to support serviceclasses A, B, C and D, respectively But as the AALs were developed it becameclear that there was a lot of commonality between AAL3 and AAL4, and it waseventually decided to combine them into a single AAL, now referred to as 3/4

At the same time it was realised that a lot of the services in classes C and D didnot need the complexity and associated overheads of AAL3/4 and a simpleand efficient adaptation layer was developed to support them Originallyknown as SEAL, it has now been standardised as AAL5

The main provisions of these AALs are outlined below

There are three variations on AAL1 supporting three specific services: circuittransport (sometimes called circuit emulation); video signal transport; andvoice-band signal transport The segmentation and reassembly sublayerfunctions are the same for all three, but there are differences in what theconvergence sublayer does AAL1 is in principle also applicable to high-qualityaudio signal transport, but specific provisions for this have not yet beenstandardised

Circuit transport can carry both asynchronous and synchronous signals.Asynchronous here means that the clock of the constant bit rate source is notfrequency-locked to a network clock, synchronous means that it is G.702signals at bit rates up to 34 368 kbit/s are examples of asynchronous signals;I.231 signals at bit rates up to 1920 kbit/s are examples of syncronous signals.Video signal transport supports the transmission of constant bit rate videosignals for both interactive services, which are comparatively tolerant toerrors but intolerant to delay, and distribution services, which are lesstolerant to errors but more tolerant to delay As we will see the convergencesublayer (CS) functions needed for the interactive and distributive servicesare not quite the same

Voice-band signal transport supports the transmission of A-law and
-lawencoded speech at 64 kbit/s

Operation of AAL1

For each cell in the send direction the convergence sublayer (CS) accumulates

47 octets of user information which is passed to the SAR sublayer togetherwith a 4-bit sequence number consisting of a 3-bit sequence count and a 1-bitCSindication (CSI) If the constant bit rate signal has a framing structure (such

as the 8 kHz structure on an ISDN circuit-mode bearer service) the CSsublayer indicates the frame boundaries to the remote peer CSby inserting a1-octet pointer as the first octet of the payload of selected segments, and usesthe CSI-bit to indicate to the far end that this pointer is present This reducesthe user information payload of these segment to 46 octets As outlinedbelow, the CSI bit can also be used to carry clock recovery information.The SAR protects against corruption of the sequence number in transmission

by calculating and adding a 4-bit error code designated the sequence numberprotection (SNP) This error code enables single-bit errors to be corrected andmultiple-bit errors to be detected at the far end of the connection The 47 octets

of user information (or the 46 octets of user information plus the 1-octetpointer) together with the octet containing the sequence number andsequence number protection are passed to the ATM Layer as the completepayload for a cell (Note that for simplicity of illustration Figures 5.13 and 5.14

do not distinguish the CSfrom the SAR sublayers.)

At the receiving end, shown in Figure 5.14, the AAL receives the 48-octetpayload of cells from the ATM layer The SAR sublayer checks the sequence