Mạng và viễn thông P26

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Asynchronous Transfer M o d e ( ATM) ATM looks set to become the first universal telecommunication technology, capable of switching and transporting all types of telecommunication connection (e.g. voice,data video, multimedia). It willformthebasisofthefuture broadbandintegratedservicesdigitalnetwork(B-ISDN). Because of the anticipated importance of ATM, wediscussherethetechnicalprinciples and terminology in depth, defining the main jargon and explaining what marks out ATM from its predecessors. In particular, discuss the principlesof statistical multiplexing and the specifics of we cell switching. ...

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  1. Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 26 Asynchronous Transfer M o d e ( ATM) ATM looks set to become the first universal telecommunication technology, capable of switching and transporting all types of telecommunication connection (e.g. voice,data video, multimedia). It willformthebasisofthefuture broadbandintegratedservicesdigitalnetwork(B-ISDN). Because of the anticipated importance of ATM, wediscussherethetechnicalprinciples and terminology in depth, defining the main jargon and explaining what marks out ATM from its predecessors. In particular, discuss the principlesof statistical multiplexing and the specifics of we cell switching. 26.1 A FLEXIBLE TRANSMISSIONMEDIUM An ATM-equipped transmission line or telecommunication network is able to support 0 usage by multipleuserssimultaneously 0 differenttelecommunicationneeds(e.g.telephone, datatransmission,LANinter- connection, videotransmission, etc.) 0 each application running at different transmission speeds (i.e. with differing band- width needs) However, these capabilities are also offeredby predecessor technologies, so why bother with ATM, you might ask? What distinguishes ATM from its predecessors is that it performsthesefunctionsmore efficiently. ATM is capable of aninstant-by-instant adjustment in the allocation of the available network capacity between the various users competing for its use. Rather than allocatingfixed capacity between the two communi- cating parties for the durationof a call or session, ATM ensures that the line capacity is optimally used on a moment-by-momentbasis, by carrying only the needed, or ‘useful’, information. 451
  2. 452 ASYNCHRONOUS TRANSFER MODE (ATM) The dynamic allocation of bandwidth is achieved by ATM using a newly developed technique called cell switching. relay Understanding principles its is the key to understanding ATM, its strengths and limitations. In our discussion, we shallrefer frequently to the work of the ATM Forum, an industry-wide common interest group, comprisingtelecommunications equipment manufacturers, network operatorsand users who have combined to speed the process of developing and agreeing technical standards for ATM. 26.2 STATISTICALMULTIPLEXINGANDTHEEVOLUTION OF CELLRELAY SWITCHING ATM is based upon a statistical multiplexing technique called cellrelay switching. Statistical multiplexing, we discussed in as Chapters 9 and 18,is widely used to improve the efficiency of data networks. As shall see, it can also used effectively to carryspeech we be connections, provided extra provisions are made to control the propagation delay. The majorbenefit of statistical multiplexing that the useful carrying capacityof the is line is maximized by avoiding the unnecessary transmission of redundant information (i.e. pauses in speech or idle periods on data lines). In addition, as the full capacityof the line (i.e. its full speed) ismade available for each individual connection for carriage of information, the transmission time (propagation time) may be reduced. In the example of Figure 26.1, we recap the technique of statistical multiplexing. Three separate users (represented by sources A, B and C ) are to communicate over the same transmission line, sharing the line resources by means of statistical multiplexing. The three separate source circuits are fed into a statistical multiplexor, which is con- nected by a single line to the demultiplexor thereceiving end. (A similar at arrangement, using a second line for the receive channel, but with multiplexor on the right and demultiplexor on the left, will also be necessary but this is not shown). The statistical multiplexor sends whatever it receives from any of the source channels directly onto the transmission line. Why is it called statistical multiplexing? Simply because it relies on the statistical unlikelihood of all three channels wanting to send information simultaneously. Actually, for a short period of time, the multiplexor is designed to be able to cope simultaneous transmission from of the sources. This with all separate source transmission line arrival re-sorted circuits sianals source A 'L demultiplexor source B Figure 26.1 The principle of statistical multiplexing
  3. RELAYCELL PROBLEMS TO BE THE BY SOLVED 453 is done by sending the most important signal directly to line and storing the lesser important signal in a bufSer for an instant until the first signal has been transmitted. This causes a slight variation in the time taken by signal packets to traverse the network. a data Variation in the time packets take to traverse networkis not important.So long as theaverage delay is not great, computer users do notnotice whether some typed char- acters appear imperceptibly faster or slower than others. As a result, packet-switched networking techniques have dominated the world of data transmission. Voice networks meanwhilehaveremainedcircuit switched networks, because they cannottolerate variable delays, because they tend to ‘chop up’ the signal. The strength of circuit switching is the guaranteed throughput and fast and constant signal propagation time over the resulting connection. This critical so that acceptable is voice quality can be achieved (any variation in the signal propagation time manifests itself to the listener as a rather broken up form the original signal,an effect known as of jitter). A telephone call connected in a circuit-switched manner is much like an empty pipe between two telephone users. Whatever one speaker talks into the pipe comes out at the other end in an identical format, but only one pair of callers can use the pipe during any particular call. And so it came to pass... that there were networks ford a t a . . . and separate networks for telephones. The cell relay technique of statisticalmultiplexing used in ATM is designed to contain the variation in propagation delay experienced by delay-sensitive signals such as voice and video. It is heralded as the first technique capable of efficient voice and data integration within a single network. 26.3 THEPROBLEMS TO BE SOLVED BYCELLRELAY Returning to the statistical multiplexing example of Figure 26.1, a typical data packet contains between 1 and 256 characters (i.e. between 8 and 2048 bits), and the linespeed is typically 9600 bit/s. Therefore the delay at a time when twosourcestry to send simultaneously and one of the packets hasbe temporarily stored in a buffer will be up to to 200ms (2048/9600 S) longer than when there is no simultaneous transmission. In other words,theremay be up to 200ms of jitter. This is unacceptablefor speech transmission, but this is not the only difficulty to be overcome. Another problem is in minimizing the loss of line bandwidthtothemanagement overhead of statistical multiplexing, as we discuss next. To allow a statistical demultiplexor (Figure 26.1) to sort out the various packets belonging to the different logical connections, andforward them to correct the destinations (A to A , B to B, C to C, etc.) there needs to be a header attached to each packetto say which logical connection (i.e.telephoneconversation ordata com- munications session) itbelongs to. The header (Figure 26.2) is crucial, but hasthe disadvantage that it adds to the information which must be carried between multi- plexor and demultiplexor. At the demultiplexor, the header is removed so it does not disturb the receiver, but meanwhile it has generated an overhead load for the trans- mission line. It is thus impossible using statistical multiplexing techniques to achieve 100% loading with rawuser information. Some of the capacity needs to be given up to carry the overhead.
  4. 454 ~ ASYNCHRONOUS TRANSFER MODE (ATM) seDarate source transmissiorr line re-sorted arrival circuits sianals source A .. . . source B source C 7 Figure 2 3 Statistical multiplexing headers and overhead 6 The major challengesfor ATM developers are to minimize thejitter experienced by speech, video and other delay-sensitive applications, while simultaneously optimizing line efficiency by minimizing networkoverhead. As we shall see, these demands contend with one another. 26.4 THE TECHNIQUE OF CELL RELAY Cell relay is a form of statistical multiplexing that is similar in many ways to packet switching, except that the packets are instead calledcells. Each of the' cells is of a fixed rather than a variable size. The fixed cell size defined by ATM standards is 48 octets (bytes) plus a 5 octet header (i.e. octets in all, see Figure 26.3). The transmission line 53 speeds currently foreseen used are either 2 to-be Mbit/s, 12Mbit/s, 25 Mbit/s, 34 Mbit/s, 45 Mbit/s, 52Mbit/s, 155Mbit/s or 622Mbit/s. We may thus conclude that 0 the overhead is at least 5 bytes in 53 bytes, i.e. >9% 0 the duration of a cell is at most (i.e. at 2 Mbit/s line rate) 53 X 8 bit42 Mbits-' =0.2ms (12ps at 34Mbits-') As the cell duration is relatively short, prhvided that a priority scheme is applied to allow cells from delay-sensitive signal sources (e.g. speech, video, etc.) to have access to the next cell slot, then the jitter (variation. in signal propagation delay) can be kept very low (i.e. of orderof 0.2ms); not zero.asis possible with circuit switching, at the but least low enough to give a subjectively acceptable quality for telephone listeners or video watchers. Jitter-insensitive r ~ sources (e.g. datacommunication channels) can t c be made to wait for the allocation of the next low priority slot. The jitter could be reduced still further by reducing the cell size, but this would increase the proportion of the line capacity neededto carry the cell headers, and thus reduce the line efficiency. 48 octet (byte) infomfion & or cell payload I d 5s Figure 2 . ATM 53 byte cell format 63
  5. THE ATM 455 ~. ...................... free slot 1 cell cell Figure 26.4 The cells and slots of cell relay 26.5 THE ATM CELLHEADER The cell header carries informationsufficient to allow the ATM network to determine to which connection (and thus to which destination port and end user) eachshould be cell delivered. We could draw a comparison with a postal service and imagine each of the cells to be a letter of 48 alphanumeric characters contained in an envelope on which a five-digit postcode appears. You simply drop your letters (cells) in in the right order and they come out in the same order at the other end, though maybejittered in time. slightly Just like a postal service has numerous vans, lorries and personnel to carry different lettersoverdifferentstretchesandsorting offices todirectthelettersalongtheir individual paths, so an ATM network can comprise a mesh of transmission links and switches to direct individual cells by inspecting the address contained in the header (Figure 26.5). The ATM ‘switch’ acts inmuch the sameway as a postal sorter. On its incomingside is a FIFO (first in-first out) buffer, like a pile of letters. At the front of the buffer (like the top letter in the pile) is the cell which has been waiting longest to be switched. New cells arriving are added to the back of the buffer. The switchingprocessinvolves looking at each in turn, and determining from the address in the header which cell held outgoing line should be taken. The is then added to the cell FIFO outputbuffer which is on queueing cells waiting to be transmitted this line. The cell then proceeds to thenext exchange. You may think that afive-digit postcode is rather inadequate as a means address- of ing all the likely users of an ATM network, and it might be, were it not for several provisions of the ATM specifications. First, the ‘digits’ are whole octets (base 256) rather than decimal digits (base 10). This means that the header has the range for over 10l2combinations (40 bits), though only a maximum 28 bits (2.7 X 108combinations) of are ever used for addressing. Second, the addresses (correctly called identiJers) are only allocated to active connections. 101
  6. 456 (ATM) MODE TRANSFER ASYNCHRONOUS ATMconnectionsareallocatedan identiJier during call set-up,andthis is re- allocated to another connection when the connection is cleared. In this way the number of different identifiers need not directly reflect the number of users connected to the network(whichmay be manymillions),onlythenumberofsimultaneouslyactive connections. In addition, various subregionsof the network may use different identifier schemes, thus multiplying the available capacity, but then demanding the ability of network nodes to translate (i.e. amend) identifiers in the five-octet header. By highly efficient usage of the information carried in the header, the length of the header can be kept to a minimum. As a result, the network overhead is minimized. 26.6 THECOMPONENTS OF ANATMNETWORK There are four basic types of equipmentwhich go to make up an ATM network. These are e customer equipment (CEQ),also called B-TE (broadband-ISDN terminal equipment) e ATM switches m ATM crossconnects e ATM multiplexors TheseelementscombinetogethertomakeanetworkasshowninFigure 26.6. A number of standard interfaces are also defined by the ATM specifications as the basis for the connections between the various components. The most important interfaces are e the User-Network Interface (UNZ) e the Network-Node Interface (NNZ) m the Inter-Network Interface (ZNI) These are also shown in Figure 26.6. customer I equipment 2 ; ATM I cross- customer j multiplexor ; connect equipment 7 W Q ) ; ATM j ATM ; customer switch switch equipment ; W Q ) inter-network UN/ NNI user network interface network interface node second ATM network Figure 26.6 The components of an ATM network (ITU-T network reference model for ATM)
  7. THE COMPONENTS OF AN ATM NETWORK 457 ATM customer equipment ( C E Q ) is any item of equipment capable of communicat- ing across an ATM network. Oneof the most popular of today’s visions is the concept of multimedia applications, devices capable of enabling their users simultaneously to transmit synchronized video, electronic mail, data applications and telephone messages over the same line at the same time. TheATM user network interface (UNZ) is thestandard technical specification allowing ATM customer equipment (CEQ) from various different manufacturers to communicate over a network provided by yet another manufacturer. It is the interface employedbetween ATMcustomerequipmentand either ATM multiplexor, ATM crossconnect or ATM switch. It consists of a set of layered protocols aswe shall discuss later. Customer equipments communicate with one another across an ATM network by meansofa virtualchannel ( V C ) . The VC mayeither be set-up and cleared down on a call-by-call basis similar to a telephone network, in which case the connection is a switched virtual circuit ( S V C ) or it may be a permanently dedicated connection (like a leaseline or private wire), in which case it is a permanent virtual circuit ( P V C ) . An ATM multiplexor allows different virtual channels from different ATM UN1 ports to be bundled for carriage over the same physical transmission line. Thus two or three customers outlying from the main exchange (Figure 26.6) could share a common line. Returning to our analogy with the postal system, the ATM multiplexor performs a similar function to a postal sack; it makes easier the task of carrying a number of different messages to the sorting station (ATM switch)by bundling a number of virtual channels into a single container, a virtual p a t h ( V P ) . More about virtual channels and virtual paths later in the chapter (Figures 26.10-26.13). An ATM crossconnect is a slightly more complicated device than the ATM multi- plexor. It is analogous to a postaldepot, where the various vanloadsof mail are unloaded, the various sacks sorted and adjusted into different van loads. At the postal depot, the individualsacksremainunopened, and at the ATM crossconnect,the virtual path contents, the individual virtual channels remain undisturbed. The ATM crossconnect appears again in Figure 26.12. A full ATM switch is the most complex and powerful of the elements making up an ATM network. It is capable not only of cross connecting virtual paths, but also of sortingand switchingtheircontents,the channels (Figure26.13). It is the virtual equivalent of a full postal sorting office, where sacks can eitherpass through unopened, or can be emptied and each letter individually re-sorted. It is the only type of ATM node device capable of interpreting and reacting upon user or network signalling for the establishment of new connections or the clearing of existing connections. The ATM network node interface (NNZ) is the interface used between nodes within the network or between different sub-networks. A standardized NNI gives the scope to build an ATM network from individual nodes or sub-networks supplied by different manufacturers. The inter-network interface (ZNZ) allows not only for intercommunication, but also for clean operational and administrative boundaries between interconnected networks. It is based on the NNI but includes more fetaures for ensuring security, control and proper administration of inter-carrier connections (i.e. where networks of two different operators are interconnected). ATM Forum calls this interfaceB-ZCZ (broadband inter- carrier interface).
  8. 458 (ATM) MODE TRANSFER ASYNCHRONOUS 26.7 THE ATM ADAPTATION LAYER (AAL) An extra functionality added to basic ATM network (correctly is a called theA TMLayer) to accommodate the carriage of various different types of connection-oriented and connectionless network services (Chapter Figure 26.6). This functionalityis contained 25, in theA T M adaptation layer.The ATMadaptation layer ( A A L ) lays out a of ruleson set how the 48 byte cell payload can be used, and how it should be coded. These special codings enable the end devices which are communicating across the A T M Layer to communicate with one another using any of the possible connection-oriented or con- nectionless service as desired. The servicesoffered by the ATM adaptationlayer ( A A L ) are classified into four classes or types (the standards use both terminologies). The distinguishing parameters of the various classes are as illustrated in Table 26.1. AnexampleofaClassAservice is circuitemulation (i.e. aconnection service providing for ‘clear channel’ connections like hard-wired digital circuits). In the ATM specifications such services are referred to as constant bit rate ( C B R ) or circuit emula- tion services (CES). Thus a constant bit rate video or speech signal would be an AAL Class A service and would use AAL1. Variable bit rate ( V B R ) video and audio is an example of a class B service. Thus an audio speech signal which sends information duringsilent periodsis an exampleof a no Class B VBR service and would use AAL2. Class C and Class D cover the connection-oriented and connectionless data transfer services. Thus anX.25 packet switching service would supported by a Class C service, be and connectionless data services like electronic mail and certain types of LAN router service would be Class D. Both classes C and D use AAL types AAL3/4 or AAL5. 26.8 ATM VIRTUAL CHANNELSAND VIRTUAL PATHS A virtual channelextended all the way across an ATMnetwork (ATM Layer) actually is a virtual channel connection ( V C C ) . Thisconnection is composed of anumber of shorter length virtual channel links, which when laid end-to-end make up the VCC. Table 26.1 Service classification of the ATM adaption layer (AAL) Transmission characteristic Class A Class B Class C Class D AAL Type AAL Type 1 AAL Type 2 AAL Type 314 AAL Type 314 (AALl) (AALZ) (AAL3/4), (AAL3/4), AAL Type 5 AAL Type 5 (AAL5) (AAL5) Timing relation between Required Not required source and destination I Bit rate Constant Variable Connection mode Connection-oriented I Connectionless
  9. ONTROL USER, AND MANAGEMENT PLANES 459 virtual channel connection (VCC) virtual channel link ATM multiplexor or switch function I R. customer .': connection path virtual ... equipment B GEQ) crass- ATM ... connectfunction '... "\* * physical transmission path Figure 26.7 The relationship between virtual channels, virtual paths and physical transmission paths A virtual channel link is a part of the overallVCC, and shares the same endpoints as a virtualpath connection ( V P C )(Figure 26.7). The ideaof a virtualpath ( V P )is valuable in the overall design and operationATM networks.As we have already discovered the of in earlier part of a chapter, a virtual path has a function rather like a postal sack. In the same way that a postal sack helpsto ease the handling letters which all share a similar of destination, so a virtual pathhelps to ease the workload of the ATM network nodes by enabling them to handle bundled groups of virtual channels. Thus a virtual path ( V P ) carries a numberof different virtual channel links, which in theirown separate ways may be concatenated with other virtual channel links to make VCCs. Just like virtual channels, virtual paths can be classified into virtual path connections (VPCs) and virtual path links, where a VPC is made up by the concatenation of one ormorevirtualpathlinks. A virtual path link is deriveddirectlyfrom a physical transmission path. 26.9 USER, CONTROL AND MANAGEMENT PLANES Before two customer equipments ( C E Q ) may communicate with one another (i.e. trans- fer information) across the plane (U-plane) of an ATM network, a connection must user first be established. The connection is established by means of a control or a manage- ment communication between theCEQ and the network. This communication may take one of five forms (Figure 26.8) 0 control plane communication (access) 0 control plane communication (network) 0 management plane communication type l 0 management plane communication type 2 0 management plane communication type 3
  10. 460 ASYNCHRONOUS TRANSFER MODE (ATM) management plane communication .................. NMC (network management centre) customer equipment management planet management planet type- : 2 communication type3 ; VP or VC VP or VC crossconnect crossconnect 1 control plane communication (network) ............................. control plane communication (access) ..................................................................... information transfer across user plane the ............................................................................... Figure 26.8 User, control and management planes of an ATM network (ITU-T recommenda- tion 1.3 1 1) A control plane communication (access) is a one conducted between CEQ (customer equipment) andanATM switch. During suchacommunication,which uses UNI signalling, a connection is established or released (in the case of SVCs, switched virtual circuits) much like dialling a telephone number in a telephone network. Control plane communications (network) follow, as theATM switch communicates (using network will signalling) withother nodes in the network to establish the complete network connection. Once the connection is established, the user transfers information (i.e. communicates) across the user plane. The connection could alsohave been established manually by the service technicians at the network management centre (a PVC, permanent virtual circuit). In this case, the user uses a management plane communication type I from his CEQ to the NMC to request the establishment of a permanent connection. This could be carried by UN1 signalling or could simply be a telephone call. The various switches and other network elements are then configured from the NMC by means of messages sent by management plane communication type 2. Management plane communication type3 is initiated by ATM switches which require to refer to the NMC for information, authority or other assistance in the process of connection set-up. (It may be, for example, that certain high bandwidth connections require authorization from the NMC to prevent network congestion at peak times). 26.10 HOW IS A VIRTUAL CHANNEL CONNECTION (VCC) SET UP? A UN1 signallingvirtualchannel (SVC, but not to be confused with SVC, switched virtual connection) is a virtual channel or virtual path connection at a UN1 dedicated specifically to UNI signalling. Signallingvirtualchannelsmayalsoexist at an N N I interface.
  11. SIGNALLING CHANNELS VIRTUAL AND META-SIGNALLING CHANNELS VIRTUAL 461 A signallingmessagesentoverthe SVC (signalling V C ) might be‘set up virtual connection number one between user A anduser B’. Another message might be ‘clear the connection between A and ATM uses a dedicated channel for signalling information B’. (common channel signalling as we discussed in Chapter 7). Drawing a parallel between ATM signalling and narrowband ISDN signalling,the UN1 signallinginterface is equivalent to the narrowband ISDN signallingdefined by ITU-Trecommendation Q.931 (and indeed is based on it and specified in Q.2931), and ATM NNI signalling is based on signalling system 7 ( S S 7 ) as used in narrowband ISDN. At the time when a CEQ requests to set up a new SVC (switched virtual circuit) connection across an ATM network, it must first negotiate with the network over the UNI signalling VC, declaring the required peak cell rate, quality of service ( Q O S ) class and other parameters needed. The connection admission control ( C A C ) function at the ATM switch then decides whether sufficient resources are available to allow immediate connection. If so, the connection is set up. If not, the connection request is rejected to protect the quality of existing connections. (The analogyis the telephone user’s receipt of busy tone when no more lines are available). During the negotiation, virtual paths and connectionsbetween the various nodes and other equipments are allocated, and the referencenumbers of theseconnections,thecombination of virtual path identifiers ( VPIs) andvirtual channel identifiers ( VCIs),are confirmed over the signalling channel. These values (VPI and VCI) then appear in the header of any cells sent, to identify all those cells which relate to this Connection. 26.11 SIGNALLING VIRTUAL CHANNELSAND META-SIGNALLING VIRTUAL CHANNELS Both management and control communication in an ATM network take place via signalling virtual channels (SVCs). At NNI interfaces, SVCs are usually permanently configured between the various servers (i.e. control processors) controlling a particular B-ISDN service (e.g. video on demand, picture telephone service, etc.). However, unlike narrowband ISDN, signallingvirtualchannels ( S V C ) are not normally permanently available at UNI.Instead, they are established on demand means of ameta-signalling by virtual channel ( M S V C ) . This is a permanently allocated UN1 signalling channel of a fixed bandwidth. It is found in the virtual path VPI 0 and has aVC1 value standard to = the particular network. By meansthe of meta-signalling virtual channel, the end device (CEQ) can establish an SVC (signalling VC) to the ATM switch (c-plane) or to the network man- agementcentre(m-plane) for signallingcommunication.A serviceprojileidentifier ( S P I D ) carried in the meta-signalling determines which service the user requires, and enablesasignallingvirtualchannel to the appropriate signalling point server to be established. The functionality or device which existsat the end of a signalling virtual channeland conducts the act of signalling is called a signalling point ( S P ) . Such functionality exists in customer equipment (CEQ) and in ATM switches. A signalling transfer point ( S W ) is a switching point for the information carried in signalling virtual channels. Using a single signalling VC via an STP, an SPmaycommunicatesignalling messages to
  12. 462 (ATM) MODE TRANSFER ASYNCHRONOUS STP SP SP UN1 I A C NNI D B CEQ CEQ (customer equipment) Figure 26.9 Signalling points (SPs) and signalling transfer points (STPs) numerous other SPs using eitherassociated-mode or quasi-associated mode signalling, as we discussed in Chapter 12. STPs improve the efficiency and reliability of the signal- ling network (Figure 26.9). 26.12 VIRTUAL CHANNEL IDENTIFIERS (VCIs) AND VIRTUAL PATH IDENTIFIERS (VPIs) As we saw in Figure 26.7, virtual channel connections comprise concatenated virtual path connections. Each is identijied by reference numbers carried by the cell headers of active connections called virtual channel identlJiers ( VCZs) and virtual path identifiers ( VPZS). Figure 26.10 illustrates how a physical connection may be subdivided into a number of different virtual paths, each with a unique VPI. Each VPI inturn may be subdivided into several virtual channels, each with a separate VCI. The combination of VPI and VC1 values is unique to each UN1 or NNI and is sufficient to identify any active connec- tion at the interface (i.e. on the same physical connection). An ATM multiplexor, as we discussed earlier, allows a number of virtual channels from separate virtual paths to be combined over a single virtual path. Thus the virtual channels carried by physical links 1,2 and3 of Figure 26.11 are combined together into a single virtual path carried by physical link 4. In this way three separate end user devices use separate virtual channels to share a single physical connection line from ATM multiplexor to the ATM network. An ATM crossconnect allows rearrangement of virtual paths without disturbance of the virtual channels which they contain. Thus in Figure 26.12, the contents of incoming VCI, physical VClb connection 4 Figure 26.10 Virtual path and virtual channel identifiers
  13. VIRTUAL CHANNEL IDENTIFIERS (VCIS) AND VIRTUAL PATH IDENTIFIERS (VPIS) 463 physical link 1, VPI=l. VCI=l 0 physical link 2,VPI=2, VCI=l C 3 physical link 4, VPI=4. VCls 1 , 2 8 3 physical link 3, VPI=3, VCI=l C , Figure 26.11ATM multiplexor VPI=l, VCls 1 & 2 VPI=4, VCIS 3 & 4 7 r VPI=2, VCls 3 & 4 VPI=5, VCls 5 & 6 3 JC >L VPI=3, VCls 5&6 1 VCls VPI=6, &2 7 2 J ' L Figure 26.12ATM crossconnect virtual path VPI = 1 are crossconnected to outgoing virtual path VPI = 6, the VCIs remaining unchanged. An ATM crossconnect thus a simple form of ATM switch, but is one which need only process (and translate (i.e. amend)) VPI values. A full ATM switch (Figure 26.13) must have the capability not only to crossconnect virtual paths, but also to switch virtual channels between different virtual paths. This requires the additional ability to process and translate VCIs held in cell headers. It is thus a relatively complex device. Good performance depends onvery fast processing of both VPIs and VCIs in the cell headers. A full ATM switch is consequently a costly device. V 1 24 C V1 2 C 1 V1 2 C 2 VC1 23 I 1 V1 C 21 I I VC1 22 T VP crossconnect I Figure 26.13 Full ATM switch
  14. 464 ASYNCHRONOUS TRANSFER MODE IATM) 26.13 INFORMATIONCONTENTAND FORMAT OF THE ATMCELL HEADER The main function of the ATM cell header is to carry the VPI and VC1 information whichallowstheactivenetworkelements to switch the cells of activeconnections through the network. The exact format of the cell header is as shown in Figure 26.14. The cell header comprises 40 bits, of which 24 (UNI) or 28 (NNI) are used for the virtual path and virtual channel identifiers. Together, the VPIjVCI fields are called the routingfield. There are four otherfieldswhich occupy the remainder of the header. The PT (payload type)field is occupied by the payload type identijier (PTZ). This identifies the contentsof the cell (the informationfield or payload) as either a user data cell, a cell containing network management information, or a resource management cell. The cell loss priority ( C L P )bit (when set to value 1 is used to identify less important cells which may be discarded first at a time of network or link congestion. The genericPOW control (GFC) field is used to control the cell transmission between the customer equipment (CEQ) and the network (Figure 26.15). Finally, the header error control ( H E C ) field serves to detect errors in the cell header caused during cell transmission. When there is no trunk congestion (i.e. there is no appreciable accumulation of cells waiting in the multiplexor buffer to be transmitted over the trunk) then the GFC field is set to the uncontrolled transmission mode. However, if there is a sudden surge of cells from all of the CEQ devices and the multiplexor experiences congestion (i.e. the filling of its cell buffers to a critical threshold level) then the GFC field is used to subject the 8 7 6 5 4 3 2 1 octet GFC (at UNI). VPI (at NNI) VPI VPI VC1 VC1 VC1 PT I CLP 4 HEC 5 GFC = Generic Flow Control PT = PayloadType VPI = Virtual Path Identifier CLP = Cell Loss Priority VC1 = Virtual Channel Identifier HEC = Header Error Control Note: the GFC is used only at the UNI, at the NNI bits 5-8of octet 1 are used as VPI :3%, Figure 26.14 Structure of the ATM cell header device A device B multiplexor deviceC I CEQ Figure 26.15 Generic flow control regulates trunk congestion at a multiplexor
  15. ATM 465 Table 26.2 The functional layers ofATM Layer name Sublayer name Further sublayer Higher layers ATM adaptation layer (AAL) Convergence sublayer (CS) Service specific (SS) Common part (CP) Segmentation and reassembly (SAR) sublayer ATM layer VC level VP level Physical layer Transmisssion convergence (TC) sublayer Physical medium (PM) 0 Service access point (SAP) ~ an imaginary point between functional layers. cell flow from the various CEQ devices to controlled transmission. This limits the rate at which the CEQ devices may continue to send cells of one or more different types to the network. An ATMcell is transmitted in the orderof octets (i.e. octet 1 first, followed by octet 2, then octet 3, etc.). The most signiJicant bit ( M S B ) of each octet (i.e. bit 8) is transmitted first. Thus first the header and then the payload is transmitted, MSB first. 26.14 ATM PROTOCOL LAYERS Table 26.2 illustrates the protocol layers of ATM. We use this in the discussion which follows to define the terminology of ATM and to explaintherelationships of the various layers to one another. 26.15 THE ATM TRANSPORT NETWORK The foundation of the various protocol layers (the protocol stack, the set of functions which together make information transfer possible) is the physical medium used for the carriage of electrical or optical signals. The physicallayer is a specification which defines what electrical or optical signals and voltages, etc., should be used. In addition, it sets out a procedure for transferring data information across the line, providing for clocking of the bits sent and the monitoring of the equipment. The physical layer of ATM is similar function in tothe physical layer (layer 1) of the open systems inferconnection ( O S I ) protocol stack (Chapter 9). The physical layer is divided into two sublayers. These are the physical medium sub- layer and the transmission convergence ( K ) sublayer. The physical medium sublayer defines the exact electrical and optical interface, the line code and the bit timing. The TC sublayer provides for framing of cells, for cell delineation, for cell rate adaption to
  16. 466 ASYNCHRONOUS TRANSFER MODE (ATM) the information carriage capacity of the line, and for operational monitoring of the various line components (regenerator section ( R S ) , digital section ( D S ) or transmission p a t h ( T P ) ) . Preferred physical media defined for use with ATM include optical fibre and coaxial cable. There is also some scopeto use twisted pair cable. It is the ATM layer which controls the transport of cells across an ATM network, setting up virtual channel connections and controlling the submission rate (generic flow control) of cells from user equipment. The service provided to the ATM layer by the physical layer is the physical transport of a valid flow of cells. This is 'delivered' at a conceptual 'point' called the physical layerservice access point ( P L - S A P ) . The flow of cells is correctly called a service data unit ( S D U ) , in fact the PL-SDU (physical layer service data unit). Figure 26.16 Maintenance test tool for ATM (Courtesy of Siemens A G ) . The operation and maintenance (OAM) cells of ATM provide for advanced measurement of network performance without affecting live connections.
  17. CAPABILITY OF THE ATM ADAPTION LAYER (AAL) 467 The ATM layer controls the service provided to it by the physical layer by means of service primitive commands. These are standardized requests and commands exchanged between the control function within the ATM layer (in thejargon called the A T M layer entity ( A T M - L E ) ) and the physical layer entity ( P L - L E ) . They allow, for example, a particular ATM-LE torequest transfer of a flow of cells (service data unit). Conversely, the physical layer may wish temporarily to halt the transfer of cells to it by the ATM layer because of a problem with the physical medium. The transmissionconvergencesublayer receives informationintheform of cells provided to it by the ATM layer. This is the PL-SDU, or more specifically, the TC- SDU. These cells are supplemented by further information, including PL-cells (physical layer cells) and OAM cells (operations and maintenance cells). The extra information, an example of protocol control information (PCZ) turns the TC-SDU into a TC-PDU (protocol data unit). It ensuresthecorrecttransmission of informationacrossthe physical medium. The TC-PDU is passed to the physical medium sublayer, where it is called the PM-SDU (physical medium service data unit). Finally, the PM-SDU is converted to thePM-PDU by addition of furtherPCZ and is passed tothe mediumitself.Theform of thePM-PDU(andthusthe conversion performed by the physical medium sublayer) is dependent upon the type of medium used (e.g. electrical, optical, etc.). To accommodate a change of the physical medium, onlythe physicalmedium sublayer need be swapped.Otherhardware and software components (e.g. corresponding to the ATM layer) can be re-used. Together the ATM layer, the physical layer and the physical medium are called itself an A T M transport network. An A T M transport network is capable of conveying information in the form of cells between network end-points. However, so that the information content carried by an ATM transport network can be correctly interpreted by the receiver, there are further higher layer protocols defined. The most important of these is the A T M adaptation layer ( A A L ) . 26.16 CAPABILITY OF THE ATM ADAPTATION LAYER(AAL) As the name suggests, the A T M adaptation layer ( A A L ) provides for the conversion of the higher layer information into a format suitable for transport by an ATM transport network. The higherlayers are information, devices or functions of unspecific type whichrequiretocommunicateacrosstheATMnetwork. Higherlayerinformation carried by the ATM network may be either 0 user information (user plane)of one of a number of different forms (e.g. voice, data, video, etc.) as categorized by the AAL service classes (Table 26.1) 0 control information (control plane) for setting up or clearing connections 0 network management information (management plane) for monitoring and config- uring network elementsor for sending requests between network management staff. Like the other layers,the AAL accepts AAL-SDUs from the higher layers and passes an AAL-PDU to the layer below it (the ATM layer), where it is known as an ATM-SDU. However, unlike the ATM and physical layers a number of different alternative services
  18. 468 (ATM) MODE TRANSFER ASYNCHRONOUS can be madeavailabletothe higherlayers abovetheAAL,thusallowing differ- ent types of information to be adapted for carriage across a common ATM transport network. It is the ATM adaptationlayer which gives ATM networks their capability to transfer all sorts ofdifferentinformationtypes.It is splitintotwosublayers:the convergence sublayer, CS (where the alignment of the various information types into a common format takes place and division into cells occurs); and the segmentation and reassemblysublayer, S A R (where the cells are numbered sequentiallyto allow reconstruction in the right order at the receiving end). 26.17 PROTOCOL STACK WHEN COMMUNICATING VIA AN ATM TRANSPORT NETWORK Figure 26.17 illustrates the peer-to-peer communications which take place when two user end devices communicate with one another by means of an ATM transport switch. The ATM switch supports only the lowest three protocol layers, and ‘speaks’ peer-to- peer with each of the ends, translating protocol information such as VPIs andVCIs as necessary andrelaying user information. Meanwhile, at the ATM adaptationlayer ( A A L ) and the higher layers, the two end devices communicate peer-to-peer directly over the connection established by the lower three layers. This information remains uninterpreted and passes ‘transparently’ through the network. user actual communications path _ _ _ _ _ _W imaginary peer-to-peercommunication Figure 26.17 Protocol layer representation of two end devices communicating via ATM layer switch
  19. OTOCOL ATM 469 If we were to monitor the wire between either of the end devices and the ATMswitch of Figure 26.17 then we would observe communication at each of the layers. What we actually observe are cells, but structured a little like a Russian doll. The smallest doll (right inside) is the information that we want to carry between the users (the higher layer information). All the other dolls are the protocol information (PCI), one doll for each of the lower layers, each providing a function critical to the reliable carriage and correct interpretation of the message. 26.18 ATM PROTOCOL REFERENCE MODEL(PRM) Strictly, Figure 26.17 illustrates only the protocol stack for the user plane (i.e. for the transfer of information between end devices once the connection has been established. The AAL protocolsused on the control and management planes will usually differ from the AAL protocols used on the user plane of the same connection, though identical protocols will be used at the ATM transport layers. This is illustrated schematically in the ATM protocol reference model ( P R M ) as shown in Figure 26.18. In the case of the control and management planes, the network itself must interpret the higher layer information and react to it. The control plane AALs (for the user- network signalling in setting up a switched virtual circuit, SVC) will typically need to be suited for data information transfer. Similar protocols will also be necessary for the management plane. In contrast, in the case of communication across the user plane, the higher layer information may take any number of different forms (speech, data, etc.), but the individual network elements themselves (e.g. switches) may be incapable of recognizing and interpreting these various forms. In real switches and ATM end user devices,common or duplicate hardware and soft- ware may thus be used for management, control and user planes at ATM and physical /l / /. ................ management plane.................... 2 Figure 26.18 The B-ISDN protocol reference model (Courtesy o L W ) f
  20. 470 ASYNCHRONOUS TRANSFER MODE (ATM) layers, but distinct hardware and software will be necessary for signalling and user information transfer at the AAL and higher layers of these planes. The different types of user, control and management services are carried by the AAL by means of service-spec@ convergence services. Examples of specific user plane services offered by the ATM adaptation layer ( A A L ) are 0 frame relay SSCS (service speciJic convergence sublayer) service SMDS (switched multimegabit data service) SSCS service 0 reliable data delivery SSCS service (a packet-network like data network service) 0 LAN emulation SSCS service 0 desktopquality video SSCS service 0 entertainmentquality video SSCS service 0 further services still in the stage of development by ATM forum UN1 NNI *----, communication or signalling DSS2 digital subscriber signalling system2 B-ISUP broadband integrated services user part MTP3 message transfer protocol layer3 SSCS service specific convergence sublayer SSCF service specific coordination function SSCOP service specific connection-oriented protocol CP common part (convergence sublayer) SAR segmentation and reassembly sublayer AAL5 AAL service type 5 UN1 user--network interface NNI network-node interface Figure 26.19 UN1 and NNI protocols used on the control plane
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