Mạng và viễn thông P18

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Packet Switching Packet switching emerged in the 1970s as an efficient means of data conveyance. It overcame the inability of circuit-switched (telephone) networks to provide efficiently for variable bandwidth connections for bursty-type usage as required between computers, terminals and storagedevices.

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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) PART 3 MODERN DATA NETWORKS
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) Packet Switching Packet switching emerged in the 1970s as an efficient means of data conveyance. It overcame the inability of circuit-switched (telephone) networks to provide efficiently for variable bandwidth connections for bursty-type usage as required between computers, terminals and storagedevices. In this chapter we discuss the basics of packet switching and ITU-T’s X.25 recommendation, nowadays the worldwide technical standard interface to packet-switched networks. We then also go on todiscuss the IBM company’s SNA (systems network architecture), a proprietary form of packet switching, important because of its dominant role in IBM computer networks. 18.1 PACKET SWITCHING BASICS Packet switching is so-called because the user’s overall message is broken up into a number of smaller packets, each of which is sent separately. We illustrated the concept in Figure 1.10 of Chapter 1. Each packet of data is labelled to identify its intended destination, and protocol control information (PCZ) is added, as we saw in Chapter 9, before it is sent. The receiving end re-assembles the packets in their proper order, with the aid of sequencenumbers and the other PC1 fields. Each packet is carried across the network in a store-and-forward fashion, taking the most efficient route available at the time. Packet switching is a form of statistical multiplexing, as we discovered in Chapter 9. Figure 18.1 illustrates how a link within a packet switching network is used to carry the jumbled-up packets of various different messages and the use of the information carried in the packet header to sort arriving packets at the destination end into the separate logical channels, virtual circuits ( VCs) or virtual calls (VCs). Transmissioncapacity between pairs of nodesinapacket-switchednetwork is generally not split up into rigidly separate physical channels, each of a fixed bandwidth. Instead,theentireavailablebandwidth between two nodalpoints (switches) inthe network is bundled together as a single high bitrate pipe, and all packets to be sent between the two endpointsof the link share the same pipe (Figure 18.1). In this way, the entire bandwidth (i.e. full bitspeed)can be used momentarily by any of the logical channels sharing the connection. This means that individual packets are transported more quickly and bursts of transmission can be accommodated. 341
342 PACKET SWITCHING mm n U packet may switch Figure 18.1 The statistical multiplexing principle of packet switching A problem arises when more than one or all logical channels try to send packets at once. This is accommodated by buffers at sending and receiving ends of the connection as shown in Figure 18.2. These delay some of the simultaneous packets for an instant until the line becomes free. By use of buffers as shown in Figure 18.2, it is possible to run thetransmission link at very close to100% utilization.This is achieved by sharingthecapacity between a number of end devices (each with a logical channel). The statistical average of the total bitrate of all the logical channels must be slightly lower than the line bitrate so that all packetsmay be carried,butatany individualpoint in timethe buffers may be accumulating packets or emptying their contents to the line. Packet switching is able to carry logical channels of almost any average bitrate. Thus a 128 kbit/s trunk between two packet switches might carry 6 logical channels of mixed and varying bitrates 5.6 kbit/s, 11.4 kbit/s, 12.3 kbit/s,22.1 kbit/s, 28.7 kbit/s, 43.0 kbit/s and still have capacity to spare. This compares with the two channels which a telephone network would be able to carry using the same trunk capacity. (The excess capacity of the telephone channels simply has to be wasted, and the other four channels cannot be carried.) packet switch buffer Figure 18.2 The use of a buffer to accommodate simultaneous sending of packets by different logical channels
TRANSMISSION DELAY IN PACKET-SWITCHED NETWORKS 343 18.2 TRANSMISSION DELAY IN PACKET-SWITCHED NETWORKS When using the trunksin a packet-switched networkat very close to full utilization, very large buffers are required for each of the logical channels,to smooth out the bursts from individual channels into a smooth output for carriage by the line. (This is rather like having a very large water reservoir,collecting water during showersof rain, and varying in water depth, butalways capableof outputting a constant volume of water for munic- ipal use (Figure 18.3). The water reservoir is analagous to the data buffers, the showers of rain to the bursts of data information, and the constant output to the information carried by the line.) We can make sure that the packets accumulated in the buffer are despatched on a jirst-in-jirst out (FIFO) basis to fairly share out the queueing delays which result, but it is critical to ensure that the queueing delay does not become unacceptably long. The chance of a very long delay is much greater when close to 100% utilization of the line is expected. (Imagine waiting in line for a bus, all of the seats of which had to be full before it pulled away; either the bus doesn’t come very often, or there is a very long queue to ensure that all the seats can be filled). A certainamount of queueing delaycaused by buffering is not noticeable to computer users (a $ second is a very long queueing delay in packet switching network terms). Even if a typed character did not appear on the computer screen until a f second afterhittingthekeyboard,the user is unlikely to notice. A variation inthedelay (sometimes a $ second, and sometimes no delay) is also unimportant. (The fact that some characters appear on the screen more quickly than the f second maximum delay will not be noticed.) On the other hand, once the average delay becomes much longer, then computer work may become frustrating, so that much longer queueing delays are unacceptable. There is an entirestatistical science used to estimatequeueingdelays. Themost important formulais the Erlang call-waiting formula, which we willdiscuss in Chapter 30. In simple terms, however, the unacceptability of long queueing delays means that the
344 PACKET SWITCHING trunks in packet-switching networks may not be utilized at 100%. Typical acceptable maximum utilization is around 50%. Despite this fact, they are still more efficient than circuit-switched networks to carry data tra@c (i.e. information between computers). 18.3 ROUTINGIN PACKET-SWITCHED NETWORKS Packets are routed across the individual paths within the network according to the prevailing traffic conditions, the link error reliability, and the shortest path to the desti- nation, according to one of two main techniques, so-called path-oriented routing and dutagram routing. The routes chosen are controlled by the (layer 3) software of the packet switch, together with routing information pre-set by the network operator. Path-orientedrouting is nowadaysthemostcommontechnique.In path-oriented routing, a fixed path is chosen for a given logical channel (i.e. virtual circuit, V C ) at the time of call set up. The pathitself is chosen based on the current loadingof the network and the available topology. In the case of the virtual call (i.e. switchedvirtualcon- nection) service, the required destination of the path is indicated using an X.121 address carried by a layer 3 packet, called the call set-up packet. This packet has the equivalent function to the dialled digits of a telephone number in setting up a telephone call. Should any link in the path become unavailable during the course of the call (say because of a transmission failure), then an altenative path is sought, without breaking the connection. The packets are stored for a short while, while the new path is found and then sent over this path. (Figure 18.4). The advantage of path-orientedrouting is that the packets pertaining to a given logical connection all take the same path, all suffer about the same amount of queue- ing delay in the buffers and arrive pretty much in the same order as they were sent (allowing for any lost on the way). This makes the job of re-sequencing the packets at the receiving end much easier, as well as the job of directing the packets through the network. It alsoleads tomorepredictable delayperformancefortheend user or computerapplication. The packetswitchingnetworkcomponents can therefore be relatively simple and cheap. The disadvantage of path-oriented routing is the inability of the network on a more immediate basis to employ alternative routing to better utilize the network as a whole (A31ppq-W packet C-path back-up packet fcl p (c31 c1 1 4 * normal A-connection-path switch 4q m p 1 ," normal C-path (currently failed) A' Figure 18.4 Circumventing a transmission link failure using path-oriented routing
ROUTING IN PACKET-SWITCHEDNETWORKS 345 m L U U) 2
346 PACKET SWITCHING during periodsof sudden surge in demand resulting from simultaneous packet bursts by many logical channels sharing the same path. The second type of routing, datagram routing, allows for more dynamic routing of individual packets (Figure 18.5), and thus has the potential for better overall networkefficiency. The technique, however, requires more sophisticated equipment, and powerful switch processors capable of determining routes for individual packets. Packet switching gives good end-to-end reliability, with well-designed switches and networks it is possible to bypass network failures (even during the progress of a call). Packet switching is also efficient in its use of network links andresources, sharing them between a number of calls, thereby increasing their utilization. 18.4 ITU-T RECOMMENDATION X.25 Most packet-switched networks use the protocol standards set by ITU-T’s recommen- dation X.25. Thissets out the mannerin which adata terminal equipment ( D T E )should interact with a data circuit terminating equipment ( D C E ) , forming the interface to a packet-switched network. The relationship is shown in Figure 18.6. The X.25 recommendation defines theprotocolsbetween DTE (e.g. personal computer computer or terminal controller (e.g. IBM 3174)) and DCE (i.e. the connection point to a wide area network, W A N )corresponding to OS1 layers 1, 2 and 3 (Figure 18.7) which we learned about in Chapter 9). The physical connection may either be X.21 (digital leaseline) or X.21 bis (V.24/V.28 modem in conjunction with an analogue leaseline: Chapter 9). Alternatively, the X.31 recommendation (Chapter 10) specifies how the physical connection (DTE/DCE) may be achieved via an ISDN (integrated digital services network). Finally, recommenda- tion X.32 specifies the use of a dial-up connection for apacket mode connection via the telephone or ISDN network to an X.25 packet exchange. The X.25 recommendation itself defines theOS1 Iayer 2 and layer 3 protocols. These are called the link access procedures (LAPB and L A P ) and thepacket level interface. The link access procedureassures the correct carriage data across the of link connecting DTE DTE DCE DCE DTE I I I I l *X25 --U I c X 2 5 - W Packet switched network Figure 18.6 The X.25 interface to packet switched networks
ITU-T X.25 347 E layer 3 protocol (packet layer) * X.25 packet level interface - layer 2 protocol X.25 LAPB (link access procedure) (NETWORK) (link layer) layer 1 protocol X.21, X.Slbis, X.31 or X.32 (physical layer) DTE DCE Figure 18.7 OS1 layered model representation of ITU-T recommendation X.25 to DCE and for multiplexing of logical channels; the packet level interface meanwhile guarantees the end-to-end carriage of information across the network as a whole. The LAPB (link access procedure balanced) protocol provides for the I S 0 balanced class o procedure and also allows for use of multiple physical circuits making up a f single logical link. LAP is an olderand simpler procedure only suitable for single physical circuits without balanced operation.The link access procedures use the principles and terminology of high-level data link control ( H D L C ) as defined by ISO. This procedure ensures the correct and error-free transmission of data information across the link from DTE to DCE. Itdoes not, however, enable the DCE (in the form of a dataswitchingexchange, D S E ) to determinewheretheinformationshould be forwarded to withinthenetwork or ensure its correct and error-free arrival at the distant side of the packet network. Thisis the job of the OS1 layer 3 protocol, the X.25 packet level interface. During the set-upof a switched virtual circuit (SVC, also called a virtual call ( V C ) )it is a level 3 call set up packet which delivers the DSE the data network address of the remote DTE. Level 3 packets confirm the set-upof the connection to the initiating DTE and then passend to end through the network, allowing user data tobe carried between the DTEs. A packet of data carried by the X.25 protocol may be anything between three and about 4100octets (bytes: 8 bits). Up to 4096 alphanumeric characters of user information may be carried in a single packet. In slang usage, many people refer to ‘X.25 networks’. In general they mean packet switching networks to which X.25 compliant DTEsmay be connected, forrecommenda- tion X.25 describes only the U N I (user-network interface, see Chapter 7). The X.25 protocol allows DTEs madeby any manufacturer to communicate across the network. You should not be tempted into believing that the protocol used between the various packet data switching exchanges (DSEs) within the network is also X.25. Generally, packet-switched data networks arebuilt from a number of individual exchanges, but all of them provided by the same manufacturer. The protocol used for the carriage of the data between the exchanges is normally an X.25-like, but enhanced ‘proprietary’ protocol. Examples include those used by Northern Telecom (Nortel), Telenet, BBS, Tymnet and France’s Transpac. Proprietary trunk protocols typically allowthecarriage of sophisticatednetwork management and charging information back to the network control centre. In addition,
348 PACKET SWITCHING they may allow for dynamic adjustment of the traffic paths taken through the network, so giving better overall network performance during heavy traffic loading. Where separate packet-switched sub-networks(provided by different manufac- turers) need to be interconnected, the X.75 (NNI) protocol is used. We discuss this later in the chapter. 18.5 THE TECHNICAL DETAILS OF X.25 X.25 was one of the first data protocols to be well defined and standardized. As such it has formed the basis on which later data transport protocols have been developed. Understanding the principles in detail will give the reader a very good understanding of all other dataswitching protocols, all which use similar principles. Therethus follows of a very detailed description. 18.6 X.25 LINK ACCESSPROCEDURE(LAPAND LAPB) The link access procedure can be performed either in the basic mode (B = 0, called LAP) or in the more advanced balanced mode (B = 1, called LAPB). Nowadays the LAPB mode is more common. There are two formsof LAPB; the basic form is called LAPB modulo 8,the extended form is called LAPB modulo 128. Only the modulo 8 form is universally available. The difference between the two forms is only the maximum value of the sequence number given to consecutive packets before resetting to value ‘0’. LAPB allows for dataframes to be carried across a physical layer connection between a DTE and a DCE. The frame is structured in the manner shown in Figure 18.8. The j a g is a delimiter between frames. The address (perhaps confusingly named, as we discovered in Chapter 9) is a means of indicating whether the frame a command or is a response frame, and whether controlis with the DTE or DCE. It coded as shown in is Table 18.1. Flag Address Control Information Frame Check Flag 01111110 (8 bits) (8 bits) (N bits) Sequence (of next frame) (16 bits) Figure 18.8 X.25 LAPB modulo 8 frame Table 18.1 X.25 LAPB address field coding Single link procedure Multiple link procedure command from DCE to DTE address A - 1l000000 address C - 11110000 response of DTE to DCE address A - 1 l000000 address C - 11 110000 command from DTE to DCE address B - 10000000 address D - 11l00000 response of DCE to DTE address B - 10000000 address D - 11l00000
ACCESS LINK X.25 (LAP PROCEDURE AND LAPB) 349 Table 18.2 X.25 LAPB modulo 8 control field command and response coding ~ ~ ~ ~ ~ ~~~~~~~~~~~~ Format Command Response bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 bit 8 Information I (information) 0 P transfer Supervisory (receive RR ready) RNR (receive not ready) REJ (reject) Unnumbered SABM (set asynchronous balanced mode) DISC (disconnect) 1 1 0 0 P 0 1 0 DM (disconnect l l l l F l l O mode) UA (unnumbered l l O O F l l O acknowledgement) FRMR l l l O F O O l (frame reject) The controlfieldcontains either command or a response, and sequence numbers where a applicable as a reference when acknowledging receipt of a previous frame. There three are types of control field format,correspondingtothenumberedinformationtrans- fer of I-frames (I format), numbered supervisory functions(S format) and unnumbered control functions (U format). The control field is coded as detailed in Table 18.2. The frame check sequence ( F C S )field is a string of bits which help to determine at the receiving end whether the data in the frame has in any been corrupted. Itis a 16 bit way field, created using the properties of a cyclic code, hence the term cyclic redundancy check. The exact 16 bit sequence sent is the ones complement of the sum (in binary) of the following two parts. (1) The remainder of xk(xI5+ xI4 + xI3 + XI* xl1+ x10+ x9 + x8 + x7 + x6 + +X5 + + + + + + X4 x3 X2 XI X 1) divided (in binary) by x16 + x L 2 x5 + 1 + where k is the number of bits in frame the excluding flag and FCS. (excluding flag and FCS) and xI6, (2) The remainderof the productof the frame content when divided by X'6 + X12 + X5 + 1 There are six configurable parameters when setting up LAPB connections. These, their meaning and typical value settings are given in Table 18.3.
350 PACKET SWITCHING Table 18.3 Optional parameter settings for X.25 LAPB Meaning Parameter Frame window size This is the maximum permitted number of 7 outstanding frames which may be sent without receiving an acknowledgement N1 parameter This is the maximum number of bits in an I-frame 2096 N2 parameter This is the maximum number of attempts which 10 attempts may be used to complete a transmission Timer T1 The timer T1 is the timeout period after which a 3 seconds retransmission may be initiated Parameter T2 This is the maximum time allowed before a n 0.2 seconds acknowledging frame must be initiated Timer T3 Should the channel remain idle longer than this time, 0 (not used) then the link shall be assumed to be non-active 18.7 X.25 PACKET LEVEL INTERFACE (LAYER 3 PROTOCOL) The information carried within a LAPB I-frame will be a packet of user data informa- tion structured in t h e format as defined by the X.25 packet level interface. The general format of a packet is as s h o w n i n Figure 18.9. Octet 8 7 6 5 4 3 2 1 1 General format identifier Logical channel groupnumber I 2 Logical channel number clearing cause, resetting cause interrupt data or 31 4 diagnostic code I 5 Packet type identifier I 41 Address digit 1 I I Address digit 2 I I I X length Facility (1 octet) Figure 18.9 X.25 packet format (layer 3)
X.25 PACKET LEVEL INTERFACE (LAYER 3 PROTOCOL) 351 Table 18.4 X.25 packet-coding of the general format identifier General format identifier bit 8 bit 7 bit 6 bit 5 Call set-up sequence modulo packets number 8 X X 0 1 sequence number modulo I28 X X 1 0 Clearing number packets modulo sequence 8 X 0 0 1 sequence number modulo 128 X 0 1 0 Flow control, interrupt, sequence number modulo 8 0 0 0 1 reset, restart, registration sequence number modulo 128 0 0 1 0 and diagnostic packets Data packets sequence number modulo 8 X X 0 1 sequence number modulo 128 X X 1 0 general format identifier extension 0 0 1 1 reserved for other applications * * 0 0 The minimum number of octets a packet is three, the general format identifier, the in logical channel identifier and the packet typeidentifier. The other packettypes are added as required. As is normal, the least significant bits of each octet (i.e. bit 1) is transmitted first. The general format identijier field provides information about the nature the rest of of the packet header (the first three octets of the packet). It is coded according to the values set out in Table 18.4. The logical channel number is a reference number allowing the DTE and DCE to distinguish to which logical connection (or statistically multiplexed virtual channel) the packet belongs. Thus, in theory, up to 4096 logical channels may be supported by a single DTE /DCE connection simultaneously. The packet type identijier is coded according to Table 18.5. As can be seen from the table, certain packet types are needed for virtual calls (VCs, also called switched virtual channels, or SVCs), and a lesser number of packet types are required to sup- port permanent virtualchannels (or PVCs). The difference between an SVC and a PVC is essentially the difference between an ordinarytelephone line and apoint- to-point leaseline. With an ordinary telephone line and an SVC each connection is set up on demand by dialling the number of the desired destination. With a telephone leaseline or a PVC the connection is permanently connected between the same two endpoints. In clear request and clear indication packets, the clearing cause is included at octet 4. In reset request packets the reset cause appears in octet 4, while in interrupt data packets. Interrupt data, when sent, also appears in octet 4. This is data which is not subject to normal flow control,in effect allowingthe DTEto overrideprevious commandstothedistantDTE (like a ‘break’ or ‘escape’ key). Callrequest, call accepted and call connected packets contain from octet 4 onwards the address block field, andoptional facilitylength and facility $eld andthe calleduser data. The
352 PACKET SWITCHING Table 18.5 X.25packetidentifiercoding Packet type From to From to DCEDTE DTE DCE VC PVC bit bitbitbit bitbit bitbit 8 1 6 5 4 3 2 1 ~ Call set-up and clearing questcall callincoming J 0 0 0 0 1 0 1 1 accepted call connected call J 0 0 0 0 1 1 1 1 request indication clear clear J 0 0 0 1 0 0 1 1 DCE clear confirmation clear DTE confirmation J 0 0 0 1 0 1 1 1 Data and interrupt DCE data DTE data J J x x x x x x x o DCE DTE interrupt interrupt J J 0 0 1 0 0 0 1 1 DCE interrupt confirmation DTE interrupt confirmation J J O O 1 0 0 1 1 1 Flow control and reset DCE RR (modulo 8) DTE RR (modulo 8) J J x x x 0 0 0 0 1 DCE RR (modulo DTE 128) RR (modulo128) J J 0 0 0 0 0 0 0 1 DCE RNR (modulo 8) DTE RNR (modulo 8) J J x x x 0 0 1 0 1 DCE RNR (modulo 128) DTE RNR (modulo 128) J J 0 0 0 0 0 1 0 1 DTE REJ (modulo 8) J J x x x 0 1 0 0 1 DTE REJ (modulo 128) J J 0 0 0 0 1 0 0 1 request reset indication reset J J O O O 1 1 0 1 1 DCE reset confirmation reset DTE confirmation J J O O O 1 1 1 1 1 Restart indication request restart restart J J 1 1 1 1 1 0 1 1 DCE restart confirmation restart DTE confirmation J J 1 1 1 1 1 1 1 1 Diagnostic J J 1 1 1 1 0 0 0 1 Registration registration request J J l 1 1 1 0 0 1 1 registration confirmation J J 1 1 1 1 0 1 1 1 diagnostic code isincluded indiagnostictypepackets andalsooptionallyin clear request and reset request packets. The address block field is coded as shown in Figure 18.10. The calling DTE and called DTE address length fields are coded in binary. The address digits themselves, however, are in digit codedfour blocks, coded binary decimal,according to ITU-T recommendation X.121. Ifnecessary,bits 1-4 of the last octet are filled with Os to maintain octet alignment. Thefacility length field merely indicates as a binary number the length of the facility field which follows. The facilities field allows the calling DTE to request a number of optional services. These (where made available) are as listed in Table 18.6.
(LAYER PACKET INTERFACE LEVEL X.25 3 PROTOCOL) 353 8 7 6 5 4 3 2 1 called DTE address length calling DTE address length called DTE address l I I I I I calling DTE address I I I 0 0 0 0 I I I Figure 18.10 Format of the X.25 packet address block Table 18.6 X.25networkfacilities Description X.25 Optional facilities on-line facility registration if subscribed to, allows user to request facility registration extended packet sequence numbering packets numbered modulo 128 rather than normal modulo 8 D bit modification support of D-bit procedure packet retransmission allows DTE to request DCE to restransmit packets incoming calls barred prevents incoming calls being presented to DTE outgoing calls barred prevents DCE from accepting outgoing calls one-way logical channel outgoing restricts logical channel use to originating outgoing virtual calls only one-way logical channel incoming restricts logical channel use to incoming virtual calls only non-standard default packet sizes provides for the selection of non-default packet sizes non-standard default window sizes provides for the selectionof non-default window sizes default throughput classes assignment provides for the selection of default throughput classes flow control parameter negotiation allows the DTE to alter window and packet sizes for each virtual call throughput class negotiation allows throughput class (i.e. bitspeed) negotiation for each call closed user group restricts incoming calls to the DTE to be from other specific DTEs which are members of a closed user group bilateral closed user group a closed user group of only two DTEs fast select allows the call request packet to contain up to 128 octets of user information, thereby speeding the sending of short user messages fast select acceptance authorizes DCE to forward fast select packets to the DTE
354 PACKET Table 18.6 (continued) X.25 Optional facilities Description reverse charging allows calls to be requested for charging to recipients rather than callers reverse charging acceptance DTE authorization to the DCE that it is willing to accept reverse charge calls local charging prevention prevents the DCE from allowing virtual callsto be set up for which the local DTE will be charged network user identification (NUI) an authorisation procedure allowing for the dial-into an X.25 network port across a public telephone network charging information DTE may request charging information RPOA related facilities allows the calling DTE to specify transit networks through which the call should be routed hunt group allows several separate DTE/DCE network connections to share the same network address. Incoming calls are routed to any free logical channel within at any of the DTEs call redirection and call deflection allows deflectionon busy or redirection on no answer called line address modified notification notifies the calling DTE that the called address has been modified (diverting to a new destination) transit delay selection and indication allows the calling DTE to specify a desired transit delay TOA/NPi address subscription DTE/DCE to use TOA/NPi address format 18.8 TYPICAL PARAMETER DEFAULT SETTINGS USED IN X.25 NETWORKS A typical public X.25 network might be set with thefollowing default settings for the up packet level interface window size 2 packet size 128 window negotiation allowed packet negotiation allowed throughput negotiation allowed lowest logical channel number 1 highest logical channel number 16 customer DTE to select logical channel number, from highest number descending
DISASSEMBLERS PACKET 355 18.9 PACKET ASSEMBLER/DISASSEMBLERS (PADS) The X.25 protocol is suitable for the connection of synchronous communication devices (i.e. computers) to a packet switching network. It is widely used, and many devices are available worldwide (both DTEs and packet DSEs (data switching exchanges) which supportit. However,directly X.25 protocol is unsuitableforconnecting slow and relatively unintelligent asynchronous computer terminals to an X.25 packet network. Instead a procedure is defined by ITU-T recommendation X.28. Necessary interworking is achieved by means of a piece of equipment provided in place of the DCE at the packet-switched exchange. This equipment is called a packet assembler/disassembler or P A D for short. As its name suggests, it assembles packets from the asynchronous terminal data for onward transmission, and it disassembles packets received fromthefarendDTE, conveyingthem ontotheasynchronous terminal as individual characters. Three ITU-T recommendations define the operation of PADS. 0 X.3 defines the functions of the PAD (i.e. the conversion of asynchronous characters into synchronous packets). 0 X.28 defines the control interface betweenPAD and asynchronous terminal. The X.28 parameters (Figure 18.18) define how thePAD is to react to the characters sent to it, how often to despatch groups of characters as apacket, and which characters typed by the end terminal are be interpreted directlyby the PAD itself as control characters. to 0 X.29 defines the interface between PAD and remote X25 DTE (usually some form of host computer). Figure 18.1 1 illustrates the inter-relationship of the three recommendations and Table 18.7 lists the PAD control parametersof recommendation X.28. In the final column of packet-switched network functions definedby X.3 L PAD DCE (packet- V.24 I I I interface ! I 4 8 -I X.28 - - + -H X.29: - protocol Figure 18.11 Thepacketassembler/disassembler (PAD)
356 PACKET Table 18.7 PAD control parameters defined by ITU-T recommendation X.28 Purpose Parameter Simple values Permitted standard reference profile parameter 1 PAD recallusing character 0 = no escape 1 1 = escape 32-126 = ASCII code of ‘escape’ character parameter (i.e. Echo character 2 is 0 = no echo 1 returned to bedisplayed on 1 =echo screen) parameter 3 Selectionof data fowarding 0 =forwarded only on ‘poll’fromnetwork 126 characters (e.g. ‘carriage 2 = forward on ‘carriage return’ return’ key) 8 =forward on (CR), (ESC), (DEL), (ENQ), WK) 18=forward on (CR), (EDT), (ETX) 126 = forward on all control characters parameter 4 Selection of idletimerdelay 0 = no delay 1-255 (delay, multiple of 10ms) parameter 5 Ancillarydevice control 0 = X-ONand X-OFF not use in 1 =X-ON and X-OFF in use 2 = X-ON and X-OFF in use during data transfer parameterControl 6 of PAD service 0 = n o signals sent to DTE 1 and command signals 1 =signals sent to DTE other values define which signals may be used parameter 7 Selection of reaction of 0 = n o reaction to ‘break’ 2 PAD to the‘break’key 1 = assembledcharactersand‘interrupt’sent to host 2 = reset on ‘break’ other values define other actions parameter 8 Discard output 0 = deliver data 0 l = discard data parameterPadding 9 after carriage 0-255 (number characters of to be inserted 0 return’) ‘carriage after return parameter 10 Line 0-255 length folding (definesaof characters) line in 0 parameter 11Binaryspeed 12 = 2 400 bit/s - 13 = 4 800 bit/s 14= 9 600 bit/s 16= 19200bit/s 19= 14400bit/s 20 = 28 800 bit/s 21 = 38 400 bit/s other values give other speeds
DISASSEMBLERS PACKET 357 Table 18.7 (continued) Parameter Purpose Simple values Permitted standard reference profile parameter 12Flow control of the PAD 0 = n o use of X-ON and X-OFF for flow 1 control by the DTE 1 = use of X-ON and X-OFF for flow control parameter 13 ‘Linefeed’insertion after 0 =no linefeed insertion 0 ‘carriage return’ 1 = linefeed after return’ ‘carriage other values define other actions parameter 14 Linefeed 0-255 characters padding (number of inserted after linefeed) parameter 15 Editing 0 = editing command only in mode 1 = editing in command and data transfer modes 2 =extended editing parameter16Characterdelete 0-127 (ASCII code of character used as 127 character delete) parameter 17 Linedelete 0-127 (ASCII code of character used as line 24 delete) parameter 18 Linedisplay 0-127 (ASCII code of character used to 18 request line display) parameter 19 Editing PAD service 0 = no editing 1 signals 1 =editing for print terminals 2 =editing for display terminals 8, 32-126 this character replaces deleted characters parameter 20Echomask 0 = no echo mask 0 1 = no echo of ‘carriage return’ 2 = no echo of ‘linefeed’ other values define other control characters not echoed parameter 21 Paritytreatment 0 =parity not generated or checked 0 1 =parity checked of input from DTE 2 =parity generated for output to DTE 3 =parity generated and checked (combination of 1 and 2) parameter 22Pagewait 0-255 (number of line feeds which may be 0 generated before an ‘abort’) parameter 23 Size of input field 0-255 (number of characters) 0 parameter 24 End of frame signal 0 = not used 0 1-127 ASCII code of character to signify end of frame
358 PACKET SWITCHING Table 18.7 (continued) Parameter Purpose Simple Permitted values standard reference profile parameter 25 Additional data forwarding 0 =no extra data forwarding 0 character 1-127 = ASCII code of extra forwarding character parameter 26 Display ‘interrupt’ character 0 = no abort character 0 1-127 = ASCII code of ‘interrupt’ character parameter 27 Display interrupt 0 = no confirmation 0 confirmation (‘prompt’ 1-127 = ASCII code of ‘interrupt’ character) confirmation character parameter 28 Diacritic character coding 0 =not used 0 scheme parameter 29 Extended echo mask 0 = not used 0 Table 18.7, the parameter settings are given for the simple standard profile. This is a standard X.28 parameter setting set for emulating a low speed ‘transparent’ leaseline between an asynchronous computer terminal and its terminal controller. In caseyou are left wondering,havingstudiedTable 18.7, what exactly ‘X-ON’ and‘X-OFF’are, these arethealternativestates of a given control leadwithina standard DTE-to-DCE interface(such as V.24 as defined in Chapter 9). When the lead is set ‘X-ON’ then the DTE may send transmit data. When instead the lead is set ‘X-OFF’ then no data may be transmitted. This ensures that the DCE is ready to receive data before the DTE may send. Before call set up the lead is set X-OFF, then X-ON aftercallset-up.Duringthecall,theleadmaybe reset to X-OFF to slow up the receipt of data (i.e. for flow control) if the DCE or network becomes overloaded. A call can be set up from an asynchronous DTE (so-called startlstop terminal)simply by typing in the X.121 data network address (Chapter 29). This procedure for call set- up from a start-stop terminal is also defined by X.28. 18.10 ITU-T RECOMMENDATION X.75 To connect different packet switched networks together, a network-network interface (NNZ) or gateway protocol is defined by ITU-T in its recommendation X.75. X.75 can be thought of as a super set of the X.25 protocol (Figure 18.12). It wasoriginally developed for international interconnection of packet networks). Like X.25, recommendation X.75 defines all the protocols forlayers 1-3. At layer 1, a 64 kbitls bearer conforming to Recommendation G.703 is stipulated. At layer 2, the
ITU-T X.75 359 m W c D -I ln U 2 W U 0 N Y - U z r