Mạng và viễn thông P20

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Frame Relay For datatransfer, X.25-based packet switching has established itself worldwide as a standard and very reliable means. However, X.25 is not a technique suited to the higher quality and speeds of so by modern data communications networks, and it is beginning to be supplanted new techniques, among them ‘frame relay’. In this chapter we start by discussing the shortcomings of X.25-based packet switching in carrying highspeed bitrates and explain how frame relay was designed to overcome these problems....

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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) 20 Frame Relay For datatransfer, X.25-based packet switching has established itself worldwide as a standard and very reliable means. However, X.25 is not a technique suited to the higher quality and speeds of so modern data communications networks, and it is beginning to be supplanted new techniques, by among them ‘frame relay’. In this chapter we start by discussing the shortcomings of X.25-based packet switching in carrying highspeed bitrates and explain how frame relay was designed to overcome these problems. We conclude with a more detailed review of the frame relay protocols themselves. 20.1 THE THROUGHPUT LIMITATIONS OF X.25 PACKET SWITCHING Thereliability of X.25 packet switching resulted worldwide has from accepted standards and the hugeavailability of compatible hardware and software products enabling computer devices made by different manufacturers and strewn around the world to intercommunicate without difficulty. X.25 was the universal first data communication protocol and it stimulated rapid growth in data communication traffic volumes, because of its reliability and its robustness. Paradoxically, its robustness is nowleading to thedemise of X.25,becauseone of the main limitations of packet switchingbased on X.25 is itsunsuitabilityforcarriage of highspeedinformation channels andits relative inefficiency when used in conjunctionwithhighquality transmission networks. When X.25 was developed in the late 1970s, the relative speed of the communicating 9600 devices was very low (in comparison with today’s devices, typically under bit/s) and the quality of wide area digital lines was comparatively poor. As a result (and to their credit) X.25 packet networks are highly robust against poor line quality. X.25 networks are able to survive and even recover from even extensive bit errors on digital lines. The problem is that the cost of this robustness is the very limited linespeeds which are possible, and the relative inefficiency of line utilization in the case higher quality lines. of The problems which arise when attempting to operate X.25 protocol at high speeds areduetothe windowing techniqueemployed by X.25to helpavoid errors. To 379
380 FRAME RELAY illustrate the problemwe consider trying to use a 2 Mbit/s line to carry X.25 dataover a distance of 1000 km. As Figure 20.1 illustrates, on a high speed data transmission line there are always a large number of bits in transit on the line any point in time (because of its length), in at our example around 000 bits or 2500 bytes (line lengthX bitrate/speed of light). (That 20 should blow any preconception you might have had that electricity travels so fast that we can consider sender and transmitter to be in synchronism with one another!) These bits in transit on the line must be considered when designing high speed data networks, if the network is to operate efficiently. X.25 lays a very high priority on the safe arrival of bits, in the correct order and withouterrors.One of themethods used to ensuresafearrival is the use of an acknowledgement window. Only so manypackets(asdefined by the window size, typically 7) may be transmitted by the sending device before an acknowledgement is received confirmingsafearrival. As thetypicalmaximumpacket size is defined as 256 bytes, this means that a maximum of 1792 bytes (7 X 256) may be transmitted by thesenderbefore an acknowledgement is returned by the receiver to confirmsafe arrival. This compares with the2500 bytes actually on the line, so that even before con- sidering the inefficiencies caused by packet overheads (the protocol control information in the X.25 header) the X.25 window will constrain the efficiency of the line of Figure 20.1 to a maximum of 1792/2500 (maximum bits allowed in transit/available bits in transit) or around 70%! Simple, you might say: increase the maximum window size! Unfortunately this only generates new problems. First, the end devices need to provide much greater storage buffers forretainingcopies of thesentbutunacknowledgedinformation.Second, because the window size is greater, so is the likelihood of errors within a window. The probability of the need for a retransmission of the informationeliminate the errorsis to thus also greater. Also, because of the increased window size, the time required for retransmission is longer. So increasing the window size may actually reduce throughput! Today's digital transmission is several orders of magnitude better quality than thatof the 1970s, so that the heavy duty error detection and correction techniques used by X.25 (a) n ~ pulse travels at around 10' m/s 4- 1 bit length = speedm/S in bitrate = 2X 1 0' 50 m sender network (2048 kbitls) receiver number of bits in transit on the line = line length I bit length = 1000 km I 5 0 m = 20 000 bits = 2500 bytes Figure 20.1 Bits in transit in an X.25 packet-switched data network
FOR NEED RESPONSE THE FASTER DATA NETWORKS 381 have become redundant. Windowing and acknowledgement are now largely super- fluous. Framerelay (or frame relaying) was one of the first techniques designed to dispense with heavy duty error detection and correctiontechniques. Instead of it being undertaken by the network, thejob of error detection and correction or recovery is left to higher layer protocols (i.e. the end user’s device, computer or software) to sort out. The frame relay network, meanwhile, may concentrate on the raw information carriage and is thus more efficient and also more capable of higher information throughput. Thusfor widearea computerandLAN-to-LANconnection needs of 64 kbit/sor greater, frame relay is today’s preferred method. However, for bitrates above2 Mbit/s, native ATM (see Chapter 26) should be considered. 20.2 THE NEED FORFASTER RESPONSE DATANETWORKS Althoughthe basic need tocarry high bandwidth signals drovethe need forthe development of the frame relay protocols, so did the need for faster responding net- works. It may not immediately obvious, but the time required to propagate even low be bandwidth information across a digital network dependent on thebitspeed employed: is the higher the bitspeed, the lower the propagation delay. Even data applications with limited information transport needs appear to run faster when carried by a high speed network, even though sufficient bandwidth may havepreviously already been available. The following discussion gives two reasons why. Let us imagine two rainwater conduits, one small bore and oneof large bore. Let us of assume that the first has throughput capacity of 5litres/s and the second of 10litres/s. Now let us assume that therainfall rate is 4 litre+. Why should I bother with the large bore conduit? The answer that therainfall rate is not constant.Over the courseof time is the rate may vary between, say 2 and 6 litres per second, so during moments time that in when the rate of rainfall exceeds 5 litres/s, water will be accumulating in the roof gutter rather than flowing down the conduit. The accumulation clears when the rainfall rate drops momentarilybelow 5 litresis. As aresult of the momentary accumulation, some of the rainwateris delayed slightly, thereby increasing the propagation delay. The analogy is relevant to data transmission, where the rate of generation of typed characters or other data to be transferred is not constant, but can fluctuate wildly. The first reason why high speed lines give better performance is that they cope with short high speed bursts better. The second reason is that frames can be conveyed more quickly, as we explain next. Figure 20.2 illustrates a more detailed example of data transmission across a tele- communicationtransmission line. In this case, thecarriage of the electrical signals (i.e. the waveform pulses representing individual bits of a digital signal pattern) is at a speed close to thespeed of light. Thus theleading edge of an individual pulse traverses the network at around108metres/s (Figure 20.2(a)). The time required, however, to transmit an entire frameof 1 byte (8 bits) is sensitive to thetransmission bitspeed.The propagation time in this case is equal to the sumof the raw propagation time and the signal duration (Figure 20.2(b)). Taking the example of a 9600 bit/s dataline of 100 km length (as might be employed in a corporate packet switching network today), we can calculate propagation times for both cases (a) and (b) of our example:
382 FRAME RELAY (a) .-DL-, pulsetravelsataround 10’ m h sender network receiver travels (b) pattern signal at 108m l S l+----+ signal duration = number of bitslbitspeed Figure 20.2 Signal propagation time across a data network 0 single bit (pulse) propagation time = 105 m/lOs ms-’= 10-3 S = 1 ms 0 byte propagation time =pulse propagation time + signal duration = 1 ms + 8 bits/9600 bitS-’ = 1.8ms In other words, before enough bits (8) have been received to interpret the frame (in our case a data or ASCII character), 1.8 ms have elapsed. This compares with the 1 ms needed for conveyance of a pulse across the line. Despite the fact that the average throughput required from the line may be far less than the 9600 bit/s available (say 2400 bit/s or 300 characters/s), the effective propagation time of characters across the line is much longer than the 1 ms that you might expect. No human being will notice the extra 0.8ms, you might say? Indeed they will not where a simple one-way transmission is involved with a human end user. However, where an interactive dialogue is taking place between two computers (question-answer- question-answer), then this will take around 80% longer to conduct. A human waiting for the computer’s response may a response inaround 4seconds, where previously it see was around 2 seconds. Such intercomputer dialogues are the main cause of delays for modern computer software. (Typical dialogues run ‘please send first character’ - ‘first character’ - ‘received firstcharacter OK, please next send character’ - ‘second character’ - ‘received second character OK. . .’ etc.) The perhaps surprisingreality is that it may indeed make sense to use a network with 64 kbit/stransmissionlinksratherthan 9600 bit/slinks even though theaverage throughput is only 4000 bit/s ! (Reduction in byte propagation time from 1.S ms to 1.1 ms). The following points are thuscritical in the response time performance of data networks and associated computer applications software 0 transmission line bitspeed (e.g. 9600 bit/s, 64 kbit/s, 2 Mbit/s, etc.) 0 message, packet or frame length in number of bits 0 the number of inter-computerinteractions (request response and dialogue) necessary to complete an action before responding to the human user. Though our example is of a lowspeed data application, similar principles apply for all types of data applications. Thus the higher the bitspeeds employed in the network, the faster the application response time.
THE EMERGENCE AND USE OF FRAME RELAY 383 router frame relay switch wide-area frame relay G- Figure 20.3 Typicaluseofframerelay to improvethewideareaefficiencyof LAN/router networks 20.3 THEEMERGENCEAND USE OF FRAME RELAY The recent explosion in the number LANs (local area networks,networks connecting of personal computers withinoffice buildings) and theneed to interconnectLANs, as well as the growing numberof clientlserver computing (UNIX) environments, have created the demand for high speed networking in data networks. Frame relay provides for relatively cheap wide area data communication at rates between 9600 bit/s and 2 Mbit/s, and has proved a viable alternative to leaselines, particularly in router networks. Initially, frame relay networks only supported PVC (permanent virtual channel) ser- vice between pairs of fixed end-points. The service to the user was therefore somewhat akin to a 64 kbit/s leaseline, but without the full costs of a leaseline, because statistical multiplexing in the wide area part of the network allowed resources to shared across be a number of users and therefore costs to be saved by each of them (Figure 20.3). The pricing strategyof public network operators has also encouraged the of frame relay use service as a leaseline replacement service where high speed but relatively low volume usage is required, because flat rate charging based on the committed information rate (CIR) become the industry standard. has 20.4 FRAME RELAY UN1 In the arrangement of Figure 20.3, which is typical for a frame relay network, each of the routers is connected to the frame relay network usingsingle connection, typically a of 64 kbit/s, employing the Frame relay VNI (user-network interface). Over this single physical connection, up to1024 (21°) logical channels(PVCs, in the correct frame relay terminology, data links) may be connected, each to a separate end destination. These
384 FRAME RELAY channels are available on a permanent basis, but the capacity of the wide area part of the network is only used when there are actually frames requiring to be relayed from one end of the data link to the other. In theframe header (like the HDLC header of X.25) is a numbered value identifying the logical connection to which the frame belongs. This is called the data link channel identifier (DLCZ). Unlike X.25,there are no real end-to-end networkfunctions supported at OS1 layer 3. These are the functions which provide within the network for reliable end-to-end transfer. Saving these functions simplifies the frame relay protocol (in comparison with X.25), making it more efficient and faster running. As is also shown in Figure 20.3, it is common in frame relay networks to build fully meshed networks of logical connections (PVCs) between individual routers (a triangle in our case). This circumvents theneed for the routers themselves act as transit nodes to for inter-router traffic, and so improves the overall performance perceived by the LAN users without having mucheffect on the overall cost of either the network hardware or the transmission lines needed in the wide area network ( W A N ) . 20.5 FRAME RELAY SVC SERVICE The main drawback of the arrangement shown in Figure 20.3 the management effort is needed to establish and maintain the large number of PVC connections within the network. Potentially, each time a link in the network fails or a new link is added, administrative work may be necessary to reconfigure some or all of the PVCs to new, more efficient paths. To get around this problem, the Europeans in particular have driventhedevelopment of an enhancement of the frame relay UN1 to includethe capability for on-demand establishmentof datalinks (i.e. switchedvirtualcircuits, SVCs) using a dial-up procedure. Although this adds layer 3 functions to the frame relay protocol stack, these are only connection and clearing functions, which do not affect thesubsequentend-to-endcarriagecharacteristics(highspeed, low delay as discussed). The main benefit of an SVC network is that individual data links need only be established when needed and may be cleared afterwards. This simplifies the network and its management, and in addition has the effect of automatically optimizing the routing of connections each time they are newly established. 20.6 CONGESTION CONTROL IN FRAME RELAYNETWORKS The high speed of the computer devices using frame relay networks leads the need for to special measures to control network congestion, because the layer 3 protocol is almost non-existent. Figure 20.4 illustrates a case in which oneof the intermediate links within the wide area or backbone part of the network is in congestion. As a result, frames are accumulating rapidly (and unabated) the buffer immediately preceding the congested in link, as they wait to be transmitted over the link. Ultimately, the buffer will overflow and frames will be lost, so affecting all thedata links sharing the congested link. Worse still,oncetheenduser devices detect the loss of information,retransmission will commence, and the load on the network only further increases.
CONGESTION CONTROL IN FRAME RELAY NETWORKS 385 end user end user device device (sending) (receiving) excess frames overflowing the buffer and being discarded Figure 20.4 Congestion in a frame relay network As the connection from the sending device of Figure 20.4 to the network is not con- gested, the sending of frames would continue unabated, were it not for the congestion notzjication procedures within the frame relay protocols. Any time a frame underway within a frame relay network encounters congestion (the exceeding of a given waiting time or a given buffer queue length) then the frame header is tagged with a notification message, called the forward explicit congestion notijica- tion ( F E C N ) . This notifies the receiving device and the switch to which it is connected of the congestion, which may then be communicated back to the source end by means of a backward explicit congestion notijication( B E C N ) message. The sending device may voluntarily respond to receipt of the BECN by reducing its transmitted output, or may be forced by the network to do so. By reducingtheframetransmissionratefrom relevant causal sources, the congestion will ease, and the transmission rate restriction may be removed. Meanwhile, further service degradation within the networkas a whole is avoided. A further refinement of the congestion control procedure of framerelay is provided by the committed information rate ( C I R ) and excess informationrate ( E I R ) parameters.The committedinformationrate ( C I R ) is theagreedminimum bitrate to be provided by the network between the two ends of the frame relay data link. The CIR is agreed at the time of setting up the connection. Provided the frame transmission rate of the sending device is at or below the CIR, then the network is not permitted to force a reduction in the frame sending rate of the sending device, and is not permitted wilfully to discard frames. However, where the sending device is exceeding theCIRata time when the BECN message is received, then the network may first request reduction in the rate of frametransmission to the CIR. Shouldthereductionnot be undertaken(forexample,becausethesending device cannot respond to the request), then the network is permitted to discard the excess frames. At times of no congestion, sending devices are permitted for defined short periodsof time (called the excess burst, or excess burst duration, B,) to transmit at bitrates higher than the CIR. The maximum bitrate at which the device may send is termed the E I R (excess information rate). The EIR is always greater or equal to theCIR. Itis a manage- ment decision for the network operator how high EIR and CIR may be set for a given connection, and usually these values are included in the contract or order for the user’s connection (for a PVC)or negotiated at connection set-up time foran SVC. The ability
386 FRAME RELAY to handle short bursts of high speed information (above theCIR) is what makes frame relaynetworksattractivetodataapplicationsrequiringfastresponsetimes,as we discussed earlier in the chapter. 20.7 FRAME RELAY NNI As in packet-switched data networks, it is common for all the switches within a given operator’s network to be purchased from and supplied by a single manufacturer. The leadingmanufacturers of framerelaynetworkcomponents arethe Stratacom and Cascade companies and Northern TeIecom (NorteI). As the switches are supplied by a single manufacturer, it is not necessary to use a standardized interface between the nodes within the network. As a result, the individual manufacturers have tended to develop extra congestion controls, network management and service features over and above those required by the frame relay standards, to try to improve the market value of their products. All very well; except, of course when a given frame relay connection needs to be switchedacrosstwodifferentnetworks or sub-networks,supplied by different manufacturers (as in Figure 20.5). For this a standardized interface, the NNZ (network-network interface) is required. Although frame the relay NNI allows for interconnection of sub-networks of switchessupplied by different manufacturers, and although reliable data transfer is possible, it is true that the congestioncontrol and management capabilities of the combined network are much more restricted than the capabilities available within each of the sub-networks independently. This reflects the relative youth of the frame relay NNI standard. 20.8 FRAME FORMAT Figure 20.6 illustrates the formatof a single frame. It consists of five basic information fields, much like the data link layer format of X.25 (i.e. HDLC). The flag marks the beginning of the frame, delineating it from the previous frame. The address field carries A frame relay network frame relay networkB (manufacturerA) (manufacturer B) UN1 NNI UN1 Figure 20.5 Frame relay NNI (network-networkinterface)
ADDRESS FIELD FORMAT 387 Flag Address Field Control Information field F r a m e check s e q u e n c e (FCS) Figure 20.6 Frame format for frame relay the DLCI (data link connection identifier), the equivalent the logical channel number of ( E N ) of HDLC (i.e. is an OS1 layer 2 address). In addition, the address field also contains control information (command/response),the forward and backward explicit congestionnotification (FECN and B E C N ) discussedpreviouslyinthis chapter, the discard eligibility ( D E ) indication and some extra fields used for extended addressing. The controlfield contains supervision indication for the connection like receiver ready ( R R ) ,receiver not ready ( R N R ) , etc. For user information frames, this field indicates the length of the frame. Such controls discussed morefully in chapter 18 on X.25. These were enable the two end devices to coordinate one another for the communication. The informationJield contains the user information. This may be up 65 536 bytes in to length. Finally, the frame check sequence ( F C S ) is a cyclic redundancy check ( C R C ) code providing for error detection. We discussed CRC codes in Chapter 9. 20.9 ADDRESS FIELD FORMAT Figure 20.7 illustrates the two octet (i.e. two byte) address field used in frame relay. The main function of the address field is to carry the data link connection identifier (DLCZ), which identifies the end device to which the frame is to be sent (OS1 layer 2 address). The DLCI is a 10 bit field, allowing up to 1024 separate virtual connections to share the same physical connection. 1 2 3 4 5 6 l 8 (transmitted first BECN = backward explicit congestion notification C/R = command/response bit DE = discard eligibility DLCI = data link connection identifier EA = extended addressing lsb = least significant bits msb = most significant bits Figure 20.7 Address field format for frame relay
388 FRAME RELAY Higher layer information Higher layer information network layer 4.933 (signalling) 4.933 link layer 4.922 (core aspects) 4.922 (Core aspects physical layer physical layer Note: 4.933 protocol is the network layer protocol used for establishing switched connections (SVCs). It is not used in the PVC (permanent virtual connection) service. Figure 20.8 Protocol stack for frame relay Should congestion be encountered by a given frame during its transit through the network, then the affected intermediate switch will toggle the FECN (forward explicit congestion notijication) as we discussed earlier in the chapter. This alerts the receiving device of the congestion. In response, returned frames are marked using the BECN (backward explicit congestion notijication). This allows the flow control procedures to be undertaken. Should congestion become so serious that frames need to be discarded, then frames marked with thediscard eligibility ( D E ) set to ‘1’ will be discarded first. The DE bit is set to ‘1’by the firstframerelayswitch(neartheorigin) on excess frames (i.e. those causing the informationrate to exceed the committed information rate( C I R ) ) . 20.10 ITU-T RECOMMENDATIONSPERTINENTTO FRAME RELAY The following ITU-T recommendations define frame relay. 0 Recommendation 1.233 describestheframerelayservice. 0 Recommendation 1.122 defines the framework of recommendations which specify frame relay, referring to the complete list of relevant recommendations. e Recommendation Q.922 is perhaps the most important. Itdefines the core aspects of frame relay, specifically the data link procedure (i.e. frame format, address field etc.). 0 Recommendation 1.370 defines the congestion management procedures. e Recommendation 4.933 defines the signalling procedures to set up switched virtual connections. Thisrecommendation is not relevant for permanent virtual circuit ( P V C ) service. Figure 20.8 shows the layered protocol structure of frame relay. 20.11 FRAD (FRAME RELAY ACCESS DEVICE) Frame relay has historically been offered by public telecommunication. operators as a cheapalternativetoaleaselineincaseswherehighbitrates were desirablebutthe To number of hours of usage per day was relatively low. access a frame relay network, a
ICE) FRAD (FRAME RELAY ACCESS 389 leaseline eauivalent frame relay UN1 Figure 20.9 A frame relay access device (FRAD) DTE (suchas a router)musteitherincorporateaframe relayinterfacecard,or alternatively use some other standard interface and an external conversion device. The most important of the available external conversion devices is the FRAD. Frame relay access devices (FRADs) provide for the conversion of continuous bit stream oriented (CBO) data signals into a frame format, in much the same way that a PAD (packet assembler/disassembler) converts the data stream from a ‘dumb’ computer terminal into the packet format requiredby an X.25 network. AFRAD may thus be used to convert acontinuousdatastreamfromanend user device normally expecting to use a ‘transparent’ leaseline-like connection into a format suitable for carriage by a frame relay network. Figure 20.9 illustrates the principle. Figure 20.9 illustrates the equivalent a data leaseline, created using two of frame relay access devices (FRADs) and a frame relay network. This is usually more economical than the equivalentleaseline where the CIR (committed information rate) lower than is the maximumspeed of the leaseline. Usually the EIR setto the maximum speed the is of equivalent leaseline, and CIR is set somewhat above average the actual user information throughput rate. This virtually guarantees throughput of the user data (at the CIR). Bursts up to the maximum leaseline speed (the EIR) are still possible for short periods, but at more ‘quiet’ other users may share the trunk resources within times the network on a ‘statistical’ basis. This is reflected in the lower costs of frame relay service compared to ‘transparent’ leaseline service. Also shown in Figure 20.9 is the local management interface (LMI). This allows status andother network,management informationtobesharedbetweentheend terminal and the network in an SNMP-like format Chapter 27), so enabling overall (see monitoring and management of the network.