Mạng và viễn thông P20

Frame RelayFor 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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Mạng và viễn thông P20 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 379380 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 ...