tailieunhanh - Mạng và viễn thông P18

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. | 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 18 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 storage devices. In this chapter we discuss the basics of packet switching and ITU-T s recommendation nowadays the worldwide technical standard interface to packet-switched networks. We then also go on to discuss the IBM company s SNA systems network architecture a proprietary form of packet switching important because of its dominant role in IBM computer networks. 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 of Chapter 1. Each packet of data is labelled to identify its intended destination and protocol control information PCI 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 sequence numbers and the other PCI 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 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