ability. SDH will have an overall effect of simplifying the transmitted to all network elements immediately, providing rapid network. An SDH terminal performs several levels of multiplexing response to the end user. Packet switching Jim Costello Telecommunications Consultant Paul Dyer Northern Telecom David L Jeanes Bell Northern Research Contents 54.1 History and underlying concepts 54/3 54.2 Packet switched call 54/4 54.3 The packet switched network 54/5 54.4 Packet switching and the OSI model 54/7 54.5 Datagram networks 54/12 54.6 Routeing over packet switched networks 54/12 54.7 Packet switching equipment 54/15 This page intentionally left blank History and underlying concepts 54/3 History and underlying concepts As networks have grown in size and importance, functions such as routeing, error correction and flow control have also become essential. Packet switching is a mature, secure method of transferring data Packet switching therefore evolved to distribute these compo- across a network. It divides data into segments, each of which is nents throughout a network. It is a subset of traditional message wrapped in an envelope to form a packet; a typical message com- switching, in which data is transmitted in blocks, stored by the first prises one or more packets. Each packet contains the actual user data switching node it meets in the network and forwarded to the next plus information helpful to its movement across the network, such and subsequent downstream nodes until it reaches the destination. as addressing, sequencing and error control. At its heart is the concept of the virtual circuit: a fixed path A tried and tested, robust technique, packet switching attracts a through the network from sender to destination is defined at the large and growing installed base. It is ideal for transaction-orien- beginning of the session or call. The path remains unchanged for the tated applications such as banking, point-of-sale retail and elec- duration of the connection. (Figure 54.1). tronic mail. Investment in packet 