Publisher Summary This chapter discusses packet switching. Telephone and telex systems use circuit switching. Message telegram systems use message switching, in which a message is sent to a switching center, where it is stored. In message switching, messages longer than 1000 bits are broken up into separate messages or “packets,” hence the term “packet switching.” Each packet must have some additional bits added for address and administrative purposes. Packet switching is effective when most of the messages handled fit in a small number of packets, preferably one. The critical points of comparison for data transmission systems are speed of operation and cost. Recommendation X.25 transfers packets in frames in the High (level) Data Link Control format. The line from the X.25 DTE (Data Terminal Equipment) to the Datapac network is time division multiplexed to provide the capability of handling traffic on a number of virtual circuits. A “virtual circuit” is a bidirectional association between a pair of DTEs over which all data transfers take the form of packets.