The Camp-On Problem for Multiple-Address Traffic

1405 1410 1412 1417 1417 1420 1421 1422 The frequency of conference calls in voice telephony is very low; but many of the messages carried by some data communication systems are directed from a single transmitter to two or more destinations. In the mathematical analysis of some types of data traffic, it is the presence of these multiple-address (MA) messages that raises problems essentially different from those encountered in the classical congestion theory of telephone systems. Methods of coping with MA traffic fall into three classes. One involves message switching (also called storage, or the store-and-forward method), in which a transmitter sends a message to a switching center or other central location at which it is stored. Copies of the message are then retransmitted more or less independently to the desired receivers. A second class makes use of selective calling. In the simplest example, all stations are connected to a single channel, which may be thought of as a loop without a central switch; a message on this channel may be directed to any or all of the receivers. The number of simultaneous transmissions that is possible in a system of this class is limited by the number of space-, time-, or frequency-division channels in the loop. The third class of methods is based on line switching, in which a switching center merely establishes connections between the terminals of a system instead of providing storage for messages in transit. Line switching for MA traffic has itself two extreme forms, sequential and simultaneous, between which lie many other types of line switching.