an be sequenced at the ultimate destination. In centralized networks the opposite ap- proach is often taken. The usual solution to keep data blocks in or- der is to assign a sequence number to each block for use in detecting missing and duplicate blocks. 3.5 BISYNC PROTOCOL While bit-oriented protocols represent the wave of the future, the most common link control procedure remains IBM's binary syn- chronous communications protocol, or bisync, which has been used since 1968 to control transmission between IBM computers and ter- 40 TELECOMMUNICATIONS minals. Since bisync uses special characters to delineate the various fields of a message, it is known as a "character-oriented protocol." In point-to-point bisync, data blocks have a simple structure. They begin with a special two-character sequence (DLE-STX) and end with a special sequence (DLE-ETX). As the data is being transmitted, a cyclic redundancy checksum is computed and sent at the end of the block (seeFig.3-5A). Thereceiver also constructs the checksum, which it compares with the transmitted version to determine if transmission errors occurred. If the two checksums match, the receiver sends a positive acknowledgment and the transmitter can send its next block. If there is a discrepancy, the receiver sends a negative ac- knowledgment, and the previous block is retransmitted. For multipoint configurations, the protocol functions by desig- nating one device as master and the other devices as slaves. The mas- ter then sends a special block to each slave in turn to determine if the station has any data to transfer. This block also contains an iden- tification sequence, or address of the slave being polled. If the slave is ready to transmit, it sends a positive acknowledgment followed by a data block. If it has nothing to send, it transmits a negative ac- knowledgment and the polling continues with the next slave in the sequence. Despite its popularity, bisync operates in half-duplex mode and requires each block to be acknowledged before the next one can be trans