Early synchronous protocols were byte-oriented protocols, where synchronization was maintained by transmitting a sequence of synchronous idle characters when the line was not actively transmitting data or transparently within a long transmission block. A certain number of idles were sent prior to each transmission. The IBM Binary Synchronous protocol (Bisync) is still in use, Other examples of byte-oriented protocols are IBM's Synchronous transmit-receive (STR), and Digital Data Communications Message Protocol (DDCMP) from Digital Equipment Corporation. Other computer manufacturers often offered similar protocols, differing mainly in small details.
Bit-oriented protocols are synchronous protocols that view the transmitted data as a stream of bits with no semantics, or meaning. Control codes are defined in terms of bit sequences instead of characters. Synchronization is maintained on an idle line by transmitting a predefined sequence of bits. Synchronous Data Link Control (SDLC) specifies that a station continue transmitting a sequence of '1' bits on an idle line.4 Data to be transmitted on an idle line is prefixed with a special bit sequence '01111110'b, called a flag. SDLC was the first bit-oriented protocol developed, and it was later adopted by the International Organization for Standardization (ISO) as High-Level Data Link Control (HDLC). Other examples of bit-oriented protocols are Logical Link Control (LLC)—IEEE 802.2, and ANSI Advanced Data Communication Control Procedures (ADCCP).
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