Synchronous serial communication describes a serial communication

protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technolog ...

in which "data is sent in a continuous stream at constant rate." Synchronous communication requires that the

clocks A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and the ...

in the transmitting and receiving devices are ''synchronized'' – running at the same rate – so the receiver can sample the signal at the same time intervals used by the transmitter. No start or stop bits are required. For this reason "synchronous communication permits more information to be passed over a circuit per unit time" than

asynchronous serial communication Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Instead of a common synchronization signal, the data stream conta ...

. Over time the transmitting and receiving clocks will tend to drift apart, requiring ''resynchronization''.

Byte-oriented protocols

Early synchronous protocols were

byte-oriented protocol Byte-oriented framing protocol is "a communications protocol in which full bytes are used as control codes. Also known as character-oriented protocol." For example UART communication is byte-oriented. The term "character-oriented" is deprecated, ...

s, 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 Synchronous transmit-receive (STR) was an early IBM character-oriented communications protocol which preceded Bisync. STR was point-to-point only, and employed a ''four-of-eight'' transmission code, communicating at up to 5100 characters per se ...

'' (STR), and '' Digital Data Communications Message Protocol'' (DDCMP) from

Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...

. Other computer manufacturers often offered similar protocols, differing mainly in small details.

Bit-oriented protocols

Bit-oriented protocol A bit-oriented protocol is a communications protocol that sees the transmitted data as an ''opaque'' stream of bits with no semantics, or meaning. Control codes are defined in terms of bit sequences instead of characters. Bit oriented protocol can ...

s 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 Synchronous Data Link Control (SDLC) is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture (SNA). SDLC supports multipoint links as well as error correction. It also runs under the assumption t ...

'' (SDLC) specifies that a station continue transmitting a sequence of '1' bits on an idle line. 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 The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...

(ISO) as ''

High-Level Data Link Control High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both c ...

'' (HDLC). Other examples of bit-oriented protocols are ''Logical Link Control'' (LLC)—

IEEE 802.2 IEEE 802.2 is the original name of the ISO/IEC 8802-2 standard which defines logical link control (LLC) as the upper portion of the data link layer of the OSI Model. The original standard developed by the Institute of Electrical and Electronics En ...

, and ANSI ''

Advanced Data Communication Control Procedures In telecommunication, Advanced Data Communication Control Procedures (or Protocol) (ADCCP) is a bit-oriented data link layer protocol developed by the American National Standards Institute. It is functionally equivalent to the ISO High-Level Dat ...

'' (ADCCP).

Byte-oriented protocols

Bit-oriented protocols

References

See also