Synchronous serial communication describes a

serial communication In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits a ...

protocol , "In synchronous transmission, groups of bits are combined into frames,and frames are sent continuously with or without data to be transmitted." Synchronous communication requires that the clocks 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. Synchronization ( clock recovery) is done by data-embedded signal ...

. Over time the transmitting and receiving clocks will tend to drift apart, requiring ''resynchronization''. Synchronous RS-232 used additional pins on the DB-25 cable: the DCE (generally the modem or other peripheral) provided two clock signals to the DTE (generally the host computer or terminal), transmitter clock (pin 15, TCK) and receiver clock (pin 17, RCK). Some systems supported an alternative mode of operation in which the transmitter clock signal was provided by the DTE instead, called transmitter timing (pin 24, TT). Note the smaller DE-9 connector commonly adopted in later systems does not have these additional signal lines, and hence cannot be used with synchronous RS-232.

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'' (STR), and ''

Digital Data Communications Message Protocol Digital Data Communications Message Protocol (DDCMP) is a byte-oriented communications protocol devised by Digital Equipment Corporation in 1974 to allow communication over point-to-point network links for the company's DECnet Phase I network pr ...

'' (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 until ...

. 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 serial communication, serial communications protocol first introduced by IBM as part of its Systems Network Architecture (SNA). SDLC is used as layer 2, the data link layer, in the SNA protocol s ...

'' (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 independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. M ...

(ISO) as ''

High-Level Data Link Control High-Level Data Link Control (HDLC) is a communication protocol used for transmitting data between devices in telecommunication and networking. Developed by the International Organization for Standardization (ISO), it is defined in the standard ...

'' (HDLC). Other examples of bit-oriented protocols are ''Logical Link Control'' (LLC)— IEEE 802.2, and ANSI ''

Advanced Data Communication Control Procedures In telecommunications, 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 D ...

'' (ADCCP).

References

{{Reflist

Byte-oriented protocols

Bit-oriented protocols

References

See also