Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

In any communication session between devices, control codes are used to control another device or provide information about the status of the session. Byte- or character-oriented protocols use full bytes (8 bits) to represent established control codes such as those defined by ASCII (American Standard Code for Information Interchange). Thus, a character-oriented protocol can only be used with its native character set because that character set has the specific control characters. In contrast, bit-oriented protocols rely on individual bits for control information and are the preferred method for transmitting data. Most data link protocols like those used for local area networks are bit oriented.

In a bit-oriented transmission, data is transmitted as a steady stream of bits. Before actual data transmission begins, special sync characters are transmitted by the sender so the receiver can synchronize itself with the bit stream. This bit pattern is usually in the form of a specially coded 8-bit string. IBM's SDLC (Synchronous Data Link Control) protocol is bit oriented. The sync character is the bit string 01111110, and this is followed by an 8-bit address, an 8-bit control field, and the data. Once the receiving system receives these start frames, it begins reading eight bits at a time (a byte) from the bit stream until an error check and an ending flag appear.

IBM's SDLC and HDLC (High-level Data Link Control) are bit-oriented protocols that control synchronous communication. HDLC is used in X.25 packet-switching networks; SDLC is a subset of HDLC.

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