Control procedures are protocols that are based on the link layer and initialize and terminate a connection between the communication partners. Depending on the connection type, whether point-to-point connection or multipoint connection, a distinction is made between contention mode and call mode or between character-oriented and bit-oriented control methods. In the case of character-oriented procedures, the control and protocol functions are byte-encrypted, while in the case of bit-oriented procedures they are bit-encrypted.
According to DIN 66019, the IA-5 alphabet is prescribed as the transmission code for character-oriented control methods. In practice, however, the EBCDIC code is frequently used. The standardized character-oriented control procedures are internationally referred to as "Basic Mode Control Procedure for Data Communication Systems". Character-oriented procedures include Binary Synchronous Communication (BSC) and Medium Speed Version (MSV).
Bit-oriented control procedures are code-transparent. The control function for data transmission is bit-coded and therefore code-independent. A transmission code is specified for the coding of the data characters, depending on the system and also on the application. The standardized bit-oriented control methods are internationally called High Level Data Link Control (HDLC). Manufacturer-specific versions are Synchronous Data Link Control (SDLC) from IBM, Byte Data Link Controller (BDLC) from Burroughs and Universal Link Control (UDLC) from Unisys. In addition, there are several HDLC procedures. The best known of these are the LAP protocols, Link Access Procedure (LAP).