Data Link Protocols

1. Sets of specifications used to implement the data link layer.

2. For a protocol to be useful, it must be transparent -- it must be able to carry any combination of bits as data without their being confused with control information.
   • Asynchronous protocols treat each character in a bit stream independently and are employed mainly in modems.

   • Synchronous protocols
     • Character-oriented protocols insert control information in the data stream, in the form of an existing character encoding system.

     • Bit-oriented protocols interpret a transmission frame or packet as a succession of individual bits, made meaningful by their placement in the frame and by their juxtaposition with other bits.
3. High-level data link control (HDLC) is the basis for all bit-oriented protocols.
   • The flag field identifies both the beginning and end of a frame, and to guarantee that a flag does not appear anywhere else in a frame, bit stuffing is used to insert an extra 0 in transmissions that have more than five consecutive 1s. This prevents premature termination while reading an HDLC frame.

   • Link access procedure (LAP) is a subset of HDLC

   • Link access procedure, balanced (LAPB) provides only those basic control functions required for communication between a DTE and a DCE.

   • Link access procedure for D channel (LAPD) is used for the D channel in ISDN.

   • Link access procedure for modems (LAPM) is designed to do asynchronous-synchronous conversion, error detection, and retransmission for modems.