HDLC Modes of Operation HDLC Modes of Operation The HDLC protocol is a general purpose data link control protocol capable of supporting a range of modes of operation. The two most prevalent modes are: The best-effort or datagram service. In this mode, the packets are carried in a UI frame, and a best-effort delivery is performed (i.e. there is no guarantee that the packet carried by the frame will be delivered.) The link layer does not provide error recovery of lost frames. This mode is used for point-to-point links carrying a network protocol which itself uses datagram packets (e.g. IP). The control field of HDLC follows the address field and is the second part of all HDLC frames. The best-effort service is provided through the use of U (un-numbered) frames consisting of a single byte with the value of 0x03. The Asynchronous Balanced Mode (ABM). This provides a reliable data point-to-point data link service and may be used to provide a service which supports either a datagram or reliable network protocol. In this mode, the packets are carried in numbered I-frames, which are acknowledged by the receiver using numbered supervisory frames. Error recovery (e.g. checkpoint or go-back-n error recovery) is employed to ensure a well-ordered and reliable flow of frames. Be-aware of transmission bit-order Many books describing HDLC follow the ISO standard which defines the protocol. This describes the format of the HDLC control word according to the order in which the bits are transmitted at the physical layer. (i.e. least significant bit (lsb) transmitted first). To present a consistent view of all protocols, this course will describe the format in terms of the bit-order seen by the computer and used inside the computer network interface card. See separatesection on bit order. See also: HDLC Control Field