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