HDLC Modes of Operation
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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
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