Quote: http://www.erg.abdn.ac.uk/users/gorry/eg3567/dl-pages/hdlc-framing.html
HDLC Framing
The High Level Link Control (HDLC) protocol
defined by the International Standards Organisation
(ISO) provides a transparent transmission service at the data
link layer of the OSI reference
model
. Many protocol suites use an HDLC (or HDLC-based) link
layer, including X.25, the IP point-to-point protocol (PPP) and
SNA.
The users of the HDLC service provides PDUs which are encapsulated
to form data link layer frames. These frames are separated by
HDLC "flags" and are modified by "zero bit insertion"
to guarantee transparency. These terms are defined below:
Flags
HDLC is a data link protocol which uses a unique bit sequence
to delimit the start and end of each PDU transported by the data
link layer service. In HDLC, frames are delimited by a sequence
of bits known as a "flag". The flag sequence is a unique
8-bit sequence of the form 0111 1110. The way in which this is
performed is described in the text and diagrams which follow.
The flags before and after an HDLC frame indicate
the start and end of the frame
Transparency
The flag sequence must never occur within the content of a
frame otherwise it could be confused with an intentionally sent
flag. A technique known as 0-bit insertion is used to prevent
random data synthesising a flag. The technique is said to make
HDLC transparent, since any stream of bits may be present between
the open and closing flag of a frame. The transparency is achieved
by encoding the data by inserting a 0-bit after any sequence of
5 consecutive 1's within the payload, as shown:
Insertion of a "zero-bit" into the content
of a frame to ensure transparency
Hunt Mode
Normally a HDLC receiver starts in the idle state, waiting
for the start of a frame. This is called "Hunt" mode,
since the receiver is said to be hunting for a non-flag sequence.
This may be achieved through a shift register and combinational
logic as shown (a Finite State Machine (FSM) is usually used to
implement this):
Detection of a start of frame
Abort & Idle Patterns
Valid frames are terminated by a closing flag. If the link
layer protocol needs to transmit a higher priority frame before
the current frame has been fully sent, it may "Abort"
the frame. An "aborted" frame is terminated by an "abort
sequence": 0111 1111 instead of the normal "flag sequence".
Abort sequences may also be caused by bit errors which occur while
the frame is travelling from the transmitter to the receiver.
(For instance a flag may suffer a 1-bit inversion of the least
significant bit, resulting in a bit sequence identical to an "abort").
A frame which is terminated by an abort is received by the
receiver in the normal way, but marked as being "aborted".
The frame is then discarded without further processing.
This is
shown below:
An HDLC frame terminated by an "abort"
is discarded by the HDLC receiver
An abort sequence is often followed by a series of 1's. The
sequence of all 1's may be used to fill the gaps between frames
(or alternatively a continuous series of flags may be transmitted).
The all 1's sequence is known as the "idle" sequence,
since the line becomes idle (N.B. represented at the physical
layer by 0 Volts). This is useful for applications requiring half duplex
operation (also
known as Two Way Alternate (TWA) operation).
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