PacComm TINY-2 - Page 48

FLAG bytes are being transmitted, guarantees that a logic level
transition occurs at least once every 5 bit times. These frequent
transitions allow the receiver to synchronize its clock with the
transmitter.
While the HDLC format supplied by the communications chips
is used by the AX.25 protocol, there are several other Layer Two
concerns. These are duplicate frame detection, connection and
disconnection of the level two layers on different packet con-
trollers, and buffer overrun avoidance. The AXD5 protocol
solves these problems as described below.
AX.25 Level Two
AX.25 is based on the Balanced Link Access Procedure (LAPB)
of the CCITT X.25 standard. LAPB in turn conforms to the
HDLC standard. Two extensions are made to LAPB in AX.25.
These are the extended address field, and the unnumbered
information (OD frame. In LAPB, addresses are limited to eight
bits, while AX.25 uses from 112 to 560 bits, containing the
originator’s call sign, the destination call sign and an optional
list of one to eight digipeater (simplex digital repeater) call
signs.
The UI frame is used to send information bypassing the normal
flow control and acknowledgment protocol. The UI frame is not
acknowledged but can be transmitted at layer two without fear
of disturbing higher layers. It is used for beacon frames, for
automatic identification packets, and for sending information
frames when the packet controller is not connected to another
packet controller, e.g., CQ and OST activities.
The following table lists the frame types used by AX.25 and
describes their purpose. This material is provided to give a
general understanding of the protocol, and is not intended to
replace the published specification. The byte fields are given as
they appear in memory after data is received, i.e., the high order
bit Is at the left and the low order bit is at the right. This is also
the format of the display provided by the Ce command.
Some
texts,
including
the
AX.25
protocol
specification,
list
the
bits
in
the
order
in
which
they
are
transmitted,
which
is
low
order
it first.
The
table
also
lists
the
format
of
the
display
of
the
interpretation
of
the
command
byte
in
each
monitored
frame
when
MCOM
is
turned
ON.
The
basic
structure
of
the
MCOM
information
is:
<
cmd_type
[(C|R)
(P|F)]
[Sn]
[Rn].
>
The “<” and “>” characters are always present, and serve to
delimit the new MCOM information. Cmd_type may take any
Operating Manual, 4th Ed. © PacComm, 1990 41