MFJ MFJ-1270C, Системное руководство

Страница: 234 / 264

MFJ-1270C/1274C MANUAL PACKET PROTOCOL
Page 225
with the Bell 202 standard should be compatible with the MFJ-TNC, at least at
level one of the ISO reference model.
Data Link Layer
The duty of the Data Link layer is to supply an error-free stream of data to higher
levels. Since level one simply passes any bits received to level two and is
unaware of the content or overlying structure of the data, transmission errors are
not detectable at level one. Level two carries the responsibility of detecting and
rejecting bad data, retransmitting rejected data, and detecting the reception of
duplicate data.
Level two accomplishes this task by partitioning data to be transferred by level
one into individual frames, each with its own error detection field and frame
identification fields. The MFJ-TNC supports two versions of a level-two layer,
AX.25 version 1.0 and AX.25 version 2.0. Each of these protocols is based on
HDLC, the
H igh-Level D ata L ink C ontrol protocol defined by the ISO.
HDLC Frames
Exact knowledge of the format of HDLC frames has been made largely
unnecessary by the advent of LSI and VLSI communications chips which
interface directly with the level one hardware. The level two software need only
supply data to fill in various fields and the chip takes care of the rest. For
completeness however, an HDLC frame looks like this:
| FLAG | ADDRESS | CONTROL | PID & DATA | FCS | FLAG |
FLAG A unique bit sequence (01111110) used to detect frame boundaries.
A technique called "bit stuffing" is used to keep all other parts of the frame from
looking like a flag.
ADDRESS A field normally specifying the destination address. AX.25 uses
a minimum of 14 bytes and a maximum of 70 bytes containing the actual call
signs of the source, destination, and optionally up to eight digipeaters.
CONTROL A byte which identifies the frame type. In the AX.25 protocol,
the control field may include frame numbers in one or two 3-bit fields.
PID A Protocol Identification byte appears as the first byte of the HDLC
DATA field in AX.25 Level Two information frames, and identifies which Level