PK-232 TECHNICAL MANUAL
APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87
A-9
Page 54
It should be noted that various timers (see
A.4.7
, below) may have to be adjusted to accommo-
date the additional delays encountered when a frame must pass through a multiple-repeater
chain, and the return acknowledgement must travel through the same path before reaching the
source device.
It is anticipated that multiple-repeater operation is a temporary method of interconnecting stations
over large distances until such time that a layer 3 protocol is in use. Once this layer 3 protocol be-
comes operational, repeater chaining should be phased out.
A.3
Elements of Procedure
A.3.1
The elements of procedure are defined in terms of actions that occur on receipt of frames.
A.3.2
Control-Field Formats and State Variables
A.3.2.1
Control-Field Formats
The control field is responsible for identifying the type of frame being sent, and is also used to
convey commands and responses from one end of the link to the other in order to maintain proper
link control.
The control fields used in AX.25 use the CCITT X.25 control fields for balanced operation (LAPB),
with an additional control field taken from ADCCP to allow connectionless and round- table opera-
tion.
There are three general types of AX.25 frames. They are the Information frame (I frame), the
Supervisory frame (S frame), and the Unnumbered frame (U frame). Fig. 5 shows the basic format
of the control field associated with these types of frames.
Fig. 5 – Control-field formats
Control-Field
Type
Control-Field Bits
7
6
5
4
3
2
1
0
I Frame
N(R)
P
N(S)
0
S Frame
N(R)
P/F
S
S
0
1
U Frame
M
M
M
P/F
M
M
1
1
Where
:
1. Bit 0 is the first bit sent and bit 7 is the last bit sent of the control field.
2. N(S) is the send sequence number (bit 1 is the LSB).
3. N(R) is the receive sequence number (bit 5 is the LSB).
4. The "S" bits are the supervisory function bits, and their encoding is discussed in
A.3.4.2
.
5. The "M" bits are the unnumbered frame modifier bits and their encoding is discussed in
A.3.4.3
.
6. The P/F bit is the Poll/Final bit. Its function is described in
A.3.3
. The distinction between com-
mand and response, and therefore the distinction between P bit and F bit, is made by address-
ing rules discussed in
A.4.1.2
.
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