AEA PK-232 Technical Reference Manual page 55

PK-232 TECHNICAL MANUAL
A.3.2.1.1 Information-Transfer Format
All I frames have bit 0 of the control field set to zero. N(S) is the sender's send sequence number
(the send sequence number of this frame). N(R) is the sender's receive sequence number (the se-
quence number of the next expected received frame). These numbers are described in A.3.2.4. In
addition, the P/F bit is to be used as described in A.4.2.
A.3.2.1.2 Supervisory Format
Supervisory frames are denoted by having bit 0 of the control field set to one, and bit 1 of the con-
trol field set to zero. S frames provide supervisory link control such as acknowledging or request-
ing retransmission of I frames, and link- level window control. Since S frames do not have an infor-
mation field, the sender's send variable and the receiver's receive variable are not incremented for
S frames. In addition, the P/F bit is used as described in A.4.2.
A.3.2.1.3 Unnumbered Format
Unnumbered frames are distinguished by having both bits 0 and 1 of the control field set to one. U
frames are responsible for maintaining additional control over the link beyond what is accom-
plished with S frames. They are also responsible for establishing and terminating link connections.
U frames also allow for the transmission and reception of information outside of the normal flow
control. Some U frames may contain information and PID fields. The P/F bit is used as described in
A.4.2.
A.3.2.2 Control-Field Parameters
A.3.2.3 Sequence Numbers
Every AX.25 I frame shall be assigned, modulo 8, a sequential number from 0 to 7. This will allow
up to seven outstanding I frames per level 2 connection at a time.
A.3.2.4 Frame Variables and Sequence Numbers
A.3.2.4.1 Send State Variable V(S)
The send state variable is a variable that is internal to the DXE and is never sent. It contains the
next sequential number to be assigned to the next transmitted I frame. This variable is updated
upon the transmission of each I frame.
A.3.2.4.2 Send Sequence Number N(S)
The send sequence number is found in the control field of all I frames. It contains the sequence
number of the I frame being sent. Just prior to the transmission of the I frame, N(S) is updated to
equal the send state variable.
A.3.2.4.3 Receive State Variable V(R)
The receive state variable is a variable that is internal to the DXE. It contains the sequence num-
ber of the next expected received I frame. This variable is updated upon the reception of an error-
free I frame whose send sequence number equals the present received state variable value.
PK232TM Rev. A 5/87
APPENDIX A – AX.25 LEVEL 2 PROTOCOL
A-10 Page 55