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Subj: AX.25 Doc : Part 4/9
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AX25.DOC PART 4/9
AX.25 Amateur Packet-Radio Link-Layer Protocol
Version 2.0 October 1984
2.3 Elements of Procedure
2.3.1
The elements of procedure are defined in terms of actions
that occur on receipt of frames.
2.3.2 Control-Field Formats and State Variables
2.3.2.1 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 operation.
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.
Control-Field Control-Field Bits
Type 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
Fig. 5 -- Control-field formats
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 2.3.4.2.
5. The "M" bits are the unnumbered frame modifier bits
and their encoding is discussed in 2.3.4.3.
6. The P/F bit is the Poll/Final bit. Its function is
described in 2.3.3. The distinction between command
and response, and therefore the distinction between P
bit and F bit, is made by addressing rules discussed
in 2.4.1.2.
2.3.2.1.1 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 sequence number of the next expected received frame).
These numbers are described in 2.3.2.4. In addition, the P/F bit
is to be used as described in 2.4.2.
2.3.2.1.2 Supervisory Format
Supervisory frames are denoted by having bit 0 of the
control field set to one, and bit 1 of the control field set to
zero. S frames provide supervisory link control such as
acknowledging or requesting retransmission of I frames, and link-
level window control. Since S frames do not have an information
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 2.4.2.
2.3.2.1.3 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
accomplished 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
2.4.2.
2.3.2.2 Control-Field Parameters
2.3.2.3 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.
2.3.2.4 Frame Variables and Sequence Numbers
2.3.2.4.1 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.
2.3.2.4.2 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.
2.3.2.4.3 Receive State Variable V(R)
The receive state variable is a variable that is internal
to the DXE. It contains the sequence number 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.
2.3.2.4.4 Received Sequence Number N(R)
The received sequence number is in both I and S frames.
Prior to sending an I or S frame, this variable is updated to
equal that of the received state variable, thus implicitly
acknowledging the proper reception of all frames up to and
including N(R)-1.
2.3.3 Functions of Poll/Final (P/F) Bit
The P/F bit is used in all types of frames. It is used
in a command (poll) mode to request an immediate reply to a
frame. The reply to this poll is indicated by setting the
response (final) bit in the appropriate frame. Only one
outstanding poll condition per direction is allowed at a time.
The procedure for P/F bit operation is described in 2.4.2.
2.3.4 Control Field Coding for Commands and Responses
The following commands and responses, indicated by their
control field encoding, are to be use by the DXE:
2.3.4.1 Information Command Frame Control Field
The function of the information (I) command is to
transfer across a data link sequentially numbered frames
containing an information field.
The information-frame control field is encoded as shown
in Fig. 6. These frames are sequentially numbered by the N(S)
subfield to maintain control of their passage over the link-layer
connection.
Control Field Bits
7 6 5 4 3 2 1 0
N(R) P N(S) 0
Fig. 6 -- I frame control field
END AX25.DOC PART 4/9
----------------------
73's Nacho EA2CMN @EA2CMN.EAZ.ESP.EU
QTH: Zaragoza (Spain)
[27-Sep-1994 15:41]
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