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EA2CMN > AX25 29.09.94 06:24l 149 Lines 5169 Bytes #-11689 (0) @ EU
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Subj: AX.25 Doc : Part 5/9
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Sent: 940927/1450Z @:EA2CMN.EAZ.ESP.EU #:41776 [Zaragoza] Bid:41776_EA2CMN FBB5
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AX25.DOC PART 5/9
AX.25 Amateur Packet-Radio Link-Layer Protocol
Version 2.0 October 1984
2.3.4.2 Supervisory Frame Control Field
The supervisory frame control fields are encoded as shown
in Fig. 7.
Control Field Bits 7 6 5 4 3 2 1 0
Receive Ready RR N(R) P/F 0 0 0 1
Receive Not Ready RNR N(R) P/F 0 1 0 1
Reject REJ N(R) P/F 1 0 0 1
Fig. 7 -- S frame control fields
The Frame identifiers:
C or SABM Layer 2 Connect Request
D or DISC Layer 2 Disconnect Request
I Information Frame
RR Receive Ready. System Ready To Receive
RNR or NR Receive Not Ready. TNC Buffer Full
RJ or REJ Reject Frame. Out of Sequence or Duplicate
FRMR Frame Reject. Fatal Error
UI Unnumbered Information Frame. "Unproto"
DM Disconnect Mode. System Busy or Disconnected.
2.3.4.2.1 Receive Ready (RR) Command and Response
Receive Ready is used to do the following:
1. to indicate that the sender of the RR is now able to
receive more I frames.
2. to acknowledge properly received I frames up to, and
including N(R)-1, and
3. to clear a previously set busy condition created by an RNR
command having been sent.
The status of the DXE at the other end of the link can be
requested by sending a RR command frame with the P-bit set to
one.
2.3.4.2.2 Receive Not Ready (RNR) Command and Response
Receive Not Ready is used to indicate to the sender of I
frames that the receiving DXE is temporarily busy and cannot
accept any more I frames. Frames up to N(R)-1 are acknowledged.
Any I frames numbered N(R) and higher that might have been caught
between states and not acknowledged when the RNR command was sent
are not acknowledged.
The RNR condition can be cleared by the sending of a UA,
RR, REJ, or SABM frame.
The status of the DXE at the other end of the link can be
requested by sending a RNR command frame with the P bit set to
one.
2.3.4.2.3 Reject (REJ) Command and Response
The reject frame is used to request retransmission of I
frames starting with N(R). Any frames that were sent with a
sequence number of N(R)-1 or less are acknowledged. Additional I
frames may be appended to the retransmission of the N(R) frame if
there are any.
Only one reject frame condition is allowed in each
direction at a time. The reject condition is cleared by the
proper reception of I frames up to the I frame that caused the
reject condition to be initiated.
The status of the DXE at the other end of the link can be
requested by sending a REJ command frame with the P bit set to
one.
2.3.4.3 Unnumbered Frame Control Fields
Unnumbered frame control fields are either commands or
responses.
Fig. 8 shows the layout of U frames implemented within
this protocol.
Control Field Type Control Field Bits
7 6 5 4 3 2 1 0
Set Asynchronous Balanced Mode-SABM Cmd 0 0 1 P 1 1 1 1
Disconnect-DISC Cmd 0 1 0 P 0 0 1 1
Disconnected Mode-DM Res 0 0 0 F 1 1 1 1
Unnumbered Acknowledge-UA Res 0 1 1 F 0 0 1 1
Frame Reject-FRMR Res 1 0 0 F 0 1 1 1
Unnumbered Information-UI Either 0 0 0 P/F 0 0 1 1
Fig. 8 -- U frame control fields
2.3.4.3.1 Set Asynchronous Balanced Mode (SABM) Command
The SABM command is used to place 2 DXEs in the
asynchronous balanced mode. This is a balanced mode of operation
known as LAPB where both devices are treated as equals.
Information fields are not allowed in SABM commands. Any
outstanding I frames left when the SABM command is issued will
remain unacknowledged.
The DXE confirms reception and acceptance of a SABM
command by sending a UA response frame at the earliest
opportunity. If the DXE is not capable of accepting a SABM
command, it should respond with a DM frame if possible.
2.3.4.3.2 Disconnect (DISC) Command
The DISC command is used to terminate a link session
between two stations. No information field is permitted in a
DISC command frame.
Prior to acting on the DISC frame, the receiving DXE
confirms acceptance of the DISC by issuing a UA response frame at
its earliest opportunity. The DXE sending the DISC enters the
disconnected state when it receives the UA response.
Any unacknowledged I frames left when this command is
acted upon will remain unacknowledged.
END AX25.DOC PART 5/9
73's Nacho EA2CMN @EA2CMN.EAZ.ESP.EU
QTH: Zaragoza (Spain)
[27-Sep-1994 - 15:49]
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