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1994-11-27
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Subject: 2.3.4.3.3 Frame Reject (FRMR) Response
2.3.4.3.3.1
The FRMR response frame is sent to report that the receiver
of a frame cannot successfully process that frame and that the
error condition is not correctable by sending the offending frame
again. Typically this condition will appear when a frame without
an FCS error has been received with one of the following
conditions:
1. The reception of an invalid or not implemented command or
response frame.
2. The reception of an I frame whose information field exceeds
the agreed-upon length. (See 2.4.7.3, below.)
3. The reception of an improper N(R). This usually happens
when the N(R) frame has already been sent and acknowledged,
or when N(R) is out of sequence with what was expected.
4. The reception of a frame with an information field where one
is not allowed, or the reception of a U or S frame whose
length is incorrect. Bits W and Y described in 2.3.4.3.3.2
should both be set to one to indicate this condition.
5. The reception of a supervisory frame with the F bit set
to one, except during a timer recovery condition (see
2.4.4.9), or except as a reply to a command frame sent with
the P bit set to one. Bit W (described in 2.3.4.3.3.2)
should be set to one.
6. The reception of an unexpected UA or DM response frame. Bit
W should be set to one.
7. The reception of a frame with an invalid N(S). Bit W should be
set to one.
An invalid N(R) is defined as one which points to an I
frame that previously has been transmitted and acknowledged, or
an I frame which has not been transmitted and is not the next
sequential I frame pending transmission.
An invalid N(S) is defined as an N(S) that is equal to
the last transmitted N(R)+k and is equal to the received state
variable V(R), where k is the maximum number of outstanding
information frames as defined in 2.4.7.4 below.
An invalid or not implemented command or response is
defined as a frame with a control field that is unknown to the
receiver of this frame.
2.3.4.3.3.2
When a FRMR frame is sent, an information field is added to
the frame that contains additional information indicating where
the problem occurred. This information field is three octets
long and is shown in Fig. 9.
Information Field Bits
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 Z Y X W V(R) C V(S) 0 Rejected Frame
R Control Field
Fig. 9 -- FRMR frame information field
Where:
1. The rejected frame control field carries the control field
of the frame that caused the reject condition. It is in
bits 0-7 of the information field.
2. V(S) is the current send state variable of the device
reporting the rejection (bit 9 is the low bit).
3. The CR bit is set to zero to indicate the rejected frame
was a command, or one if it was a response.
4. V(R) is the current receive state variable of the device
reporting rejection (bit 13 is the low bit).
5. If W is set to 1, the control field received was invalid or
not implemented.
6. If X is set to 1, the frame that caused the reject
condition was considered invalid because it was a U or S
frame that had an information field that is not allowed.
Bit W must be set to 1 in addition to the X bit.
7. If Y is set to 1, the control field received and returned
in bits exceeded the maximum allowed under this
recommendation in 2.4.7.3, below.
8. If A is set to 1, the control field received and returned
in bits 1 to 8 contained an invalid N(R).
9. Bits 8, and 20 to 23 are set to 0.
2.3.4.3.4 Unnumbered Acknowledge (UA) Response
The UA response frame is sent to acknowledge the
reception and acceptance of a SABM or DISC command frame. A
received command is not actually processed until the UA response
frame is sent. Information fields are not permitted in a UA
frame.
2.3.4.3.5 Disconnected Mode (DM) Response
The disconnected mode response is sent whenever a DXE
receives a frame other than a SABM or UI frame while in a
disconnected mode. It is also sent to request a set mode
command, or to indicate it cannot accept a connection at the
moment. The DM response does not have an information field.
Whenever a SABM frame is a received, and it is determined
that a connection is not possible, a DM frame shall be sent.
This will indicate that the called station cannot accept a
connection at that time.
While a DXE is in the disconnected mode, it will respond
to any command other than a SABM or UI frame with a DM response
with the P/F bit set to 1.
2.3.4.3.6 Unnumbered Information (UI) Frame
The Unnumbered Information frame contains PID and
information fields and is used to pass information along the link
Ωoutside the normal information controls. This allows information
fields to go back and forth on the link bypassing flow control.
Since these frames are not acknowledgeable, if one gets
obliterated, there is no way to recover it. A received UI frame
with the P bit set shall cause a response to be transmitted.
This response shall be a DM frame when in the disconnected state
or a RR (or RNR, if appropriate) frame in the information
transfer state.
2.3.5 Link Error Reporting and Recovery
There are several link-layer errors that are recoverable
without terminating the connection. These error situations may
occur as a result of malfunctions within the DXE, or if
transmission errors occur.
2.3.5.1 DXE Busy Condition
When a DXE becomes temporarily unable to receive I
frames, such as when receive buffers are full, it will send a
Receive Not Ready (RNR) frame. This informs the other DXE that
this DXE cannot handle any more I frames at the moment. This
condition is usually cleared by the sending of a UA, RR, REJ, or
SABM command frame.
2.3.5.2 Send Sequence Number Error
If the send sequence number, N(S), of an otherwise error-
free received frame does not match the receive state variable,
V(R), a send sequence error has occurred, and the information
field will be discarded. The receiver will not acknowledge this
frame, or any other I frames, until N(S) matches V(R).
The control field of the erroneous I frame(s) will be
accepted so that link supervisory functions such as checking the
P/F bit can still be performed. Because of this updating, the
retransmitted I frame may have an updated P bit and N(R).
2.3.5.3 Reject (REJ) Recovery
REJ is used to request a retransmission of I frames
following the detection of a N(S) sequence error. Only one
outstanding "sent REJ" condition is allowed at a time. This
condition is cleared when the requested I frame has been
received.
A DXE receiving the REJ command will clear the condition
by resending all outstanding I frames (up to the window size),
starting with the one indicated in N(R) of the REJ frame.
2.3.5.4 Time-out Error Recovery
2.3.5.4.1 T1 Timer Recovery
If a DXE, due to a transmission error, does not receive
(or receives and discards) a single I frame or the last I frame
in a sequence of I frames, it will not detect a send-sequence-
number error, and therefore will not transmit a REJ. The DXE
which transmitted the unacknowledged I frame(s) shall, following
the completion of time-out period T1, take appropriate recovery
action to determine when I frame retransmission should begin as
described in 2.4.4.9, below. This condition is cleared by the
reception of an acknowledgement for the sent frame(s), or by the
link being reset. See 2.4.6.
2.3.5.4.2 Timer T3 Recovery
Timer T3 is used to assure the link is still functional
during periods of low information transfer. Whenever T1 is not
running (no outstanding I frames), T3 is used to periodically
poll the other DXE of a link. When T3 times out, a RR or RNR
frame is transmitted as a command and with the P bit set. The
waiting acknowledgement procedure (2.4.4.9, below) is then
executed.
2.3.5.5 Invalid Frame or FCS Error
If an invalid frame is received, or a frame is received
with an FCS error, that frame will be discarded with no action
taken.
2.3.5.6 Frame Rejection Condition
A frame rejection condition occurs when an otherwise
error-free frame has been received with one of the conditions
listed in 2.3.4.3.3 above.
Once a rejection error occurs, no more I frames are
accepted (except for the examination of the P/F bit) until the
error is resolved. The error condition is reported to the other
DXE by sending a FRMR response frame. See 2.4.5.
2.4 Description of AX.25 Procedures
The following describes the procedures used to setup,
use, and disconnect a balanced link between two DXE stations.
2.4.1 Address Field Operation
2.4.1.1 Address Information
All transmitted frames shall have address fields
conforming to 2.2.13, above. All frames shall have both the
destination device and the source device addresses in the address
field, with the destination address coming first. This allows
many links to share the same RF channel. The destination address
is always the address of the station(s) to receive the frame,
while the source address contains the address of the device that
sent the frame.
The destination address can be a group name or club call
sign if the point-to-multipoint operation is allowed. Operation
with destination addresses other than actual amateur call signs
is a subject for further study.
2.4.1.2 Command/Response Procedure
AX.25 Version 2.0 has implemented the command/response
information in the address field. In order to maintain
compatibility with previous versions of AX.25, the
command/response information is conveyed using two bits.
An upward-compatible AX.25 DXE can determine whether it
is communicating with a DXE using an older version of this
protocol by testing the command/response bit information located
in bit 7 of the SSID octets of both the destination and source
address subfields. If both C bits are set to zero, the device is
using the older protocol. The newer version of the protocol
always has one of these two bits set to one and the other set to
zero, depending on whether the frame is a command or a response.
The command/response information is encoded into the
address field as shown in Fig. 10.
Frame Type Dest. SSID C-Bit Source SSID C-Bit
Previous versions 0 0
Command (V.2.0) 1 0
Response (V.2.0) 0 1
Previous versions 1 1
Fig. 10 -- Command/Response encoding
Since all frames are considered either commands or
responses, a device shall always have one of the bits set to one,
and the other bit set to zero.
The use of the command/response information in AX.25
allows S frames to be either commands or responses. This aids
maintenance of proper control over the link during the
information transfer state.
2.4.2 P/F Bit Procedures
The next response frame returned by the DXE to a SABM or
DISC command with the P bit set to 1 will be a UA or DM response
with the F bit set to 1.
The next response frame returned to an I frame with the P
bit set to 1, received during the information transfer state,
will be a RR, RNR, or REJ response with the F bit set to 1.
The next response frame returned to a supervisory command
frame with the P bit set to 1, received during the information
transfer state, will be a RR, RNR, or REJ response frame with the
F bit set to 1.
The next response frame returned to a S or I command
frame with the P bit set to 1, received in the disconnected
state, will be a DM response frame with the F bit set to 1.
The P bit is used in conjunction with the time-out
recovery condition discussed in 2.3.5.4, above.
When not used, the P/F bit is set to zero.
2.4.3 Procedures For Link Set-Up and Disconnection
2.4.3.1 LAPB Link Connection Establishment
When one DXE wishes to connect to another DXE, it will
send a SABM command frame to that device and start timer (T1).
If the other DXE is there and able to connect, it will respond
with a UA response frame, and reset both of its internal state
variables (V(S) and V(R)). The reception of the UA response
frame at the other end will cause the DXE requesting the
connection to cancel the T1 timer and set its internal state
variables to 0.
If the other DXE doesn't respond before T1 times out, the
device requesting the connection will re-send the SABM frame, and
start T1 running again. The DXE should continue to try to
establish a connection until it has tried unsuccessfully N2
times. N2 is defined in 2.4.7.2, below.
If, upon reception of a SABM command, the DXE decides
that it cannot enter the indicated state, it should send a DM
frame.
When receiving a DM response, the DXE sending the SABM
should cancel its T1 timer, and not enter the information-
transfer state.
The DXE sending a SABM command will ignore and discard
any frames except SABM, DISC, UA, and DM frames from the other
DXE.
Frames other than UA and DM in response to a received
SABM will be sent only after the link is set up and if no
outstanding SABM exists.
2.4.3.2 Information-Transfer Phase
After establishing a link connection, the DXE will enter
the information-transfer state. In this state, the DXE will
accept and transmit I and S frames according to the procedure
outlined in 2.4.4, below.
When receiving a SABM command while in the information-
transfer state, the DXE will follow the resetting procedure
outlined in 2.4.6 below.
2.4.3.3 Link Disconnection
2.4.3.3.1
While in the information-transfer state, either DXE may
indicate a request to disconnect the link by transmitting a DISC
command frame and starting timer T1 (see 2.4.7).
2.4.3.3.2
A DXE, upon receiving a valid DISC command, shall send a UA
response frame and enter the disconnected state. A DXE, upon
receiving a UA or DM response to a sent DISC command, shall
cancel timer T1, and enter the disconnected state.
2.4.3.3.3
If a UA or DM response is not correctly received before T1
times out, the DISC frame should be sent again and T1 restarted.
If this happens N2 times, the DXE should enter the disconnected
state.
2.4.3.4 Disconnected State
2.4.3.4.1
A DXE in the disconnected state shall monitor received
commands and react upon the reception of a SABM as described in
2.4.3.1 above and will transmit a DM frame in response to a DISC
command.
2.4.3.4.2
In the disconnected state, a DXE may initiate a link set-up
as outlined in connection establishment above (2.4.3.1). It may
also respond to the reception of a SABM and establish a
connection, or it may ignore the SABM and send a DM instead.
2.4.3.4.3
Any DXE receiving a command frame other than a SABM or UI
frame with the P bit set to one should respond with a DM frame
with the F bit set to one. The offending frame should be
ignored.
2.4.3.4.4
When the DXE enters the disconnected state after an error
condition or if an internal error has resulted in the DXE being
in the disconnected state, the DXE should indicate this by
sending a DM response rather than a DISC frame and follow the
link disconnection procedure outlined in 2.4.3.3.3, above. The
DXE may then try to re-establish the link using the link set-up
procedure outlined in 2.4.3.1, above.
2.4.3.5 Collision Recovery
2.4.3.5.1 Collisions in a Half-Duplex Environment
Collisions of frames in a half-duplex environment are
taken care of by the retry nature of the T1 timer and
retransmission count variable. No other special action needs to
be taken.
2.4.3.5.2 Collisions of Unnumbered Commands
If sent and received SABM or DISC command frames are the
same, both DXEs should send a UA response at the earliest
opportunity, and both devices should enter the indicated state.
If sent and received SABM or DISC commands are different,
both DXEs should enter the disconnected state and transmit a DM
frame at the earliest opportunity.
2.4.3.5.3 Collision of a DM with a SABM or DISC
When an unsolicited DM response frame is sent, a
collision between it and a SABM or DISC may occur. In order to
prevent this DM from being misinterpreted, all unsolicited DM
frames should be transmitted with the F bit set to zero. All
SABM and DISC frames should be sent with the P bit set to one.
This will prevent any confusion when a DM frame is received.
2.4.3.6 Connectionless Operation
In Amateur Radio, there is an additional type of
operation that is not feasible using level 2 connections. This
operation is the round table, where several amateurs may be
engaged in one conversation. This type of operation cannot be
accommodated by AX.25 link-layer connections.
The way round-table activity is implemented is
technically outside the AX.25 connection, but still using the
AX.25 frame structure.
AX.25 uses a special frame for this operation, called the
Unnumbered Information (UI) frame. When this type of operation
is used, the destination address should have a code word
installed in it to prevent the users of that particular round
table from seeing all frames going through the shared RF medium.
An example of this is if a group of amateurs are in a round-table
discussion about packet radio, they could put PACKET in the
destination address, so they would receive frames only from
others in the same discussion. An added advantage of the use of
AX.25 in this manner is that the source of each frame is in the
source address subfield, so software could be written to
automatically display who is making what comments.
Since this mode is connectionless, there will be no
requests for retransmissions of bad frames. Collisions will also
occur, with the potential of losing the frames that collided.
2.4.4 Procedures for Information Transfer
Once a connection has been established, as outlined
above, both devices are able to accept I, S, and U frames.
2.4.4.1 Sending I Frames
Whenever a DXE has an I frame to transmit, it will send
the I frame with N(S) of the control field equal to its current
send state variable V(S). Once the I frame is sent, the send
state variable is incremented by one. If timer T1 is not
running, it should be started. If timer T1 is running, it should
be restarted.
The DXE should not transmit any more I frames if its send
state variable equals the last received N(R) from the other side
of the link plus seven. If it were to send more I frames, the
flow control window would be exceed, and errors could result.
If a DXE is in a busy condition, it may still send I
frames as long as the other device is not also busy.
If a DXE is in the frame-rejection mode, it will stop
sending I frames.
2.4.4.2 Receiving I Frames
2.4.4.2.1
If a DXE receives a valid I frame (one with a correct FCS
and whose send sequence number equals the receiver's receive
state variable) and is not in the busy condition, it will accept
the received I frame, increment its receive state variable, and
act in one of the following manners:
1. If it has an I frame to send, that I frame may be sent with the
transmitted N(R) equal to its receive state variable V(R) (thus
acknowledging the received frame). Alternately, the device may
send a RR frame with N(R) equal to V(R), and then send the I
frame.
2. If there are no outstanding I frames, the receiving device will
send a RR frame with N(R) equal to V(R). The receiving DXE may
wait a small period of time before sending the RR frame to be sure
additional I frames are not being transmitted.
2.4.4.2.2
If the DXE is in a busy condition, it may ignore any
received I frames without reporting this condition other than
repeating the indication of the busy condition.
If a busy condition exists, the DXE receiving the busy
condition indication should poll the sender of the busy
indication periodically until the busy condition disappears.
A DXE may poll the busy DXE periodically with RR or RNR
frames with the P bit set to one.
The reception of I frames that contain zero-length
information fields shall be reported to the next level but no
information field will be transferred.
2.4.4.3 Reception of Out of Sequence Frames
When an I frame is received with a correct FCS, but its
send sequence number, N(S), does not match the current receiver's
receive state variable, the frame should be discarded. A REJ
frame shall be sent with a receive sequence number equal to one
higher (modulo 8) than the last correctly received I frame if an
uncleared N(S) sequence error condition has not been previously
established. The received state variable and poll bit of the
discarded frame should be checked and acted upon, if necessary,
before discarding the frame.
2.4.4.4 Reception of Incorrect Frames
When a DXE receives a frame with an incorrect FCS, an
invalid frame, or a frame with an improper address, that frame
shall be discarded.
2.4.4.5 Receiving Acknowledgement
Whenever an I or S frame is correctly received, even in a
busy condition, the N(R) of the received frame should be checked
to see if it includes an acknowledgement of outstanding sent I
frames. The T1 timer should be cancelled if the received frame
actually acknowledges previously unacknowledged frames. If the
T1 timer is cancelled and there are still some frames that have
been sent that are not acknowledged, T1 should be started again.
If the T1 timer runs out before an acknowledgement is received,
the device should proceed to the retransmission procedure in
2.4.4.9.
2.4.4.6 Receiving Reject
Upon receiving a REJ frame, the transmitting DXE will set
its send state variable to the same value as the REJ frame's
received sequence number in the control field. The DXE will then
retransmit any I frame(s) outstanding at the next available
opportunity conforming to the following:
1. If the DXE is not transmitting at the time, and the channel
is open, the device may commence to retransmit the I
frame(s) immediately.
2. If the DXE is operating on a full-duplex channel
transmitting a UI or S frame when it receives a REJ frame,
it may finish sending the UI or S frame and then retransmit
the I frame(s).
3. If the DXE is operating in a full-duplex channel
transmitting another I frame when it receives a REJ frame,
it may abort the I frame it was sending and start
retransmission of the requested I frames immediately.
4. The DXE may send just the one I frame outstanding, or it may
send more than the one indicated if more I frames followed
the first one not acknowledged, provided the total to be
sent does not exceed the flow-control window (7 frames).
If the DXE receives a REJ frame with the poll bit set, it
should respond with either a RR or RNR frame with the final bit
set before retransmitting the outstanding I frame(s).
2.4.4.7 Receiving a RNR Frame
Whenever a DXE receives a RNR frame, it shall stop
transmission of I frames until the busy condition has been
cleared. If timer T1 runs out after the RNR was received, the
waiting acknowledgement procedure listed in 2.4.4.9, below,
should be performed. The poll bit may be used in conjunction
with S frames to test for a change in the condition of the
busied-out DXE.
2.4.4.8 Sending a Busy Indication
Whenever a DXE enters a busy condition, it will indicate
this by sending a RNR response at the next opportunity. While
the DXE is in the busy condition, it may receive and process S
frames, and if a received S frame has the P bit set to one, the
DXE should send a RNR frame with the F bit set to one at the next
possible opportunity. To clear the busy condition, the DXE
should send either a RR or REJ frame with the received sequence
number equal to the current receive state variable, depending on
whether the last received I frame was properly received or not.
2.4.4.9 Waiting Acknowledgement
If timer T1 runs out waiting the acknowledgement from the
other DXE for an I frame transmitted, the DXE will restart timer
T1 and transmit an appropriate supervisory command frame (RR or
RNR) with the P bit set. If the DXE receives correctly a
supervisory response frame with the F bit set and with an N(R)
within the range from the last N(R) received to the last N(S)
sent plus one, the DXE will restart timer T1 and will set its
send state variable V(S) to the received N(R). It may then
resume with I frame transmission or retransmission, as
appropriate. If, on the other hand, the DXE receives correctly a
supervisory response frame with the F bit not set, or an I frame
or supervisory command frame, and with an N(R) within the range
from the last N(R) received to the last N(S) sent plus one, the
DXE will not restart timer T1, but will use the received N(R) as
an indication of acknowledgement of transmitted I frames up to
and including I frame numbered N(R)-1.
If timer T1 runs out before a supervisory response frame
with the F bit set is received, the DXE will retransmit an
appropriate supervisory command frame (RR or RNR) with the P bit
set. After N2 attempts to get a supervisory response frame with
the F bit set from the other DXE, the DXE will initiate a link
resetting procedure as described in 2.4.6, below.
2.4.5 Frame Rejection Conditions
A DXE shall initiate the frame-reset procedure when a
frame is received with the correct FCS and address field during
the information-transfer state with one or more of the conditions
in 2.3.4.3.3, above.
Under these conditions, the DXE will ask the other DXE to
reset the link by transmitting a FRMR response as outlined in
2.4.6.3, below.
After sending the FRMR frame, the sending DXE will enter
the frame reject condition. This condition is cleared when the
DXE that sent the FRMR frame receives a SABM or DISC command, or
a DM response frame. Any other command received while the DXE is
in the frame reject state will cause another FRMR to be sent out
with the same information field as originally sent.
In the frame rejection condition, additional I frames
will not be transmitted, and received I frames and S frames will
be discarded by the DXE.
The DXE that sent the FRMR frame shall start the T1 timer
when the FRMR is sent. If no SABM or DISC frame is received
before the timer runs out, the FRMR frame shall be retransmitted,
and the T1 timer restarted as described in the waiting
acknowledgement section (2.4.4.9) above. If the FRMR is sent N2
times without success, the link shall be reset.
2.4.6 Resetting Procedure
2.4.6.1
The resetting procedure is used to initialize both
directions of data flow after a nonrecoverable error has
occurred. This resetting procedure is used in the information-
transfer state of an AX.25 link only.
2.4.6.2
A DXE shall initiate a reset procedure whenever it receives
an unexpected UA response frame or an unsolicited response frame
with the F bit set to one. A DXE may also initiate the reset
procedure upon receipt of a FRMR frame. Alternatively, the DXE
may respond to a FRMR by terminating the connection with a DISC
frame.
2.4.6.3
A DXE shall reset the link by sending a SABM frame and
starting timer T1. Upon receiving a SABM frame from the DXE
previously connected to, the receiver of a SABM frame should send
a UA frame back at the earliest opportunity, set its send and
receive state variables, V(S) and V(R), to zero and stop T1
unless it has sent a SABM or DISC itself. If the UA is correctly
received by the initial DXE, it resets its send and receive state
variables, V(S) and V(R), and stops timer T1. Any busy condition
that previously existed will also be cleared.
If a DM response is received, the DXE will enter the
disconnected state and stop timer T1. If timer T1 runs out
before a UA or DM response frame is received, the SABM will be
retransmitted and timer T1 restarted. If timer T1 runs out N2
times, the DXE will enter the disconnected state, and any
previously existing link conditions will be cleared.
Other commands or responses received by the DXE before
completion of the reset procedure will be discarded.
2.4.6.4
One DXE may request that the other DXE reset the link by
sending a DM response frame. After the DM frame is sent, the
sending DXE will then enter the disconnected state.
2.4.7 List of System Defined Parameters
2.4.7.1 Timers
To maintain the integrity of the AX.25 level 2
connection, use of these timers is recommended.
2.4.7.1.1 Acknowledgement Timer T1
The first timer, T1, is used to make sure a DXE doesn't
wait forever for a response to a frame it sends. This timer
cannot be expressed in absolute time, since the time required to
send frames varies greatly with the signaling rate used at level
1. T1 should take at least twice the amount of time it would
take to send maximum length frame to the other DXE, and get the
proper response frame back from the other DXE. This would allow
time for the other DXE to do some processing before responding.
If level 2 repeaters are to be used, the value of T1
should be adjusted according to the number of repeaters the frame
is being transferred through.
2.4.7.1.2 Response Delay Timer T2
The second timer, T2, may be implemented by the DXE to
specify a maximum amount of delay to be introduced between the
time an I frame is received, and the time the resulting response
frame is sent. This delay may be introduced to allow a receiving
DXE to wait a short period of time to determine if there is more
than one frame being sent to it. If more frames are received,
the DXE can acknowledge them at once (up to seven), rather than
acknowledge each individual frame. The use of timer T2 is not
mandatory, but is recommended to improve channel efficiency.
Note that, on full-duplex channels, acknowledgements should not
be delayed beyond k/2 frames to achieve maximum throughput. The
k parameter is defined in 2.4.7.4, below.
2.4.7.1.3 Inactive Link Timer T3
The third timer, T3, is used whenever T1 isn't running to
maintain link integrity. It is recommended that whenever there
are no outstanding unacknowledged I frames or P-bit frames
(during the information-transfer state), a RR or RNR frame with
the P bit set to one be sent every T3 time units to query the
status of the other DXE. The period of T3 is locally defined,
and depends greatly on level 1 operation. T3 should be greater
than T1, and may be very large on channels of high integrity.
2.4.7.2 Maximum Number of Retries (N2)
The maximum number of retries is used in conjunction with
the T1 timer.
2.4.7.3 Maximum Number of Octets in an I Field (N1)
The maximum number of octets allowed in the I field will
be 256. There shall also be an integral number of octets.
2.4.7.4 Maximum Number of I Frames Outstanding (k)
The maximum number of outstanding I frames at a time is
seven.
END.