Rejected command control field is the control field of the received
command which caused the command reject.
.parag
\(em
V(S) is the current send state variable value at the DCE or DTE reporting the rejection condition (bit 10\|=\|low\(hyorder bit).
.parag
\(em
V(R) is the current receive state variable value at the DCE or DTE
reporting the rejection condition (bit 14\|=\|low\(hyorder bit).
.parag
\(em
W set to 1 indicates that the control field received and returned in bits 1 through 8 was undefined or not implemented.
.parag
\(em
X set to 1 indicates that the control field received and returned in bits 1 through 8 was considered invalid because the frame contained an information field which is not permitted with this frame or is a supervisory or unnumbered frame with an incorrect length. Bit W must be set to 1 conjunction with this
bit.
.parag
\(em
Y set to 1 indicates that the information field received exceeded the
maximum established capacity of the DCE or DTE reporting the rejection
condition.
.parag
\(em
Z set to 1 indicates the control field received and returned in bits 1
through 8 contained an invalid N(R).
.parag
\fINote\fR
\ \(em\ Bits 9, 13 and 21 to 24 shall be set to 0.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 13/X.25 [T11.25], p.
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.sp 1P
.LP
2.6.5
\fIException condition reporting and recovery\fR
.sp 9p
.RT
.PP
The error recovery procedures which are available to effect
recovery following the detection/occurrence of an exception condition at the
Data Link Layer are described below. Exception conditions described are
those situations which may occur as the result of transmission errors,
DCE or DTE
malfunction, or operational situations.
.RT
.sp 1P
.LP
2.6.5.1
\fIBusy condition\fR
.sp 9p
.RT
.PP
The busy condition results when the DCE or DTE is temporarily
unable to continue to receive I\ frames due to internal constraints,
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR frame.
.PP
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SARM command), RR, REJ or SARM
frame.
.bp
.RT
.LP
.sp 1P
.LP
2.6.5.2
\fIN(S) sequence error condition\fR
.sp 9p
.RT
.PP
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.PP
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frames which may follow, until an I\ frame with the correct N(S)
is received.
.PP
A DCE or DTE which receives one or more valid I frames having sequence
errors but otherwise errorless shall accept the control information contained
in the N(R) field and the P\ bit to perform data link control functions,
e.g.\ to receive acknowledgement of previously transmitted I\ frames and
to cause the DCE or DTE to respond (P\ bit set to\ 1). Therefore, the retransmitted
frame may
contain an N(R) and a P\ bit that are updated from, and therefore different
from, those contained in the originally transmitted I\ frame.
.PP
The methods specified in \(sc\(sc\ 2.6.5.2.1 and\ 2.6.5.2.2 shall be available
for initiating the retransmission of lost of errored I\ frames following
the
occurrence of an N(S) sequence error condition.
.RT
.sp 1P
.LP
2.6.5.2.1
\fIREJ recovery\fR
.sp 9p
.RT
.PP
The REJ frame is used by a receiving DCE or DTE to initiate a
recovery (retransmission) following the detection of an N(S) sequence error.
.PP
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
requested I\ frame is received.
.PP
A DCE or DTE receiving an REJ frame initiates sequential
(re)transmission of I\ frames starting with the I frame indicated by the N(R)
obtained in the REJ frame.
.RT
.sp 1P
.LP
2.6.5.2.2
\fITime\(hyout recovery\fR
.sp 9p
.RT
.PP
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit an REJ frame. The DTE or DCE, which transmitted the
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.7.4.8 and 2.7.7.1 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin.
.RT
.sp 1P
.LP
2.6.5.3
\fIInvalid frame\fR \fIcondition\fR
.sp 9p
.RT
.PP
Any frame which is invalid will be discarded, and no action will be taken
as the result of that frame. An invalid frame is defined as one
which:
.RT
.LP
a)
is not properly bounded by two flags;
.LP
b)
contains fewer than 32 bits between flags;
.LP
c)
contains a Frame Check Sequence (FCS) error; or
.LP
d)
contains an address other than A or B.
.PP
For those networks that are octet\(hyaligned, a detection of
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
validity check that requires the number of bits between the opening flag and
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length. Otherwise the frame is considered invalid.
.sp 1P
.LP
2.6.5.4
\fICommand rejection\fR \fIcondition\fR
.sp 9p
.RT
.PP
A command rejection condition is established upon the receipt of an error\(hyfree
command frame with one of the conditions listed in \(sc\ 2.6.4.8
above.
.PP
At the DCE or DTE, this command rejection exception condition is
reported by a CMDR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination
of the P bit. The CMDR response may be repeated at each opportunity, as
specified in \(sc\ 2.7.6.5, until recovery is effected by the DTE, or until
the DCE initiates its own recovery.
.bp
.RT
.sp 1P
.LP
2.6.5.5
\fIExcessive idle channel state condition on the incoming channel\fR
.sp 9p
.RT
.PP
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall not take any action for a period
T3 (see \(sc\ 2.7.7.3\ below), while waiting for detection of a return
to the active channel state (i.e.\ detection of at least one flag sequence).
After the period T3, the DCE shall notify the Packet Layer of the excessive
idle channel state condition, but shall not take any action that would
preclude the DTE from
establishing the data link by normal data link set\(hyup procedures.
.PP
\fINote\fR \ \(em\ Other actions to be taken by the DCE at the Data Link
Layer upon expiration of period T3 is a subject for further study.
.RT
.sp 2P
.LP
2.7
\fIDescription of the LAP procedure\fR
.sp 1P
.RT
.sp 1P
.LP
2.7.1
\fILAP procedure for addressing\fR
.sp 9p
.RT
.PP
The address field identifies a frame as either a command or a
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.PP
Frames containing commands transferred from the DCE to the DTE
will contain the address\ A.
.PP
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B.
.PP
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B.
.PP
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A.
.PP
A and B addresses are coded as follows:
.RT
.LP
Address
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.LP
\ \ A
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.LP
\ \ B
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.PP
\fINote\fR \ \(em\ The DCE will discard all frames received with an address
other than A or B; the DTE should do the same.
.sp 1P
.LP
2.7.2
\fILAP procedure for the use of the P/F bit\fR
.sp 9p
.RT
.PP
The DCE or DTE receiving an SARM, DISC, supervisory command or
I\ frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next
response
frame it transmits.
.PP
The response frame returned by the DCE to an SARM or DISC command with
the P\ bit set to\ 1 will be a UA response with the F\ bit set to\ 1. The
response frame returned by the DCE to an I\ frame with the P\ bit set to\
1, received
during the information transfer phase, will be an RR, REJ, RNR or CMDR
response with the F\ bit set to\ 1. The response frame returned by the
DCE to a
supervisory command frame with the P\ bit set to\ 1, received during the
information transfer phase, will be an RR, RNR, REJ or CMDR response with
the F\ bit set to\ 1.
.PP
The P bit may be used by the DCE in conjunction with the timer
recovery condition (see \(sc\ 2.7.4.8 below).
.PP
\fINote\fR \ \(em\ Other use of the P bit by the DCE is a subject for further
study.
.RT
.sp 2P
.LP
2.7.3
\fILAP procedures for data link set\(hyup and disconnection\fR
.sp 1P
.RT
.sp 1P
.LP
2.7.3.1
\fIData link set\(hyup\fR
.sp 9p
.RT
.PP
The DCE will indicate that it is able to set up the data link by