Transmitter send sequence number (bit 2 = low\(hyorder bit).
.parag
N(R)
Transmitter receive sequence number (bit 6 = low\(hyorder bit).
.parag
S
Supervisory function bit.
.parag
M
Modifier function bit.
.parag
P/F
Poll bit when issued as a command, final bit when issued as a
response (1 = Poll/Final).
.parag
P
Poll bit (1 = Poll).
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 3/X.75 [T3.75], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T4.75]\fR
.ce
TABLE\ 4/X.75
.ce
\fI\fIa)\ Control field formats (modulo 128)\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(36p) | cw(12p) sw(12p) sw(12p) sw(12p) sw(12p) sw(12p) sw(12p) sw(12p) | cw(12p) sw(12p) sw(12p) sw(12p) sw(12p) sw(12p) sw(12p) sw(12p) , ^ | c | c | c | c | c | c | c | c | c | c | c | c | c | c | c | c.
Rejected frame control field 0 V(S) C/R V(R) W X Y Z 0 0 0 T{
0
Rejected frame control field is the control field of the received frame which caused the frame reject.
.parag
V(S)\| is the current send state variable value at the STE reporting the
rejection condition (bit 10 = low\(hyorder bit).
.parag
C/R\| set to 1 indicates the rejected frame was a response.
.parag
C/R\| set to 0 indicates the rejected frame was a command.
.parag
V(R)\| is the current receive state variable value at the STE reporting the
rejection condition (bit\ 14 = low\(hyorder bit).
.parag
W\| set to 1 indicates that the control field received and returned in bits 1
through 8 was invalid or not implemented.
.parag
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 incorrect length. Bit W must be set to 1 in conjunction with this
bit.
.parag
Y\| set to 1 indicates that the information field received exceeded the maximum established capacity.
.parag
Z\| set to 1 indicates the control field received and returned in bits 1
Rejected frame control field 0 V(S) C/R V(R) W X Y Z 0 0 0 T{
0
Rejected frame control field is the control field of the received frame which caused the frame reject. When the rejected frame is an unnumbered frame, the
control field of the rejected frame is positioned in bit positions 1\(hy8, with
9\(hy16 set to 0. If, however, the interim solution mentioned in \(sc\ 2.3.2.1.3 is
adopted, the 2\(hyoctet control field will be placed in bit positions 1\(hy16.
.parag
V(S) is the current send state variable value at the STE reporting the
rejection condition (bit 18\ =\ low order bit).
.parag
C/R set to 1 indicates the rejected frame was a response. C/R set to 0
indicates the rejected frame was a command.
.parag
V(R) is the current receive state variable value at the STE reporting the
W set to 1 indicates that the control field received and returned in bits 1
through 16 was invalid or not implemented.
.parag
X set to 1 indicates that the control field received and returned in bits 1
through 16 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 incorrect length. Bit W must be set to 1 in conjunction with this
bit.
.parag
Y set to 1 indicates that the information field received exceeded the maximum established capacity.
.parag
Z set to 1 indicates the control field received and returned in bits 1 through 16 contained an invalid N(R).
.parag
Bits 17 and 37 through 40 shall be set to 0.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 8/X.75 [T8.75], p.21\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
2.3.5.2
\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\ frame which may follow until an I\ frame with the correct\ N(S)
is received.
.PP
An STE which receives one or more valid I frames having sequence
errors
or subsequent supervisory frames (RR,\ RNR and REJ) shall accept the control
information contained in the\ N(R) field and the P/F\ bit to perform link
control functions; e.g.,\ to receive acknowledgement of previously transmitted
I\ frames, and to cause the STE to respond (P\ bit sent to\ 1).
.RT
.sp 1P
.LP
2.3.5.2.1
\fIREJ recovery\fR
.sp 9p
.RT
.PP
The REJ frame is used by a receiving STE to initiate a recovery
(retransmission) following the detection of an N(S) sequence error.
.PP
With respect to each direction of transmission on the link, only one \fIsent
REJ\fR exception condition from an STE is established at a time. A
\fIsent REJ\fR exception condition is cleared when the requested I\ frame is
received.
.PP
An STE receiving REJ initiates sequential (re\(hy)transmission of
I\ frames starting with the I\ frame indicated by the N(R) obtained in the REJ
frame.
.PP
The retransmitted frame(s) may contain an N(R) and a P bit that is
updated from, and therefore different from, the ones contained in the
originally transmitted I\ frame(s).
.RT
.sp 1P
.LP
2.3.5.2.2
\fITime\(hyout recovery\fR
.sp 9p
.RT
.PP
If an STE, due to a transmission error, does not receive (or
receives and discards) a single I\ frame or the last I\ frames 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 STE which transmitted the unacknowledged
I\ frame(s) shall, following the completion of a system specified time\(hyout
period (see \(sc\(sc\ 2.4.5.9 and\ 2.4.8.1\ below), take appropriate recovery
action to determine at which I\ frame retransmission must begin. The retransmitted
frames may contain an N(R) and a P\ bit that are updated from, and therefore
different from, the ones contained in the originally transmitted I\ frames.
.RT
.sp 1P
.LP
2.3.5.3
\fIInvalid frame condition\fR
.sp 9p
.RT
.PP
Any frame which is invalid will be discarded, and no action is
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)
in non\(hyextended (modulo 8) operation, contains fewer than
32\ bits between flags; in extended (modulo\ 128) operation,
contains fewer than 40\ bits between flags of frames that contain
sequence numbers or 32\ bits between flags of frames that do not
contain sequence numbers.
.LP
\fINote\fR \ \(em\ Or fewer than 40 bits (modulo 128) if 2\(hyoctet
control field is used as alternative\ b during the interim
period (see \(sc\ 2.3.2.1.3).
.LP
c)
contains a Frame check sequence (FCS) error;
.LP
d)
contains an address other than A or B (for single link
operation) or other than\ C or\ D (for multilink operation).
.PP
For those networks that are octet aligned, a detection of
non\(hyoctet alignment may be made at the 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, or the frame considered invalid.
.bp
.sp 1P
.LP
2.3.5.4
\fIFrame rejection condition\fR
.sp 9p
.RT
.PP
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.PP
This frame rejection exception condition is reported by sending an
FRMR response for appropriate STE action.
.PP
Once an STE has established a frame rejection condition, no
additional\ I or S\ format frames are accepted until the condition is reset
except for examination of the P\ bit. The FRMR response may be repeated
at each opportunity, as specified in \(sc\ 2.4.7.3 until recovery is effected
by the other STE or until the STE initiates its own recovery in case the
other STE does not respond.
.RT
.sp 1P
.LP
2.3.5.5
\fIExcessive idle channel state condition on 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 STE shall wait for a period\ T3 (see
\(sc\ 2.4.8.3 below) without taking any specific action, 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 STE shall notify the MLP or the packet
layer of the excessive idle channel state condition, but shall not take any
action that would preclude the other STE from establishing the link by
normal link set\(hyup procedures.
.PP
The value of T3 is a system parameter and is agreed
bilaterally.
.RT
.sp 2P
.LP
2.4
\fIDescription of the procedures\fR
.sp 1P
.RT
.sp 1P
.LP
2.4.1
\fIExtended and non\(hyextended modes of operation\fR
.sp 9p
.RT
.PP
Changing from non\(hyextended operation to extended operation,
or vice versa, requires bilateral agreement and is not supported
dynamically.
.PP
Table 5/X.75 indicates the command and response control field formats used
with the non\(hyextended (modulo\ 8) service. The mode setting command
employed to initialize (set up) or reset the non\(hyextended mode is the SABM
command. Table 6/X.75 indicates the command and response control field
formats used with the extended (modulo\ 128) service. The mode setting
command employed to initialize (set up) or reset the extended mode is the
SABME command.
.RT
.sp 1P
.LP
2.4.2
\fIProcedure for addressing\fR
.sp 9p
.RT
.PP
Commands are sent with the remote STE address and responses are
sent with the local STE address.
.PP
In order to allow differentiation between single link operation and
multilink operation for diagnostic and/or maintenance reasons, different
address pair encodings shall be assigned to links operating with the multilink
procedure (MLP) compared to links operating with the single link procedure