Information Technology Reference
In-Depth Information
Address
Control
Info
FCS
01111110
01111110
0
N(S)
P/F
N(R)
Information frame
1
0
S
P/F
N(R)
Supervisory frame
1
1
M
P/F
M
Unnumbered frame
b
7
b
6
b
5
b
4
b
3
b
2
b
1
b
0
Figure I.5
Format of an 8-bit control field
Supervisory frame
Supervisory frames contain flow control data. They confirm or reject previously received in-
formation frames and also can indicate whether a station is ready to receive frames.
The
N(S)
field is used with the S bits to acknowledge, or reject, previously transmitted
frames. Responses from the receiver are set in the S field, these are receiver ready (RR),
ready not to receive (RNR), reject (REJ) and selectively reject (SREJ). Table I.1 gives the
format of these bits.
RR informs the receiver that it acknowledges the frames sent up to
N(R)
. RNR tells the
transmitter that the receiver cannot receive any more frames at the present time (RR will can-
cel this). It also acknowledges frames up to
N(R)
. The REJ control rejects all frames after
N(R)
. The transmitter must then send frames starting at
N(R)
.
Table I.1
Supervisory bits
b
5
b
4
Receiver status
Receiver ready (RR)
0
0
1
0
Receiver not ready (RNR)
Reject (REJ)
0
1
1
1
Selectively reject (SREJ)
Unnumbered frame
If the first two bits of the control field are 1's then it is an unnumbered frame. Apart from the
P/F
flag the other bits are used to send unnumbered commands. When sending commands,
the
P/F
flag is a poll bit (asking for a response), and for responses it is a flag bit (end of re-
sponse).
The available commands are SARM (set asynchronous response mode), SNRM (set nor-
mal response mode), SABM (set asynchronous balance mode), RSET (reset), FRMR (frame
reject) and Disconnect (DISC). The available responses are UA (unnumbered acknowledge),
CMDR (command reject), FRMR (frame reject) and DM (disconnect mode). Bit definitions
for some of these are:
