Information Technology Reference
In-Depth Information
Address
Control
Info
FCS
01111110
01111110
16 bits
P/
F
0
N(S)
N(R)
Information frame
P/
F
1
0
S
N(R)
Supervisory frame
-
P/
F
P/
F
1
1
M
M
Unnumbered frame
-
Figure I.8
Extended control field.
Figure I.9 shows an example conversation between a sending station (node A) and a re-
ceiving station (node B). Initially three information frames are sent numbered 2, 3 and 4
(
I[N(S)=2]
,
I[N(S)=3]
and
I[N(S)=4, P=1]
). The last of these frames has the poll bit
set, which indicates to node B that node A wishes it to respond, either to acknowledge or re-
ject previously unacknowledged frames. Node B does this by sending back a supervisory
frame (
S[RR, N(R)=5]
) with the receiver ready (RR) acknowledgement. This informs node
A that node B expects to receive frame number 5 next. Thus it has acknowledged all frames
up to and including frame 4.
In the example in Figure I.9 an error has occurred in the reception of frame 5. The recipi-
ent informs the sender by sending a supervisory frame with a reject flow command (
S[REJ,
N(R)=5]
). After the sender receives this it resends each frame after and including frame 5.
If the receiver does not want to communicate, at the present, it sends a receiver not ready
flow command. For example
S[RNR,
N(R)=5]
tells the transmitter to stop sending data, at
the present. It also informs the sender that all frames up to frame 5 have been accepted. The
sender will transmit frames once it has received a receiver ready frame from the receiver.
Figure I.9 shows an example of data flow in only the one direction. With ABM both sta-
tions can transmit and receive data. Thus each frame sent contains receive and send counter
values. When stations send information frames the previously received frames can be ac-
knowledged, or rejected, by piggy-backing the receive counter value. In Figure I.10, node A
sends three information frames with
I[N(S)=0,N(R)=0]
,
I[N(S)=1, N(R)=0]
, and
I[N(S)=2,N(R)=0]
. The last frame informs node B that node A expects to receive frame 0
next. Node B then sends frame 0 and acknowledges the reception of all frames up to, and in-
cluding frame 2 with
I[N(S)=0,N(R)=3]
, and so on.
I.5 Derivatives of HDLC
There are many derivatives of HDLC, including:
