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listing the data will be essentially unidirectional and the receiver in the
network needs to be able to acknowledge the receipt of Info frames
without sending Info frames back. This is achieved with the Receiver
Ready (RR) frame which just sends the N(R) value to the sender. Fig.

1.10 shows an example of this.

Unless the data is unidirectional, or unidirectional for a period, then
there is no need for the use of the RR frame. However, most X.25
implementations require that frames are acknowledged within a short
time of being sent, and the RR frame is very common.

The frame numbering scheme gets a little more complex when the
numbers wrap around. The same logic still applies though. See Fig. 1.11.

We now need to look at what happens if there is a loss of data on the
link. Consider the conversation shown in Fig. 1.12. Clearly something
has gone awry. The most likely cause is that Info frame 4 from the
lefthand side has been corrupted in transit, thus when it arrives at the

Fig. 1.10 Use of the RR frame to acknowledge receipt of data without sending more data in return

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