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receiver has to acknowledge receipt of the Info frames, and the sender
cannot send any new frames if seven are outstanding and awaiting
acknowledgement.

The acknowledgement is carried in the N(R) field of returning Info
frames, and indicates the next frame that is expected.

Suppose a frame is sent with N(R) of 3 and N(S) of 7:

• The Receiver of the frame must check whether frame seven is the
next one that it was expecting.
• The Receiver may now use an N(S) of 3 again.

It is not necessary for every frame to be individually acknowledged. Fig.
1.9 shows an example conversation where only some of the frames are
acknowledged, but the acknowledgement includes earlier frames as well.

Suppose that a user has requested that a file be listed out on screen
and that the conversation is over a packet switched network. During the

Fig. 1.9 A layer two conversation; note that it is not necessary for every frame to be individually acknowledged

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