6100 ADCCP Programming Manual

6100 ADCCP Concepts and Context
Introduction to 6100 ADCCP
1–14 069225 Tandem Computers Incorporated
With the Ns/Nr scheme of sequence checking, two communicating stations do not
need to acknowledge each frame individually. An acknowledgment must be issued at
least once for every window size. Because the status of an unacknowledged frame is
not known (the frame might have to be sent again), the sending station retains each
frame until it has been acknowledged.
The use of frame sequence numbers is illustrated in Figure 1-7. Notice that once
transmission begins, the Nr values specify the number of the next frame to be received
and that a single Nr value can acknowledge the receipt of several successive frames.
Also note that the acknowledgment pattern shown in Figure 1-8 is somewhat
simplified: it assumes a half-duplex, point-to-point environment. In a multipoint
environment, the primary station must use separate sets of sequence numbers for each
secondary station.
Figure 1-8. Frame Sequence Checking
006
Primary Secondary
Ns
0
1
2
3
Nr
0
0
0
0
Ns
0
1
2
Nr
4
4
4
Ns
4
5
6
7
0
Nr
3
3
3
3
3
Ns
3
4
5
Nr
1
1
1
Supervisory-Frame Control Field. An S-frame reports whether a station wants to receive
data, and in some cases, which data the station wants to receive. The format of the
control field is similar to the format of an I-frame except that the Ns value (which is
unnecessary because supervisory frames are unnumbered) is replaced by a 2-bit
command code, and the frame identifier is two bits long instead of one. The format