Introduction to Tandem X.25 Capabilities

Data-Link-Layer Components
What is X.25?
065307 Tandem Computers Incorporated 1–29
Figure 1-20 illustrates the data transfer portion of a communications session between a
DTE and a DCE. Keep in mind that only information frames (I-frames) can contain
packets.
Figure 1-20. Data Transfer of Information Frames
System X
System X System Y
System X System Y
System Y
System X
System X System Y
System X
System X System Y
System X System Y
System X
System X System Y
System Y
DTE DCE
A, I, S0, R0
B, I, S0, R1 A, I, S1, R0
B, I, S1, R2 A, I, S2, R1
B, I, S2, R2
A, I, S3, R2
A, I, S4, S3
B, I, S3, R5 A, I, S5, R3
A, RR, R4, P
A, RR, R6, F
LINK
Legend
A or B = station address
Frame Type:
I = Information
RR = Receive Ready
Sequence Count:
S = N(S)
R = N(R) values. Note that number after RR is N(R) value.
P = poll-bit set to 1, F = final bit set to 1
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