Introduction to Tandem X.25 Capabilities

Frame Types
What is X.25?
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In Figure 1-22, the DCE returns an RR response with an N(R) of 4. However, the DTE
had only sent I-frames up to an N(S) of 2 and would therefore expect an N(R) of 3 in
response for acknowledgement. To recover from this situation, the DTE sends the
FRMR command with appropriate diagnostic information. Then, the DCE responds
with a SABM to reset the link.
Figure 1-22. FRMR Frame Transmission
System_Y
System_X
System_Y
System_X
System_X
System_X System_Y
System_Y
System_Y
System_Y
System_X System_Y
System_Y
System_Y
System_Y
B, I, S=0, R=0
B, I, S=1, R=0
B, RR, R4
RESET
A, SABM
B, I, S=0, R=0
B, I, S=1, R=0
DTE DCE
B, I, S=2, R=0
INVALID N(R)
A, FRMR, Z
A, UA
LINK
Legend
A or B = station address
Frame Type:
FRMR = Frame Reject
Z = Diagnose Field
SABM = Set Asynchronous Balanced Mode
UA = Unnumbered Acknowledgement
I = Information
RR = Receive Ready
Sequence Count:
S = N(S)
R = N(R) values. Note that number after RR and REJ
is N(R) value.
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