Introduction to Tandem X.25 Capabilities

Frame Types
What is X.25?
1–32 065307 Tandem Computers Incorporated
Figure 1-21 shows how REJ frames are used for error recovery. When the DTE
attempts to send its third transmission of an I-frame, the DCE responds with REJ and
N(R) of 1. This sequence number notifies the DTE that the DCE has successfully
received only one I-frame, S0. Thus, the DTE resends I-frames, starting with S1.
Figure 1-21. REJ Frame Transmission
System_Y
System_X System_Y
System_Y
System_X
System_X System_Y
System_Y
System_X
System_X
System_Y
System_Y
System_X
System_X
System_Y
System_X System_Y
System_Y
B, I, S0, R0
B, I, S1, R0
B, I, S1, R0
B, I, S2, R0
B, RR, R3
DTE DCE
B, I, S2, R0
SEQUENCE
ERROR
B, REJ, R1
LINK
Legend
A or B = station address
Frame Type:
I = Information
REJ = Reject
RR = Receive Ready
Sequence Count:
S = N(S)
R = N(R) values. Note that number after
RR and REJ is N(R) value.
017