Information Technology Reference
In-Depth Information
X.25 Network
Figure 5-1
Router (DTE)
Router
PVCs
Switch (DCE)
X.25 Network
X.25 switch
Router
Router
PVCs
X.121 Address Format
Figure 5-2
DNIC
Country
PSN
NTN
X.25 Framing
X.25 uses HDLC LAPB as the Layer-2 protocol to manage communication between DTE and
DCE devices. The LAPB frame has the format, as shown in Figure 5-3.
LAPB Frame Format
Figure 5-3
Bytes:
1
1
1
Variable
1
1
Flag
Address
Control
Data
FCS
Flag
Flag is the bit stream 01111110, which indicates the beginning or end of the frame. The address
field indicates command or response. The control field indicates if the frame is an information,
supervisory, or unnumbered frame. The frame check sequence (FCS) handles error checking.
LAPB takes care of error detection and link recovery. In contrast, Frame Relay lets the upper-
layer protocol detect the error and handle the retransmission.
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