System information

Troubleshooting X.25 Connections 19-411
X.25 Frame Format
Figure 19-2 X.25 and the OSI Reference Model
End-to-end communication between DTEs is accomplished through a bidirectional association
called a virtual circuit. Virtual circuits permit communication between distinct network elements
through any number of intermediate nodes without the dedication of portions of the physical
medium that characterizes physical circuits. Virtual circuits can be either permanent or switched
(temporary). Permanent virtual circuits (PVCs) are typically used for the most often used data
transfers, whereas switched virtual circuits (SVCs) are used for sporadic data transfers. Layer 3 X.25
is concerned with end-to-end communication involving both PVCs and SVCs.
When a virtual circuit is established, the DTE sends a packet to the other end of the connection by
sending it to the DCE using the proper virtual circuit. The DCE looks at the virtual circuit number
to determine how to route the packet through the X.25 network. The Layer 3 X.25 protocol
multiplexes between all the DTEs served by the DCE on the destination side of the network, and the
packet is delivered to the destination DTE.
X.25 Frame Format
An X.25 frame is composed of a series of fields, as shown in Figure 19–3. Layer 3 X.25 fields make
up an X.25 packet and include a header and user data. Layer 2 X.25 (LAPB) fields include
frame-level control and addressing fields, the embedded Layer 3 packet, and a frame check sequence
(FCS).
User-
defined
process
Packet-
switching
network
X.25
packet
level
X.25
frame
level
X.25
physical
level
Packet interface
Frame interface
Physical interface
DTE/DCE
interface
DTE
OSI Reference
Model
DCE
X.25
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6
5
4
3
2
1