Information Technology Reference
In-Depth Information
Figure19-2 X.25 and the OSI Reference Model
X.25
OSI reference
model
7
6
User-
defined
process
Packet-
switching
network
5
4
X.25
packet
level
X.25
frame
level
X.25
physical
level
Packet interface
3
Frame interface
2
Physical interface
1
DTE
DTE/DCE
interface
DCE
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
among 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).
