Nortel Networks 8000 - Manual (Page 207)

7 LAPB and X.25 configuration
Nortel Secure Router 8000 Series
Configuration -WAN Access
7.1.2 X.25 Principle
Virtual Circuit
By using the X.25, you can connect two DETs through the existing telephone network.
A virtual circuit (VC) is the created connection in X.25 to ensure reliable communication
between two DTEs. A VC only exists logically, which is different from physical circuit in
circuit switching.
The following are the two types of VCs:
• Permanent VC (PVC): PVCs are permanently established connections used for frequent
and consistent data transfers.
• Switched VC (SVC): SVCs are temporary connections used for sporadic data transfers.
Once you set up a VC between a pair of DTEs, you assign it with a unique VC number. When
one DTE has to send a packet to the other, it numbers the packet (with a VC number) and
sends it to the DCE. According to the number on the packet, DCE determines the method to
switch this packet within the switching network so that the packet can reach the correct
destination.
X.25 Protocol layer
According to the OSI (Open System Interconnection) reference model, the X.25 protocol suite
maps the lowest three layers from physical layer to packet layer. See Figure 7-2.
Figure 7-2 X.25 protocol Layer
OSI reference model
X.25
7
6
5
4
3
X.25
( __ Packet layer _ _ _ _ _
y
X.25
Packet layer interface Packet layer
2
X.25
Link layer
(_ _ _ Link layer _______
y
interface
X.25
Link layer
1
X.25
^
Physical layer ^
X.25
Physical layer
’ interface ^ Physical layer
DTE DCE
X.25 layer 1 (physical-layer protocol): It defines certain physical and electrical
characteristics in the connection between DTE and DCE.
X.25 layer 2 (link-layer protocol): It defines the format and procedure of interactive
frames between DTE and DCE. That is LAPB.
X.25 layer 3 (packet-layer protocol): It describes the format of packet used by the packet
layer and the procedure of packet switching between two 3-layer entities.
7-4 Nortel Networks Inc. Issue 5.3 (30 March 2009)