3Com MSR 50 Series Configuration Manual page 284

284 C 14: X.25
HAPTER AND
LAPB C
ONFIGURATION
Figure 56 X.25 network model
DTE DCE
DTE
DCE
PSE
PSN
The X.25 protocol defines the lowest three layers of the OSI (Open System
Interconnection) reference model. As shown in the following figure, layer 3
(packet layer) provision of X.25 describes the packet format used by the packet
layer and the procedure of packet switching between two layer-3 entities. Layer 2
(link layer) provision of X.25, also known as Link Access Procedure Balanced
(LAPB), defines the frame format and procedure adopted in the DTE-DCE
interaction. Layer 1 (physical layer) of X.25 defines some physical and electrical
characteristics in the connection between DTE and DCE.
Figure 57 DTE/DCE interfaces
OSI reference model
7
6
5
4
X.25 packet layer
3
X.25 data link layer
2
X.25 physical layer
1
DTE
The connection established via X.25 protocol between two DTEs is called virtual
circuit (VC), which exists logically and is distinct from the physical circuit in circuit
switching in nature. VCs involve Permanent Virtual Circuit (PVC) and Switched
Virtual Circuit (SVC). PVC is used for transmitting traffic that is generated in a
frequent but stable way and SVC for transmitting traffic that is generated in a
burst way.
Once a virtual circuit is established between a pair of DTEs, it is assigned a unique
virtual circuit number. When one DTE is to send a packet to the other, it numbers
this packet (with virtual circuit number) and sends it to DCE. According to the
number on the packet, DCE determines how to switch this packet within the
switching network, so that this packet can reach the correct destination. A link
PSE DCE
PSE
PSE DCE
PSN
Data terminal equipment
Data circuit-terminating equipment
Packet switching equipment
Packet switching network
X.25
Packet layer
interface
Data link
interface
Physical layer
interface
DTE
DTE
X.25 packet layer
X.25 data link layer
X.25 physical layer
DCE