196CHAPTER 16: CONFIGURING LAPB AND X.25

Figure 58 X.25 network model

DTE

DTE

DCE

PSE

PSE

PSE

DCE

DCE

DTE

PSN

DTE: Data Terminal Equipment

DCE: Data Circuit-terminating Equipment

PSE: Packet Switching Equipment

PSN: Packet Switching Network

The X.25 protocol suite maps to the lowest three layers of the OSI (Open System Interconnection) reference model. The following protocols are typically used in X.25 implementations: Packet-Layer Protocol (PLP), Link Access Procedure Balanced (LAPB), and other physical-layer serial interfaces. X.25 layer 3 (packet-layer protocol) describes the format of packet used by the packet layer and the procedure of packet switching between two layer 3 entities. X.25 layer 2 (link-layer protocol), also called LAPB (Link Access Procedure Balanced), defines the format and procedure of interactive frames between DTE and DCE. X.25 layer 1 (physical-layer protocol) defines some physical and electrical characteristics in the connection between DTE and DCE. The above relation is shown in the following diagram.

Figure 59 DTE/DCE interface

OSI reference model

7

 

6

 

5

 

4

 

3

X.25

Packet layer

 

2

X.25

Link layer

 

 

1

X.25

Physical layer

 

 

 

 

DTE

X.25

Packet layer interface

Link layer interface

Physical layer interface

X.25

Packet layer

X.25

Link layer

X.25

Physical layer

DCE

A virtual circuit is a logical connection created to ensure reliable communication between two network devices. A virtual circuit denotes the existence of a logic, bi-directional path from one DTE device to another across an X.25 network. Two types of X.25 virtual circuits exist: permanent virtual circuit (PVC) and switched virtual circuit (SVC). PVCs are permanently established connections used for frequent and consistent data transfers, whereas SVCs are temporary connections used for sporadic data transfers.

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