4.7 X.25 Protocol
The X.25 protocol is defines the connection between data terminal equipment (DTE) and circuit-terminating equipment (DCE). X.25 is the protocol of point-to-point interaction between DTE and DCE equipment.
DTE usually refers to the host or terminal at the user side and DCE usually refers to the synchronous modem. DTE is connected with DCE directly. DCE is connected to a port of packet switching exchange, and some connections are established between the packet switching exchanges, thus forming the paths between different DTE.
With X.25, two DTE is able to communication to each other. Once a DTE device contacts another to request a communication session then it means session communication is established. If the request is accepted, the two systems begin full-duplex information transfer. The following datagram is shown the relation between entities in X.25 network
图1-1 x.25网络模型PSEDCEDTEDTE 
DCE 
PSEPSN
DTE 数据终端设备DCE 数据电路终接设备PSE 分组交换设备PSN 分组交换网The X.25 packet-switching protocol suits map to the lower three layers of the OSI (Open system Interconnection) model. 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 3-layer 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.
VC (virtual Circuits) is logic connection between two network devices. VC is a logic and bi-directional path from one DTE device to another cross an X>25 network. There are two