X.25 Network Model

As shown in Figure 8-2, an X.25 network consists of the following devices:

• Data terminal equipment (DTE): a communication terminal such as a telephone, personal computer, or network host on the X.25 network. DTE devices are used at the user side.
• Data circuit-terminating equipment (DCE): a communication device such as a modem or packet switch on the X.25 network. DCE devices connect DTE devices and packet switching exchanges (PSEs), and are often used at the carrier side.
• Packet switching exchange (PSE): a device used to transmit data from DTE devices. Most devices on a carrier's X.25 network are PSEs.

Figure 8-2 X.25 network model

X.25 Protocol Layers

As shown in Figure 8-3, the X.25 protocol stack includes the packet layer, data link layer, and physical layer. The three layers correspond to the network layer, data link layer, and physical layer of the Open Systems Interconnection (OSI) reference model respectively.

Figure 8-3 X.25 protocol layers

• Physical layer: defines the specifications of electrical interfaces connecting DTE and DCE devices and the process of setting up physical information transmission channels.

• Data link layer: uses the Link Access Procedure Balanced (LAPB) protocol. LAPB defines the frame transmission process on links between DTE and DCE devices and the frame format. For details about LAPB, see Understanding LAPB.

• Packet layer: defines the packet format and packet exchange process between entities at the packet layer. It also defines how to implement procedures such as flow control and error processing.

X.25 is an independent protocol stack, and X.25 packets can transmit packets of multiple upper-layer protocols, such as IP.