X.25
X.25 is an ITU-T standard protocol suite for packet switched WAN communications. It defines how connections are established and maintained between user devices and network devices.
The first draft of X.25 was released in 1974. It incorporated the experience and recommendations from Telenet and Tymnet in the USA and the Datapac packet-switched network in Canada. It has evolved to include more feature options and facilities.
An X.25 network comprises three types of devices: DTE, DCE, and PSE. DTEs are user devices communicating across the X.25 network. They are usually terminals, personal computers, or network hosts. DCEs are communications devices, such as modems and packet switches. PSEs are switches in the carrier's network cloud. They transfer data from one DTE device to another through the X.25 network. Figure 120 shows the relationships among the three types of X.25 devices.
Figure 120: X.25 network model
DTE | Data terminal equipment |
DCE | Data circuit-terminating equipment |
PSE | Packet switching exchange |
PSN | Packet switched network |
As shown in Figure 121, X.25 defines three layers identical to the lowest three layers of the OSI reference model.
Figure 121: X.25 protocol layers
The X.25 physical layer defines the physical and electrical specifications between DTE and DCE.
The X.25 data link layer defines the format and specifications of the frames exchanged between DTE and DCE. Link Access Procedure, Balanced (LAPB) is the data link protocol of X.25.
The X.25 packet layer defines the packet format and rules for Layer 3 packet switching.
The X.25 packet layer can multiplex multiple virtual circuits over the Layer 2 link between DTE and DCE to provide separate point-to-point connections for subscribers.
Figure 122 shows the data forms at different X.25 layers.
Figure 122: X.25 packet and LAPB frame
