X.25 Network

X.25 is an ITU-T standard protocol suite for packet switched wide area network (WAN) communication.  X.25 defines how connections between user devices and network devices are established and maintained. It typically used in packet switched networks of common carriers.

X.25 originally defined three basic protocol levels or architectural layers. In the original specifications, these were referred to as levels and had a level number, whereas all ITU-T X.25 recommendations and ISO 8208 standards released after 1984 refer to them as layers.

The following figure illustrates a typical X.25 network. X.25 network consist three typical components. DTE (Data Terminal Equipment), DCE (Data Communication Equipment) and PSE (Packet Switching Exchange). The user end of the network is known as Data Terminal Equipment and the carrier's equipment is Data Circuit-terminating Equipment. X.25 routes packets across the network from DTE to DTE by using Packet Switching Equipments. X.25 supports two types of virtual circuits, virtual calls (VC) and permanent virtual circuits (PVC). Virtual calls are established on an as-needed basis.

X.25 Network

Physical layer:  This layer specifies the physical and other characteristics to control the physical link between a DTE and a DCE. Physical layer commonly use X.21, EIA-232, EIA-449 or other serial protocols.

Data link layer: The data link layer consists of the link access procedure for data interchange on the link between a DTE and a DCE.  It uses Link Access Procedure, Balanced (LAPB) as data link protocol. It is a bit-oriented protocol provides error correction and orderly delivery.

Packet layer: This layer specifies packet layer protocols for exchanging data.

Relation to the OSI Reference Model

The following figure illustrates the relationships between OSI layers and X.25 layers.

Relationships between OSI layers and X.25 layers

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