X.25 Protocol

Layer 2: Frame Level Layer

The Frame Level layer is an implementation of the ISO HDLC standard called Link Access Procedure Balanced (LAPB). This layer provides an error-free link between two connected devices.

Layer 3: Network Layer

The Network layer provides communication between devices that are connected to a common network. In the case of X.25, this layer is referred to as the X.25 packet.

Logical Channels

The user end of the network is known as Data Terminal Equipment (DTE) and the carrier's equipment is Data Circuit- terminating Equipment (DCE). The X.25 protocol layer permits a DTE user on an X.25 network to communicate with a number of remote DTEs simultaneously.

Connections occur on logical channels of two types:

•Switched virtual circuits (SVCs)

•Permanent virtual circuits (PVCs)

SVCs

SVCs are much like telephone calls; a connection is established, data is transferred, and then the connection is released. Each DTE on the network is given a unique DTE address, which can be used much like a telephone number.

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