Telecommunications - X25 information


X25 is a packet based protocol.

X25 is divided into 3 layers, for more specific information about each layer see:

Physical Layer - includes topics such as -

Link Layer - includes topics such as -

Packet (network) Layer - includes topics such as -

A protocol such as X.25 has many features to provide:

it defines,

To define one single protocol to do all this would be very complicated, also it would be very inflexible, to add any new features might require major changes to the software of equipment connected to the network.

To minimize these problems X.25 in defined in 3 layers, each layer is, as far as possible, independent of the others.

Each layer builds on the services provided by the layer below it and adds new services of its own.

The lowest layer of X.25 is the physical layer, it defines the physical interface between the Public Network customers equipment and the first point on the network (the modem). The physical layer of X.25 is a subset of V.24/RS232.

The next layer up is the Link layer. This layer turns the error susceptible bitstream, provided by the physical layer, into an error free link between adjacent nodes.

The packet layer sits on top of the link, providing facilities for multiplexing several calls over one link and provides the mechanism for setting up and releasing calls and signaling facilities.

 


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