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 -
- Electrical Interfaces
Link Layer - includes topics such as -
Packet (network) Layer - includes topics such as -
- packet types
- clear causes
- facilities (per call facilities)
- facilities (subscription time facilities)
- Througput issues
A protocol such as X.25 has many features to provide:
- a method of physically connecting to the network
- procedures to detect and correct line errors
- how many calls to be established simultaneously
- how a call is established and cleared
- how facilities such as reverse charging are requested, etc. etc.
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
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.