LAYER 1 The Physical Layer is concerned with electrical or signaling. It includes several standards such as V.35, RS232 and X.21bis.
LAYER 2 The Data Link Layer, which is an implementation of the ISO HDLC standard called Link Access Procedure Balanced (LAPB) and provides an error free link between two connected devices.
LAYER 3 The Network Layer, which provides communications between devices, connected to a common network. In the case of X.25, this layer is referred to as the X.25 Packet Layer Protocol (PLP) and is primarily concerned with network routing functions and the multiplexing of simultaneous logical connections over a single physical connection.
Switched Virtual Circuits (SVC) A SVC is very 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 (This can be used much like a telephone number).
Permanent Virtual Circuits (PVC) A PVC is similar to a leased line in that the connection is always present. The logical connection is established permanently by the Packet Switched Network administration. Therefore, data may always be sent, without any call setup.
    1. The calling DTE sends a Call Request Packet, which includes the address of the remote DTE to be contacted. The destination DTE decides whether or not to accept the call (the Call Request packet includes the sender's DTE address, as well as other information that the called DTE can use to decide whether or not to accept the call).
    2. A call is accepted by issuing a Call Accepted Packet, or cleared by issuing a Clear Request Packet.
    3. Once the originating DTE receives the Call Accepted packet, the virtual circuit is established and data transfer may take place. When either DTE wishes to terminate the call, a Clear Request Racket is sent to the remote DTE, which responds with a Clear Confirmation Packet.


X.25 is a DATA PUMP: There has to be some higher level that is making sense of the bits.


<None yet>

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Maintained by Mohan Atreya , Last update April 8, 1999

First created on November 10, 1998