The Open Systems Interconnection Model
The OSI model provides a framework which can be used to better understand the nature of ISDN. The OSI model defines 7 layers of protocols, each of which is used in some way to facilitate communications across a network. Lower layers relate to physical and bit level networking protocols (e.g. physical connector and bit-level error detection definitions), while higher layers are usually associated with protocols which allow applications to communicate with each other across a network (e.g. the "language" used between an e-mail server and client).
(7) defines the protocols used for application connectivityAlthough ISDN is a technology that is defined at the 3 lowest layers of the model, it also considered as merely a physical connection, a high-speed "pipe" through which protocols at layer 2 or higher may be transferred. The reason for these dichotomous views is related to the fact that the protocols used on the different types of ISDN channels are different, D-channels being defined at layers 2 and 3, and B-channels able to carry anything at layer 2 or higher.
(6) defines a set of non-application specific protocols
(5) specifies process to process communications
(4) defines a means to communicate across a network (more than a single physical connection) in an error-free fashion
(3) specifies the protocol used to link devices that span multiple connections
(2) defines the protocol used to transfer data in an error free fashion across a single physical connection
(1) defines electrical and mechanical characteristics of the protocol used to transfer bits between devices
Ali
hussain(c 1999)