# ISDN protocols and protocol layering

ISDN communications can be described at many levels, from the way bits are transferred from machine to machine to the sets of messages computers pass to one another. A scheme for communication at a certain level is called a protocol.

In the late 1970's, the International Standards Organization (ISO) established the Open Systems Interconnect (OSI) model for communication. ISDN is based on this model. In OSI, seven separate levels, or layers, of communication are defined. The first three layers, called the chained layers, are the lowest levels. The following table describes the chained layers:

Layer | Description
Physical layer (layer 1) | The electrical and mechanical layer. Protocols for this layer describe, from an electrical and mechanical perspective, the methods used to transfer bits from one device to another. A protocol used at this layer is CCITT recommendation I.430/I.431.
Data link layer (layer 2) | The layer above the physical layer. Protocols for this layer describe methods for error-free communication between devices across the physical link. A protocol used at this layer is CCITT recommendation Q.921, also known as Link Access Procedures on the D channel (LAPD).
Network layer (layer 3) | The layer above the data link layer. Protocols for this layer describe methods for transferring information between computers. They also describe how data is routed within and between networks. A protocol used at this layer is CCITT recommendation Q.931.

Layers higher than these are end-to-end layers. They describe how information is exchanged and delivered end-to-end. They also define process-to-process communication, and describe application-independent user services, as well as user interfaces and applications.

The following illustration shows the OSI protocol layering model:

The functionality provided by a layer includes the services and functions of all of the layers below it. A service access point (SAP) is the point at which a layer provides services to the layer directly above it. Each SAP is associated with a unique service access point identifier (SAPI).