# Integrated Services Digital Network (ISDN)

Integrated Services Digital Network (ISDN) is a continually evolving international standard for networking services, including voice and non-voice services. The network is completely digital from one end to the other. Voice information is digitized and sent in digital form. Signaling information is sent separately from voice information, using a method called common channel signaling (CCS)
.

This topic describes:

- ISDN protocols and protocol layering

- Information exchange between layers

## 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 chained layers are:

Name | Number | Description
Physical layer | Layer 1 | The electrical and mechanical layer. Protocols for this layer describe, on an electrical and mechanical basis, the methods used to transfer bits from one device to another. One 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. One 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. One 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, user interfaces, and applications.

The following illustration shows the hierarchy of layers:

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. A unique service access point identifier (SAPI) is associated with each SAP.

## Information exchange between layers

Cooperation between entities on the same layer is governed by a peer-to-peer protocol specific to the layer and the entity. To exchange information between two or more layer entities, a connection must be established between the layer entities using the protocol of the layer directly below. Connections are provided by a layer between two or more SAPs.

Data message units are conveyed between peer-to-peer entities at the lowest layer by means of a physical connection. Layer (n+1) requests services from layer n through primitives. These primitives allow the logical exchange of information and control between two adjacent layers.

The following illustration shows the message primitives exchanged between layers:

Four types of primitives are exchanged between adjacent layers:

Primitive type | Example | Description
REQUEST (RQ) | ACU_CONN_RQ | A layer issues this type to request a service from the layer directly below it.
INDICATION (IN) | ACU_CONN_IN | A layer providing a service issues this primitive type to notify the layer above it of any specific activity that is related to the service. An INDICATION that a layer receives may be the result of an activity performed by the layer directly below it that is related to a REQUEST given by a peer entity.
RESPONSE (RS) | ACU_CONN_RS | A layer issues this primitive type to acknowledge the receipt of an INDICATION from a lower layer.
CONFIRM (CO) | ACU_CONN_CO | A layer providing a requested service issues this primitive type to confirm that the activity completed.