# ISDN

Integrated services digital network (ISDN) is a continually evolving international standard for networking a wide range of 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 presents the following information:

- ISDN protocols and protocol layering

- ISDN carriers

- Specificity of BRI boards

## 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 (refer to the following illustration). The first three layers, called the chained layers, are the lowest levels. The chained layers are:

Layer | Description
--- | ---
1 - Physical | 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.
2 - Data link | 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).
3 - Network | 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.

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

The layer 3 protocol is not a symmetric protocol. There is a network side and a terminal side. Typically, applications that act as a network node must use ISDN network side stacks. Applications that act as a terminal (for example, IVR, or any application which is connected on the PSTN) must use ISDN terminal side stacks.

## ISDN carriers

ISDN is transmitted over T1 carriers, E1 carriers, and BRI (basic rate interface) carriers. These are typically four-wire digital transmission links.

With basic-rate ISDN, the channels are usually used as follows:

- Two channels carry data digitized at 64 Kbps: voice, audio, data and/or video signals. These channels are called bearer channels (B channels).

- One channel carries signaling information for the two B channels. This is called the D channel.

AG 2000-BRI configuration

## Specificity of BRI boards

The AG 2000-BRI board is designed to work with two types of CCITT I.430 BRI configurations, generally known as S-bus.

The following illustration shows the first type of S-bus configuration which is called point-to-point configuration. The AG 2000-BRI board can be connected either at the terminal or at the network side.

Point-to-point configuration

The following illustration shows the second type of S-bus configuration which is called point-to-multipoint configuration. One of the four BRI trunks of the AG 2000-BRI board shares the S-bus with other terminal equipment devices (such as other AG 2000-BRI board trunks or ISDN phones, fax machines, PC for data transfer). The S-bus supports up to eight terminal equipment devices.

When several terminals share the S-bus, they share the two B channels available on the S-bus. Only two calls can be handled simultaneously. When calls arrive from the network, the terminals are notified but only one establishes the incoming call. In point-to-multipoint configurations, bus contention issues must be addressed.

Note: In this example, each BRI trunk of an AG 2000-BRI board is terminal equipment. It provides a standardized connector called the S-interface as a connection to the network. Since the S-bus is standard, all terminal equipment providing an S-interface (a standardized connector to connect to the network), can be connected to a PBX or network.

Point-to-multipoint configuration