Integrated Services Digital Network (ISDN)

Integrated Services Digital Network

In contrast to Signaling System 7, the ISDN primary rate interface (PRI) is designed as an access protocol, and is therefore also known as primary rate access (PRA).

Figure 1. ISDN as an access protocol
The graphic shows a caller connecting to Blueworx Voice Response via an ISDN trunk. The telephone company's network consists of three switches linked by voice circuits to each other, to the caller and to Blueworx Voice Response. Each switch also connects with a service control point through the SS7 protocol.

An ISDN trunk is a T1 or E1 trunk with one of the timeslots dedicated to signaling: this is called the D-channel. (The D-channel signaling protocol is based on the ITU-T Q.921 and Q.931 recommendations.) The remaining timeslots are used for voice and are called B-channels (see Figure 2). ISDN trunks are generally used between an external subscriber and a switch within a network. ISDN will eventually displace channel associated signaling (that is, T1 and E1 trunks) as the means by which primary-rate subscribers attach to SS7-controlled networks.

Figure 2. ISDN B-Channels and D-Channels. This example shows one digital trunk. There are 30 B-channels on an E1 trunk; 23 B-channels on a T1 trunk.
The channels are shown as direct links by one D-channel and several B-channels from a switch to Blueworx Voice Response.
Figure 3. Attaching Blueworx Voice Response as an intelligent peripheral in North America
In this Figure the caller is shown connecting to the telephone company's network via CAS or ISDN access protocol, or in analog mode. The switches within the network are linked with a service control point through the SS7 protocol and relay voice circuits to Blueworx Voice Response through ISDN.

In addition to being used as an access protocol, in the U.S., primary rate ISDN is used to attach intelligent peripherals within an operating company's advanced intelligent network (as shown in Figure 3). Blueworx Voice Response can either function as an intelligent peripheral or provide voice processing function as part of a larger service node.

Advantages of ISDN

The usual justification for ISDN is that a single digital channel can be used for several different types of communication services, for example, voice, X.25, video, and so on. However, for Voice Response Units such as Blueworx Voice Response, ISDN offers a further set of benefits even if the Voice Response Unit is only ever used for voice:

ISDN standards supported

The optional ISDN features support:

There is an optional feature of Blueworx Voice Response to interface to the network over an ISDN primary rate interface that conforms to each of the following standards:

For more detailed information about how Blueworx Voice Response supports ISDN protocols, refer to the Blueworx Voice Response for AIX: General Information and Planning information.

Parent topic: Common channel signaling