ISDN Interface Overview

The Integrated Services Digital Network (ISDN) technology is a design for a completely digital telecommunications network. ISDN can carry voice, data, images, and video across a telephony network, using a single interface for all transmissions.

ISDN Channels

ISDN uses separate channels to transmit voice and data over the network. Channels operate at bandwidths of either 64 Kbps or 16 Kbps, depending on the type of channel.

Bearer channels (B-channels) use 64 Kbps to transmit voice, data, video, or multimedia information. This bandwidth is derived from the fact that analog voice lines are sampled at a rate of 64 Kbps (8,000 samples per second using 8 bits per sample).

Delta channels (D-channels) are control channels that operate at either 16 Kbps or 64 Kbps. D-channels are used primarily for ISDN signaling between switching equipment in an ISDN network.

ISDN Interfaces

ISDN provides two basic types of service, Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Services Routers support both ISDN BRI and ISDN PRI.

ISDN BRI is designed for high-bandwidth data transmissions through existing telephone lines. The copper wires that make up much of the existing telephony infrastructure can support approximately 160 Kbps, which provides enough bandwidth for two B-channels and a D-channel, leaving 16 Kbps for any data framing, maintenance, and link control overhead.

ISDN PRI is designed for users with greater capacity requirements than can be met with ISDN BRI. In the United States, the most common PRI supports 23 B-channels and 1 D-channel, totalling 1,536 Kbps, which is roughly equivalent to a T1 link. In Europe, the most common PRI supports 30 B-channels and 1 D-channel, totalling 1,984 Kbps, which is roughly equivalent to an E1 link.

Typical ISDN Network

Figure 17 shows a typical ISDN network.

Figure 17: ISDN Network

In Figure 17, two types of end-user devices are connected to the ISDN network:

• Terminal equipment type 1 (TE1) device—Designed to connect directly through an ISDN telephone line.
• Terminal equipment type 2 (TE2) device—Not designed for ISDN. TE2 devices—for example, analog telephones or modems—must connect to the ISDN network through a terminal adapter (TA).

A terminal adapter allows non-ISDN devices on the ISDN network.

NT Devices and S and T Interfaces

The interface between the ISDN network and a TE1 device or terminal adapter is called an S interface. The S interface connects to a network termination type 2 (NT2) device such as a PBX, or directly to the TE1 device or terminal adapter, as shown in Figure 17. The NT2 device is then connected to a network termination type 1 (NT1) device through a T interface. The S and T interfaces are electrically equivalent.

An NT1 device is a physical layer device that connects a home telephone network to a service provider carrier network. ISDN devices that connect to an NT1 device from the home network side use a 4-wire S/T interface. The NT1 device converts the 4-wire S/T interface into the 2-wire U interface that telephone carriers use as their plain old telephone service (POTS) lines.

In the United States, NT1 devices are user owned. In many other countries, NT1 devices are owned by the telephone service providers.

U Interface

The U interface connects the ISDN network into the telephone switch through line termination (LT) equipment. The connection from LT equipment to other switches within the telephone network is called the exchange termination (ET).

ISDN Call Setup

Before traffic can pass through an ISDN network, an ISDN call must be set up. ISDN call setup requires a Layer 2 connection to be initialized and then a Layer 3 session to be established over the connection.

To specify the services and features to be provided by the service provider switch, you must set service profile identifiers (SPIDs) on TE1 devices before call setup and initialization. If you define SPIDs for features that are not available on the ISDN link, Layer 2 initialization takes place, but a Layer 3 connection is not established.

Layer 2 ISDN Connection Initialization

The TE device and the telephone network initialize a Layer 2 connection for ISDN as follows:

1. The TE device and the telephone network exchange Receive Ready (RR) frames, to indicate that they are available for data transmission. A call cannot be set up if either the TE device or telephone network does not transmit RR frames.
2. If both ends of the ISDN connection are available to receive data, the TE device sends an Unnumbered Information (UI) frame to all devices on the ISDN link.
3. When it receives the UI frame, the network responds with a message containing a unique terminal endpoint identifier (TEI) that identifies the endpoint on the ISDN link for all subsequent data transmissions.
4. When the TE device receives the TEI message, it sends out a call setup message.
5. The network sends an acknowledgement of the call setup message.
6. When the TE device receives the acknowledgement, a Layer 2 connection is initialized on the ISDN link.

Layer 3 ISDN Session Establishment

The caller, switch, and receiver establish a Layer 3 ISDN connection as follows:

1. When a Layer 2 connection is initialized, the caller sends a SETUP message to the switch in the telephone network.
2. If the setup is message is valid, the switch responds with a call proceeding (CALL PROC) message to the caller and a SETUP message to the receiver.
3. When the receiver receives the SETUP message, it responds with an ALERTING message to the telephone switch.
4. This ALERTING message is then forwarded to the caller.
5. The receiver then accepts the connection by sending a CONNECT message to the switch.
6. The switch forwards the CONNECT message to the caller.
7. The caller responds with an acknowledgement message (CONNECT ACK).
8. When the CONNECT ACK message is received by the receiver, the ISDN call is set up and traffic can pass.