Understanding ISDN

Integrated Services Digital Network (ISDN) is a set of standards for digital transmission over different media created by the Consultative Committee for International Telegraph and Telephone (CCITT) and International Telecommunication Union (ITU). ISDN is an all-digital dialup (on-demand) service that carries voice, data, and video transmissions over telephone lines. As a dial-on-demand service, it has fast call setup and low latency as well as the ability to carry high-quality voice, data, and video transmissions. ISDN is also a circuit-switched service that can be used on both multipoint and point-to-point connections. ISDN can carry voice, data, images, and video across a telephony network, using a single interface for all transmissions.

ISDN connectivity is supported as a backup for a primary Internet connection. J Series devices can be configured to fail over to an ISDN interface when the primary connection experiences interruptions in Internet connectivity.

You can also use ISDN at the central office to terminate calls that originate at branch office routers and for central office callback for security, accounting, or cost savings at the branch office.

This topic contains the following sections:

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:

Typical ISDN Network

Figure 8 shows a typical ISDN network.

Figure 8: ISDN Network

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In Figure 8, two types of end-user devices are connected to the ISDN network:

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 8. 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.

A service profile identifier (SPID) is a number that specifies the services available to you on the service provider switch and defines the feature set ordered when the ISDN service is provisioned.

A terminal endpoint identifier (TEI) is a number that identifies a terminal endpoint, an ISDN-capable device attached to an ISDN network through an ISDN interface on the device. The TEI is a number between 0 and 127. The numbers 0–63 are used for static TEI assignment, 64–126 are used for dynamic assignment, and 127 is used for group assignment.

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 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.

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