Networking / Beginners

ISDN Devices

The phrase ISDN standard refers to the devices that are required to connect the end node to the network. Although some vendors provide devices that include several functions, a separate device defines each function within the standard. The protocols that each device uses are also defined and are associated with a specific letter. Also known as reference points, these letters are R, S, T, and U. ISDN standards also define the device types. They are NT1, NT2, TE1, TE2, and TA.

ISDN Reference Points

Reference points are used to define logical interfaces. They are, in effect, a type of protocol used in communications. The following list contains the reference points:

  • R Defines the reference point between a TE2 device and a TA device.
  • S Defines the reference point between TE1 devices and NT1 or NT2 devices.
  • T Defines the reference point between NT1 and NT2 devices.
  • U Defines the reference point between NT1 devices and line termination equipment. This is usually the central switch.

Network terminator 1 (NT1) is the device that communicates directly with the central office switch. The NT1 receives a U-interface connection from the telephone company and puts out a T-interface connection for the NT2. NT1 handles the physical layer portions of the connection, such as physical and electrical termination, line monitoring, and multiplexing.

Network terminator 2 (NT2) is placed between an NT1 device and any adapters or terminal equipment. Many devices provide the NT1 and NT2 devices in the same physical hardware. Larger installations generally separate these devices. An example of an NT2 device is a digital private branch exchange (PBX) or ISDN router. An NT2 device provides an S interface and accepts a T interface from an NT1. NT2 usually handles data link and network layer functions, such as contention monitoring and routing, in networks with multiple devices.

Terminal equipment 1 (TE1) is a local device that speaks via an S interface. It can be directly connected to the NT1 or NT2 devices. An ISDN telephone and an ISDN fax are good examples of TE1 devices.

Terminal equipment 2 (TE2) devices are common, everyday devices that can be used for ISDN connectivity. Any telecommunications device that is not in the TE1 category is classified as a TE2 device. A terminal adapter is used to connect these devices to an ISDN and attaches through an R interface. Examples of TE2 devices include standard fax machines, PCs, and regular telephones.

A terminal adapter (TA) connects TE2 devices to an ISDN. A TA connects through the R interface to the TE2 device and through the S interface to the ISDN. The peripheral required for personal computers often includes an NT1 device, better known as ISDN modems.

ISDN modems are used in PCs to connect them to an ISDN network. The term modem is used incorrectly here. ISDN passes data in a digital format, so there is no need to convert the digital data from the computer the way a traditional modem converts the digital data to analog so that it can travel. Conventional modems convert analog to digital and vice versa.


Standard telephone lines use a ten-digit identifier, better known as a telephone number, which is permanently assigned. ISDN uses similar types of identifiers, but they are not as easily used as a telephone number. ISDN uses five separate identifiers to make a connection. When the connection is first set up, the provider assigns two of these: the service profile identifier (SPID) and the directory number (DN). These are the most common numbers used because the other three are dynamically set up each time a connection is made. The three dynamic identifiers are the terminal endpoint identifier (TEI), the service address point identifier (SAPI), and the bearer code (BC).

The directory number (DN) is the ten-digit phone number the telephone company assigns to any analog line. ISDN services enable a greater degree of flexibility in using this number than analog services do. Unlike an analog line, where a one-toone relationship exists, the DN is only a logical mapping. A single DN can be used for multiple channels or devices. In addition, up to eight DNs can be assigned to one device. Because a single BRI can have up to eight devices, it can support up to 64 directory numbers. This is how offices are able to have multiple numbers assigned to them. Most standard BRI installations include only two directory numbers, one for each B channel.

The service profile identifier (SPID) is the most important number needed when you are using ISDN. The provider statically assigns the SPID when the ISDN service is set up. It usually includes the DN plus a few extra digits. The SPID usually contains between 10 and 14 characters and varies from region to region. SPIDs can be assigned for every ISDN device, for the entire line or for each B channel.

The SPID is unique throughout the entire switch and must be set up correctly. If it is incorrect, the result is like dialing the wrong phone number-you will not be able to contact the person you are trying to reach. When an ISDN device is connected to the network, it sends the SPID to the switch. If the SPID is correct, the switch uses the stored information about your service profile to set up the data link. The ISDN device will not send the SPID again unless the device is disconnected from the network.

A terminal endpoint identifier (TEI) identifies the particular ISDN device to the switch. This identifier changes each time a device is connected to the ISDN. Unlike the SPID or the DN, the TEI is dynamically allocated by the central switch.

The service address point identifier (SAPI) identifies the particular interface on the switch to which your devices are connected. This identifier is used by the switch and is also dynamically updated each time a device connects to the network.

The bearer code (BC) is an identifier made up of the combination of TEI and SAPI. It is used as the call reference and is dynamic, like the two identifiers included within it. It changes each time a connection is established.

Advantages of ISDN

ISDN offers several major advantages over conventional analog methods. First, it has a speed advantage over normal dial-up lines. Normal dial-up lines use a modem to convert the digital signals from a PC into analog. This enables data to be transferred over public phone lines. This technology does, however, have speed limitations. The fastest standard modem connection that is currently available is 56 Kbps. Given that this is an analog connection, many modems cannot reach this speed, because they are limited by the quality of the connection. This fact accounts for your connecting at different speeds each time you dial in to a remote network. Because phone lines cannot actually transmit at 56 Kbps, a special kind of compression is used to enable these speeds. Two standards are used currently. In order to satisfy all users, ISPs must support both standards, which quickly becomes expensive.

ISDN enables you to use multiple digital channels at the same time to pass data through regular phone lines. The connection made from your computer, however, is completely digital; it is not converted to analog. You can use other protocols that enable you to bind channels together to get a higher bandwidth rate. In addition, ISDN makes a connection in half the time of an analog line.

In addition to speed, ISDN supports multiple devices set up in one link. In an analog system, a single line is required for each device that is attached. For example, a separate phone line is needed for a normal phone, a fax machine, or a computer modem. Since ISDN supports multiple devices, you can use each one of these items on a single line. The connection will also be clearer because the data is being passed in digital format.

Because ISDN uses a separate channel-the D channel for signaling information- it removes the administrative overhead from the B channel so that it can focus on carrying just the data signals. This means that the data is not hindered by the session setups and the signaling information that maintains the session that is required by the devices for communication. The D channel keeps all this information off the data streams (also known as "out of band signaling"). Because of this separation, the setup and takedown of each session is much faster. In addition, ISDN equipment is able to handle calls more efficiently.

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