This article introduces the main components of ISDN. This includes, but is not limited to, the following topics:

• What are integrated services?

• Advantages of ISDN

• ISDN services

• ISDN bandwidth and channels

What Are Integrated Services?

Since the 1960s, the telecommunication networks backbone has been converting to digital. The end-user access, however, such as the telephone and modem connections, has remained mostly analog. ISDN takes advantage of the digital telecommunications backbone and replaces some of the analog service devices with new higher-speed digital equipment. So the beauty of ISDN is that it makes use of the existing backbone technology while enhancing it with cost-effective higher-speed services that were previously unavailable or unjustifiably expensive.

When the digital network is extended end-to-end by ISDN, it eliminates the need to translate (or sample) the analog waveform into a digital pattern. This allows any application, whether voice, video, or data, to transparently transmit over the backbone, because there is no longer a need to differentiate between the various types of network traffic. As a result, diverse sets of services can be integrated into one cost-effective solution.

Advantages of ISDN

ISDN provides a viable alternative to various forms of communication while allowing reliable high-speed access to the Internet and other services. The table below demonstrates how ISDN compares to a few of these forms of communication.

ISDN Services

As mentioned, ISDN can provide a number of different services:

• Data A widely used ISDN service, referring to the payload type of the ISDN packet. Has an end-to-end synchronous signal.

• Rate adaptation Allows incompatible equipment to use the ISDN network for data communication. For instance, devices that do not support synchronous connections or 64 kbps speeds nonetheless can use ISDN services. The two rate adaptation standards are as follows:

  - V.110 Can be applied to synchronous and asynchronous applications. It has no error detection or correction. It is based on TDM technology. The frame format lets flags and control bits accommodate different source speeds.
  
  

  - V.120 Can be applied to synchronous and asynchronous applications. Unlike V.110, it allows error detection and correction. It is based on STDM (HDLC) technology.
  
  

• Voice Analog or asynchronous data transfer over asynchronous modems.

• DNIS Identifies a called party number.

• CLID Identifies a calling party number.

ISDN Bandwidth and Channels

The discussion of ISDN revolves around two variations: BRI and PRI. Before we begin, let's examine the North American digital signal standards and their "T" assignments, because BRI and PRI adhere to those standards. You will also learn the European equivalents of their North American counterparts.

The table below shows the DS level, its corresponding maximum speed, the "T" designation, and the number of channels for each level.

ISDN-BRI

BRI specifies the following components:

• It is made up of three DS0s.

• It has two B channels at 64 kbps each, used for data.

• It has one D channel at 16 kbps, used for signaling.

• The remaining 48 kbps is used for framing and synchronization.

• The total speed is measured as follows:

  64 + 64 + 16 + 48 = 192

ISDN-PRI

North American PRI specifies the following components:

• It is made up of DS1 (T1) with 24 channels.

• It has 23 B channels at 64 kbps each, used for data.

• It has one D channel at 64 kbps, used for signaling, carried in timeslot 24.

• The remaining 8 kbps is used for framing and synchronization.

• The total speed is measured as follows:

  (23 * 64) + 64 + 8 = 1544 kbps

• Two encoding schemes are possible: AMI and B8ZS.

• Two separate types of framing are defined: Super-Frame (SF) and Extended Super-Frame (ESF).

European and other countries' PRI specifies the following components:

• It is made up of E1, the equivalent of T1, with 32 channels.

• It has 30 B channels at 64 kbps each, used for data.

• It has one D channel at 64 kbps, used for signaling, carried in timeslot 16.

• The remaining 64 kbps is used for framing and synchronization.

• The total speed is measured as follows:

  (30 * 64) + 64 + 64 = 2048 kbps

• Encoding is HDB3.

• Framing is multiframe.

BRI Functional Groups

BRI defines the following functional groups (ISDN devices):

• TE1 Terminal equipment 1. Specifies an ISDN-compatible device. Can connect to an NT1 or NT2 device (described in this list). Examples of a TE1 device include

  - Router with a native ISDN interface

  - Digital telephone

  - Digital fax

• TE2 Terminal equipment 2. Specifies a device that is not ISDN-compatible. Requires a terminal adapter (described next) for compliance with ISDN. TE2 equipment examples include

  - Router with no native ISDN interface

  - Devices with X.21, X.25, or EIA/TIA-232 interfaces

• TA Terminal adapter. Used with TE2 to convert electrical signals into the kind recognized by ISDN.

• NT1 Network Termination 1. Links four-wire ISDN customer wiring to the two-wire provider facility.

• NT2 Network Termination 2. Specifies a device that manages traffic to and from subscriber devices and the NT1. Performs switching and concentrating.

• LT Line Termination. Specifies a provider's side. Functions as an NT1.

• ET Exchange Termination. Specifies a line card of a subscriber in the ISDN exchange.

• LE Local Exchange. Specifies LT and ET. It is a provider's ISDN switch.

Which Devices Represent the BRI Reference Points

Reference points are interfaces between functional groups. They might or might not manifest in actual physical interfaces. Reference points include the following:

• U User reference point. Between NT1 and LT.

• T Terminal reference point. Between NT1 and NT2, or between NT1 and TE1 (or TA) if no NT2 is present.

• S System reference point. Between NT2 and TE1 (or TA). Has the same characteristics as the T interface.

• R Rate reference point. Between TA and TE2.

Let's spend a few moments discussing how functional groups and reference points work together.

First, you connect the wall jack to the NT1 with a standard two-wire cable. Then you connect the NT1 to an ISDN terminal or a terminal adapter with a four-wire connector. An eight-wire connector is used for the S/T interface because it requires both NT and TE capabilities.

An S/T interface is a combination of the S and T interfaces. It defines a reference point between a TE1 (or TA) and an NT. You can think of it as a point-to-multipoint bus that multiple ISDN devices can share.

The U interface is a two-wire interface between the NT and the provider cloud normally terminated with an eight-pin RJ-48 connector. In this case, the NAS has built-in NT1 functionality. U interface termination is mostly used in North America.

As far as the Cisco IOS is concerned, there is no real difference between the S/T or U termination when it comes to BRI operation. What you have to keep in mind is that BRI consists of a single D channel for signaling and two B channels for data.