Describe the ISDN

Q 5 (a). Describe the ISDN ?

Integrated Services Digital Network (ISDN)

Integrated Services Digital Network (ISDN) is a telephone system network. It is a wide area network which is widely available. Before to the ISDN, the telephone system was viewed as a way to transport only voice, with some special services available for data. The key feature of the ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system.

Another major market application is Internet access, where ISDN typically provides a maximum of 128 Kbit/s in both upstream and downstream directions (which can be considered to be broadband speed, since it exceeds the narrowband speeds of standard analog 56k telephone lines). 

There are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates divide their capacity across a number of channels:

• B-channels carry payloads (e.g., data or voice streams) 
• D-channels carry control and signaling information.

Basic Rate Interface

The entry level interface to ISDN is the Basic Rate Interface (BRI), a 128 kbit/s service delivered over a pair of standard telephone copper wires. The 144 kbit/s payload rate is broken down into two 64 kbit/s bearer channels ('B' channels) and one 16 kbit/s signaling channel ('D' channel or data channel). This is sometimes referred to as 2B+D.

The interface specifies the following network interfaces:

• The U interface is a two-wire interface between the exchange and a network terminating unit, which is usually the demarcation point in non-North American networks. 
• The T interface is a serial interface between a computing device and a terminal adapter, which is the digital equivalent of a modem. 
• The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on a Network termination 1 (NT1). 
• The R interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.

Primary Rate Interface

The other ISDN access available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 'B' channels of 64 kbit/s, one 'D' channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s. This is often referred to as 30B+2D. 

In North America PRI service is delivered on one or more T1 carriers (often referred to as 23B+D) of 1544 kbit/s (24 channels). A PRI has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1). Inter-changeably but incorrectly, a PRI is referred to as T1 because it uses the T1 carrier format. A true T1 (commonly called "Analog T1" to avoid confusion) uses 24 channels of 64 kbit/s of in-band signaling. Each channel uses 56 kb for data and voice and 8 kb for signaling and messaging. PRI uses out of band signaling which provides the 23 B channels with clear 64 kb for voice and data and one 64 kb 'D' channel for signaling and messaging. In North America, Non-Facility Associated Signalling allows two or more PRIs to be controlled by a single D channel, and is sometimes called "23B+D + n*24B". D-channel backup allows for a second D channel in case the primary fails. NFAS is commonly used on a T3.

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