Integrated Services Digital Network

What Is ISDN?

ISDN  is the abbreviation of Integrated Services Digital Network which is a set of communications standard for instantaneous digital transmission of data, audio, video, and other services related to network, in excess of the conventional circuits of the community switched telephone network. These standards were created by the Consultative Committee for International Telegraph and Telephone (CCITT) and International Telecommunication Union. ISDN applications include high-speed image applications, additional telephone lines in home to serve the telecommuting industry, high speed file transfer, and video conferencing. ISDN also a circuit-switched service that can be used on both multipoint and point-to-point connections. And can carry voice, data, images, and video across a telephony network, using a single interface for all transmissions.

Two types of ISDN Channels:

• The B-Channel – This is known as the Bearer ("B") channel, which is a 64 Kbps channel used for voice, video, data or multimedia transfer. These can be aggregated together to give higher bandwidth utilization.

• The D-Channel – This is known as the Delta ("D") channel, which can be either 16 Kbps or 64 Kbps and is used primarily for the signaling between the switching equipment. Some say that this adds to the security of ISDN because the controlling and data channels are separate.

BRI (Basic Rate Interface)

BRI can also be known as BA (Basic Access). This operates a single 16 Kbps D channel and two 64 Kbps B channels. Although it isn't usually pointed out, the BRI total speed is 192 Kbps; this is because you have an additional 48 Kbps overhead for framing and synchronization on the D channel (64 * 2) + (16 + 48) = (128 + 64) = 192 Kbps.

PRI (Primary Rate Interface)

PRI can also be known as PA (Primary Access). This can operate in two different modes, depending on geographic location. For European locations, PRI is made up of 30 x 64 Kbps B channels and a single 64 Kbps D channel, giving a total of 2.048 Mbps, which is also known as an E1 line (or DS1). For American and Japanese locations, PRI is made up of 23 x 64 Kbps B channels and a single 64 Kbps D channel, giving a total of 1.544 Mbps, which is also known as a T1 line (or DS1). Framing and Synchronization is at 8 Kbps for T1 or 64 Kbps for E1. T1 PRI is commonly referred to as "23B+D," and E1 PRI is commonly referred to as "30B+D.

Typical ISDN Network

Figure 1: ISDN Network

In Figure 1, 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).

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 private branch exchange (PBX), or directly to the TE1 device or terminal adapter, as shown in Figure 1. 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.

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 Advantages

• The basic advantage of ISDN is to facilitate the user with multiple digital channels. These channels can operate concurrently through the same one copper wire pair.
•  ISDN provides high data rate because of digital scheme which is 56kbps.
•  ISDN network lines are able to switch manifold devices on the single line such as faxes, computers, cash registers credit cards readers, and many other devices. These all devices can work together and directly be connected to a single line.
•  ISDN takes only 2 seconds to launch a connection while other modems take 30 to 60 second for establishment.

ISDN Disadvantages

• The disadvantage of ISDN lines is that it is very costly than the other typical telephone system.
• ISDN requires specialized digital devices just like Telephone Company.

Sources:

                                http://searchenterprisewan.techtarget.com

                                http://www.tech-faq.com

                                http://www.techrepublic.com

                                http://en.wikipedia.org

                                http://public.swbell.net

http://hea-www.harvard.edu