What is ISDN?

     ISDN stands for Integrated Services Digital Network. It is a design for a completely digital telephone/telecommunications network. It is designed to carry voice, data, images, video, everything you could ever need. It is also designed to provide a single interface for hooking up your phone, your fax machine, your computer, your videophone, your video-on-demand system , and your microwave. ISDN is about what the future phone network, and information superhighway, will look like

When we say:

a.) "Integrated Service"  refers to ISDN ability to deliver two simultaneous connection, in any combination of voice,fax,data,and  video over a single line.

b.)"Digital" refers to the fact that it is purely digital transmission , as opposed to the analog transmission method used by conventional transmission lines.

c)"Network" refers to the fact that  ISDN is not simply a POINT-to-POINT connection like a lease telephone line. ISDN network extends from the local telephone exchange to the remote user.

ISDN devices:

Terminal Adapter (TA) - Converter device that converts standard electrical signals into the form used by ISDN - allows non-ISDN devices to operate on an ISDN network.


Terminal Equipment Type 1 (TE1) - Compatible with the ISDN network.  Example:Telephones, personal computers, fax machine or videoconferencing machine.


Terminal Equipment Type 2 (TE2) - Not compatible with the ISDN network. Example: Analog phone or modem, requires a TA (TE2 connects to TA).


Network termination type 1 & 2 (NT1 and NT2) - A  small connection box that physically connects the customer site to the telco local loop, provides a four-wire connection to the customer site and a two-wire connection to the network (PRI – CSU/DSU).

ISDN Components and Reference Points:

U - Two wire cable that connects the customer’s equipment to the telecommunications provider

R - Point between non-ISDN equipment (TE2) and the TA

S - Four-wire cable from TE1 or TA to the NT1 or NT2

T - Point between NT1 and NT2

NT-1 (Network Terminator-1)
An NT-1 is an interface box that converts ISDN data into something a PC can understand (and vice versa). It works a little like a cable TV descrambler for ISDN signals, and is often built into ISDN adapters.


TA (Terminal Adapter)
This chunk of hardware converts the data it receives over ISDN to a form your computer can understand.  Sometimes mistakenly called an ISDN modem or a digital modem, a terminal adapter handles data digitally and does not need to modulate or demodulate an analog signal. Terminal adapters can be an internal board or an external board that connects to the computer through the serial port.


ISDN and the OSI Reference Model :



*The ISDN Physical Layer
*The ISDN Data Link Layer
*The ISDN Network Layer 


ISDN Protocols



*E-series protocols—Telephone network standards for ISDN.
*I-series protocols—Specify ISDN concepts and interfaces.
*Q-series protocols—Standards for ISDN switching and signaling.
*Operate at the physical, data link, and network layers of the OSI reference model








How Does ISDN work?


ISDN allows data to be transmitted and received around the world using end-to-end digital connectivity. With ISDN, voice and data are carried by 64Kbps bearer channels (B channels). Multiple B channels can cooperate to provide higher data rates. Common ISDN installations can easily be configured to use two (2) B channels, providing a true data rate of 128Kbps. In all ISDN installations, a data channel (D channel) handles control signalling at 16 kbps or 64 kbps, depending on the service type.






ISDN Protocols:



a.) E - series for Telephone network and ISDN
b.) I -  series for ISDN concepts, aspects and     interfaces





Types of Channels:


A.) Bearer channel (B-channel=64 kb/s) clear pipe for data
B.) Delta channel (D-channel, 16 kb/s or 64 kb/s) call signaling information:
   1.) who is calling
   2.) type of call
   3.) calling what number

How it is configured?

          The figure illustrates a sample ISDN configuration and shows three devices attached to an ISDN switch at the central office. Two of these devices are ISDN-compatible, so they can be attached through an S  reference point to NT2 devices. The third device (a standard, non-ISDN telephone) attaches through  the

reference point to a TA. Any of these devices also could attach to an NT1/2  device,  which  would  replace

both the NT1 and the NT2. In addition, although they are not shown, similar user stations are attached to the far-right ISDN switch.

ADVANTAGES:

:

a.)  One of the key advantage of ISDN is it's flexibility- you can use it in several different ways , depending on your needs.

for example:

* if you want simultaneous voice and data, simply assign a b- channel to each.

*if you want higher bandwidth for ypur data combine b-channels to increase the bandwidth.

b.)The Speed – whereas the normal line runs at 56kbs the ISDN can run on 64K and can also use its dual channels bursting up to 128 K

c.)The Multiple Devices  that is used- Before you needed a phone line for every device you use such as a telephone, fax and computers. With the ISDN you can use all of them on the same ISDN device

DISADVANTAGES:

a.)  It is very costly than the other typical telephone system.

b.) Requires specialized digital devices just like Telephone Company.

references: 

http://www.pulsewan.com/data101/pdfs/isdn.pdf

http://www.tandberg.com/collateral/white_papers/whitepaper_intro_to_ISDN.pdf

http://www.conceptron.com/articles/pdf/isdn.pdf