The Indian corporate scenario is increasingly getting networked and most of us have heard about ISDN, an abbreviation for Integrated Services Digital Network.

ISDN has been a much-maligned telecommunications service. It has been saddled with secondary meanings of “I Still Don’t Need it”, and “I Still Don’t Know”.

In simple terms, ISDN is a replacement for the plain old telephone service (POTS), which was never designed to meet the needs of the information age. ISDN incidentally uses the same wiring that currently serves homes and business environment.

ISDN has been available globally for almost more than a decade. But it was introduced only about two years ago in India. Things have moved rapidly since then and ISDN is fast becoming the connectivity of choice, especially for the networked corporate user in the metros.

In India, it is the Department of Telecom Services (DTS), that manages policy related to high speed telecommunication lines. The DTS claims to have more than 15,000 ISDN lines installed to date and these are expected to grow multifold.

What exactly is ISDN?

Let’s first understand what ISDN means and how it works. ISDN refers to communication over telephone lines using digital signals. In plain and simple terms, it is a totally digital phone line. Unlike normal analog phone lines that use voice signals to transfer data, ISDN uses a digital communications line that enables voice, data, video, and graphics to be sent over a single, ordinary telephone line, at speeds up to 128 Kbps. ISDN is thus ideal for high-speed access to the Internet that is about 4 times faster than a 28.8 Kbps modem. Even voice calls in an ISDN system use a method of converting voice to digital signals before passing them through the channels.

Lets see the technical side

The ISDN line consists of the same twisted pair of wires which were traditionally used for analog telephones.

Two types of ISDN channels are defined: B and D. The B channel is a 64 Kbps clear channel that can carry any digitized data, voice, text, image, or video. A clear channel means no signaling information is sent; it is an open communication line. The D channel, used for signaling information, can operate at either 16 Kbps or 64 Kbps.

B-Channels

B-channels are logical “pipes” in a single ISDN line. Two B-channels can be combined to download data at very high speeds on to a PC. One of the B-channels can also be made free for another device, such as a telephone or fax machine, with the remaining B-channel used for data transfer. Each provides a 64 Kbps clear channel, whose entire bandwidth is available for data, since the call setup and other signaling, is done through a separate D-channel.

D-channel

The ISDN D-channel is used mostly for administrative signaling to instruct the carrier to set up or terminate a B-channel’ call, to ensure that a B-channel is available to receive a call, or to provide signaling information for such features as caller identification. The D-channel uses packet switched connections, which are best suited to the intermittent but latency sensitive nature of signaling traffic, thus accounting for the vastly reduced call setup time of one to two seconds on ISDN calls, as against 10-40 seconds using an analog modem. The D-channel provides for either 16 Kbps (for BRI service) or 64 Kbps (for PRI service).

Basic Rate Interface (BRI)

In its most basic form (called BRI or Basic Rate Interface), ISDN consists of two 64 Kbps “Bearer channels” and a single 16 Kbps delta (or data) channel that is used for line signaling, synchronization, etc (hence the term 2B+D). The two B- (bearer) channels are equivalent to having two normal analog phone lines. The BRI thus provides a total of 144 Kbps of bandwidth. When an user requests the telecommunications company (popularly known as a ‘telco’ in the US) for an ISDN line, he always gets a BRI. The user actually gets two phone lines. These can either be used simultaneously for the same job (for example, getting a 128 Kbps connect to the Internet) or for two totally different jobs (for example, a 64 Kbps connect to the Net, and a fax being sent at the same time). Naturally, if you are using both lines simultaneously you have to pay for two calls, not one. Please note that the user cannot utilize the 16 kbps delta (or data) channel - It’s dedicated for the use of the telco. Up to eight ISDN devices can be connected to a single BRI line (but practically users connect to not more than two or three devices), and can all share the B -channels and D -channels.

Primary Rate Interface (PRI)

The ISDN PRI includes 23 B-channels in North America & Japan plus one 64 Kbps D-channel. The number of B -channels is limited by the size of the standard trunk line, which is T1 in North America & Japan and E1 in Europe and India. In the United States and Japan, the PRI (Primary Rate) access is equivalent to a 1,544 Mbps T1 circuit, and it supports 23 B-channels plus one 64 Kbps D-channel, usually written as 23B+D.

In Europe, the PRI is 30-B -channels plus one D -channel, since the European equivalent to T1 is a 2,048 Mbps E1 circuit. The European PRI is typically written as 30B+D and it is this system that the Dept. of Telecom Services has chosen to implement in India.

Does a user require special equipment for ISDN?

The short and straight answer is “Yes”. Your 33.6 or 56.6 Kbps modem is quite useless for working on a digital line. So are the normal telephone instruments. If you are working with an EPABX, that too will not work on ISDN directly. However, all of them can be modified to work with ISDN. In case you wish to use your ISDN for connecting to the Interent, you will need a device called Terminal Adapter (TA). This TA performs a function similar to the modems, only difference is the fact that whereas the modem converts digital to analog and vice-versa, the TA converts digital in one format to digital in another.

We will now look at some commonly used ISDN devices in more detail All devices that connect to an ISDN line are called TE-I’s (Terminal Equipment - I). All analog devices like telephones and fax machines are called TE-II (Terminal Equipment II). These analog devices can be used on an ISDN line through TE-I devices. For example, you could connect an analog telephone to an ISDN line through a Terminal Adapter.

Terminal adapter

A Terminal Adapter (TA) is to ISDN what a modem is to a telephone line. A TA connects your system to the ISDN line. But, unlike the modem, a TA does not need to convert analog signals into digital and vice versa, since ISDN is already a digital connection. Like a modem, a TA connects to the Net by connecting to your machine’s serial port, and has drivers that need to be installed. A TA uses simple dial-up networking. However, if you’re using a 128 Kbps TA, then you’ll need to enable the multi-link option in your DUN (Dial-Up Networking).

Network Termination Device (NT1)

Once you get your ISDN connection, your local telecom department will extend an ISDN line called a U-interface, upto your premises. This interface connects to a switch at the telephone exchange and has a single pair of wires that can handle full-duplex communication for a single ISDN device. At your end you need to terminate the line with an NT1. Usually, it is not recommended to directly connect another ISDN device to the U-interface. ISDN wiring and connection beyond the NT1, in your premises, is referred to as S/T or S-bus interface. The NT1 converts the two-wire, U-interface into 4-wire S/T interface or the S-bus. This provides two pairs of wires for transmitting and receiving, so that multiple devices can be attached for performing full-duplex communication.The telecom department usually provides this device, but you can also purchase your own. In effect, the NT1 box actually serves as a multiplexer that allows multiple ISDN devices to share the line. Without this device, your ISDN line functions like any ordinary phone line.

ISDN Router

In case you want to share your ISDN connection with other machines on a network, one way is to use a terminal adapter and a software proxy server. However, on a network it is advisable to use an ISDN router because of better manageability and lower costs.

An ISDN router is similar to a hardware proxy server using analog telephone lines. It performs all the functions of a TA and goes beyond that. Like a TA, it acts as a gateway to the Internet. The other advantage is that the router connects directly to your network and does not need a dedicated machine for maintaining it. The ISDN router is used often when a corporate LAN has to be provided remote access over ISDN.

Videoconferencing equipment & other devices

There are various other products that you can connect to your ISDN line. These include a feature phone, a video telephone, G4 fax terminal, an ISDN PC card, etc. As ISDN is digital, it would not be compatible with your ordinary analog telephone instruments. So you will need an ISDN feature phone. This ISDN phone will offer you features like Call Divert, Call Identify (CLI), Call Wait, and answering machine features etc. You could also attach videoconferencing instruments to your ISDN line. Videoconferencing equipment harness the capability of ISDN giving you the benefit of seeing the person you are talking to over the phone instrument itself, provided the other party has similar equipment or a WebCam. To send and receive faxes, you could also connect a digital fax machine called the G4 fax machine to your line, which lets you transmit high-resolution images too. It is an internal ISDN terminal adapter that fits into one of the slots in your PC. As the popularity of ISDN increases, more and more ISDN- ready devices are slated to hit the market in the foreseeable future.

The ISDN scenario in India

With the recent liberalization and rationalization of the telecom tariffs & policies, you are now able to get an ISDN connection almost on demand. One can apply for an ISDN connection with either DOT (Department of Telecom), or MTNL depending on whether you are located in the metros or elsewhere. The forms are also available online on the MTNL and DOT websites, which you can print and fill in. For DOT, you will have to find out whether your area’s telephone exchange offers ISDN connectivity before applying.

How much does it all cost?

When ISDN was first introduced in India way back in 1996-97, VSNL and MTNL used to charge a phenomenal Rs 12 lakhs per annum for just half circuit access (ie. 64 kbps), way above the international standards. Since then, the charges have been significantly reduced. Assuming you can get ISDN, the next question is “What does it cost?” With DOT, ISDN rates known as tariffs could vary for different areas. You will need to get these details from your local telephone exchange.

Here is the break-up of how much you would pay for an ISDN line

ISDN pricing generally has two components - Installation Charges and Usage Charges.

Installation Charge

This is a one time charge required to have the ISDN service installed.

The following is the approximate cost of a single ISDN BRI line

(If NT1s are not in stock, you will have to buy one. The cost is approx.- Rs 15,000. NT1s are available from Philips, Siemens and Fujitsu etc.)

Usage Charges

This is a charge associated with use of the ISDN line. The minimum usage charge per channel, which earlier used to be Rs 5,000/- per month, has since been waived off. Since May 1999, the same number of free calls as are available for a normal DOT POTS line are applicable, which is an additional benefit.

Applications in the Real World

Internet Access over ISDN

Getting an ISDN line alone is not sufficient. If you want to gain Net access, then you will have to get an ISDN account from an ISP. The charges for this would again vary for each ISP. For example, an ISDN dial-up account from VSNL costs only Rs.2850/- for 100 hours, with an initial registration fee of just Rs.100/-. Today, all major ISPs in the country are providing ISDN accounts, including VSNL, Satyam-Online, MTNL and Dishnet.

Do I actually get to surf at 128 kbps?

Again, the answer is “Yes”. Of course, you will have to throw away your modem and buy a TA instead. Also your PC may not be able to handle that kind of throughput (the serial port has a 115.2 Kbps limit). Moreover, the ISP has to be able to provide that kind of throughput. Also, the ISP should have hardware and software that supports either Bonding (Bandwidth-ON-Demand) or Multilink-PPP. This is because you are actually using two separate 64 Kbps lines. Both Bonding and Multilink-PPP have a system that uses only one single channel at the beginning and utilize the second channel only as the first fills up. This ensures lower bills for the customer, since the second channel is used only if that kind of throughput is really required.One of the most frequent complaints among modem users who surf the World Wide Web is the slow movement of information. Surfing the web with ISDN as compared to surfing on a dial-up line is like catching that perfect wave. The thrill of seamless connections and quick responsive graphics will keep you coming back for more. ISDN gives you reliable, high-speed access to the Internet, and the dramatic increase in data transfer speeds decreases the amount of time you spend waiting for web documents & files to download, which lets you get more work done in less time.

Videoconferencing

While videoconferencing has been around for a while, widespread videoconferencing is just beginning to come into its own. The ability to transmit quality video and voice across distances required very expensive equipment and leased lines, that were justified only by the lack of other options for the larger companies. As such, for companies dependent on leased lines, those videoconferencing solutions were point-to-point and a headquarters facility could be linked to satellite facilities, for example, but videoconferencing on the scale of teleconferencing was simply impractical. Both desktop videoconferencing (where a participant joins from a PC equipped with ISDN, a video camera, and a microphone), and conference room videoconferencing (where more sophisticated equipment such as remote control cameras allow group participation) have become as easy to set up as voice conferencing. Also because of ISDN’s versatility, videoconferences can include the sharing of documents, images, and other files with all participants, through file transfers or screen display. For general business use, ISDN videoconferencing can make meetings much more practical and productive. There is far less travel time and expense involved, as participants no longer need to physically congregate at one place. Since participants are typically at their own offices, videoconferencing enables everyone to have access to their own computers and file cabinets for any information that may be required during the meeting.

Some Alternatives to ISDN

Looking at the scene objectively, there really aren’t a lot of alternatives today especially if you want more bandwidth at home or at a small business. There are a variety of faster network choices available to larger organizations, but these solutions tend to be priced beyond the reach of most individuals and SOHO businesses. For Internet access or remote access to a corporate LAN, ISDN is the only high- speed option available for most people.

Looking at the scene objectively, there really aren’t a lot of alternatives today especially if you want more bandwidth at home or at a small business. There are a variety of faster network choices available to larger organizations, but these solutions tend to be priced beyond the reach of most individuals and SOHO businesses. For Internet access or remote access to a corporate LAN, ISDN is the only high- speed option available for most people.

With its BRI and PRI services, ISDN has the flexibility to meet the bandwidth needs of a home office, branch office, or the company headquarters. A small office can use ISDN BRI to support all its voice and data communications requirements. In a larger office, multiple ISDN BRI lines can be divided among multiple users and applications via a server or a PBX. Users in a very large office can benefit from the capacity of ISDN PRI to provide a large quantity of multiple B channels.

In conclusion, with the technology currently available, ISDN provides extremely cost effective, and high bandwidth connectivity for a wide diaspora of applications.

JAIDEEP CHITNIS is responsible for the Web Administration at C-DAC. Besides this, he also looks after the planning, design & implementation of C-DAC’s Wide Area Network. His current interests include high speed networking, Web Technologies & Network Security. Click here to send an eMail.