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ISDN: An Alternative to Dial-Up?
By Mahesh Rathod

ISDN is the faster alternative to dial-up Net access. We give you a lowdown on this technology and the issues involved in implementing it

For many years now, the primary means of access to the Internet has been the dial-up method. But the limitations of speed, low quality

transmission and the longer time required to setup a connection have been major constraints for users.

Integrated Services Digital Network (ISDN) resolves some of the problems associated with dial-up Net access. ISDN is faster than an analog modem connection to the Internet, though it costs a little more.

What is ISDN?
Integrated Services Digital Network (ISDN) is a set of communication

standards allowing a single wire or optical fibre to carry simultaneous voice, digital network services and/or video transmissions.

The telephone network is almost entirely digital, with the notable exception of the segment, which links the customer and the local exchange. ISDN uses existing copper wire or optical fibre to render this segment functionally digital as well.

With the ability to run on the regular copper phone wire, ISDN divides digital signals into bearer channels (B channels) for voice and data transmission, and a data channel (D channel) for signaling. B channels will typically have a bandwidth of 64 Kbps, while the D channel will have a bandwidth in the range of 16 to 64 Kbps, depending on the type of ISDN service.

The two types of ISDN services are Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI is ideal for most individual users and small or low-traffic LANs, while PRI is intended to meet greater capacity requirements, such as that of small Internet Service Providers or other organizations providing dialup access.

BRI comprises two B channels and one 16 Kbps D channel. PRI will typically have 23 B channels and one 64 Kbps D channel. In addition, multiple PRI lines can be supported with a single 64 Kbps D channel using Non-Facility Associated Signaling (NFAS).

What are the speeds?
The nature of an analog modem connection requires that some of its bandwidth be dedicated to error correction, with a resulting reduction in throughput. The digital connection of ISDN, on the other hand, is able to dedicate its full bandwidth to data transmission.

With a regular analog connection, the maximum speed that can be attained on the wire is 56 Kbps, and for all practical purposes is usually much lesser than this. With ISDN, a digital connection is established, and your speed increases dramatically. BRI ISDN gives you 64 Kbps per B channel, of which you have two. The two channels can be bound together for a speed of 128 Kbps.

The signaling is done on the D channel at 16 Kbps. This may sound slow by comparison, but what you have is essentially the same as a 14.4 modem that does nothing but handle the establishing of the connection, with virtually no error correction required. While an analog modem can take up to a minute to find a protocol and establish the connection, ISDN signaling usually takes less than two seconds.

PRI ISDN is also 64 Kbps per B channel, but you have 23 of them. And your D channel is also 64 Kbps. This means a total bandwidth of 1.5 Mbps, the same as that of a T1 line.

Advantages and Disadvantages
Besides the obvious advantage of a significant increase in speed (up to five times faster than a standard analog connection), the other really big advantage of ISDN is its flexibility. It gives you the ability to perform multiple tasks simultaneously. Using the multiple channels of ISDN, you can fax or speak on the phone while you surf the Net all on the same phone wire.

Not only is ISDN flexible, it's also fairly intelligent. For example, with BRI ISDN, you could be using both B channels for a total bandwidth of 128 Kbps while you surf, and suddenly you get an incoming call. Your ISDN terminal adapter can automatically drop you down to a single 64 Kbps B channel for data and let the other channel carry the voice call. Of course, this function must be supported by the hardware you are using, but most ISDN products do have such capability.

The single biggest disadvantage is likely to be your physical location. If you're not in an area that's reasonably close to a telephone company's central office (one with the required equipment already installed), ISDN may not be an option for you. If you live in a metro or near a city, ISDN is an ideal choice. But at other remote locations ISDN is not available.

One more thing: as a digital service, ISDN is subject to interference. Although not generally vulnerable to line noise in the same way as analog, ISDN is extremely sensitive to unusual or anomalous wiring.

Gears you need for ISDN
There are certain things you need to know before you can get going with your ISDN connection.

ISDN Wiring
In India, the local telephone company does the installation of the ISDN line. However, the internal wiring has to be done by you or any third party contractor.

Interface Equipment
There are two types of ISDN interfaces: the U Interface, which carries the digital signals between the customer location and the phone company central office over a single pair of wires; and the Subscriber/Termination Interface (S/T Interface), which carries the signals between your terminal adapter and the wall jack on two pairs of wires.

The ISDN equipment you use will support one interface or the other. If it supports the S/T Interface, you will require a Network Termination 1

(NT-1) to convert between the U Interface and the S/T Interface. This device will have an external power supply, and one jack for the U Interface with the wall, and one or more jacks for the S/T Interface with your computer.

If your ISDN equipment supports the U Interface directly, it may be described either as a U Interface ISDN adapter or as an ISDN adapter with a built-in NT-1. These are usually easier to install than the S/T Interface adapters.

Terminal Adapters (ISDN Modems) - Internal vs. External
Internal terminal adapters generally allow for better performance, enabling your system to take complete advantage of the ISDN connection. Of course, you'll need an available slot in your computer that will support the same bus type as your ISDN adapter. The disadvantage is that internal terminal adapters are generally more difficult to install and configure than externals. Some plug and play types can do the configuration for you.

External terminal adapters tend to be much easier to install, and typically do not require specialized software. They look and install like an external analog modem. On the down side, the performance will not be of the same caliber as with an internal adapter. The reason for this is primarily because an external adapter connects through your computer's serial port, and on most PCs the serial ports cannot transmit information beyond 115 Kbps, somewhat less than the 128 Kbps data speed of BRI ISDN. It is still, nevertheless, significantly faster than the best analog connection. Another thing about external adapters is that they can tie up your CPU, slowing things down even more.

Operating System Software for ISDN
It is of course important to have an operating system, which will support ISDN hardware, allowing your software applications to communicate with and take full advantage of your ISDN terminal adapter.

First of all, Point-to-Point Protocol (PPP) is the standard Internet access protocol, and it is required for a

proper ISDN connection. Serial Line Internet Protocol (SLIP) is an older and less efficient protocol, but it is still fairly common. SLIP won't work with ISDN so make sure you have a PPP account. Of course, if your ISP can provide you with ISDN connectivity, they should know this and it probably won't be an issue.

Mahesh Rathod can be contacted at rathodmp@hotmail.com

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