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Terminal Adapters
By Mahesh Rathod

Terminal adapters allow you to leverage the benefits offered by ISDN. Here are some tips on buying these devices

One has to be careful when purchasing an external TA because vendors claim to give you speeds of up to 64K

For all those looking for a speedy and direct connection to the Internet and long for something better than 14.4 or 28.8 Kbps modem connection, ISDN (Integrated Services Digital Network) is a good and relatively cheap alternative.

What is ISDN?
Integrated Services Digital Network (ISDN) is basically a telecommunication technology that delivers all types of communications (voice, fax, data, video, sound, etc.) over a single or common high-speed digital network using the existing copper phone lines.

There are two types of ISDN services:

Basic Rate Interface (BRI): BRI consists of one 16 Kbps D channel and two 64 Kbps B channels for a total of 144 Kbps. This is the main service provided by all ISPs and usually meets the needs of most individual users.

Primary Rate Interface (PRI): PRI is intended for organizations with greater capacity requirements such as ISPs.

What is an ISDN modem?
ISDN modem or a Terminal Adapter (TA) is a device that connects a computer to an external digital communications line, such as an ISDN line. A terminal adapter is similar to a modem, whereas a modem needs to convert analog signals to digital and vice versa, a terminal adapter just needs to pass along digital signals. There are two main types of ISDN terminal adapters. You can either buy an internal or external terminal adapter. It really depends on what features you want supported with your ISDN line. But as ISDN becomes more common, future computers will probably have terminal adapters built in.

External terminal adapters
External adapters are better if you are going to use your ISDN line for "voice" (phone, fax, analog modem, etc). One has to be careful when purchasing an external TA because vendors claim to give you speeds of up to 64K. However, many external TAs cannot convert synchronous 64Kbps data into 57.6 Kbps asynchronous data. These TAs can only communicate in asynchronous mode at 38.4 Kbps.

Internal terminal adapter
Internal terminal adapters usually go inside your computer like any other internal card. If you are going to be using ISDN strictly for Internet access, then an internal adapter is the right choice. The internal models are normally cheaper than external, because the manufacturers do not need to include a power supply or enclosure. The internal adapters do not have the serial port bandwidth constraints that the external ones do; hence you do not need to buy any special accessories to squeeze the maximum out of them. However, the internals are not set up well to provide ringers, and they need an external power connection (or your PC powered on) in order to use them to make a voice call.

Some of the features to look out for in a TA

• Easy to install and use: Installation of TAs is easy and there should be an installation wizard that walks you through the configuration and setup, making the TA up and running in minutes with plug and play support for Windows 95/98/2000 and NT.
• Support for WAN Protocol: TAs should support a full-range of WAN protocols, including X.75, CLEAR (synchronous), CHAP, MD5, PPP, ML-PPP, MP+™, V.120, X.75 and PAP so that you can connect to a variety of servers on the corporate LAN or Internet.

• Features	External TAs	Internal TAs
  To use voice	Works as long as TA is	PC must be on, or
  	powered	additional equipment
  		purchased
  Ringer for other	Can usually provide for	Often requires
  devices (fax, phone,	six devices	additional equipment
  etc)
  DTE speed	Serial port speed	Much higher speed
  	constraint
  Cost	More expensive	Less expensive

  Call Bumping: TAs should support call bumping features that automatically adjust a data call from 128K to 64K. It should be able to reduce the data link to one channel to make or receive a phone call while communicating data with two B channels (at 128K).
• Battery back up: They should provide a built-in battery which supplies power for 2-3 hours in case of power failure.
• Simultaneous Voice and Data: TAs should be capable of transmitting and receiving data on your computer over one ISDN B-channel and use the other B-channel for your phone or fax machine using the analog port.
• Call line identification: For incoming calls from digital lines, the caller's number appears in the telephone display when the phone rings. This feature of TAs will enable to trace anonymous calls over ISDN.

Mahesh Rathod can be reached at rathodmp@hotmail.com