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.
|
Company |
Indicative prices in Rupees |
|
Dlink |
12,000 |
|
Multitech |
12,000 |
|
Zyxel TA 128 |
14,000 |
|
Zyxel Omni Net |
11,000 |
|
Ace TA 128k |
10,000 |
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
