An Alternative to Dial-Up?
is the faster alternative to dial-up Net access. We give
you a lowdown on this technology and the issues involved
in implementing it
many years now, the primary means of access to the Internet
has been the dial-up method. But the limitations of speed,
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?
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
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
What are the speeds?
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
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
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.
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.
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
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