| ISDN can provide data
transfer rates of up to 128 kilobits per second.
These speeds are slower than those of local area
networks (LANs) supported by high-speed data communications
technology. However, they are faster than those
of analog telephone lines, which provide transfer
rates of up to 28.8 kilobits per second. In addition
to the difference in data transfer rates, ISDN calls
can be set up much faster than analog phone calls.
While an analog modem can take up to a minute to
set up a connection, you usually can start transmitting
data in about two seconds with ISDN. Because ISDN
is fully digital, the lengthy handshaking process
of analog modems (those piercing, screeching noises)
is not required.
ISDN service is available in several configurations
of multiple channels that provide simultaneous
voice and digital communications. In addition
to increasing data throughput, multiple channels
eliminate the need for separate voice and data
phone lines.
Connection Quality
ISDN transmits data digitally and, as a result,
is less susceptible to static and noise than analog
transmissions are. Analog modem connections must
dedicate some bandwidth to error correction and
retransmission. This overhead reduces the actual
throughput. In contrast, an ISDN line can dedicate
all its bandwidth to data transmission. Because
ISDN is a digital service, it is sensitive to
outside interference, and certain factors affect
the quality of ISDN connections to the telephone
network and the Internet. The computer connecting
to the ISDN line must be within a given distance
of the telephone company's central office equipment
that serves the computer for the ISDN connection
to work. The maximum distance allowed between
the computer and the central telephone office
is typically 18,000 feet. Additionally, ISDN connections
can be adversely affected by telephone lines that
are not wired using twisted-pair wiring.
Two types of ISDN service are currently available:
Basic Rate Interface (BRI) and Primary Rate Interface
(PRI) The most appropriate type of service for
small Internet Information Server sites is ISDN-BRI.
The most appropriate type of service for medium
to large Internet Information Server sites is
ISDN-PRI.
ISDN - Basic Rate Interface
ISDN-BRI divides the telephone line into three
digital channels: two bearer channels, commonly
referred to as B channels, and one D channel.
All three channels can be used simultaneously.
The B channels can be used to send voice, circuit-switched
data, or packet-switched data at rates of 56 to
64 kilobits per second. The maximum available
transfer rate is determined by your local telephone
company. The B channels function independently
of one another. They can be used simultaneously
for separate calls or combined to provide a total
bandwidth of 128 kilobits per second.
The D channel can transfer data at 16 kilobits
per second and is used for signaling and control
information sent between the computer and the
telephone office. D channel signaling provides
functions such as call setup, call monitoring,
call termination, and enhanced telephony features.
When using ISDN-BRI, it is possible to perform
several types of connections. For example, it
is possible to have a voice conversation on one
B channel, a circuit-switched data call on the
second B channel, and a packet-switched data call
on the D channel. The D channel can simultaneously
complete all necessary signaling and call control
functions.
ISDN - Primary Rate Interface
The U.S. standard for ISDN-PRI consists of 24
digital channels: 23 B channels and 1 D channel,
commonly referred to as a 23B+D connection. ISDN-PRI
provides a total bandwidth of 1.544 megabits per
second and is designed for transmission over a
North American - standard T1 line connection.
The European standard for ISDN-PRI provides a
total of 31 or 32 digital channels. Each of these
channels is a 64 - kilobit per second B channel,
except one that is the D channel for the entire
group. In Europe, ISDN-PRI is designed for transmission
over a European-standard E1 line connection. A
PRI D channel can be used to control one or multiple
T1 or E1 lines.
Workstation Hardware
There are two types of ISDN hardware devices that
you can use to connect your computer to an ISDN
line. Internal ISDN devices are adapter cards
that you put inside your computer. External ISDN
devices are adapters that you connect to an external
port on the back of your computer.
Internal vs. External ISDN Adapters
Internal ISDN cards can take the fullest advantage
of your ISDN line. However, internal adapters
require you to open your computer to install the
card. In addition, in order to install an internal
adapter, you need a slot free in your computer
that supports the same type of bus (for example,
ISA, EISA, or PCI) as the card you want to install.
External ISDN adapters are easy to install and
do not require any special software, but they
do not provide the same level of performance that
internal adapters do. External ISDN adapters are
similar to a modem, and the computer communications
program attempts to control the external ISDN
adapter just as it controls an analog modem, typically
with AT commands.
An external ISDN adapter plugs into a computer's
serial or parallel port. These ports impose certain
limitations. Most computer serial ports do not
transmit information faster than 115 kilobits
per second, which is less than ISDN's maximum
data speed of 128 kilobits per second. These serial
ports also impose overhead on the transfer of
information between the computer and the external
adapter, further slowing data speeds.
An external adapter can also adversely affect
the performance of your computer, because an external
adapter places heavy requirements on the CPU.
To use an external ISDN adapter, it is recommended
you use a computer with a 486/33 or faster processor.
Also, interoperability issues can potentially
arise with external adapters, because higher-level
protocols such as Point-to-Point Protocol (PPP)
or authentication are implemented in the modem
itself. These protocols are evolving quickly and
can be difficult to update in an external adapter.
Adapters and the U and S/T Interfaces
The U Interface carries ISDN signals over a single
pair of wires between your computer and the telephone
central office. This interface is designed to
carry ISDN signals over long distances. The S/T
Interface uses two pairs of wires to deliver the
signal from the wall jack to the ISDN adapter
in a computer or other ISDN equipment.
If your ISDN adapter supports the S/T Interface,
you need to get a device known as a Network Termination
1 (NT1) that converts traffic between the U Interface
and the S/T Interface. The NT1 supplies a jack
for the U Interface coming from the wall and one
or more jacks for the S/T Interface connection
to the computer and other ISDN or analog devices,
in addition to a connection to an external power
supply.
Some ISDN adapters sold in North America connect
directly to a U Interface. If a single computer
is the only equipment to be connected to an ISDN
line, this type of adapter is the easiest to install.
Manufacturers may describe such an adapter as
having built-in NT1 or simply as a U Interface
ISDN adapter.
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