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Integrated
Services Digital Network (ISDN)
ISDN stands
for Integrated Services Digital Network - the name for digital
telephone service that works over existing copper telephone
wiring. There are several types of ISDN service, but the most
appropriate type for individual computer users, and the type
that this site focuses on, is the ISDN
| Basic
Rate Interface (BRI). Basic Rate ISDN divides the telephone
line into 3 digital channels: 2 "B" channels
and one "D" channel, each of which can be used
simultaneously. The B channels are used to transmit data,
at rates of 64k or 56k (depending on your telephone company).
The D channel does the administrative work, such as setting
up and tearing down the call and communicating with the
telephone network. With two B channels, you can make two
calls simultaneously. |
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Most
of the world's existing telephone network is already digital.
The only part that typically isn't digital is the section
that runs from the local exchange to your house or office.
ISDN makes that final leg of the network digital.
Availability
Unlike
analog telephone service, ISDN service is not yet available
everywhere. Your telephone company must have installed the
necessary equipment in the central office that serves you.
Because
ISDN is a digital service, it is very sensitive to outside
interference. You must be within a given distance of the telephone
company equipment that serves you (typically 18,000 feet).
Further, there can not be any other anomalies near the wiring
that might interfere with the transmission. As a result, even
if the equipment is installed at the central office, it is
possible you will not be able to get ISDN due to line interference
or your distance from the central office. To determine whether
your particular wiring will support ISDN, the telephone company
will perform what is known as a line qualification.
Some telephone
companies offer what is called "ISDN Anywhere",
which means if you order ISDN, they will find a way to get
it to you. In cases where the telephone company does not have
the right equipment in the local central office that serves
you, they can use "line extension" technology to
serve you from another exchange. The use of line extension
technology may significantly increase the cost of your ISDN
service.
Generally,
ISDN has better availability in urban and suburban areas,
and is harder to get in rural areas.
Pricing
Assuming
you can get ISDN, your next question is probably "What
does it cost?" ISDN rates (also called “tariffs”),
vary by phone company, and ISDN pricing generally has three
components:
- Installation
Charge - this is a one time charge to have the ISDN service
installed. Part of this charge may be waived if you commit
to keep your ISDN line for a period of time.
- Monthly
Charge - this is a recurring monthly charge.
- Usage
Charges - this is a charge associated with use of the ISDN
line. Typically it is not more than a couple of cents per
minute. The monthly charge may include a certain number
of hours of free usage each month. Some packages have no
usage charges at all, or may waive usage charges during
evenings and weekends.
If you
expect to make a lot of local ISDN calls within your organization,
you can sometimes get CENTREX ISDN service, which has no usage
charges for calls to other local numbers belonging to the
same organization.
Provisioning
"Provisioning"
is a telephone company term for the configuration of your
line on the telephone company's end. A normal analog phone
line comes with a few options like Call Waiting or Caller
ID, but it is not necessary to choose any of them. An ISDN
line, on the other hand, has many, many options, and choices
for all of them must be defined in order for the line to function.
Configuration:
Switch Type and SPIDs
In addition
to the configuration the telephone company must do at their
end of your ISDN line, there is also some configuration you
must do at your end. You need to know three pieces of information
supplied by the telephone company to make your ISDN service
work:
- Switch
type
- Phone
numbers
- SPID(s)
- Service Profile Identifier(s)
Most ISDN
hardware adapters need to know what type of switch they are
connected to. The switch type simply refers to the brand of
equipment and software revision level that the telephone company
uses to provide you with ISDN service. There are only a few
types of switches in the world and usually just one in countries
other than the United States.
The second
type of information is your phone number or numbers. In some
cases, each B channel on an ISDN line has its own number,
while in other cases both B channels share a single phone
number. Your telephone company will tell you how many numbers
your ISDN line will have. Separate numbers may be useful if
you plan to take incoming calls on your ISDN line.
The last
type of information is the Service Profile Identifier (SPID),
which is only used in the United States and Canada. The SPID
usually consists of the phone number with some additional
digits added to the beginning and end. The SPID helps the
switch understand what kind of equipment is attached to the
line, and if there are multiple devices attached, helps route
calls to the appropriate device on the line.
Alternatives
If you
are looking for a continuous connection, such as connecting
a Web server to the Internet, a dedicated frame relay or T-1
line may make more sense. ISDN is a circuit-switched service,
which means it is only connected when it is being used. Leaving
ISDN connected around the clock can end up being more expensive
than a dedicated line or "leased" line, which has
a flat monthly price.
Over the
next several years, the bandwidth bottleneck residential and
small business users face will be alleviated as new technologies
such as Digital Subscriber Line (DSL) are deployed. These
new technologies work over regular telephone wires. These
services will offer much greater speeds, but are deployed
in limited areas and have caveats that often make them inappropriate
for business applications.
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