ISDN Connectivity
by Jeff Newman
and
David
Willis
Why ISDN Instead of POTS or Dedicated Leased Lines?
ISDN's digital nature allows it to transfer data not only at high speeds,
but also to do it reliably with a relatively negligible amount of data loss.
Modem connections using POTS technology can be clumsy and unreliable because
of poor connection quality. The only people who manage to get a full-speed
connection with anything more than a 14.4-Kbps modem nowadays are the types
of people who leave Vegas counting their winnings. We hear that it happens,
but it never seems to happen to us.
Still, modems have come a long way, and they are nice solutions in ideal
situations. The fact is that they have just about pushed current POTS technology
to its limits in real-life scenarios. If your ISDN is capable of making
64-Kb
ps connections on a channel, you're going to get a 64-Kbps connection
on that channel every time. ISDN also has the advantage of near instantaneous
connections. Its logical connect time can be measured in milliseconds, as
opposed to a modem connection using POTS taking many seconds, depending
on the line quality as the modems handshake.
ISDN doesn't reach the speeds of higher-speed dedicated links, but at the
same time it doesn't cost nearly as much and has more flexibility. In most
geographical areas, the cost of using ISDN is only slightly more than POTS
connections. The initial setup can cost a bit more, but once the investment
is made, ISDN can soon pay for itself with high bandwidth and a relatively
low cost of operation, and since connections are dynamic, ISDN has more
flexibility than many dedicated point-to-point connections. Still, ISDN
tariff rates vary across the continent, and some areas cost more than others.
For example: BellSouth has some of the cheapest operational fees ($91 per
month with no usage charges) and U S West having one of the most expensive
($39 to $87 per month plus 15 cent per minute per B channel). These rates
fluctuate rapidly as well, so always check you region for the latest tariff
information. Long distance calls are also billed accordingly for ISDN. A
typical long distance connection for ISDN costs about 15 percent more per
channel than a POTS long-distance connection.
ISDN vs. DSL
Digital Subscriber Line (DSL) is an emerging technology
that is going to have some high-bandwidth ISDN in the future. Like ISDN,
DSL is a digital service that operates over the same copper wiring, only
it has very different characteristics. Variations of DSL, such as Asymmetric
DSL (ADSL), started as the telephone companies' answer to the competition
that cable companies present. ADSL is intended to give you lots of downloadable
(or downlink) bandwidth and a small amount of uploadable (or uplink) bandwidth
over short distances of up to 2,000 feet
(more in some instances and at
slower speeds). There is currently a great sacrifice of bandwidth for distance
within all DSL technology.
DSL was originally intended to allow television and voice communication
concurrently, but it has characteristics that make it ideal for Internet
access. When it becomes more widely available, it is supposed to allow the
user to receive data at speeds up to 6 Mbps over copper lines. Transmission
of data is typically 64 Kbps currently, but all flavors of DSL technology
are still evolving and many ideas are still in development. Chances are
that these speeds will increase in the future as DSL becomes more of a reality.
It is definitely a technology to watch, but don't expect it to directly
rival the services that ISDN provides.
ISDN fits in scenarios where higher bandwidth is needed to connect branch
offices or remote users, but very high-speed dedicated links aren't cost
justified. It also fits wherever newer technology such as videoconferencing
is needed that requires higher bandwidth to operate than POTS provides.
It fits into networks with large amounts of data needing to be backed up
or replicated over wide area links and has outgrown its window for overnight
transmissions using its existing POTS connections. ISDN is ideal for home
office used because of its advanced security and multiple device support.
May 16, 1996
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