ISDN Connectivity
by Jeff Newman
and
David
Willis
Problems With ISDN
Although ISDN is a handy communication standard, it has its problems--the
most prevalent is availability. Traditionally, ISDN has been very difficult
to obtain because it requires newer hardware and software within local telephone
switches before it can be delivered to your business or house. Some areas
are lucky and aren't stuck with older switch technology that was implemented
many years ago by the telephone companies. Some switches have been around
for a long time and are fine for supporting normal POTS communication. Before
that, the phone companies didn't need to fix what wasn't broken. Now the
phone companies are faced with the fact that a new technology such as ISDN
is requiring them to pay big bucks to upgrade the hardware and software
used to dri
ve their vast communications networks. Even if ISDN is available
to those switches in your area, you must be within 2,000 feet of your central
office for ISDN to work, as all signals traveling across copper wire diminish
over distances.
For the distance and equipment reasons and because many technicians need
to be educated to be able to install and maintain it, ISDN is not yet available
in many areas. This has been the downfall of ISDN for many years. Although
voice calls can be made from the ISDN connection to anywhere, data calls
can only be made to other ISDN connections, which is the reason that most
people want it. So an ISDN connection is not very useful unless your destination
site, such as a branch office or Internet service provider (ISP), can also
be equipped with ISDN. Despite their slow start, tone providers across the
continent are aggressively deploying ISDN. Many have greater than 30 percent
deployment currently and plan to have more than 50 percent deployment by
1997.
Another problem with ISDN is that the cost of equipment has traditionally
been high. This woe is quickly diminishing, as the number of vendors jumping
on the ISDN bandwagon is growing rapidly. Some equipment is starting to
rival that of newer POTS equipment such as modems.
Another traditional problem with ISDN is its complexity. Many different
configurations of BRI ISDN and services are offered. You must order the
ISDN connection that will match the requirements of the equipment that you
are placing to the ISDN line. This problem is also quickly fading as standards
organizations, in cooperation with vendors, are establishing ISDN ordering
codes, provided by a particular vendor for their equipment that customers
can give to the telephone company when ordering their lines that will handle
all of the provisioning. Most vendors will give you these codes, but it
does mean that you need to decide on your equipment and vendor before you
purchase your line. You only need to know a few
codes called service profile
IDs (SPIDs), needed by the equipment to communicate with the switch to identify
its required services. SPIDs are provided by the telephone company when
the connection is established. Some more innovative companies, such as Adtran,
(www.adtran.com)are coming out with ISDN products today that can even auto-detect
SPIDs, further reducing the effort of implementation.
Another problem with ISDN is that while you can get ISDN, your central office
may not yet have the software needed to implement a full 64 Kbps on each
B channel. Some of the older signaling software requires the signaling to
be handled in-band (over the B channels) instead of the out-of-band signaling
that the D channel provides. The 16 Kbps required to manage and carry information
relative to the connection must be allocated from the 64 Kbps available
to B channels, which is 8 Kbps per channel. Thus, many central offices are
limited to 56 Kbps on each B channel or 112 Kbps aggregated bandwidth for
a connection. This limitation also creates many 112-Kbps bridges across
the country, so even though you might have 128-Kbps ISDN locally, you might
only get a 112-Kbps connection when calling over long distances to some
remote sites. Still, this is an attractive bandwidth when you know that
you will get at least that much and it will be a reliable connection. The
phone companies are quickly solving this in-band signaling problem as well.
Still more woes. Even though ISDN is ideal for small office and home office
(SOHO) solutions, it doesn't provide power to its equipment like POTS does
to its telephones. If power is lost from the utility outlets in an office,
that office will not have voice or data communication unless some type of
UPS or generator is used to supply power to the critical components of the
ISDN connection, so you must take care to supply power to important applications
using ISDN.
May 16, 1996
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