Understanding the network.pdf - Back to Home

The problem with ISDN was its actual implementation. ISDN is a collection of
international standards developed by the ISO/ITU-T (formerly known as CCITT).
The original assumption was that by using an international standard, global
communications systems could be easily integrated. The ISDN standard, however,
like most "standards," offered no insight on real implementation, but, rather, was
an abundance of conceptual elements (such as services, access, and interfaces)
that were left to be interpreted and implemented by the various manufacturers and
service providers. The standards approach worked well in Europe and Japan, where
the telephone systems were owned by the government. In the U.S., however, ISDN
had some major implementation problems. Although the Bell System was perceived
as a single system, the reality was quite different. Each of the BOCs—and after
divestiture, the RBOCs—all used different carrier switch implementations. So,
although ISDN was a standard, and provided a standard set of interfaces and
services, the actual implementation of ISDN varied depending on the carrier switch
installed in the CO. This resulted in incompatibilities between CPE ISDN devices at
the CO and the CPE ISDN applications that needed transport outside of the
RBOCs'service area. In addition to ISDN's implementation incompatibilities, ISDN
service has a finite service distance. A maximum cable distance of 18,000 feet
(5.5Km) is permitted between the CO and the CPE. Cable distances beyond this
cannot support ISDN service.
It was not until the mid 1990s that ISDN became a useable technology. This was
largely as a result of the actual need for ISDN in the consumer market for increased
Internet access performance. Once there was a customer demand for ISDN, there
was a motivation by the carriers and switch manufacturers to develop compatibility
solutions to make ISDN an applicable technology. ISDN services are tariff-priced
based on the kind of usage. If voice services are being utilized over the circuit,
pricing is based on (local/toll) distance rates. If data is being transmitted over the
line, pricing is based on (local/toll) distance rates, plus a per minute charge based
on bandwidth. This makes ISDN very lucrative for the providers and quite expensive
for users. So, even with the technology issues resolved, ISDN has limited
application because its costs are high for general consumer use of ISDN over a BRI.
What is even more ironic is that when ISDN became stable enough for consumer use
around 1995-96, telephone carriers began to look at implementing Asymmetric
Digital Subscriber Line (ADSL), which provides T1 transport rates (and higher) over
a single wire pair and is tariffed with flat rate pricing. There are some limitations to
ADSL, however, as it can only provide a high-speed downstream transport path.
ADSL functions by connecting an ADSL modem on each end of the circuit. The line
is partitioned into three data channels: a high-speed downstream channel, a
full-duplex channel, and a 4Khz channel for POTS service. The full-duplex channel is
used to send upstream data. The downstream channel is for receiving data. ADSL is
currently being implemented as a high-speed modem technology for Internet
access in major U.S. markets. Its limited upload bandwidth makes it unsuitable for