ISDN service hoped to offer both voice and data, and perhaps video, all integrated into one
service and generally distributed throughout the local area as long as the on–premises
equipment was not more than 13,000 feet from the central office. The twisted wire that
now carries telephone service was a limiting factor. Later, as fiber-optic loops fell in cost and
became more readily available, a host of services were offered via the loop from your local
telephone company. These could include all those now found on cable TV; those services
now available to a computer user through a modem, plus video text and video telephony;
and services just now coming into widespread use, like true video on demand or your TV
connection to the Internet through a high-data-speed channel provided by either your local
cable company or telephone operating company.

Though the technology to implement ISDN has been here (it was designed over twenty-five
years ago), the implementation has lagged. Because they have a monopoly over telecommunications
and can dictate standards, many foreign governments have already established
standards for ISDN. Some of these standards clash with the technology and rationale of the
competitive systems found in the United States. One might assume (at one’s own risk) that
the political compromises necessary to realize every international standard could be achieved.
But they were not, and other, newer systems have displaced ISDN as the system for connecting
all the businesses and residences in the United States. The objective of ISDN was
point-to-point digital connectivity. For this to be accomplished, it was conceived using the
sometimes analog public telephone network. One of the key features of ISDN is out-of-band
signaling. This means that the control signals are in a different media than the messages.

Originally, there were two different ways to connect to an ISDN network: basic and primary.
In the basic configuration, three channels are distributed on the wireline. These are 2 B (for
Bearer) channels operating at 64 Kbps each, and 1 D (for Data) channel operating at 16
Kbps. This is called the 2B+D arrangement. The original intention was that the B channels
could carry either digitized voice or data on either channel and that the D channel would
provide control signals and other low-speed signaling requirements. You can tell at what
technical generation ISDN was conceived by the fact that it took 64 Kbps for subscriberquality
voice. In modern digital telephone systems, 32 Kbps will give you toll-quality voice,
and 16 Kbps will give you subscriber-quality voice.

Message traffic on the D channel uses Link Access Protocol-D (LAP-D), which is essentially
HDLC. The difference between the two lies in the address fields, where LAP-D uses a twooctet
address—one to identify its network, and the other the end point. By using the D
channel to control the switching of the B channels, a clear channel is established in which
bit patterns on the data (B) channel do not affect transmission. At the time of ISDN’s conception
64 Kbps was chosen because that was the data rate needed to support voice
digitization. That rate is quite high for voice given current technology, and in many cases is
too low for some of the digital data services. It would be hard to use an ISDN line as a LAN
port without having much reduced data rates (at least with the IEEE 802 types).
Additionally, some of the electronic switches will not support an ISDN channel at 64 Kbps.
Instead they require transmission at 56 Kbps because they extract the 16 Kbps signal by
using 8 Kbps from each of the 64 Kbps channels. This results in two 56 Kbps channels and
a 16 Kbps data channel that is in band for signal control. ISDN has achieved a degree of
popularity, not as an integrated service medium, but for relatively fast Internet access.

In primary access, many subscribers use common input trunks to a facility. These trunks are
multiplexed together into twenty-three B channels and one D channel and are able to
operate across one T1 carrier line. An application of ISDN is the private branch exchange
(PBX) facility, which is fed by the T1 trunk and distributed out to the subscribers.

Would ISDN be of interest to industrial users? It could provide a gateway between a LAN and
the telephone WAN. However, on most LANs the data-signaling rates might cause a severe
bottleneck in data transfer. Some ISDN terminal adapter vendors make a modem to utilize
the entire bandwidth (128 Kbps, but you will only run at 115 Kbps). The author’s experience
with ISDN is that it is not quite ready for prime time unless you live in a major metropolitan
area and all you want is fast access to the Internet. First, configuration is not simple. Second,
many operating systems do not have drivers for an ISDN terminal adapter through the serial
port. Worse, if you wish to connect via ISDN and are a user in a less-traveled location, more
than four miles from the central office, you may experience more than just a few problems
attempting your original installation. Due to the sharply rising demand, equipment may not
be in the central office for a while. Your installation costs (depending upon the state and the
tariffs imposed) vary widely. You are still subject to recurring line charges. Lastly, remember,
another ISDN facility must lie at the other end of your transmission in order to make good
use of the ISDN features. At present, these are not plentiful. One of the good ideas that
came out of the original ISDN is frame relay, which we discuss next.