Internet in the Sky
ISDN - Integrated Services Digital
ISDN, Integrated Services Digital Network, is a set of CCIT/ITU standards for digital
transmission over ordinary telephone copper wire or other media. Users who install ISDN
adapters in place of their modems can see graphics and other information loading very
quickly up to 128-Kbps.
ISDN requires adapters at both ends of the
transmission, so the access provider also needs an ISDN adapter.
ISDN enables customers to have one or two
pairs of wires that they can use for voice, data and video simultaneously.
ISDN standards were first published in the
mid-1980s, but were only finalized in the 1990s. ISDN has an advantage over higher speed
ATM and SONET services since it can work over existing copper wiring, and does not require
fiber, as do the other two services. ISDN rates include a number of B (bearer) channels,
which carry voice, data and other services, and a D (delta) channel, which carries control
and signaling information.
ISDN consists of two levels of service the Basic Rate Interface (BRI) intended for
the home and small enterprises, and the Primary Rate Interface (PRI) meant for larger
users. Both rates are digital, with digital connectivity to achieve consistent,
high-quality calling. They both support out-of -channel signaling, which means calls are
set up quickly. Each of the voice or data channels uses all of the bandwidth for user data
leaving the signaling channel available for packet switching. Furthermore, both rates have
standard interfaces, which enables all users with ISDN to communicate with each other,
regardless of whether they are using PRI or BRI ISDN.
An issue in acquiring ISDN has been its availability. ISDN has been gaining in popularity
and availability in recent years. Despite the fact that telephone companies report that
customer requests for BRI ISDN are increasing rapidly, and that telephone company
knowledge ability in installing ISDN is growing, overall deployment is still low.
Nevertheless, as availability continues to improve, the number of people installing this
service has grown. In order to use ISDN, a user must be within 3.4 miles from the central
Basic Rate Interface consists of only 2 bearer channels for customer voice or data at
64-Kbps and one delta channel for signaling at 16-Kbps. BRI ISDN runs over a single pair
of twisted wires between the customer and the telephone company. It is typically used for
Internet access, desktop video conferencing, downloading software and connecting remote
Using BRI ISDN to download new releases of software through the net saves time compared to
using modems. Modems do not often achieve their optimal speed due to noise and
interference on the telephone line.
An increasing number of people telecommute or run home-businesses. They are finding that
they need multiple telephone lines for voice, facsimile and Internet access. This results
in poor service to customers since existing lines might be busy, and customers cannot get
The use of ISDN is advantageous since users can share one line for voice, data and fax.
People who use an ISDN line need a terminal adapter with a network terminating device.
Non-ISDN equipment is connected with a terminal adapter with a built-in
network-terminating device. The ISDN line is converted from two wires coming into the
customers premise to 4-wire inside wiring. Each non-ISDN device plugs into the back
of the terminal adapter. ISDN cannot transmit video or data to analog lines since ISDN
circuits can only communicate with each other. However, in certain situations, BRI and PRI
users can use ISDN for voice calls to end-users that have POTs (Plain Old Telephone
BRI ISDN costs in the range of $150-$300 to install, with monthly fees ranging from $10 to
$40 over and above the residential charge for an analog line. Most telephone companies
charge business customers extra per minute usage. Residential customers are typically
charged for a number of fixed hours of data transmission at a flat rate.
One of the obvious differences between BRI and PRI is that BRI has three channels and PRI,
primary rate interface, has 24 channels in the United States and Japan, and 30 in the rest
of the world. 23 are bearer channels that have a bandwidth of 64 Kbps, while the 24th
delta channel is used for signaling at a rate of 64 Kbps.
PRI lines are the same as T-1 in respect to the number of channels, but PRI ISDN has
out-of-band signaling on the 24th channel, while on T-1 circuits, the signaling
is carried within each channel along with the user data. The signaling on data
communications leaves each of the bearer channels free for all 64000 bits. There is no
capacity used for call setups or tear down signals. The 24th channel also tells
the public network that calls should be sent over the public networks data network
instead of the voice network, leaving all channels available on-demand for voice or data.
This is significant since the channels dont have to be reserved ahead of time
strictly for video or data. The setup signals perform this function which allows more
efficient utilization of the PRI trunk. While BRI ISDN runs directly from a users
location to a telephone company office, 24-channel PRI ISDN is installed on the
"trunk" side of a private branch exchange (PBX), or into a multiplexer.
Trunk-side connections are run from the PBX to the central office. PRI ISDN line is
connected to specialized equipment within a PBX as extensions. When equipment like a video
conferencing unit attached to the BRI line dials a call, the call is programmed to go out
on the PRI ISDN trunk. This trunk may be used for video conferencing at speeds from 64
Kbps to 384 Kbps.
It is also used for sending the calling party number to large call centers, backing up LAN
to LAN connections, backing up dedicated private lines in case the lines fail, and
remotely accessing corporate and ISP sites. PBXs are used with PRI lines for large call
With ISDN, the telephone number is sent at the same time as the call, but on the separate
Delta channel for signaling. This enables the PBX to treat the telephone number
information differently than the call. The telephone number can be sent to a database that
matches the customers account number. That account number can then be sent to the
agents terminal where the call is sent. This saves the agent time since they do not
have to enter the customer account number. Some organizations use ISDN-compatible video
conferencing systems as extensions of PBX so that they do not have to pay for dedicated
BRI lines to their telephone company. Instead, they have the video equipment share the PRI
along with the voice telephone users. When the video is not in use, all the PRI channels
are available for voice.
The reason PRI is able to share lines for voice and data is because of the 24th
signaling channel. This out-of-band signaling channel sends an identifier to the network,
letting the network know whether it is a video call or a data call to be sent on a
separate part of the public network specially set aside for it. Companies with multiple
PRIs at a single location usually have the option of sharing one signaling channel among
For instance, an Internet service provider with six trunks might order two of them with D
channels and the rest with 24 bearer channels to receive calls. The reason they order more
than one D channel is because if one PRI with the signaling channel goes down, the other D
channel can handle all the signaling for every PRI line till the first channel is
Internet service providers supporting ISDN access to the Internet have PRI at their
locations with multiplexers that handle multiple PRI trunks and T-1 to handle non-ISDN
calls. This way, they do not have to install separate devices for ISDN and non-ISDN calls.
ISDN multiplexers have "bandwidth-on-demand" for applications that need multiple
bearer channels. For example, if there were a video system capable of transmitting video
at 384 Kbps, needing six bearer channels, the video quality would definitely be superior
to a video system transmitting at 128 Kbps.
However, since the video systems are not used all the time, bandwidth-on-demand allows
other data applications to make use of the six bearer channels when the system is not in
use. Multiple devices can share the PRI circuit, so it is an economical means of
transmitting large amounts of data at high speeds.