ISDN - Integrated Services Digital
Network


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.

1. ISDN requires adapters at both ends of the transmission, so the access provider also needs an ISDN adapter.

2. 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 office.


BRI ISDN

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 LANs together.

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 through.

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 customer’s 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 lines).

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.


PRI ISDN

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 24^th 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 24^th 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 24^th channel also tells the public network that calls should be sent over the public network’s data network instead of the voice network, leaving all channels available on-demand for voice or data.

This is significant since the channels don’t 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 user’s 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 centers.

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 customer’s account number. That account number can then be sent to the agent’s 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 24^th 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 many trunks.

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 restored.

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.