ISDN

The original version of ISDN employs base band transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mbps. B- ISDN requires fiber optic cables and is not widely available.

ISDN, which stands for Integrated Services Digital Network, is a system of digital phone connections which has been available for over a decade. This system allows voice and data to be transmitted simultaneously across the world using end-to-end digital connectivity.

ISDN (Integrated Services Digital Network) is a type of digital phone/data and Internet service that preceded ADSL (Asynchronous Digital Subscriber Line) and has for the most part been superseded by it.

Normal telephone lines carry analog signals that must be amplified and converted to digital signals by the phone company. This process introduces not only a slight lag time, but also distortion in the signal.

Dial-up modems and telephones are examples of equipment that use analog signals. ISDN makes use of digital signals running along existing copper lines to increase the data throughput, reduce line noise and enhance signal quality.

In the mid 1990s, ADSL was very expensive not widely available. Companies and individuals wanted a faster way to connect to the Internet, but the technology behind dial-up modems had reached its threshold.

ISDN became a viable alternative to provide speeds of up to 128 kilobits per second (kbps), versus the standard connection of 30-53 kbps with a dial-up modem.

The most common type of ISDN service for Internet connection is the Basic Rate Interface, or ISDN BRI. This technology creates two B-channels on the existing copper lines of 64 kbps each, along with a single 16 kbps D-channel for the phone line. This separates data channels from the voice channel, allowing telephone or fax use while online.

While ISDN is inexpensive and about twice as fast as dial-up service, it has been largely replaced by affordable DSL service. An inexpensive ADSL service offers speeds up to 384 kbps, while more expensive versions are improving in speed all the time. As of fall 2005, standard ADSL speeds range between 1.5 and 3.0 mbps (megabits per second), or 1536-3072 kbps.

Although ISDN may not be the best choice for packet-switching networks like the Internet, it is still widely used for professional audio and broadcast applications where digital clarity with integrated telephone services is specifically required.

Small businesses that often use two voice lines, such as phone and fax, and only require limited Internet connectivity of, say, an hour or less per day, may prefer ISDN. ISDN might also be a better choice for high-speed connections to intranets for video-conferencing, or to remote networks other than the Internet.

With ISDN, voice and data are carried by bearer channels (B channels) occupying a bandwidth of 64 kb/s (bits per second). Some switches limit B channels to a capacity of 56 kb/s. A data channel (D channel) handles signaling at 16 kb/s or 64 kb/s, depending on the service type. Note that, in ISDN terminology, "k" means 1000 (103), not 1024 (210) as in many computer applications (the designator "K" is sometimes used to represent this value); therefore, a 64 kb/s channel carries data at a rate of 64000 b/s.

A new set of standard prefixes has recently been created to handle this. Under this scheme, "k" (kilo-) means 1000 (103), "M" (mega-) means 1000000 (106), and so on, and "Ki" (kibi-) means 1024 (210), "Mi" (mebi-) means 1048576 (220), and so on.

There are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64 kb/s B channels and one 16 kb/s D channel for a total of 144 kb/s. This basic service is intended to meet the needs of most individual users.

PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s. In Europe, PRI consists of 30 B channels plus one 64 kb/s D channel for a total of 1984 kb/s.

It is also possible to support multiple PRI lines with one 64 kb/s D channel using Non-Facility Associated Signaling (NFAS).

H channels provide a way to aggregate B channels. They are implemented as:

H0=384 kb/s (6 B channels)
H10=1472 kb/s (23 B channels)
H11=1536 kb/s (24 B channels)
H12=1920 kb/s (30 B channels) - International (E1) only

To access BRI service, it is necessary to subscribe to an ISDN phone line. Customer must be within 18000 feet (about 3.4 miles or 5.5 km) of the telephone company central office for BRI service; beyond that, expensive repeater devices are required, or ISDN service may not be available at all.

Customers will also need special equipment to communicate with the phone company switch and with other ISDN devices. These devices include ISDN Terminal Adapters (sometimes called, incorrectly, "ISDN Modems") and

ISDN Routers.

The early phone network consisted of a pure analog system that connected telephone users directly by a mechanical interconnection of wires. This system was very inefficient, was very prone to breakdown and noise, and did not lend itself easily to long-distance connections. Beginning in the 1960s, the telephone system gradually began converting its internal connections to a packet-based, digital switching system.

Today, nearly all voice switching in the U.S. is digital within the telephone network. Still, the final connection from the local central office to the customer equipment was, and still largely is, an analog Plain-Old Telephone Service (POTS) line.

In the early 1990s, an industry-wide effort began to establish a specific implementation for ISDN in the U.S. Members of the industry agreed to create the National ISDN 1 (NI-1) standard so that end users would not have to know the brand of switch they are connected to in order to buy equipment and software compatible with it.

However, there were problems agreeing on this standard. In fact, many western states would not implement NI-1. Both Southwestern Bell and U.S. West (now Qwest) said that they did not plan to deploy NI-1 software in their central office switches due to incompatibilities with their existing ISDN networks.

Most recently, ISDN service has largely been displaced by broadband internet service, such as xDSL and Cable Modem service. These services are faster, less expensive, and easier to set up and maintain than ISDN. Still, ISDN has its place, as backup to dedicated lines, and in locations where broadband service is not yet available.