Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital (as opposed to analog) transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds, than is capable with analog systems. More broadly, ISDN is a set of protocols for establishing and breaking circuit switched connections, and for advanced call features for the end user.
Consumer and industry perspectives
There are two points of view into the ISDN world. The most common viewpoint is that of the end user who wants to get a digital connection into the telephone/data network from home, whose perforcemance would be a little better than a modem connection. They typical end-user's connection to the Internet is related to this point of view, and talk about the merits of various ISDN modems, carriers' offerings and tarriffing (features, pricing) are from this perspective. Much of the following discussion is from this point of view, but it should be noted that as a data connection service, ISDN has been mostly superseded by DSL..
There is however a second viewpoint, that of the telephone industry, where ISDN is not a dead issue. A telephone network can be thought of as a collection of wires strung between switching systems. The common electrical specification for the signals on these wires is T1 or E1. On a normal T1, the signalling is done with A&B bits to indicate on or off hook conditions and MF and DTMF tones to encode the destination number. ISDN is much better than this as messages can be sent much more quickly than by trying to encode numbers as long (100 ms per digit) tone sequences. This translated to much faster call setup times which is greatly desired by carriers who have to pay for line time and also by callers who hate to wait while their call hops from switch to switch.
It is also used as a smart network technology intended to add new services to the public switched telephone network (the PSTN) by giving users direct access to end-to-end circuit-switched digital services.
ISDN has never gained popularity as a telephone network in the United States and today remains a niche product. In Japan, it became popular to some extent from around 1999 to 2001, but now that ADSL has been introduced, the number of subscribers is in decline.
In Japan, NTT, a dominant telephone company, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.
In the UK, British Telecom (BT) provides Home Highway and Business Highway, which are BRI ISDN services which offer connection from analog devices (such as normal phones) as well as ISDN devices (such as PCs equipped with terminal adapters). Home Highway has been bought by many home users, usually for Internet connection. Although not as fast as ADSL, it was available before ADSL, and in places where ADSL does not reach. BT also offers PRI ISDN.
In France, France Telecom offers ISDN services under their product name Numeris (2 B+D) of which a profesional Duo and home Itoo version is available. ISDN is generally known as RNIS in France and has widespread availability. The introduction of ADSL is reducing ISDN use for data transfer and internet access, although it is still common in more rural and outlying areas.
In Germany, ISDN is very popular with an installed base of 25 mio. channels (29% of all subscriber lines in Germany as of 2003 and 20% of all ISDN channels worldwide). Due to the success of ISDN, the number of installed analog lines is decreasing. Deutsche Telekom (DTAG) offers both BRI and PRI. Competing phone companies often offer ISDN only and no analog lines.
In ISDN, there are two types of channels, B and D:
There are two kinds of access to ISDN:
- B channels are used for data--this refers both to voice and data information, and
- D channels are intended for signalling and control (but can also be used for data).
- B stands for Bearer and D stands for Delta.
Call data is transmitted over the data (B) channels, with the signalling (D) channels used for call setup and management. Once a call is set up, there is a simple 64 kbit/s synchronous bidirectional data channel between the end parties, lasting until the call is terminated. There can be as many calls as there are data channels, to the same or different end-points. Bearer channels may also be multiplexed into what may be considered single, higher-bandwidth channels via a process called B channel bonding.
- Basic Rate Interface (BRI) - consisting of two B channels, each with bandwidth of 64 kbit/s, and one D channel with a bandwitdth of 16 kbit/s. Together these three channels cna be designated as 2B+D, and,
- Primary Rate Interface (PRI) - containing a greater number of B channels and a D channel with a bandwidth of 64 kbit/s, The number of B channels varies based on the country:
North America and Japan: 23B+1D, aggregate bit rate of 1.544 Mbit/s (T1),
Europe, Australia: 30B+D, aggregate bit rate of 2.048 Mbit/s (E1).
The D channel can also be used for sending and receiving X.25 data packets, and connection to X.25 packet network. In practice, this was never widely implemented.
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