 |
|
| ||||||||||||||||||||||||||
|
|
| ||||||||||||||||||||||||||
|
|
|
|
|
| ||||||||
Integrated Services Digital Network (ISDN) is a type of telephone network system. More specifically, ISDN is a set of protocols for establishing calls and then tearing them down.
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 connection into the telephone/data network at home, something a little better than a modem connection. Most of the links that would be on the Internet are related to this point of view and talk about the merits of various ISDN modems, carrier's offerings and tarriffing (features, pricing). Much of the following discussion is from this point of view and correctly points out that from this viewpoint, ISDN is mostly superseded by DSL as a viable connection to the Internet.
There is however a second viewpoint, that of the telephone industry, where ISDN is not a dead issue. The telephone network in some ways is 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:
B stands for Bearer and D stands for Delta.
There are two kinds of access to ISDN:
Calls are made 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 users, 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.
A set of reference points are defined in the ISDN standard to refer to certain points between the telco and the end user ISDN equipment.
1 Most NT-1 devices can perform the functions of the NT-2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.
2 Inside North America, the NT-1 device is considered customer premises equipment and must be maintained by the customer, thus, the U interface is provided to the customer. In other locations, the NT-1 device is maintained by the telco, and the S/T interface is provided to the customer.
Amongst the kinds of data that can be moved over the 64 kbit/s channels are pulse-code modulated voice calls, providing access to the traditional voice PSTN. This information can be passed between the network and the user end-point at call set-up time.
In North America, ISDN is nowadays mostly used as an alternative to analog connection, most commonly for Internet access. Some of the services envisaged as being delivered over ISDN are now delivered over the Internet instead. In Europe, and in Germany in paticular, ISDN has been successfully marketed as a phone with features, as opposed to a POTS (Plain Old Telephone Service) with little to no features. In North America, POTS phones have all of the features available to them marketed (Three Way Call, Call Forwarding, Caller ID, etc), but in Germany, the ISDN lines were marketed as being the phones for people who wanted advanced services.
Where an analog connection would require a modem an ISDN connection requires a terminal adapter (TA).
The following is an example of a Primary Rate (PRI) ISDN call showing the Q.921/LAPD and the Q.931/Network message intermixed (i.e. exactly what was exchanged on the D-channel). The call is originating from the switch where the trace was taken and goes out to some other switch, possibly an end-office LEC, who terminates the call.
The first line format is <time> <D-channel> <Transmitted/Received> <LAPD/ISDN message ID>. If the message is an ISDN level message, then a decoding of the message is attempted showing the various Information Elements that make up the message. All ISDN messages are tagged with an ID number relative to the switch that started the call (local/remote). Following this optional decoding is a dump of the bytes of the message in <offset> <hex> ... <hex> <ascii> ... <ascii> format.
The RR messages at the beginning prior to the call are the keep alive messages. Then you will see a SETUP message that starts the call. Each message is acknowledged by the other side with a RR.
Specifications defining the physical layer and part of the data link layers of ISDN:
|
||||
| View Live Article | This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License | |
||
