Integrated Services Digital Network (
ISDN) is a set of communications standards for simultaneous
digital transmission of voice, video, data, and other network services over the traditional circuits of the
public switched telephone network. It was first defined in 1988 in the
CCITT red book.
Prior to ISDN, the phone system was viewed as a way to transport voice, with some special services available for data.The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as
Basic Rate Interface (BRI),
Primary Rate Interface (PRI) and
Broadband ISDN (B-ISDN).
ISDN is a
circuit-switched telephone network system, which also provides access to
packet switched networks, designed to allow digital transmission of voice and
data over ordinary
telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64
kilobit/s. A major market application for ISDN in some countries is
Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions.
Channel bonding can achieve a greater data rate; typically the ISDN B-channels of 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.
ISDN should not be mistaken for its use with a specific protocol, such as
Q.931 whereby ISDN is employed as the network, data-link and physical layers in the context of the
OSI model. In a broad sense ISDN can be considered a suite of digital services existing on layers 1, 2, and 3 of the OSI model. ISDN is designed to provide access to voice and data services simultaneously.
In a
videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.
ISDN elements
Integrated services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice, video, and fax, over a
single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone line. It also refers to Integrated Switching and Transmission
in that
telephone switching and
carrier wave transmission are integrated rather than separate as in earlier technology.
Basic Rate Interface
The entry level interface to ISDN is the Basic(s) Rate Interface (BRI), a 128
kbit/s service delivered over a pair of standard telephone copper wires. The 144 kbit/s rate is broken down into two 64 kbit/s bearer channels ('B' channels) and one 16 kbit/s signaling channel ('D' channel or delta channel).
BRI is sometimes referred to as 2B+D
The interface specifies the following network interfaces:
- The U interface is a two-wire interface between the exchange and a network terminating unit, which is usually the demarcation point in non-North American networks.
- The T interface is a serial interface between a computing device and a terminal adapter, which is the digital equivalent of a modem.
- The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on an NT1
- The R interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.
BRI-ISDN is very popular in Europe but is much less common in North America. It is also common in Japan - where it is known as INS64.
Primary Rate Interface
The other ISDN service available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 'B' channels of 64 kbit/s, one 'D' channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s. In North America PRI service is delivered on one or more T1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1).
In North America, NFAS allows two or more PRIs to be controlled by a single D channel, and is sometimes called "23B+D + n*24B". D-channel backup allows for a second D channel in case the primary fails. One popular use of NFAS is on a T3.
PRI-ISDN is popular throughout the world, especially for connection of
PSTN circuits to
PBXs.
Even though many network professionals use the term "ISDN" to refer to the lower-bandwidth BRI circuit, in North America by far the majority of ISDN services are in fact PRI circuits serving PBXs.
Data channel
The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with
G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.
Each one of these channels is known as a
DS0.
Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over
RBS lines. This was commonplace in the 20th century, but has since become less so.
Signaling channel
The signaling channel (D) uses
Q.931 for signaling with the other side of the link.
X.25
X.25 can be carried over the B or D channels of a BRI line, and over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale (credit card) terminals because it eliminates the modem setup, and because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines.
X.25 was also part of an ISDN protocol called "Always On/Dynamic ISDN", or AO/DI. This allowed a user to have a constant multi-link PPP connection to the internet over X.25 on the D channel, and brought up one or two B channels as needed.
ISDN and broadcast industry
ISDN is used heavily by the broadcast industry as a reliable way of switching low latency, high quality, long distance audio circuits. In conjunction with the appropriate
codec an ISDN BRI can be used to send stereo bi-directional audio with 20Hz-20 kHz bandwidth, although commonly the aging
G.722 codec is used over a single B channel lending the resultant audio a low-fi "ISDN sound". Where very high quality audio is required multiple ISDN BRIs can be used in parallel to provide a higher bandwidth circuit switched connection.
BBC Radio 3 commonly makes use of three ISDN BRIs to carry 320kbps audio stream for live outside broadcasts. ISDN BRI services are used to link remote studios, sports grounds and outside broadcasts into the main broadcast studio. ISDN via satellite is used by field reporters around the world. It's also common to use ISDN for the return audio links to remote satellite broadcast vehicles.
IP based streaming codecs such as the Comrex Access are starting to gain a foothold in the broadcast sector, using broadband internet to connect remote studios. However reliability is crucially important for broadcasters and the quality of service offered by ISDN has not yet been matched by packet switched alternatives.
United States and Canada
ISDN-BRI has never gained popularity as a general use telephone access technology in Canada and the US and today remains a niche product. The service was seen as
a solution in search of a problem,
[4] and the extensive array of options and features were difficult for most customers to understand and utilize. ISDN has long been known by several derogatory acronyms highlighting these issues, such as
It Still Does Nothing, Innovations Subscribers Don't Need, and
I Still Don't kNow.[5][6]
Once the concept of
broadband Internet access came to be associated with data rates incoming to the customer or 256 kbit/s or more,
[7] and alternatives like
ADSL grew in popularity, the consumer market for BRI imploded. Its only remaining positive element is that while ADSL has a functional distance limitation, BRI has a greater limit and can use repeaters. As such, BRI may be acceptable in situations where the customer is too remote for ADSL to work. Widespread use of BRI is further stymied by some small North American
CLECs such as
CenturyTel having given up on it and not providing Internet access using it.
[8] However, AT&T in most states (especially the former SBC/SWB territory) will still install an ISDN BRI line anywhere a normal analog line can be placed and the monthly charge is roughly $55.
ISDN-BRI is currently primarily used in industries with specialized and very specific needs. High-end videoconferencing made by companies such as
Sony,
Polycom,
Tandberg, and
LifeSize via the
LifeSize Networker bond up to 8 B-channels together (using a BRI circuit for every 2 channels) to provide digital, circuit-switched video connections to almost anywhere in the world. This is very expensive, and is being replaced by IP-based conferencing, but where cost concern is less of an issue than predictable quality and where a QoS-enabled IP does not exist, BRI is the preferred choice.
Most modern non-VoIP PBXs use ISDN-PRI circuits. These are connected via
T1 lines with the central office switch, replacing older analog two-way and
direct inward dialing (DID) trunks. PRI is capable of delivering
Calling Line Identification (CLID) in both directions so that the telephone number of an extension, rather than a company's main number, can be sent. It is still commonly used in
recording studios, when a voice-over actor is in one studio, but the director and producer are in a studio at another location. The ISDN protocol delivers channelized, not-over-the-Internet service, powerful call setup and routing features, faster setup and tear down, superior audio fidelity (as compared to
POTS (plain old telephone service), lower delay and, at higher densities, lower cost.
Configurations
In ISDN, there are two types of channels, B (for "bearer") and D (for "delta"). B channels are used for data (which may include voice), and D channels are intended for signaling and control (but can also be used for data).
There are two ISDN implementations. Basic Rate Interface (BRI), also called basic rate access (BRA) — consists of two B channels, each with bandwidth of 64
kbit/s, and one D channel with a bandwidth of 16 kbit/s. Together these three channels can be designated as 2B+D. Primary Rate Interface (PRI), also called primary rate access (PRA) in Europe — contains a greater number of B channels and a D channel with a bandwidth of 64 kbit/s. The number of B channels for PRI varies according to the nation: in North America and Japan it is 23B+1D, with an aggregate bit rate of 1.544
Mbit/s (
T1); in Europe, India and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (
E1).
Broadband Integrated Services Digital Network (BISDN) is another ISDN implementation and it is able to manage different types of services at the same time. It is primarily used within
network backbones and employs
ATM.
Another alternative ISDN configuration can be used in which the B channels of an
ISDN BRI line are bonded to provide a total duplex bandwidth of 128 kbit/s. This precludes use of the line for voice calls while the internet connection is in use. The B channels of several BRIs can be BONDED, a typical use is a 384K videoconferencing channel.
Using
bipolar with eight-zero substitution encoding technique, call data is transmitted over the data (B) channels, with the signaling (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 (actually implemented as two simplex channels, one in each direction) between the end parties, lasting until the call is terminated. There can be as many calls as there are bearer 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, or via use of Multi-Link PPP "bundling" or by using an H0, H11, or H12 channel on a PRI.
The D channel can also be used for sending and receiving
X.25 data packets, and connection to X.25 packet network, this is specified in
X.31. In practice, X.31 was only commercially implemented in UK, France and Japan.
