Integrated Services Digital Network
Encyclopedia
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 telephone 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 network
s, 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.
However, common use reduced ISDN to be limited to Q.931 and related protocols, which are a set of protocols for establishing and breaking circuit switched connections
, and for advanced calling features for the user. They were introduced in 1986.
In a videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.
, 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 (apart from broadband Internet access
and entertainment television
) at a much higher transmission rate, without forcing the purchase of multiple analog phone lines. 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.
s ('B' channels) and one 16 kbit/s signaling channel ('D' channel or delta channel). This is sometimes referred to as 2B+D.
The interface specifies the following network interfaces:
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.
In North America PRI service is delivered on one or more T1 carriers (often referred to as 23B+D) of 1544 kbit/s (24 channels). A PRI has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1). Inter-changeably but incorrectly, a PRI is referred to as T1 because it uses the T1 carrier format. A true T1 or commonly called 'Analog T1' to avoid confusion uses 24 channels of 64 kbit/s of in-band signaling
. Each channel uses 56 kb for data and voice and 8 kb for signaling and messaging. PRI uses out of band signaling which provides the 23 B channels with clear 64 kb for voice and data and one 64 kb 'D' channel for signaling and messaging. In North America, Non-Facility Associated Signalling
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. NFAS is commonly used on a T3
.
PRI-ISDN is popular throughout the world, especially for connecting PBXs to PSTN.
While the North American PSTN can use PRI or Analog T1 format from PBX to PBX, the POTS
or BRI can be delivered to a business or residence. North American PSTN can connect from PBX to PBX via Analog T1, T3, PRI, OC3, etc...
Even though many network professionals use the term "ISDN" to refer to the lower-bandwidth BRI circuit, in North America BRI is relatively uncommon whilst PRI circuits serving PBXs are commonplace.
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.
for signaling with the other side of the link.
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.
can operate over the D channel of BRIs and PRIs, but it is seldom, if ever, used.
from home, whose performance would be better than a 20th century analog 56K modem
connection. Discussion on the merits of various ISDN modems, carriers' offerings and tariffs (features, pricing) are from this perspective. Since the principal consumer application is for Internet access
, ISDN was mostly superseded by DSL
in the early 21st century. Inexpensive ADSL service offers speeds up to 384 kbit/s, while more expensive versions are improving in speed all the time. As of fall 2005, standard ADSL speeds are in millions of bits per second.
There is a second viewpoint: that of the telephone industry, where ISDN is a core technology. 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
. Between telephone company switches, the signaling is performed via SS7. Normally, a PBX is connected via a T1 with robbed bit signaling
to indicate on-hook or off-hook conditions and MF
and DTMF
tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode
numbers as long (100 ms
per digit) tone sequences. This results in faster call setup times. Also, a greater number of features are available and fraud is reduced.
ISDN is also used as a smart-network technology intended to add new services to the public switched telephone network
(PSTN) by giving users direct access to end-to-end circuit-switched digital services and as a backup or failsafe circuit solution for critical use data circuits.
using MPEG or various manufacturers proprietary algorithms, an ISDN BRI can be used to send stereo bi-directional audio coded at 128 kbit/s with 20 Hz-20 kHz audio bandwidth, although commonly the G.722
algorithm is used with a single 64 kbit/s B channel to send much lower latency mono audio at the expense of audio quality. 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 320 kbit/s 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.
In many countries, such as the UK and Australia, ISDN has displaced the older technology of equalised analogue landlines, with these circuits being phased out by telecommunications providers. IP based streaming codecs are starting to gain a foothold in the broadcast sector, using broadband internet to connect remote studios. However reliability and latency is crucially important for broadcasters and the quality of service offered by ISDN has not yet been matched by packet switched alternatives.
. The service was seen as a solution in search of a problem, 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.
Once the concept of broadband Internet access
came to be associated with data rates incoming to the customer at 256 kbit/s or more, 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. 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 studio
s, 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.
, Reliance Communications
and Bharti Airtel
, are the largest communication service providers, offers both ISDN BRI and PRI services across the country. Reliance Communications and Bharti Airtel uses the DLC technology for providing these services. With the introduction of broadband technology, the load on bandwidth is being absorbed by ADSL. ISDN continues to be an important backup network for point-to-point leased line customers such as banks, Eseva Centers http://www.esevaonline.com, Life Insurance Corporation of India
, and SBI
ATMs.
, a dominant Japanese telephone company
, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.
, British Telecom
(BT) provides ISDN2e (BRI) as well as ISDN30 (PRI). Until April 2006, they also offered services named Home Highway and Business Highway, which are BRI ISDN-based services that offer integrated analog connectivity as well as ISDN. Later versions of the Highway products also included built-in Universal serial bus
(USB) sockets for direct computer access. Home Highway has been bought by many home users, usually for Internet connection, although not as fast as ADSL, because it was available before ADSL and in places where ADSL does not reach.
offers ISDN services under their product name Numeris (2 B+D), of which a professional 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, and for applications such as business voice and point-of-sale terminals
.
, ISDN is very popular with an installed base of 25 million 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. However, these operators generally offer free hardware that also allows the use of POTS equipment, such as NTBAs with integrated terminal adapter
s. Because of the widespread availability of ADSL services, ISDN is today primarily used for voice and fax traffic, but is still very popular thanks to the pricing policy of German telecommunication providers.
Today ISDN (BRI) and ADSL/VDSL are often bundled on the same line, mainly because the combination of ADSL with an analog line has no cost advantage over a combined ISDN-ADSL line. Some German operators started to implement Next Generation Networking
, generally realized via DSL and unbundled local loop. However, a few operators offer the same services via the cable television infrastructure
or, in selected areas, via FTTH. Because of the popularity of ISDN, virtually all these telecommunication providers bundle their products with residential gateway
s that include both integrated analog telephony adapter
s and ISDN-NGN adapters.
, the incumbent telecommunications operator, offers ISDN BRI (BRA) services in Greece
. Following the launch of ADSL in 2003, the importance of ISDN for data transfer began to decrease and is today limited to niche business applications with point-to-point requirements.
Other ISDN providers are Reliance
, and VSNL.
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.
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 (CPE) 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. In India, service providers provide U interface and an NT1 may be supplied by Service provider as part of service offering
In North America, ISDN is now used mostly as an alternative to analog connections, most commonly for Internet access. Some of the services envisioned as being delivered over ISDN are now delivered over the Internet instead. In Europe, and in Germany in particular, ISDN has been successfully marketed as a phone with features, as opposed to a POTS phone with few or no features. Meanwhile, features that were first available with ISDN (such as Three-Way Calling, Call Forwarding
, Caller ID
, etc.) are now commonly available for ordinary analog phones as well, eliminating this advantage of ISDN. Another advantage of ISDN was the possibility of multiple simultaneous calls (one call per B channel), e.g. for big families, but with the increased popularity and reduced prices of mobile telephony this has become less interesting as well, making ISDN unappealing to the private customer. However, ISDN is typically more reliable than POTS, and has a significantly faster call setup time compared with POTS, and IP connections over ISDN typically have some 30–35ms round trip time, as opposed to 120–180ms (both measured with otherwise unused lines) over 56k or V.34/V.92 modems, making ISDN more reliable and more efficient for telecommuters.
Where an analog connection requires a modem
, an ISDN connection requires a terminal adapter
(TA). The function of an ISDN terminal adapter is often delivered in the form of a PC card with an S/T interface, and single-chip solutions seem to exist, considering the plethora of combined ISDN- and ADSL-routers.
ISDN is commonly used in radio broadcasting. Since ISDN provides a high quality connection this assists in delivering good quality audio for transmission in radio. Most radio studios are equipped with ISDN lines as their main form of communication with other studios or standard phone lines. Equipment made by companies such as Telos/Omnia (the popular Zephyr codec), Comrex, Tieline and others are used regularly by radio broadcasters. Almost all live sports broadcasts on radio are backhauled to their main studios via ISDN connections.
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:
From the point of view of the OSI architecture, an ISDN line has a stack of three protocols
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
of voice, video, data, and other network services over the traditional circuits of the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
. It was first defined in 1988 in the CCITT red book.
Prior to ISDN, the telephone 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
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface
Basic rate interface
Basic Rate Interface is an Integrated Services Digital Network configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service...
(BRI), Primary Rate Interface
Primary rate interface
The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....
(PRI) and Broadband ISDN (B-ISDN).
ISDN is a circuit-switched
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
telephone network
Telephone network
A telephone network is a telecommunications network used for telephone calls between two or more parties.There are a number of different types of telephone network:...
system, which also provides access to packet switched network
Packet switched network
A packet-switched network is a digital communications network that groups all transmitted data, irrespective of content, type, or structure into suitably sized blocks, called packets...
s, designed to allow digital transmission of voice and data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
over ordinary telephone copper wires
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
, 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
Kilobit
The kilobit is a multiple of the unit bit for digital information or computer storage. The prefix kilo is defined in the International System of Units as a multiplier of 103 , and therefore,...
/s. A major market application for ISDN in some countries is Internet access
Internet access
Many technologies and service plans for Internet access allow customers to connect to the Internet.Consumer use first became popular through dial-up connections in the 20th century....
, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions. Channel bonding
Channel bonding
Channel bonding is a computer networking arrangement in which two or more network interfaces on a host computer are combined for redundancy or increased throughput....
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
Q.931
ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...
whereby ISDN is employed as the network, data-link and physical layers in the context of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. 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.
However, common use reduced ISDN to be limited to Q.931 and related protocols, which are a set of protocols for establishing and breaking circuit switched connections
Signalling (telecommunications)
In telecommunication, signaling has the following meanings:*the use of signals for controlling communications...
, and for advanced calling features for the user. They were introduced in 1986.
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, videoVideotelephony
Videotelephony comprises the technologies for the reception and transmission of audio-video signals by users at different locations, for communication between people in real-time....
, and fax
Fax
Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...
, over a single line
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
. 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 (apart from broadband Internet access
Broadband Internet access
Broadband Internet access, often shortened to just "broadband", is a high data rate, low-latency connection to the Internet— typically contrasted with dial-up access using a 56 kbit/s modem or satellite Internet with inherently high latency....
and entertainment television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
) at a much higher transmission rate, without forcing the purchase of multiple analog phone lines. It also refers to integrated switching and transmission in that telephone switching and carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
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 payload rate is broken down into two 64 kbit/s bearer channelBearer channel
A bearer channel is a DS-0 that carries call content i.e. one that does not carry signaling.In the Common Channel Signaling scheme for telecommunications, signaling is sent out-of-band, while all other traffic rides bearer channels...
s ('B' channels) and one 16 kbit/s signaling channel ('D' channel or delta channel). This 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 pointDemarcation pointIn telephony, the demarcation point is the point at which the public switched telephone network ends and connects with the customer's on-premises wiring. It is the dividing line which determines who is responsible for installation and maintenance of wiring and equipment -- customer/subscriber, or...
in non-North American networks. - The T interface is a serial interface between a computing device and a terminal adapterTerminal adapter- ISDN :In ISDN terminology, a terminal adapter or TA is a device that connects a terminal to the ISDN network.The TA therefore fulfills a similar function to the ones a modem has on the POTS network, and is therefore sometimes called an ISDN modem...
, 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 Network termination 1Network termination 1Network Termination 1 : In Integrated Services Digital Networks , a functional grouping of customer-premises equipment that includes functions that may be regarded as belonging to OSI Layer 1, i.e., functions associated with ISDN electrical and physical terminations on the user premises.Note: The...
(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 access 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 T1 carriers (often referred to as 23B+D) of 1544 kbit/s (24 channels). A PRI has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1). Inter-changeably but incorrectly, a PRI is referred to as T1 because it uses the T1 carrier format. A true T1 or commonly called 'Analog T1' to avoid confusion uses 24 channels of 64 kbit/s of in-band signaling
In-band signaling
In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...
. Each channel uses 56 kb for data and voice and 8 kb for signaling and messaging. PRI uses out of band signaling which provides the 23 B channels with clear 64 kb for voice and data and one 64 kb 'D' channel for signaling and messaging. In North America, Non-Facility Associated Signalling
Non-Facility Associated Signalling
Non-Facility Associated Signalling or NFAS is a Primary Rate Interface configuration whereby multiple T1 carriers share a signaling channel ....
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. NFAS is commonly used on a T3
Digital Signal 3
A Digital Signal 3 is a digital signal level 3 T-carrier. It may also be referred to as a T3 line.*The data rate for this type of signal is 44.736 Mbit/s.*This level of carrier can transport 28 DS1 level signals within its payload....
.
PRI-ISDN is popular throughout the world, especially for connecting PBXs to PSTN.
While the North American PSTN can use PRI or Analog T1 format from PBX to PBX, the POTS
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
or BRI can be delivered to a business or residence. North American PSTN can connect from PBX to PBX via Analog T1, T3, PRI, OC3, etc...
Even though many network professionals use the term "ISDN" to refer to the lower-bandwidth BRI circuit, in North America BRI is relatively uncommon whilst PRI circuits serving PBXs are commonplace.
Data channel
The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711G.711
G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...
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.931Q.931
ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...
for signaling with the other side of the link.
X.25
X.25X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
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
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
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.
Frame Relay
In theory, Frame RelayFrame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
can operate over the D channel of BRIs and PRIs, but it is seldom, if ever, used.
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 networkTelephone network
A telephone network is a telecommunications network used for telephone calls between two or more parties.There are a number of different types of telephone network:...
from home, whose performance would be better than a 20th century analog 56K modem
56K modem
56k modems are voiceband modems nominally capable of download speeds up to 56 kbit/s . In the late 1990s, they were the most popular access method for personal Internet usage, but their use had declined as broadband technologies such as DSL gained wider availability.-Speed:The 56 kbit/s...
connection. Discussion on the merits of various ISDN modems, carriers' offerings and tariffs (features, pricing) are from this perspective. Since the principal consumer application is for Internet access
Internet access
Many technologies and service plans for Internet access allow customers to connect to the Internet.Consumer use first became popular through dial-up connections in the 20th century....
, ISDN was mostly superseded by DSL
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
in the early 21st century. Inexpensive ADSL service offers speeds up to 384 kbit/s, while more expensive versions are improving in speed all the time. As of fall 2005, standard ADSL speeds are in millions of bits per second.
There is a second viewpoint: that of the telephone industry, where ISDN is a core technology. 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
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
or E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
. Between telephone company switches, the signaling is performed via SS7. Normally, a PBX is connected via a T1 with robbed bit signaling
Robbed bit signaling
In communication systems robbed-bit signaling is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using Channel Associated Signaling ....
to indicate on-hook or off-hook conditions and MF
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
and DTMF
Dual-tone multi-frequency
Dual-tone multi-frequency signaling is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is...
tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode
Encode
Encode may refer to:* Can be related to "Code"* Encode ApS, a Danish software company* Encode SA, a Greek information security company* ENCODE, the ENCyclopedia Of DNA Elements...
numbers as long (100 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
per digit) tone sequences. This results in faster call setup times. Also, a greater number of features are available and fraud is reduced.
ISDN is also used as a smart-network technology intended to add new services to the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
(PSTN) by giving users direct access to end-to-end circuit-switched digital services and as a backup or failsafe circuit solution for critical use data circuits.
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 an appropriate codecCodec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
using MPEG or various manufacturers proprietary algorithms, an ISDN BRI can be used to send stereo bi-directional audio coded at 128 kbit/s with 20 Hz-20 kHz audio bandwidth, although commonly the G.722
G.722
G.722 is a ITU-T standard 7 kHz wideband speech codec operating at 48, 56 and 64 kbit/s. It was approved by ITU-T in November 1988. Technology of the codec is based on sub-band ADPCM ....
algorithm is used with a single 64 kbit/s B channel to send much lower latency mono audio at the expense of audio quality. 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
BBC Radio 3
BBC Radio 3 is a national radio station operated by the BBC within the United Kingdom. Its output centres on classical music and opera, but jazz, world music, drama, culture and the arts also feature. The station is the world’s most significant commissioner of new music, and its New Generation...
commonly makes use of three ISDN BRIs to carry 320 kbit/s audio stream for live outside broadcasts. ISDN BRI services are used to link remote studios, sports grounds and outside broadcasts
Remote broadcast
In broadcast engineering, a remote broadcast is broadcasting done from a location away from a formal television studio and is considered an electronic field production . A remote pickup unit is usually used to transmit the audio and/or video back to the television station, where it joins the...
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.
In many countries, such as the UK and Australia, ISDN has displaced the older technology of equalised analogue landlines, with these circuits being phased out by telecommunications providers. IP based streaming codecs are starting to gain a foothold in the broadcast sector, using broadband internet to connect remote studios. However reliability and latency 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 productNiche market
A niche market is the subset of the market on which a specific product is focusing; therefore the market niche defines the specific product features aimed at satisfying specific market needs, as well as the price range, production quality and the demographics that is intended to impact...
. The service was seen as a solution in search of a problem, 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.
Once the concept of broadband Internet access
Broadband Internet access
Broadband Internet access, often shortened to just "broadband", is a high data rate, low-latency connection to the Internet— typically contrasted with dial-up access using a 56 kbit/s modem or satellite Internet with inherently high latency....
came to be associated with data rates incoming to the customer at 256 kbit/s or more, 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. 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
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
, Polycom
Polycom
Polycom is a multinational corporation with approximately 3,200 employees worldwide and an annual revenue of approximately $1.2 billion in 2010. The company manufactures and sells telepresence and voice communications solutions.-Company History 1990:...
, Tandberg
Tandberg
This article is about the video-conferencing vendor, a Cisco company. Other companies with the same name such as Tandberg Television or Tandberg Data for data storage: see here and here...
, and LifeSize
LifeSize
LifeSize, a division of Logitech, is a video and audio communications company that provides high definition video conferencing endpoints and accessories, infrastructure products and a cloud-based video collaboration platform. LifeSize's worldwide headquarters is located in Austin, Texas...
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
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
-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
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
with the central office switch, replacing older analog two-way and direct inward dialing
Direct Inward Dialing
Direct inward dialing , also called direct dial-in in Europe and Oceania, is a feature offered by telephone companies for use with their customers' private branch exchange systems...
(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 studio
Recording studio
A recording studio is a facility for sound recording and mixing. Ideally both the recording and monitoring spaces are specially designed by an acoustician to achieve optimum acoustic properties...
s, 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
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
(plain old telephone service), lower delay and, at higher densities, lower cost.
India
Bharat Sanchar Nigam LimitedBharat Sanchar Nigam Limited
Bharat Sanchar Nigam Limited is a state-owned telecommunications company headquartered in New Delhi, India. It is the largest provider of fixed telephony and fourth largest mobile telephony provider in India, and is also a provider of broadband services...
, Reliance Communications
Reliance Communications
Reliance Communications Ltd. is an Indian broadband and telecommunications company headquartered in Navi Mumbai, India. RCOM is the world's 16th largest mobile phone operator with over 144 million subscribers. Established on 2004, a subsidiary of the Reliance Group...
and Bharti Airtel
Bharti Airtel
Bharti Airtel Limited , commonly known as Airtel, is an Indian telecommunications company that operates in 20 countries across South Asia, Africa and the Channel Islands. It operates a GSM network in all countries, providing 2G or 3G services depending upon the country of operation...
, are the largest communication service providers, offers both ISDN BRI and PRI services across the country. Reliance Communications and Bharti Airtel uses the DLC technology for providing these services. With the introduction of broadband technology, the load on bandwidth is being absorbed by ADSL. ISDN continues to be an important backup network for point-to-point leased line customers such as banks, Eseva Centers http://www.esevaonline.com, Life Insurance Corporation of India
Life Insurance Corporation of India
The Life Insurance Corporation of India is the largest state-owned life insurance company in India, and also the country's largest investor. It is fully owned by the Government of India. It also funds close to 24.6% of the Indian Government's expenses. It has assets estimated of...
, and SBI
State Bank of India
The State Bank of India is the largest Indian banking and financial services company with its headquarters in Mumbai, India. It is state-owned. The bank traces its ancestry to British India, through the Imperial Bank of India, to the founding in 1806 of the Bank of Calcutta, making it the oldest...
ATMs.
Japan
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. NTTNippon Telegraph and Telephone
, commonly known as NTT, is a Japanese telecommunications company headquartered in Tokyo, Japan. Ranked the 31st in Fortune Global 500, NTT is the largest telecommunications company in Asia, and the second-largest in the world in terms of revenue....
, a dominant Japanese telephone company
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.
United Kingdom
In the United KingdomUnited Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, British Telecom
BT Group
BT Group plc is a global telecommunications services company headquartered in London, United Kingdom. It is one of the largest telecommunications services companies in the world and has operations in more than 170 countries. Through its BT Global Services division it is a major supplier of...
(BT) provides ISDN2e (BRI) as well as ISDN30 (PRI). Until April 2006, they also offered services named Home Highway and Business Highway, which are BRI ISDN-based services that offer integrated analog connectivity as well as ISDN. Later versions of the Highway products also included built-in Universal serial bus
Universal Serial Bus
USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
(USB) sockets for direct computer access. Home Highway has been bought by many home users, usually for Internet connection, although not as fast as ADSL, because it was available before ADSL and in places where ADSL does not reach.
France
France TelecomFrance Télécom
France Telecom S.A. is the main telecommunications company in France, the third-largest in Europe and one of the largest in the world. It currently employs about 180,000 people and has 192.7 million customers worldwide . In 2010 the group had revenue of €45.5 billion...
offers ISDN services under their product name Numeris (2 B+D), of which a professional 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, and for applications such as business voice and point-of-sale terminals
Point of sale
Point of sale or checkout is the location where a transaction occurs...
.
Germany
In GermanyGermany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, ISDN is very popular with an installed base of 25 million 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
Deutsche Telekom
Deutsche Telekom AG is a telecommunications company headquartered in Bonn, Germany. It is the largest telecommunications company in Europe....
(DTAG) offers both BRI and PRI. Competing phone companies often offer ISDN only and no analog lines. However, these operators generally offer free hardware that also allows the use of POTS equipment, such as NTBAs with integrated terminal adapter
Terminal adapter
- ISDN :In ISDN terminology, a terminal adapter or TA is a device that connects a terminal to the ISDN network.The TA therefore fulfills a similar function to the ones a modem has on the POTS network, and is therefore sometimes called an ISDN modem...
s. Because of the widespread availability of ADSL services, ISDN is today primarily used for voice and fax traffic, but is still very popular thanks to the pricing policy of German telecommunication providers.
Today ISDN (BRI) and ADSL/VDSL are often bundled on the same line, mainly because the combination of ADSL with an analog line has no cost advantage over a combined ISDN-ADSL line. Some German operators started to implement Next Generation Networking
Next Generation Networking
Next-generation network is a broad term used to describe key architectural evolutions in telecommunication core and access networks. The general idea behind the NGN is that one network transports all information and services by encapsulating these into packets, similar to those used on the...
, generally realized via DSL and unbundled local loop. However, a few operators offer the same services via the cable television infrastructure
Cable internet
In telecommunications, cable Internet access, often shortened to cable Internet or simply cable, is a form of broadband Internet access that uses the cable television infrastructure. Like digital subscriber line and fiber to the premises services, cable Internet access provides network edge...
or, in selected areas, via FTTH. Because of the popularity of ISDN, virtually all these telecommunication providers bundle their products with residential gateway
Residential gateway
A residential gateway is a home networking device, used as a gateway to connect devices in the home to the Internet or other WAN.It is an umbrella term, used to cover multi-function networking computer appliances used in homes, which may combine a DSL or cable modem, a firewall, a consumer-grade...
s that include both integrated analog telephony adapter
Analog telephony adapter
An analog telephony adapter, or analog telephone adapter, is a device used to connect one or more standard analog telephones to a digital telephone system or a non-standard telephone system....
s and ISDN-NGN adapters.
Greece
OTEOTE
Hellenic Telecommunications Organization S.A. , usually known by its Greek initials OTE, is the dominant telecommunications provider in Greece. Formerly a state-owned monopoly, OTE was part-privatized in 1998 and is now listed on the Athens and New York Stock Exchanges...
, the incumbent telecommunications operator, offers ISDN BRI (BRA) services in Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....
. Following the launch of ADSL in 2003, the importance of ISDN for data transfer began to decrease and is today limited to niche business applications with point-to-point requirements.
Other ISDN providers are Reliance
Reliance Communications
Reliance Communications Ltd. is an Indian broadband and telecommunications company headquartered in Navi Mumbai, India. RCOM is the world's 16th largest mobile phone operator with over 144 million subscribers. Established on 2004, a subsidiary of the Reliance Group...
, and VSNL.
International deployment
A study of the German Department of Science shows the following spread of ISDN-channels per 1000 inhabitants in the year 2005:- NorwayNorwayNorway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
401 - DenmarkDenmarkDenmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
339 - GermanyGermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
333 - SwitzerlandSwitzerlandSwitzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
331 - JapanJapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
240 - UK 160
- FinlandFinlandFinland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
160 - SwedenSwedenSweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
135 - ItalyItalyItaly , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
105 - FranceFranceThe French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
85 - SpainSpainSpain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
58 - United StatesUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
47
Configurations
In ISDN, there are two types of channels, B (for "bearer") and D (for "data"). 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
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
); in Europe, India and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
). Broadband Integrated Services Digital Network
Broadband Integrated Services Digital Network
In the 1980s the telecommunications industry expected that digital services would follow much the same pattern as voice services did on the public switched telephone network, and conceived a grandiose end-to-end circuit switched services, known as 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
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
.
Another alternative ISDN configuration can be used in which the B channels of an ISDN BRI line are bonded
Channel bonding
Channel bonding is a computer networking arrangement in which two or more network interfaces on a host computer are combined for redundancy or increased throughput....
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
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
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
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
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.
Reference points
A set of reference points are defined in the ISDN standardInternational standard
International standards are standards developed by international standards organizations. International standards are available for consideration and use, worldwide...
to refer to certain points between the telco
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
and the end user ISDN equipment.
- RR interfaceR interface defines the point between a non-ISDN device and a terminal adapter which provides translation to and from such a device.Source: from Federal Standard 1037C...
- defines the point between a non-ISDN device and a terminal adapterTerminal adapter- ISDN :In ISDN terminology, a terminal adapter or TA is a device that connects a terminal to the ISDN network.The TA therefore fulfills a similar function to the ones a modem has on the POTS network, and is therefore sometimes called an ISDN modem...
(TA) which provides translation to and from such a device - SS interfaceS interface : For basic rate access in an Integrated Services Digital Network environment, a User–network interface reference point that* is characterized by a Four-wire circuit, 144 kbit/s user rate,...
- defines the point between the ISDN equipment (or TA) and a Network Termination Type 2 (NT-2) device - TT-interfaceA T-interface is used for basic rate access in an Integrated Services Digital Network environment. It is a User–network interface reference point that is characterized by a four-wire, 144 kbit/s user rate....
- defines the point between the NT-2 and NT-1Network termination 1Network Termination 1 : In Integrated Services Digital Networks , a functional grouping of customer-premises equipment that includes functions that may be regarded as belonging to OSI Layer 1, i.e., functions associated with ISDN electrical and physical terminations on the user premises.Note: The...
devices1
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 (CPE) 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. In India, service providers provide U interface and an NT1 may be supplied by Service provider as part of service offering
Types of communications
Among 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 now used mostly as an alternative to analog connections, most commonly for Internet access. Some of the services envisioned as being delivered over ISDN are now delivered over the Internet instead. In Europe, and in Germany in particular, ISDN has been successfully marketed as a phone with features, as opposed to a POTS phone with few or no features. Meanwhile, features that were first available with ISDN (such as Three-Way Calling, Call Forwarding
Call forwarding
Call forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...
, Caller ID
Caller ID
Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...
, etc.) are now commonly available for ordinary analog phones as well, eliminating this advantage of ISDN. Another advantage of ISDN was the possibility of multiple simultaneous calls (one call per B channel), e.g. for big families, but with the increased popularity and reduced prices of mobile telephony this has become less interesting as well, making ISDN unappealing to the private customer. However, ISDN is typically more reliable than POTS, and has a significantly faster call setup time compared with POTS, and IP connections over ISDN typically have some 30–35ms round trip time, as opposed to 120–180ms (both measured with otherwise unused lines) over 56k or V.34/V.92 modems, making ISDN more reliable and more efficient for telecommuters.
Where an analog connection requires a modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
, an ISDN connection requires a terminal adapter
Terminal adapter
- ISDN :In ISDN terminology, a terminal adapter or TA is a device that connects a terminal to the ISDN network.The TA therefore fulfills a similar function to the ones a modem has on the POTS network, and is therefore sometimes called an ISDN modem...
(TA). The function of an ISDN terminal adapter is often delivered in the form of a PC card with an S/T interface, and single-chip solutions seem to exist, considering the plethora of combined ISDN- and ADSL-routers.
ISDN is commonly used in radio broadcasting. Since ISDN provides a high quality connection this assists in delivering good quality audio for transmission in radio. Most radio studios are equipped with ISDN lines as their main form of communication with other studios or standard phone lines. Equipment made by companies such as Telos/Omnia (the popular Zephyr codec), Comrex, Tieline and others are used regularly by radio broadcasters. Almost all live sports broadcasts on radio are backhauled to their main studios via ISDN connections.
Sample call
The following is an example of a Primary Rate (PRI) ISDN call showing the Q.921/LAPDLink Access Procedures, D channel
Link Access Procedures on the D channel , specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.It is heavily based on HDLC.-External links:*http://www.protocols.com/pbook/pdf/isdn.pdf...
and the Q.931
Q.931
ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...
/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
Local exchange carrier
Local Exchange Carrier is a regulatory term in telecommunications for the local telephone company.In the United States, wireline telephone companies are divided into two large categories: long distance and local...
, 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.
10:49:47.33 21/1/24 R RR
0000 02 01 01 a5 ....
10:49:47.34 21/1/24 T RR
0000 02 01 01 b9 ....
10:50:17.57 21/1/24 R RR
0000 02 01 01 a5 ....
10:50:17.58 21/1/24 T RR
0000 02 01 01 b9 ....
10:50:24.37 21/1/24 T SETUP
Call Reference : 000062-local
Bearer Capability : CCITT, Speech, Circuit mode, 64 kbit/s
Channel ID : Implicit Interface ID implies current span, 21/1/5, Exclusive
Calling Party Number : 8018023000 National number User-provided, not screened Presentation allowed
Called Party Number : 3739120 Type: SUBSCRB
0000 00 01 a4 b8 08 02 00 3e 05 04 03 80 90 a2 18 03 .......>........
0010 a9 83 85 6c 0c 21 80 38 30 31 38 30 32 33 30 30 ...l.!.801802300
0020 30 70 08 c1 33 37 33 39 31 32 30 0p..3739120
10:50:24.37 21/1/24 R RR
0000 00 01 01 a6 ....
10:50:24.77 21/1/24 R CALL PROCEEDING
Call Reference : 000062-local
Channel ID : Implicit Interface ID implies current span, 21/1/5, Exclusive
0000 02 01 b8 a6 08 02 80 3e 02 18 03 a9 83 85 .......>......
10:50:24.77 21/1/24 T RR
0000 02 01 01 ba ....
10:50:25.02 21/1/24 R ALERTING
Call Reference : 000062-local
Progress Indicator : CCITT, Public network serving local user,
In-band information or an appropriate pattern is now available
0000 02 01 ba a6 08 02 80 3e 01 1e 02 82 88 .......>.....
10:50:25.02 21/1/24 T RR
0000 02 01 01 bc ....
10:50:28.43 21/1/24 R CONNECT
Call Reference : 000062-local
0000 02 01 bc a6 08 02 80 3e 07 .......>.
10:50:28.43 21/1/24 T RR
0000 02 01 01 be ....
10:50:28.43 21/1/24 T CONNECT_ACK
Call Reference : 000062-local
0000 00 01 a6 be 08 02 00 3e 0f .......>.
10:50:28.44 21/1/24 R RR
0000 00 01 01 a8 ....
10:50:35.69 21/1/24 T DISCONNECT
Call Reference : 000062-local
Cause : 16, Normal call clearing.
0000 00 01 a8 be 08 02 00 3e 45 08 02 8a 90 .......>E....
10:50:35.70 21/1/24 R RR
0000 00 01 01 aa ....
10:50:36.98 21/1/24 R RELEASE
Call Reference : 000062-local
0000 02 01 be aa 08 02 80 3e 4d .......>M
10:50:36.98 21/1/24 T RR
0000 02 01 01 c0 ....
10:50:36.99 21/1/24 T RELEASE COMPLETE
Call Reference : 000062-local
0000 00 01 aa c0 08 02 00 3e 5a .......>Z
10:50:36.00 21/1/24 R RR
0000 00 01 01 ac ....
10:51:06.10 21/1/24 R RR
0000 02 01 01 ad ....
10:51:06.10 21/1/24 T RR
0000 02 01 01 c1 ....
10:51:36.37 21/1/24 R RR
0000 02 01 01 ad ....
10:51:36.37 21/1/24 T RR
0000 02 01 01 c1 ....
Protocols
- DSS1 (ETSI "EuroEuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
-ISDN", also used in many non-European countries) - DSS2 (Digital Subscriber Signalling System No. 2)
- ETS 300 specification at ETSI
- NI-1 (USUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
National ISDN Phase 1) - NI-2 (USUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
National ISDN Phase 2) - 4ESS (Lucent 4ESS specific protocol defined in AT&T TR 41459)
- INS-NET 64/1500 (JapanJapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
ese national/NTTNippon Telegraph and Telephone, commonly known as NTT, is a Japanese telecommunications company headquartered in Tokyo, Japan. Ranked the 31st in Fortune Global 500, NTT is the largest telecommunications company in Asia, and the second-largest in the world in terms of revenue....
carrier-specific protocol) - DACSDigital Access Carrier SystemDigital Access Carrier System is the name used by British Telecom in the United Kingdom for a 0+2 Pair gain system.- Usage :...
used in the UKUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
by British Telecom it uses non standard D channel signalling for pair gainPair gainIn telephony, pair gain is a method of transmitting multiple POTS signals over the twisted pairs traditionally used for a single traditional subscriber line in telephone systems. Pair gain has the effect of creating additional subscriber lines...
- QSIGQSIGQSIG is an ISDN based signaling protocol for signaling between private branch exchanges in a Private Integrated Services Network . It makes use of the connection-level Q.931 protocol and the application-level ROSE protocol...
- Remote Operations Service Element protocolRemote operations service element protocolThe Remote Operations Service Element is the OSI service interface, specified in , that provides remote operation capabilities, allows interaction between entities in a distributed application, and upon receiving a remote operations service request, allows the receiving entity to attempt the...
(ROSE) - Q.931Q.931ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...
- FTZ 1 TR 6FTZ 1 TR 6FTZ 1 TR 6 is the standard for the obsolete German national digital signalling protocol used for the ISDN...
(obsolete GermanGermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
national protocol) - TS.013/TS.014 (obsolete AustraliaAustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
n national protocol) - VN2/VN3/VN4 (obsolete FrenchFranceThe French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
national protocols)
Specifications defining the physical layer and part of the data link layers of ISDN:
- ISDN BRI: ITU-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
I.430. - ISDN PRI: ITU-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
I.431.
From the point of view of the OSI architecture, an ISDN line has a stack of three protocols
- physical layer
- data link layer
- network layer (the ISDN protocol, properly)
Other
- CAPICommon ISDN Application Programming InterfaceThe Common ISDN Application Programming Interface is an ISDN-conform standardized software interface. With the help of the CAPI-Interface computer software intended for the use with ISDN can be provided, without knowledge of the deployed, proprietary ISDN card.CAPI was designed from 1989 by German...
- ADSLAsymmetric Digital Subscriber LineAsymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...
- ATMAsynchronous Transfer ModeAsynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
- B-ISDN
- InternetInternetThe Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
- H.320H.320H.320 is an umbrella recommendation by the ITU-T for running Multimedia over ISDN based networks. The main protocols in this suite are H.221, H.230, H.242, audio codecs such as G.711, and video codecs such as H.261 and H.263....
- ETSI
- List of device bandwidths
External links
- http://www.itu.int/rec/T-REC-I/e (list of published recommendations available in English, French and Spanish)
- http://hea-www.harvard.edu/~fine/ISDN/
- http://www.ralphb.net/ISDN/
- http://www.roblee.com/isdn.htm