Editing ISDN (section)

== Design ==

''Integrated services'' refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice, [[Videotelephony|video]], and [[fax]], over a [[local loop|single line]]. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of what were expected to be 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<ref>{{Cite journal |title= Pragmatic Introduction of Digital Switching and Transmission in Existing Networks | first1= G | last1 = Robin | first2= S | last2 = Treves |journal= IEEE Transactions on Communications |date= July 1979 |volume=27 |doi=10.1109/TCOM.1979.1094494 |issue= 7 |page = 1071}}</ref> in that [[telephone switch]]ing and [[carrier wave]] transmission are integrated rather than separate as in earlier technology.

=== 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)&nbsp;— consists of two B channels, each with bandwidth of 64 [[kbit/s]], and one D channel with a bandwidth of {{nowrap|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&nbsp;— contains a greater number of B channels and a D channel with a bandwidth of {{nowrap|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]] ([[Digital Signal 1|T1]]); in Europe, India and Australia it is 30B+2D, with an aggregate bit rate of {{nowrap|2.048 Mbit/s}} ([[E-carrier|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 backbone]]s and employs [[Asynchronous Transfer Mode|ATM]].

Another alternative ISDN configuration can be used in which the B channels of an ISDN BRI line are [[Channel bonding|bonded]] to provide a total duplex bandwidth of {{nowrap|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 [[B8ZS|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 {{nowrap|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 [[multiplexing|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 the UK, France, Japan and Germany.

=== Reference points ===

A set of ''reference points'' are defined in the ISDN [[International standard|standard]] to refer to certain points between the [[Telephone company|telco]] and the end-user equipment.

*[[R interface|R]]&nbsp;– defines the point between non-ISDN terminal equipment, ''terminal equipment 2'' (''TE2''), and a ''[[terminal adapter]]'' (TA) which provides translation to and from such a device
*[[S interface|S]]&nbsp;– defines the point between ISDN terminal equipment, ''terminal equipment 1'' (''TE1''), or a TA and a ''Network Termination Type 2'' ([[Network termination 2|NT2]]) device
*[[T-interface|T]]&nbsp;– defines the point between the NT2 and [[network termination 1]] (NT1) devices.

Most NT-1 devices can perform the functions of the NT2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.

In North America, the NT1 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 NT1 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.

=== Basic Rate Interface ===
[[File:ISDN box on wall June 21.jpg|thumb|A decommissioned BT ISDN 2e box]]

The entry level interface to ISDN is the [[Basic Rate Interface]] (BRI), a 128 [[kilobits per second|kbit/s]] service delivered over a pair of standard telephone copper wires.<ref name="Talkers 2013-03-28"/> The {{nowrap|144 kbit/s}} overall payload rate is divided into two {{nowrap|64 kbit/s}} [[bearer channel]]s ([[B channel|'B' channels]]) and one {{nowrap|16 kbit/s}} signaling channel ([[D channel|'D' channel]] or data channel). This is sometimes referred to as 2B+D.<ref name="SW Bell ISDN">{{cite web|title=What is ISDN?|url=http://public.swbell.net/ISDN/overview.html|publisher=Southwestern Bell|access-date=April 6, 2013|archive-url=https://web.archive.org/web/20130415082843/http://public.swbell.net/ISDN/overview.html|archive-date=April 15, 2013|url-status=dead}}</ref>

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 a [[Network termination 1]] (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&nbsp;— where it is known as INS64.<ref name="Wisegeek BRI">{{cite web|title=What Is Basic Rate Interface?|url=http://www.wisegeek.com/what-is-basic-rate-interface.htm|access-date=April 6, 2013|archive-date=April 28, 2013|archive-url=https://web.archive.org/web/20130428040354/http://www.wisegeek.com/what-is-basic-rate-interface.htm|url-status=live}}</ref><ref name="ISDN SwitchType">{{cite web|title=ISDN\SwitchType|url=http://msdn.microsoft.com/en-us/library/windows/hardware/ff541725%28v=vs.85%29.aspx|publisher=Microsoft|access-date=April 6, 2013}}</ref>

=== Primary Rate Interface ===

The other ISDN access available is the [[Primary Rate Interface]] (PRI), which is carried over [[T-carrier]] (T1) with 24 time slots (channels) in North America, and over [[E-carrier]] (E1) with 32 channels in most other countries. Each channel provides transmission at a {{nowrap|64 kbit/s}} data rate.

With the E1 carrier, the available channels are divided into 30 bearer (''B'') channels, one data (''D'') channel, and one timing and alarm channel. This scheme is often referred to as 30B+2D.<ref>{{cite book | first = John C. | last = Bellamy | title = Digital Telephony | url = https://archive.org/details/digitaltelephony00bell_133 | url-access = limited | edition = 3 | publisher = Wiley Interscience | date = 2000 | isbn = 978-0-471-34571-8| page = [https://archive.org/details/digitaltelephony00bell_133/page/n521 496]}}</ref>
 
In North America, PRI service is delivered via T1 carriers with only one data channel, often referred to as 23B+D, and a total data rate of {{nowrap|1544 kbit/s}}. [[Non-Facility Associated Signalling]] (NFAS) allows two or more PRI circuits to be controlled by a single [[D channel]], which 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 [[Digital Signal 3]] (DS3/T3).

PRI-ISDN is popular throughout the world, especially for connecting [[private branch exchange]]s to the [[public switched telephone network]] (PSTN).

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.

=== Bearer channel ===

The bearer channel (B) is a standard {{nowrap|64 kbit/s}} voice channel of 8 bits sampled at 8&nbsp;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 {{nowrap|64 kbit/s}} signal, but some were limited to 56K because they traveled over [[robbed-bit signaling|RBS]] lines. This was commonplace in the 20th century, but has since become less so.

=== 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.

=== Frame Relay ===

In theory, [[Frame Relay]] can operate over the D channel of BRIs and PRIs, but it is seldom, if ever, used.