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|2. Data Link Layer|
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|1. Physical Layer|
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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 1984 in the CCITT red book.
Prior to ISDN, the phone system was viewed as a way to transport voice, with some special services available for data. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI) and Broadband ISDN (B-ISDN).
ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions. ISDN B-channels can be bonded to achieve a greater data rate, typically 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 has 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 call 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.
The entry level interface to ISDN is the Basic Rate Interface (BRI), a 128 kbit/s service delivered over a pair of standard telephone copper wires. The 144 kbit/s rate is broken down into two 64 kbit/s bearer channels ('B' channels) and one 16 kbit/s signaling channel ('D' channel or delta channel).
BRI is sometimes referred to as 2B+D
The interface specifies the following network interfaces:
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.
The other ISDN service available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 'B' channels of 64 kbit/s, one 'D' channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s. In North America PRI service is delivered on one or more T1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1).
In North America, NFAS allows two or more PRIs to be controlled by a single D channel, and is sometimes called "23B+D + n*24B". D-channel backup allows for a second D channel in case the primary fails. One popular use of NFAS is on a T3.
Even though many network professionals use the term "ISDN" to refer to the lower-bandwidth BRI circuit, in North America by far the majority of ISDN services are in fact PRI circuits serving PBXs.
The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.
Each one of these channels is known as a DS0.
Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines. This was more of a problem in the past, and is not commonly encountered nowadays.
The signaling channel (D) uses Q.931 for signaling with the other side of the link.
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.
In theory, Frame Relay can operate over the D channel of BRIs and PRIs, but it is seldom, if ever, used.
There are two points of view into the ISDN world. The most common viewpoint is that of the end user, who wants to get a digital connection into the telephone/data network from home, whose performance would be better than an ordinary analog modem connection. The typical end-user's connection to the Internet is related to this point of view, and discussion on the merits of various ISDN modems, carriers' offerings and tariffs (features, pricing) are from this perspective. Much of the following discussion is from this point of view, but it should be noted that as a data connection service, ISDN has been mostly superseded by DSL.
There is 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.
ISDN is used heavily by the broadcast industry as a reliable way of switching low latency, high quality, long distance audio circuits. In conjunction with the appropriate Codec an ISDN BRI can be used to send stereo bi-directional audio with 20Hz-20 kHz bandwidth, although commonly the aging G.722 codec is used over a single B Channel lending the resultant audio a low-fi "ISDN sound". Where very high quality audio is required multiple ISDN BRIs can be used in parallel to provide a higher bandwidth circuit switched connection. BBC Radio 3 commonly makes use of three ISDN BRIs to carry 320kbps audio stream for live outside broadcasts. ISDN BRI services are used to link remote studios, sports grounds and outside broadcasts into the main broadcast studio. ISDN via satellite is used by field reporters around the world. It's also common to use ISDN for the return audio links to remote satellite broadcast vehicles.
IP based streaming codecs such as the Comrex Access are starting to gain a foothold in the broadcast sector, using broadband internet to connect remote studios. However reliability is crucially important for broadcasters and the quality of service offered by ISDN has not yet been matched by packet switched alternatives.
ISDN-BRI has never gained popularity as a general use telephone access technology in Canada and the US and today remains a niche product. The service was seen as a solution in search of a problem, 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.
Part of the difficulty is that as BRI was coming into service, the concept of what the word broadband meant was being revised upward to mean at least 256 kbit/s incoming to the customer. As ADSL grew in popularity in the United States, 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 North American CLECs such as CenturyTel having given up on it and will not provide 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 and Tandberg bond up to 6 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 is no object and perfect quality is required, BRI is the preferred choice.
Most modern non-VoIP PBXs use ISDN-PRI circuits. These are connected via T1 lines with the central office switch, replacing older analog two-way and direct inward dialing (DID) trunks. PRI is capable of delivering calling line identification (CLID) in both directions so that the telephone number of an extension, rather than a company's main number, can be sent. It is still commonly used in recording studios, when a voice-over actor is in one studio, but the director and producer are in a studio at another location. The ISDN protocol delivers channelized, not-over-the-Internet service, powerful call setup and routing features, faster setup and tear down, superior audio fidelity (as compared to POTS (Plain Old Telephone Service)), lower delay and, at higher densities, lower cost.
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. NTT, a dominant Japanese telephone company, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.
In the United Kingdom, 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. Virgin Media also use ISDN lines for customers of Virgin Broadband who live in cable TV areas.
France Telecom 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.
In Germany, 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 adapters. 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 gateways that include both, integraded analog telephony adapters and ISDN-NGN adapters.
OTE, 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 begun to decrease and is today limited to niche business applications with point-to-point requirements.
Bharat Sanchar Nigam Limited, the State owned and largest communication service provider, offers both ISDN BRI and PRI services across the country. 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 , Life Insurance Corporation of India, and SBI ATMs.
A study of the German Department of Science shows the following spread of ISDN-channels per 1000 inhabitants in the year 2005:
In ISDN, there are two types of channels, B (for "bearer") and D (for "delta"). B channels are used for data (which may include voice), and D channels are intended for signaling and control (but can also be used for data).
There are two ISDN implementations. Basic Rate Interface (BRI), also called basic rate access (BRA) — consists of two B channels, each with bandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s. Together these three channels can be designated as 2B+D. Primary Rate Interface (PRI), also called primary rate access (PRA) in Europe — contains a greater number of B channels and a D channel with a bandwidth of 64 kbit/s. The number of B channels for PRI varies according to the nation: in North America and Japan it is 23B+1D, with an aggregate bit rate of 1.544 Mbit/s (T1); in Europe, India and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1). Broadband Integrated Services Digital Network (BISDN) is another ISDN implementation and it is able to manage different types of services at the same time. It is primarily used within network backbones and employs ATM.
Another alternative ISDN configuration can be used in which the B channels of an ISDN BRI line are bonded to provide a total duplex bandwidth of 128 kbit/s. This precludes use of the line for voice calls while the internet connection is in use. The B channels of several BRIs can be BONDED, a typical use is a 384K videoconferencing channel.
Using bipolar with eight-zero substitution encoding technique, call data is transmitted over the data (B) channels, with the signaling (D) channels used for call setup and management. Once a call is set up, there is a simple 64 kbit/s synchronous bidirectional data channel (actually implemented as two simplex channels, one in each direction) between the end parties, lasting until the call is terminated. There can be as many calls as there are bearer channels, to the same or different end-points. Bearer channels may also be multiplexed into what may be considered single, higher-bandwidth channels via a process called B channel BONDING, or via use of Multi-Link PPP "bundling" or by using an H0, H11, or H12 channel on a PRI.
The D channel can also be used for sending and receiving X.25 data packets, and connection to X.25 packet network, this is specified in X.31. In practice, X.31 was only commercially implemented in UK, France and Japan.
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
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 Call, 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 Omnia (the popular Zephyr codec) and others are used regularly by radio broadcasters. Sometimes a dipswitch setting must be changed on one codec to "talk" with another made by a different manufactuer.
The following is an example of a Primary Rate (PRI) ISDN call showing the Q.921/LAPD and the Q.931/Network message intermixed (i.e. exactly what was exchanged on the D-channel). The call is originating from the switch where the trace was taken and goes out to some other switch, possibly an end-office LEC, who terminates the call.
The first line format is <time> <D-channel> <Transmitted/Received> <LAPD/ISDN message ID>. If the message is an ISDN level message, then a decoding of the message is attempted showing the various Information Elements that make up the message. All ISDN messages are tagged with an ID number relative to the switch that started the call (local/remote). Following this optional decoding is a dump of the bytes of the message in <offset> <hex> ... <hex> <ascii> ... <ascii> format.
The RR messages at the beginning prior to the call are the keep alive messages. Then you will see a SETUP message that starts the call. Each message is acknowledged by the other side with a RR.
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 ....
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
|Optical||Coaxial cable||Twisted pair||Phone line||Power line||Unlicensed terrestrial bands||Licensed terrestrial bands||Satellite|
|LAN||Ethernet||G.hn||Ethernet||HomePNA · G.hn||G.hn||Wi-Fi · Bluetooth · DECT · Wireless USB|
|WAN||PON · Ethernet||DOCSIS||Ethernet||Dial-up · ISDN · DSL||BPL||Muni Wi-Fi||GPRS · iBurst · WiBro/WiMAX · UMTS-TDD, HSPA · EVDO · LTE||Satellite|