ISDN
Integrated Services Digital Network or Isolated Subscriber Digital Network (ISDN), originally "Integriertes Sprach- und Datennetz" (German for "Integrated Speech and Data Net"), is a telephone system network. Prior to the ISDN, the phone system was viewed as a way to transport voice, with some special services available for data. The key feature of the 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 the ISDN defined:
1.Basic Rate Interface (BRI),
2. Primary Rate Interface (PRI)
3.Broadband-ISDN (B-ISDN).
ISDN is a circuit-switchedtelephonenetwork system, that also provides access to packet switched networks, designed to allow digitaltransmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone.
- It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kbit/s.
- Another major use case is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions (which can be considered to be broadband speed, since it exceeds the narrowband speeds of standard analog 56k telephone lines).
- ISDN channels may use bonding to achieve a greater data rate, typically 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.
ISDN should not be confused with any specific protocol, like Q.931. In a broad sense ISDN can be considered a digital communications medium 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 the late 1980s.[1]
In a videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.
ISDN is a system with digitized phone connections which has been pure analogue connection for a decade. This is the first protocol defines on a digital communications line that allows for the transmission of voice, data, video and graphics, at very high speeds, over standard communication lines simultaneously, carried by bearer channels (B channels) occupying a bandwidth of 64 kbits per second (some switches limit it to a capacity of 56 kb/s). A data channel (D channel) is defined handles signaling at 16 kb/s or 64 kb/s, depending on the service type. ISDN is not restricted to public telephone networks alone; it may be transmitted via packet switched networks, telex, CATV networks, etc. There are two basic types of ISDN service:
- Basic Rate Interface(BRI) - consists of two 64 kb/s B channels and one 16 kb/s D channel for a total of 144 kb/s. This basic service is intended to meet the needs of most individual users. The U interface provided by the telco for BRI is a 2-wire, 160 kb/s digital connection. Echo cancellation is used to reduce noise, and data encoding schemes (2B1Q in North America, 4B3T in Europe) permit this relatively high data rate over ordinary single-pair local loops.
- Primary Rate Interface (PRI) which is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s. In Europe, PRI consists of 30 B channels plus one 64 kb/s D channel for a total of 1984 kb/s. It is also possible to support multiple PRI lines with one 64 kb/s D channel using Non-Facility Associated Signaling (NFAS).
The CCITT (now ITU-T) study group responsible for ISDN first published a set of ISDN recommendations in 1984 . Prior to this publication, various geographical areas had developed different versions of ISDN. The use of nation-specific information elements is enabled by using the Codeset mechanism which allows different areas to use their own information elements within the data frames. Some common nation-specific ISDN variants are: National ISDNused in the USA is by Bellcore. It has four network-specific message types. It does not have any single octet information elements. Other changes are the addition of the SEGMENT, FACILITY and REGISTER message types and the Segmented Message and Extended Facility information elements. Also, some meanings of field values have changed and some new accepted field values have been added.
Due to its limitation of bandwidth and services, this traditional ISDN is called narrowband ISDN, in contrast to the BISDN (Broadband ISDN).
Protocol Structure - ISDN: Integrated Services Digital Network
Below is the general structure of the ISDN frame:
8 / 7 / 6 / 5 / 4 / 3 / 2 / 1Protocol discriminator
0 / 0 / 0 / 0 / Length of reference call value
Flag / Call reference value
0 / Message type
Other information elements as required
Protocol discriminator - The protocol used to encode the remainder of the Layer.
Length of call reference value - Defines the length of the next field. The Call reference may be one or two octets long depending on the size of the value being encoded.
Flag - Set to zero for messages sent by the party that allocated the call reference value; otherwise set to one.
Call reference value - An arbitrary value that is allocated for the duration of the specific session, which identifies the call between the device maintaining the call and the ISDN switch.
Message type - Defines the primary purpose of the frame. The message type may be one octet or two octets (for network specific messages). When there is more than one octet, the first octet is coded as eight zeros. A complete list of message types is given in ISDN Message Types below.
ISDN Information Elements - there are two types of information elements: single octet and variable length. Single octet information elements- the single octet information element appears as follows:
8 / 7 / 6 / 5 / 4 / 3 / 2 / 11 / Information element identifier / Information element
Variable length information elements - The following is format and the variable length information element:
8 / 7 / 6 / 5 / 4 / 3 / 2 / 10 / Information element identifier
Length of information elements
Information elements (multiple bytes)
The information element identifier identifies the chosen element and is unique only within the given Codeset. The length of the information element informs the receiver as to the amount of the following octets belonging to each information element.
- ISDN Message Types - The possible ISDN message types: Call Establishment, Call Information Phase, Call Clearing, and Miscellaneous.
- Codeset - Three main Codesets are defined. In each Codeset, a section of the information elements are defined by the associated variant of the protocol:
Codeset 0 / The default code, referring to the CCITT set of information elements.
Codeset 5 / The national specific Codeset.
Codeset 6 / The network specific Codeset.
CPE - Customer Premises Equipment - refers to all ISDN compatible equipment connected at the user sight. Examples of devices are telephone, PC, Telex, Facsimile, etc. The exception is the FCC definition of NT1. The FCC views the NT1 as a CPE because it is on the customer sight, but the CCITT views NT1 as part of the network. Consequently the network reference point of the network boundary is dependent on the variant in use.
ISDN Channels B, D and H - The three logical digital communication channels of ISDN perform the following functions:
B-Channel / Carries user service information including: digital data, video, and voice.D-Channel / Carries signals and data packets between the user and the network
H-Channel / Performs the same function as B-Channels, but operates at rates exceeding DS-0 (64 Kbps). They are implemented as H0 (384 kb/s (6 B channels), H10 (1472 kb/s -23 B channels), H11 (1536 kb/s - 24 B channels), and H12 (1920 kb/s for International -E1 only).
ISDN elements
- Integrated Services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice, video, and fax, over a single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.
- Digital refers to its purely digital transmission, as opposed to the analog transmission of plain old telephone service (POTS). Use of an analog telephone modem for Internet access requires that the Internet service provider's (ISP) modem converts the digital content to analog signals before sending it and the user's modem then converts those signals back to digital when receiving. When connecting with ISDN there is no digital to analog conversion.
- Network refers to the fact that ISDN is not simply a point-to-point solution like a leased line. ISDN networks extend from the local telephone exchange to the remote user and includes all of the telecommunications and switching equipment in between.
The purpose of the ISDN is to provide fully integrated digital services to the users. These services fall under three categories: bearer services, supplementary services and teleservices.
Basic Rate Interface
The entry level interface to ISDN is the Basic Rate Interface (BRI), a 144 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 three different network interfaces:
- The U interface is a two-wire interface between the exchange and the Network Terminating Unit which is usually the demarcation point in non-North American networks.
- The T interface is a serial interface between a computing device and a Terminal Adapter, which is the digital equivalent of a modem.
- The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on an NT1
- The R interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.
BRI-ISDN is very popular in Europe but is much less common in North America.
Primary Rate Interface
The other ISDN service available is the Primary Rate Interface (PRI) which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 'B' channels of 64 kbit/s, one 'D' channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s. In North America PRI service is delivered on one or more T1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1).
In North America, NFAS allows two or more PRIs to be controlled by a single D channel, and is sometimes called "23B+D + n*24B". D-channel backup allows you to have a second D channel in case the primary fails. One popular use of NFAS is on a T3.
PRI-ISDN is popular throughout the world, especially for connection of PSTN circuits to PBXs.
Even though many network professionals use the term "ISDN" to refer to the lower-bandwidth BRI circuit, in North America by far the majority of ISDN services are in fact PRI circuits serving PBXs.
Data Channel
The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.
Each one of these channels is known as a DS0 (dee-ess-zero).
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.
Signalling Channel
The signalling channel (D) uses Q.931 for signalling with the other side of the link.
X.25
X.25 can be carried over the B or D channels of a BRI line, and over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale (credit card) terminals because it eliminates the modem setup, and because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines.
X.25 was also part of an ISDN protocol called "Always On/Dynamic ISDN", or AO/DI. This allowed a user to have a constant multi-link PPP connection to the internet over X.25 on the D channel, and brought up one or two B channels as needed.
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 tarriffing (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 corporate or other PBX is connected via a T1, and the signalling was done with A&B bits to indicate on-hook or off-hook conditions and MF and DTMFtones 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 translated to much 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
In India, ISDN was very popular until the introduction of ADSL. Bharat Sanchar Nigam Limited, the largest communication service provider in India and a state owned company, is offering both ISDN BRI and PRI services across the country over its ISDN network. After the introduction of ADSL broadband technology with static IPs, the data transfer load is taken up by ADSL. But ISDN still plays a very big role as a backup network for point-to-point leased line customers and low cost reliable data network for organisations located all over India, such as banks, E-seva centres, Life Insurance Corporation of India, and so on.
Configurations
In ISDN, there are two types of channels, B (for "Bearer") and D (for "Delta"). B channels are used for data (which may include voice), and D channels are intended for signaling and control (but can also be used for data).
There are two ISDN implementations. Basic Rate Interface (BRI), also called Basic Rate Access (BRA) in Europe — 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 ISDNbasic rate interface 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.