ISDN

Broadband ISDN

ISDN

Integrated Services Digital network was developed by ITU-T in 1976. It is a set of protocols that combines digital telephony and data transport services. The whole idea is to digitize the telephone network to permit the transmission of audio, video and text over existing telephone lines. ISDN is an effort to standardize subscriber services, provide user/network interfaces, and facilitate the internetworking capabilities of existing voice and data network. The goal of ISDN is to form a wide are network that provides universal end-to-end connectivity over digital media. This can be done by integrating all of the separate transmission services into one without adding new links of subscriber lines.

The Integrated services digital network or ISDN is a set of standards for a digital network carrying both voice and data communications. Today, most residential and some business communications are carried over analogue local loops. Only midsize and larger business with high volumes of telecommunications traffic currently use T-1 carrier. ISDN extends the advantages of digital transmission to all users, large and small, by including many different sizes of digital transmission paths.

ISDN Channels

To accommodate large and small users, ISDN provides different sizes of transmission paths, know as channels. These channels can be combined in different ways to satisfy user requirements.

The B-channel, also called the bearer channel, is a 64-kbps digital channel. The B-channel can carry user data or digitized voice.

The D-channel is a digital channel used to carry signalling information. The capacity of the D-channel can be 16kbps or 64 kbps, depending on the type of circuit used. These bits are reserved for signalling information, such as on-hood and off-hook status, dialling information, busy signals and so on, and are also used in T-1 signalling. However, most T-1 transmission used in-band signalling, where the signalling bits are interspersed in the same channels as the data bits. ISDN uses out-of-band signalling or common-channel signalling, meaning that a separate channel (the D-channel) is used for the signalling bits.

Since most signalling typically occurs at the beginning and end of a connection, the D-channel may be idle most of the time. Therefore, certain low-priority user data can also be passed on the D-channel when it is free. A typical example of low-priority data for the D-channel might be electronic mail sent in a packet format; there is usually no urgent need for rush delivery, so these messages can be sent when the D-channel is available, rather than using the high-speed B-channel.

The B-channel and D-channel are the basic building blocks of ISDN transmission although there are several other channels already defined. The A-channel is similar to today's analogue telephone circuit and provides a means for analogue signals to be carried in an ISDN. Since not everyone discards his or her analogue customer-premises equipment overnight, some method of carrying analogue signals will still be necessary; hence the need for the A-channel. Another channel, known as the C-channel, provides a low-speed(up to 16 kbps) data channel for older, pre-ISDN data devices, such as many dumb terminals.

The Basic-Rate interface

ISDN provides service to residential and business users with the basic-rate interface, or BRI. This interface consists of two B-channels and single D-channel. Usually, one B-channel is used for digitized voice, the other B-channel is used for high speed data, and the D-channel is used for signalling and low speed data packets. The basic rate interface is commonly refereed to as 2B+D.

In the basic rate interface, the D-channel can carry 16 kbps of signalling information, and each B-channel can carry 64 kbps of data or digitized voice, for a total of 144 kbps. However, an addition 48 kbps is used for overhead, which includes synchronization and framing bits. Therefore, the basic-rate interface requires 192kbps of digital transmission capacity, even though the effective bandwidth is actually 144kbps.

The basic-rate interface is intended to provide voice and data transmission to the office desktop or the home for the typical individual user. A 64 kbps data transfer rate is adequate for basic terminal and PC applications, although as processor speeds increase, a higher bandwidth may be desirable. The basic-rate interface provides the user with simultaneous voice and data communications capability, along with advanced signalling capability. The 64kbps data rate is far superior to traditional data transmission, which uses modems and analogue lines; much of the signalling information in the basic-rate interface is not available to the user in today's public network. Certainly, the basic-rate interface is a market technological improvement over today's analogue local loops.

The Primary-rate interface

ISDN also provides a higher bandwidth service for business users, known as the primary rate interface or PRI. This interface normally is used to connect PBXs to central offices, PBXs to PBXs, PBXs to LANS and LANs to LANs and in other high-bandwidth applications. The primary rate interface consists of either 23 or 30 channels for user data and single 64-kbps D-channel for signalling.

In North America and Japan , where T-1 transmission is already popular, the 23B+D method is used, since the capacity of each of the 23 B-channels is 64 kbps and the D-channel capacity is also 64kbps, existing T-1 transmission equipment that transmits 24 channels of 64kbps each can also accommodate this primary-rate interface. Since a separate D-channel is used for signalling, no bits are taken from the B-channel for this purpose, and each B-channel has a full 64kbps capacity, unlike the channels in T-1. A framing arrangement similar to that found in T-1 transmission is used in this version of the primary-rate interface. Therefore, there are 24 channels each of 64 kbps and an additional 8 kbps used for framing, for a total of 1.544Mbps.

In Europe , where high-speed digital transmission is done today using 2.048 MBPS, the 30 B+D arrangement is used. Though at first this arrangement may seen to cause a fundamental incompatibility between North-American and European networks, this is not the case. Each country or continent, can use its own method for digital transmission, as long as gateways exist to connect the different networks. It certainly makes sense to maximize the use of existing digital transmission equipment between central offices and toll offices in each country, and the two different primary-rate interfaces permit this.

Other channel arrangements

The A-channel and C channel can be combined in a hybrid interface to allow users without ISDN phones and terminals to receive at least some of the ISDN features. These users will have voice and data communication, but not have access to some of the ISDN services.

Individual channels may be offered separately in some instances, to comply with regulatory requirements in the US and other countries. For example, a customer might be able to lease 35 B-channels and 6 D-channels. However, it is expected that most customer-premises equipment will be designed to take advantage of either the basic-rate or primary-rate interfaces. A comparison of typical ISDN interfaces is shown below

Broadband ISDN

The basic-rate and primary-rate ISDN interfaces were once considered extremely high-bandwidth interfaces. However, with improvements in transmission technology and the advent of fibre optic cables, much faster transmission rates are now possible. An even faster class of ISDN interface standards, referred to as Broadband ISDN or BISDN is currently being developed by the ITU(CCITT) to handle much higher transmission speeds. When compared to this new BISDN, the basic-rate and primary-rate ISDN interfaces are sometimes called Narrowband ISDN. However, Narrowband ISDN still provides a higher effective bandwidth than that of today's existing analogue telephone facilities.

BISDN is already widely available to business users. The extremely high bandwidth of BISDN permits digitized video signals to be transmitted, as well as digitized voice and data. BISDN could eventually integrate our telephone and cable television set does today. Residential applications could revolutionize entertainment. Rather than going to the video store to rent a movie, a BISDN user would call the video store's computer, scan a catalogue of available movies on the television screen, and make a selection. The rental fee would be automatically deducted from the user's bank account, and the movie would be transmitted to the home from the video store.

With BISDN, many office workers could effectively work at home and could even attend meetings through videoconferencing. Other work could be performed on their home computer and transferred to their boss, who might also be at home. The telephone would still exist for everyday voice conversations.

Compared to today's analogue service, narrowband ISDN simply adds as a data channel and signalling, uses digital voice transmission and provides additional services. BISDN, on the other hand, could actually revolutionize the way in which we communicate. For this reason, many people believe that BISDN will eventually achieve grater residential penetration than narrowband ISDN will. Many feel that narrowband ISDN will be used mainly by businesses and that most residential users will not be interested. BISDN, however, may provide such clear advantages to residential users that it will be hard to resists. On the other hand, may see BISDN as a technology primarily for use by carriers building their wide area networks.

When the ISDN was originally designed, data rates of 64kbps to 1.544 Mbps were sufficient to handle all existing transmission needs. As application using the telecommunications networks advanced, however, these rates proved inadequate to support many applications. In addition, the original bandwidths proved too narrow to carry the large numbers of concurrent signals produced by a growing industry of digital service provider.

Broadband ISDN transmission media

The trend towards deregulation in the telecommunication and cable television industries should speed up the deployment of combined voice and video networks. The timetable for widespread implementation of BISDN will be accelerated by deregulation. The infrastructure necessary to be successful in this market is significant, so the more companies and hence, the more capital committed to its deployment-the sooner BISDN will be widely deployed.

Fibre optic cable has been replacing copper cable in many of these companies' long-distance lines for years because it is an affordable method of transmitting a very high bandwidth. Recognizing that a high-bandwidth infrastructure is the key to success in the BISDN market, some local phone companies and cable companies are already extending their fibre optic networks to locations that are very near to the customer. When the fiver optic cable reaches the customer premises, it is often called fibre to the premises, or fibre to the home in residential applications.

Most networks utilize fibre optic cable for much of the distribution, but return to copper cable of some form before reaching the customer premises. Cooper cable can support high bandwidths, but for much shorter distances than fibre optic cable can. Most of the distribution schemes take advantage of this fact and do not demand the use of fibre optic cable to every phone jack and cable box. While rewiring the network is difficult enough, rewiring customer premises makes it even more difficult to sell new services Therefore, both phone and cable companies will probably use the existing copper wiring inside residences and businesses and switch to fibre only after leaving the building.

Fibre to the curb is the term used to describe one approach taken by phone companies, where fibre optic cable is used to connect the central office to a section of a particular street. A special device then converts the optical signals to electrical signals for transmission to the customer on ordinary twisted pair wire. Fibre on the neighbourhood describes a similar approach taken by cable television companies. This method is sometimes called fibre to the feeder of fibre to vault. In this scheme, the fibre optic cable reaches only as far as the neighbourhood and does not reach each customer's street. In this case, the device converting the optical signals to electrical signal to the customers over coaxial cables. There are other hybrid distribution schemes being used, and all seek to balance the benefits of fibre optic with the economic reality of already-installed copper cable in customer premises.

Services

Broadband ISDN provides two types of services: Interactive and distributed.

Interactive services

Interactive services are those that require two-way exchanges between either two subscribers or between a subscriber and a service provider. These services are of three types: conversational, messaging and retrieval

Conversational services are those, such as telephone calls, that support real-time exchange. These real-time services can be used for telephone, video telephony, video conferencing, data transfer and so on.

Messaging services are store-and-forward exchanges. These services are bidirectional, meaning that all parties in an exchange can use them at the same time. The actual exchange, however, may not occur in real time. One subscriber asking another for information may have to wait for an answer, even though both parties are available at the same time. These services include voice mail, data mail and video mail.

Retrieval services are those used to retrieve information from a central source, called an information centre. These services are like libraries; they must allow public access and allow users to retrieve information on demand. That is, information is not distributed unless asked for. An example of a retrieval service is a videotext that allows subscribers to select video data from an on-line library. The service is bidirectional because it requires action on the part of both the requester and the provider.

Distributive services

Distributed services are unidirectional services sent from a provider to subscribers without the subscriber having to transmit a request each time a service is desired. These services can be without or with user control

Distributed services without a user control are broadcast to the user without the user's having requested them or having control over either broadcast times or content. User choice is limited to whether or not to receive the service at all. An example of this type of service is commercial TV. Programming content and times are decided by the provider alone. The user can turn on the television and change the channel but cannot request a specific program or a specific broadcast time.

Distributive services with user control are broadcast to the user in a round-robin fashion. Services are repeated periodically to allow the user a choice of times during which to receive them. Which services are broadcast at which times, however, is the option of the provider alone. Example of this type of service is educational broadcasting, tale-advertising and pays TV. With a pay TV for example, a program is made available in a limited number of time slots. A user wishing to view the program must activate his or her television to receive it, but he or she has no other control.

Future of ISDN

The narrowband ISDN (N-ISDN) was designed to replace the analogue telephone system with a digital one for both voice and data transmission. The design was based on the assumption that technological advances and mass production of N-ISDN equipment would make it affordable fro the regular telephone subscriber.

In fact, N-ISDN has replaced the normal telephone in some European countries in response to the demand of the users. In the United States , however, this replacement was delayed(it is available on demand) and new technologies (such as cable modem and ADSL) evolved that make the use of N-ISDL questionable. However, ISDN can still be considered a good solution for several reasons. First, ISDN can be brought to a subscriber premise with minimum cost and the services available can satisfy the needs of many users. Second, new equipment has appeared on the market that allows a subscriber to use the entire bandwidth of an ISDN line. This makes it competitive with some other technologies. Third, the protocol is flexible enough to be upgraded to higher data rates using new technology and new transmission media. Fourth, N-ISDN can be used as a forerunner for B-ISDN, the data rate of which is sufficient for several years to come

See Also