This lecture is divided into hyperlinked sections
Introduction
Why go digital?
ISDN Principles
How ISDN was
expected to evolve
The User Interface
Narrowband ISDN
Broadband ISDN
Services offered
by N-ISDN
Conclusion
The integrated services digital network is a worldwide public telecommunications network that has been set up to replace the existing public telecommunications networks which are characterised by various analogue sections and also to deliver a broad range of services. The candidates for integration into the network are telephony, data (e.g. LAN interconnection), text and images. It offers digital end-to-end connectivity. It is defined by having a standard user interface and is implemented as a set of digital switches and their interconnection paths. The ISDN supports a wide variety of traffic types and provides value added processing services. In practice it is implemented as multiple interconnected networks but from the point of view of the user it appears as one network.
ISDN allows multiple digital channels to be operated simultaneously through the same regular phone wiring used for analogue lines. The change comes about when the telephone company's switches can support digital connections. Therefore, the same physical wiring can be used, but a digital signal, instead of an analogue signal, is transmitted across the line. This scheme permits a much higher data transfer rate than analogue lines.
Consider the existing analogue telecommunications network as depicted in figure 3.1 below. Incoming voice lines are modulated and multiplexed at the end office and sent over an FDM line. For each channel in the FDM line there is one dedicated modem at each end of the line. Because a signal may need to pass through several switching centres en route to its destination, at each switching centre the signal has to be demodulated and passed to a space division switch in which dedicated paths are used. The signals are then modulated onto another FDM carrier once more to be transmitted. This repeated conversion introduces noise onto the analogue section of the connection and also increases the cost of the system.
Fig 3.1 The analogue network
The task performed at a switching centre is to route incoming signals onto the required outgoing path. In fig 3.2 below, we see a block diagram showing the function of a simplified switching centre. Regardless of the implementation (analogue or digital) the function is the same.
Fig 3.2 The function of a switching centre
Below in fig 3.3 is how the above scheme is implemented with an analogue space division switch. Note that for each channel provided on each trunk it is necessary to have a dedicated modulator/ demodulator pair.
Fig 3.3 Implementation of an analogue space division switch
When both the transmission and switching systems are entirely digital, true integration can be achieved. Incoming voice signals are digitised using pulse coded modulation (PCM) and multiplexed using time division multiplexing TDM. This means that at the switching points along the route of the signal there is no necessity to convert the signals from digital to analogue and back again. This reduces introduced noise and also obviates the need for the large number of modems and space division switches, the switching being performed by time division digital switches as in figure 3.4.
Fig 3.4 The simplicity of
a digital time-division switch
The following principles and evolution of ISDN are based on the CCITT Recommendation I.120 (1988) for Integrated Services Digital Networks.
CCITT defines the standards for ISDN. There are six key points.
1 To support voice and non voice applications using a limited set of standardised facilities. This means that ISDN will carry telephone calls and the exchange of digital data in conformance with CCITT recommendations specifying a small number of interfaces and transmission facilities.
2 Support for switched and non-switched applications. This means that ISDN will support packet switching, circuit switching and dedicated non-switched lines (leased lines).
3 Reliance on 64kbps connections (B channels). The intention of ISDN is to provide circuit and packet-switched connections at 64kbps, the fundamental building block of ISDN. The reasoning for the use of this rate was to support digitised voice and was the standard for evolving digital networks. This data rate is now being seen as restrictive and future developments of ISDN will permit higher rates. A data channel (D channel) handles signaling at 16 kbps or 64 kbps, depending on the service type.
4 Intelligence in the network. ISDN should provide more sophisticated services than that of purely setting up circuit switched calls.
5 Layered protocol architecture. The protocols for user access to ISDN can be mapped into the OSI model. This allows standards developed for OSI to be used over ISDN e.g. x.25 level 3 for access to ISDN packet switching services. Also new ISDN standards can be based on existing standards, thus reducing implementation cost. One example here is LAPD, based on LAPB. Standards can be developed and implemented independently for each layer. New paths may be provided through a layer, allowing customers to implement ISDN at their own pace.
6 Variety of configurations. This allows ISDN to be implemented using more than one physical implementation permitting differences in national policy, competition between network providers, types of technology and types of customer equipment.
There are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64 kbps B channels and one 16 kbps D channel for a total of 144 kbps. This basic service is intended to meet the needs of most individual users.
PRI is intended for users with greater capacity requirements.
Typically the channel structure is 23 B channels plus one 64 kbps D channel
for a total of 1536 kbps. In Europe, PRI consists of 30 B channels plus
one 64 kbps D channel for a total of 1984 kbps.
To access BRI service, it is necessary to subscribe to
an ISDN phone line. Customers must be within 18000 feet (about 3.4 miles
or 5.5 km) of the telephone exchange building for BRI service; beyond that,
expensive repeater devices are required, or ISDN service may not be available
at all. Customers will also need special equipment to communicate with
the phone company switch and with other ISDN devices. These devices include
ISDN Terminal Adapters (sometimes called, incorrectly, "ISDN Modems") and
ISDN Routers.
CCITT also expressed its views on how the ISDN was expected to evolve.
1 Evolution from telephone networks. ISDN is expected to evolve from the existing telephone networks thus the digital technologies implemented and those still evolving will form the basis for the services provided by ISDN. This implies that ISDN is to be implemented using the telephone network and although it may use existing private telephone networks, satellite links and packet-switched networks, these will play a lesser role than the telephone network. This was decided because of the existing worldwide coverage of the telephone network.
2 Transition over one or more decades. Because of the huge task that conversion to ISDN presents, it is expected to be a slow process that will be done in the context of existing digital and analogue services. There will need to be periods of coexistence during which connection and protocol conversion will be required between alternative facilities.
3 Use of existing networks. This further builds on point 2 above, an example being ISDN's packet switched service which will be accessed using X.25. As fast packet switching and more sophisticated call control is implemented an alternative to X.25 may need to be used.
4 Interim user to network arrangements. The primary concern here is that the lack of digital subscriber loops may delay introduction of digital services, especially in developing countries. Using modems and other equipment, some ISDN services may be supported by existing analogue facilities.
5 Connections at other than 64 kbps. This rate was chosen to support digitised voice communications and forms the basic channel for circuit switching. Improvements in digitising techniques have made this rate unnecessarily high. However this rate is too low for high quality video and other digital applications. This means other rates will be required.
Figure 3.5 shows how the interface will appear to the user or customer. The customer's access to the ISDN will be by means of a local interface to a digital 'pipe' having a certain bit rate with varying capacity pipes being available for different uses. Some customers may require relatively low bandwidth for a telephone and teletex, while another may wish to connect his PBX to the network, requiring a much higher bandwidth.
Fig 3.5 Conceptual view of ISDN Connection Features 1
The 'pipe' provided will have a fixed capacity but may carry a dynamically varying mix of traffic types and bit rates up to the pipe's capacity. This means a user may access both packet and circuit-switched networks and other services. This will require complex control signals to be multiplexed onto the digital pipe to control the time multiplexed data and provide the services demanded.
It was also envisaged that the customer would be charged according to the data capacity used rather than the time connected but BT has chosen to ignore this, still charging for its ISDN service by time rather than data quantity. Charging per unit data would have reduced user efforts to cram as much data onto a pipe as possible with concentrators, multiplexers, packet switches and other methods of sharing a line. It would also have led to a much greater take-up of the service by businesses and the public. BT is to blame for over-charging for this technology.
This was the first generation of ISDN and was based on the use of a 64kbps channel as the basic unit of switching. It operates using circuit switching techniques. The most important feature of N-ISDN was that it introduced frame relay, a technique for speeding communications.
Broadband ISDN was created to support high definition video which requires a channel rate of around 150Mbps. This is the second generation of ISDN and supports extremely high data rates, up to around 600Mbps. This data rate would allow simultaneous support of more than one interactive service. The major technical contribution that has been provided by B-ISDN is asynchronous transfer mode (ATM) sometimes known as frame relay.
ISDN supports a variety of services including the existing
voice and data communications. It also supports a few other services namely:
The take-up of ISDN has been severely limited in the UK by the pricing regime of British Telecoms. Their ISDN service has not taken off anywhere near as well as it ought to have done. Another limiting factor has been the copper loop’s failing to be able to provide the bandwidth required to provide the service.
ISDN has not been a complete failure because searches for technologies to deliver the data at the speed required for the service to proliferate have led to ADSL and its offspring and have also led to the development and widespread use of ATM.
Frame relay was also developed due to ISDN.
References
1 Diagram courtesy of Business Data Communications, Stallings & Slyke, Prentice Hall International