Contents

Introduction

One-way v Interactive

Analogue v Digital

Circuit switching v Packet switching

Fixed v Mobile

Geo-stationary satellites v LEO satellites

Low bandwidth v High bandwidth

Electrical signal v Optical signal

Basic signal v Processed signal

Stand-alone networks v Converged offerings

Introduction

One way of understanding how telecommunications systems work and how they are changing is to consider a number of basic paired concepts. This short paper looks at nine such pairs and it should be emphasised that many of them are linked.

One-way v Interactive

Most broadcasting systems are one-way: the radio or television signal is transmitted to the listener or viewer and there is no means for the recipient to send back any information. By contrast, telecommunications systems – whether telegraph, telex, facsimile or telephony – are intrinsically two-way: both parties to the communication process have the same facility to send and receive the same kind of information. Increasingly, broadcasting systems are becoming more interactive as digital television allows viewers to use their sets to take part in electronic polling or order goods or send e-mail or access the Internet. Similarly more and more Web sites are becoming interactive allowing the user to comment on a film or book a play or order a book.

Analogue v Digital

Analogue signals have a continuous wave-like form. The further the signal travels, the more likely it is to become degraded and therefore, over anything other than short distances, these signals are boosted in strength at various points in the network and any subsequent distortions are amplified at the same time. By contrast, digital signals are discrete in form and essentially represent power on or off or – to use the binary language of computers – nought (0) or one (1). Digital signals can be recreated at their destination in exactly the form they left their starting point despite any distortion along the way, since the only two possibilities for the original signal are 0 and 1. All communications networks – whether telecommunications or broadcasting, whether fixed or mobile – are increasingly becoming digital because the quality of the signal is so much better and the capacity of the network is so much greater.

Circuit switching v Packet switching

Traditionally telecommunications networks have connected the sender and the recipient through a series of circuits, controlled by switches or exchanges, which provide a dedicated, physical connection for the duration of the call. This provides a high-quality connection, but it requires considerable capacity that is often unused for a lot of the time. By contrast, data networks – which link computers rather than people – use a method of transmission in which the information is broken up into a multitude of tiny packets. These packets are switched to their ultimate destination through a multitude of routes and then reconfigured into the original order or message at the end of the transmission. Packet switching enables much more intensive use to be made of network capacity and therefore it is much cheaper. The Internet, which was designed to carry data and therefore uses packet switching, is increasingly being used to carry voice telephony at lower cost and (currently) poorer quality than traditional circuit switched networks. Soon, however, we will have IP (Internet Protocol) networks which will be able to carry voice as well as data with quality comparable to current telephone networks.

Fixed v Mobile

Until comparatively recently, all communications networks used fixed links whether these were wire, microwave or satellite. In the last 20 years, we have seen a growing proportion of calls carried on mobile networks. The first generation of mass market mobile networks were cellular; the second generation are called Personal Communications Networks (PCN) or Personal Communications Services (PCS); and the third generation – not yet in operation – will be called Universal Mobile Communications Systems (UMTS). What makes these mobile networks possible – and increasingly cheap – is microelectronics that has enabled the development of small and light handsets and sophisticated computers that can switch calls made while moving at speed. Over time, mobile telephony may well become a larger market than fixed telephony.

Geo-stationary satellites v LEO satellites

Until very recently, virtually all communications satellites have used a particular orbit called the geo-stationary or geo-synchronous orbit which is 22,240 miles (35,780 kms) above the earth’s surface. At this distance, the satellite travels around the globe once every 24 hours, so that it appears to be stationary above a particular spot on the earth’s surface (usually around the equator). The advantage of such an orbit is that only three-six satellites can cover the entire surface of the globe. The disadvantage of this orbit is that the signal has to travel so far up to the satellite and back to the earth that there is a time delay and – more seriously – large and expensive transmitters and receivers are necessary. By contrast, low earth orbit (LEO) satellites use much lower orbits – typically around 500-1,200 miles (or 800-1,930 kms) above the earth. The disadvantage of such orbits is than many more satellites are needed to cover the earth’s surface – the ill-fated Iridium system would have used 66 satellites – but the advantage is that the signal has much less distance to travel and therefore the handsets can be so much smaller, lighter and cheaper. In the next decade, LEO systems will enable access to advanced communications facilities almost anywhere in the world however remote.

Low bandwidth v High bandwidth

Bandwidth is a measure of the information-carrying capacity of a particular form of transmission and it is usually denoted in bits per second (bit/s) where a bit is a binary digit (that is, a nought or a one in computer terms). Services with low information content – such as basic voice telephony – only require low bandwidth and are called narrowband; services with more information content – such as video – require mid bandwidth and are called midband; and services with a great deal of information content – such as high definition television – require high bandwidth and are called broadband. Even a narrowband service like lower-rate data might need midband transmission if the intention is to download large volumes of data at high speed. Therefore the trend is for all communications networks to provide more and more bandwidth and the challenge to network operators is how to provide this in a cost-effective way.

Electrical signal v Optical signal

Twisted copper pairs and coaxial cables transmit signals in electrical form as a flow of electrons. By contrast, optical fibre transmits signals in the form of pulses of light generated by miniature lasers and received by tiny diodes. Both forms of transmission travel at the fastest speed known to physics, the speed of light which is approximately 186,000 miles per second (or 300,000 km per second). Currently all switching is done electrically, so that light signals are converted into electrical signals for the switching process and then afterwards reconverted back into light signals. However, in future we can expect the development of optical switches. Indeed developments may be so profound that some have suggested that, if the last century was that of the electron (electricity), then this century will be that of the photon (photonics).

Basic signal v Processed signal

Frequently communications networks process basic signals in order to increase the capacity of the network. One effective means of increasing the range or number of services that can be carried on any particular cable or wireless system is to compress the basic signal, so that much of the redundant information is removed before transmission. For instance, the information in the 30 frames that make up a second of video can be compressed from about 27 million to 1 million bits without serious loss of quality. Another way to enhance the capacity of a transmission system is to carry different types of signal or service at the same time. This can be done by multiplexing techniques that for instance mix voice and data signals on the same transmission path. A whole variety of compression and multiplexing technologies are now massively expanding the capacity of communications networks.

Stand-alone networks v Converged offerings

Traditionally communications networks have been created to serve specific purposes, such as the broadcasting network for television, the telephone network for voice, and the Internet for data. Increasingly both physical networks and customer offerings are converging because digitalisation of networks makes this possible and competitive marketplaces make it desirable. At least three types of convergence are occurring: between voice and data through services like ISDN and networks like the Internet; between telecommunications and broadcasting through telephone companies offering video on demand and cable companies providing telephony service; and between fixed and mobile telephony as customers are offered packages combining both services and phones are developed that can be used in the home or office or on the move.

ROGER DARLINGTON

Last modified 13 August 2000