Telecommunications Traffic, Tariffs and Costs. An Introduction for Managers
Suitable for those who wish to gain a comprehensive overview of the area, this book offers guidance on planning, ordering, managing and using telecommunications facilities.
Inspec keywords: telecommunication network planning; telecommunication traffic; telecommunication network management
Other keywords: traffic-carrying capacity; traffic measurement; telecommunication costs; telecommunication traffic; underlying theory; telecommunication tariffs; telecommunication system; telecommunication network; public-network tariff option; telecommunication facility; communication problem; cost-effective solution; matching equipment; private system cost
Subjects: Administration and management; Communication network design, planning and routing; Network management; Telecommunication applications
When the handset of a public-network telephone is lifted, the circuit between telephone and switching centre known as the local circuit is immediately barred to potential incoming callers and is said to be engaged or busy. Even though a connection to the intended destination has yet to be established and no information in the form of speech can be exchanged, to all intents and purposes the local circuit is already carrying traffic since it cannot be used for any other call. The initial purpose of the local circuit is to convey signals representing the dialled or keyed digits to the switching centre, so that they may be interpreted in order to extend the path for speech communication to the required destination. Only when a complete end-to-end circuit has been esta blished between the calling and called telephones can the local circuit be used for its intended purpose of exchanging information in the form of speech the originally introduced concept of what is meant by telecommunications traffic. The sounds of speech are represented by variations imposed on a steady electric current, and it is these current variations which are conveyed as a continuous signal between the telephones to provide two-way communication.
Public telephone network local circuits, as introduced in Chapter 1, together with the cables etc. which carry them, constitute the local network, and the switching centre to which they are connected is referred to as a local switching centre. Each local circuit will usually serve one customer and must normally consist of a pair of electrically conducting wires for speech transmission pur poses. These same two wires are used to signal the calling condition and convey the dialled or keyed digits to the local switching centre, functions to which the generic term signalling applies (where a telephone is an extension on a PBX, additional wires may be provided for this purpose). The explanation of the transmission of telephone traffic given in the following paragraphs is particular to a metallic-pair local network, but radio or optical-fibre transmission systems may be appropriate in particular circumstances and can be expected to become an increasingly acceptable alternative in the future.
The method of communication used for public telephone networks, whereby independent point-to-point circuits are temporarily linked together by switching centres to establish a through connection between two users who wish to 'converse', is termed circuit switching. The calling and called users must both be available simultaneously for a call to be effective, but do not necessarily have to be human since automatic calling or answering devices may be employed. All circuits and switches involved in the connection must be held for the duration of the call. There is an initial set-up delay (which will worsen with increasing network loading) but, once set-up has been successfully completed, interaction is to all intents and purposes instantaneous and occurs at a transmission speed which is limited by the characteristics of the circuits involved. The conversion of an established analogue public telephone network to an integrated digital network (IDN) is a lengthy and costly undertaking, involving the progressive replacement by digital equipment of all switching centres and transmission systems. Primarily intended for business users, digital transmission access to the IDN will be available as an alternative to analogue local-circuit access at extra cost (see Section 4.1), but it can only be provided where a connection can be established to a digital switching centre. Even though the amount of digital transmission and switching equipment increases each year, therefore, the network will be seen by most users as effectively an analogue transmission network since the normal access to it will still be via analogue local circuits.
This chapter discusses the local network developments leading to the integrated-services digital network. The problem with single local network cable pair is that it is not, by itself, capable of supporting simultaneous bothway digital transmission because of the impossibility of separating the signals in each direction. The burst-mode technique solves the problem by sending the digital information over the local circuit in short bursts, alternately in one direction and then the other.
This chapter discussed different types of private network. Private network is that it is primarily intended to carry internal traffic originated by, and destined for, employees of the organisation it exists to serve. Most private networks also have access to public telephone and data services. These networks range in size from those which provide service to a single site to those which link many sites nationwide or even internationally. A small PBX and its extensions and private telephone network are example of a single-site private network in its simplest form; by contrast, a multisite private network may consist of many interlinked switching centres, carrying both speech and data traffic. Two unique network configurations have been developed to permit limited distance direct communication between any two stations without the need for a switching centre. Both employ (a) a common transmission path and (b) a form of distributed packet switching to route messages to the required station. The transmission path is cabled throughout the area to be served so that additional stations (up to a limit of, typically, around 100 stations per network) can be 'plugged in' to it wherever the need arises. Data networks of this type are known by the generic term local area network (LAN) since they are designed specifically for a single site or several nearby sites, where the distances over which data needs to be transmitted are normally quite short and rarely more than a few kilometres. LANs provide for high rates of error-free data transfer and are ideally suited to a general-purpose data network which includes shared-access computers, printers, etc. Given that both LANs and PBXs will be able to offer comparable standards of service integration, the choice between them will have to be made on factors which result from their differing network concepts; the LAN with its distributed switching and ring or bus network, and the PBX with its centralised switching and star network. Technically there would appear to be little to choose between the two systems, and it is not yet possible to predict which might offer the most economic solution. An installed cabling base might well prove to be the deciding factor by determining the network structure and thus the system choice.
This chapter discusses traffic measurement, forecasting and equipment quantities. The idea of measuring traffic in terms of traffic-flow units was first conceived with the sole objective of ensuring the cost-efficient handling of telephone calls on a circuit-switched network. Other types of telecommunications traffic are now carried on the same circuit-switched network in addition to telephone calls but, since all traffic is in the form of separate successive calls, traffic measurement on the basis of erlang or CCS units remains a valid concept.
A telecommunications switching centre is constructed from an integrated range of equipment modules collectively termed a switching system. The task of deciding which and how many of these modules are needed for a particular application is generally referred to as switching-centre dimensioning. The dimensioning process is, however, constrained by the pattern and extent of the breakdown into equipment modules in other words, by the overall conceptual design of the switching system itself. A switching system is designed for the cost-effective handling of one particular type of traffic; a system intended for telephone and other call-based traffic is significantly different from those for message or packet-based data traffic.
The basic components of a telephone network have already been identified; telecommunications users are connected by local circuits to a switching centre and, where the users are sufficiently numerous and widely located, there are a number of switching centres interconnected by traffic routes. Earlier chapters have also drawn attention to various traffic-related factors which affect network performance. These and other factors now need to be considered from the point of view of their collective influence on the planning of a telephone network.
Throughout this book, emphasis has been placed on the direct relationship between the traffic-carrying efficiency and the cost of a telecommunications system or network. The traffic-handling performance of every system and net work is limited by its configuration, the concepts employed to transmit, switch and route traffic, and the design of the manufactured products which implement those concepts. To achieve a given standard of performance, therefore, requires careful attention to system choice and network design. The situation can arise where the required performance standard is beyond the technical capability of a proposed system or network solution, giving rise to the need either to find an alternative solution or to accept a lower standard of performance. Traffic-hand ling performance can be estimated by a network designer with sufficient accuracy for most dimensioning/sizing purposes, but there will be an occasional need for a detailed evaluation of system/network performance by a specialist performance engineer. Telecommunications managers should, therefore, have an understanding of what can be achieved in this field.
The term traffic-carrying performance, as applied to any telecommunications network, is used to describe how effectively that network is able to transfer information between users. Earlier chapters have shown that traffic-carrying performance is inversely related to the percentage of calls lost and call set-up delay experienced with a circuit-switched telephone network, and to information-transfer delay across all types of data network. Call losses or delays can only be reduced by providing more or, with data, faster circuits and more equipment, at greater expense; thus traffic-carrying performance is also directly related to overall network cost. By matching the traffic-carrying performance of a private telecommunications network as closely as possible to the needs of the business concerned, it is possible to ensure that network costs are no greater than absolutely necessary. This objective can be achieved by designing the network to meet a performance target.
A number of different solutions may meet the operational and technical needs of a telecommunications requirement with a sufficient degree of success to warrant consideration. Among these, the 'right' solution is the one that offers the most effective answer to that communications need at the lowest cost. Choosing the right solution involves the operational, technical and financial evaluation of each alternative. The busy manager who wishes to update a private telecommunications facility may decide to place the entire responsibility for finding the right solution upon a firm of consultants or even one supplier, but this is not necessarily the most cost-effective course of action. The recommended solution could prove to be a less-than-optimum compromise, especially if the choice has been restricted to the products of one manufacturer. Even a busy manager, however, can assume at least some of this responsibility personally. At the very least, personal involvement will reduce the amount and cost of expert advice needed and help to ensure a satisfactory outcome to the project.
