TABLE OF CONTENTS Previous chapters have concentrated on the communication of voice, primarily in the form of the telephony service as carried over the PSTN. We are now in a position to consider the various non-voice services, generally referred to as data, and the networks that carry them. At the end of this chapter we will investigate the way that these data networks can also convey voice, and that leads us to consider the new generation of communication networks.
Data is intrinsically different to voice. By definition, data is information which generally originates in the form of a digital representation normally binary 1s and 0s and it therefore does not need to be converted to digital within the network, unlike voice which originates from the telephone microphone as an analogue signal. Data originates from terminals, such as temperature measuring devices, or from a laptop or computer, consoles, control monitors and ATMs (i.e. 'automatic teller machines', not the data service described later in this chapter!) in the wall of a shop or bank. Applications generating data from these devices include the sending of e-mails, transfer of information between files and other forms of enquiry, remote control of machinery and other forms of surveillance, etc. A data service is one that conveys data between the terminals and other devices. There are several key differences between the characteristics of voice and data and hence the different requirements for successful communications, as summarised below.
Data applications may be tolerant...
