etwork, we have to understand what types of signals are transmitted through the telecommunications network and their requirements. In this chapter we look at the requirements of vari- ous applications, characteristics of analog voice channels, fundamental differ- ences between analog and digital signals, analog-to-digital conversion, and a logarithmic measure of signal level, the decibel. 3.1 Types of Information and Their Requirements Modern digital networks transmit digital information transparently; that is, the network does not necessarily need to know what kind of information the data contain. This information that is transmitted through the network may be any one of the following: Speech (telephony, fixed, or cellular); Moving images (television or video); Printed pages or still picture (facsimile or multimedia messaging); Text (electronic mail or short text messaging); Introduction to Telecommunications Network Engineering Music; All types of computer information such as program files. For digital transmission, analog signals such as speech are encoded into digital form and transmitted through the network as a sequence of bits in the same way computer files are transmitted. However, although all information is coded into digital form, the transmission requirements are highly depend- ent on the application; because of these different requirements, different net- works and technologies are in use. Video and e-mail applications, for example, require different architectures. Network technologies have taken two main development paths: one for speech services and another for data services. The telephone network and ISDN have been developed for constant-bit-rate voice communication that is well suited to speech transmis- sion. Data networks such as LANs and the Internet have been developed for bursty data transmission. The constant-bit-rate requirement for speech follows from the princi- ple that digitized voice signals have traditionally been transmitted in digital form as samples at regular intervals, as we wil