 between service requirements and network performance re- quirements is being studied within CCITT (1989a). 30.2 Purpose of a transmission plan To illustrate the approach adopted the three most common service groups are used: The transmission plan is the means of controlling the end to end transmission impairments accumulated on any connection through Analogue telephony. the network. The plan should state the end to end limiting values and 2. Voiceband data. show the apportionment of these values to the constituent parts of 3. Digital services. what may be a complex network. For example Figure 30.1 shows a possible analogue network structure with transmission loss appor- 30.3.1 Analogue telephony tioned across the network. The maximum overall transmission loss (point A to point B in this figure) is 42.5dB. The key customer requirement is for clarity and adequate volume of Primary Centre received speech together with acceptable noise levels. The relation between this customer view of quality and the relevant transmission 1.5dB characteristics is not straightforward (Richards, 1973). It is necess- ary to consider a range of parameters including transmission loss, noise, distortion, echo and delay and loss/frequency distortion, and, 1.5dB 1.5dB Primary in general it is important to have a plan for the apportionment of Centre each of these parameters. 30.3.2 Voiceband data 4.5dB 4.5dB Sometimes called modem data, these services involve the trans- mission of data over the same nominal 3kHz bandwidth path as for 1.5dB 1.5dB analogue telephony. The performance of this path is characterised Local Switch by similar impairments as for analogue telephony. Modern high speed modems (see Chapter 39) employ adaptive techniques to provide equalisation of the transmission path hence the modems can Local work over a wide range of path characteristics. However adaptive techniques are generally not able to work well over paths with significant non-linearity or with characteristics that have significant short term variations e.g. gai