networks supporting higher speed modem services as the telephony Figure 30.1 Example of analogue transmission loss plan network performance specification tend not to include the par- ameters important for these services. 30/4 Transmission planning 30.3.3 Digital services resultant acoustic power at the output of the telephone receiver. If the service offering does not include the telephone instrument then This class covers the range of services that would be provided over the end to end loss will be defined in terms of a ratio of electrical ISDN networks i.e. networks providing end to end 64kbit/s paths. power in, to electrical power out, and be expressed in decibels (dB). The potential degradations introduced by the network will include Loudness ratings are a convenient method of expressing the end errors in the bit stream and possible timing inaccuracies. The per- to end loudness loss (the acoustic to acoustic loss) in a form that can mitted end to end impairment needs to take into account the infor- be related to customer opinion. Loudness ratings provide the trans- mation being carried on the network. For example encoded voice mission planner with a single value that indicates the overall loss of signals are relatively tolerant of errors whereas the throughput of a particular connection. In practice it is convenient to divide overall digital data services can be drastically reduced by error bursts. loudness rating into the sum of send, receive and circuit loudness ratings as shown in Figure 30.2 and given by Equation 30.1. 30.3.4 Impact on transmission plan Local Local Exchange Exchange It is important for the transmission planner to know what services CUSTOMER A CUSTOMER B will be carried on the network and the allowable end to end perform- ance for those services. Ideally the transmission planner would take CIRCUIT LR the most demanding service requirement for each parameters, derive an appropriate limit and develop a plan for a network capable OVERALL LOUDNESS RATING (OLR) of supporting all services. In pr