e is a function of a number of variables within the network and these variables need to be identified and their effect on end to end performance quantified. Only when this step has been done can the planner claim to 'control the levers' of the transmission plan and begin to specify requirements for the parts of the network. Figure 30.8 Example of a digital network structure 30.5.1 Economic aspects From Figure 30.8 it is clear that the end to end performance of any Transmission planning cannot be divorced from the economic real- connection is a function of the individual system performances ities of telecommunication networks. Broadly, it is necessary for the together with the actual routing through the network. For example transmission planner to demonstrate that the solutions he proposes the end to end transmission loss of any connection would be depend- will contribute to providing the required return on investment. ent on the individual transmission losses of all the individual sys- The financial aspects are covered in more detail later. For the items in that connection i.e. the telephone instrument, 2 wire local purposes of this section it will be assumed that the financial aspects line. digital local exchange and any digital attenuation introduced. Key influences on end to end performance 30/9 However the impedances presented by the transmission systems are As error distributions tend to be non uniform the resultant noise also important as the resulting mismatch losses could have a signi- can appear impulsive in nature. ficant impact on end to end losses. This impedance aspect can be Noise can be injected or induced into connections by crosstalk considered as an interface requirement that also needs considering mechanisms from adjacent systems, non linearities in cable joints in any requirements document. and also lack of immunity from external interference. Such inter- In summary the end to end performance for any parameter is ference is minimised by controlling impedance balance about earth determined by t