g opportunities for network optimisation. As one of the functions of the BSC is to Cost. It may not be possible to achieve line of sight between provide concentration of traffic, it can be envisaged that the utilisa- each BTS and the BSC and therefore it would be necessary to tion of possible hubbing methods in linking BTS to BSC will ensure rent capacity, which is more expensive. that traffic volumes between the BSC's and MSC's will be very 2. Reliability. Only one route exists between each BTS and the high. BSC which means that if we lose the link, we lose the BTS. Given the high volume of traffic and the distances involved, protected high capacity microwave links or leased capacity will be Ring configuration used. However, given the economics involved, it can be expected that route diversity will need to be utilised. Figure 48.4 shows a ring configuration. This has the main advantage in that each BTS has two routes to the BSC, thus giving an increase in reliability. However, as demand increases, unsuspected hot spots 48.6.2 Cost savings may develop. The voice encoder used in the PCN operates at a data rate of 13kbit/s. Therefore prior to interworking with the ISDN/PSTN, the digitised voice signal must be converted to 64kbit/s A law. This is achieved via a transcoder. The PCN architecture allows the network operator the option of positioning the transcoder at the BTS, BSC or even at the MSC. If one positions the transcoder at the MSC, this enables traffic to be carried through the radio subsystem on 16kbit/s channels. 48.6.3 Trunk network The Trunk network is utilised for carrying inter-MSC traffic and for interconnecting the PCN to other networks, e.g. the national PSTN/ISDN. The interconnection of MSC's, given the distances involved, will be achieved by using high capacity bearers. Routeing schemes are put in place to ensure that costs are opti- Figure 48.4 Ring configuration of BTS's mised whilst maintaining the required quality of service. 48.6.3.1 Location of MSC Ring - star combination The location of the