he interference levels affect the capacity of individual TSLs. Re-transmissions take place in the data flow at the air interface. As the loading of the network (i.e. the number of subscribers on one TSL) increases, re- transmissions are reduced, SO degrading the quality of the network. With an increased number of subscribers, the TSL capacity decreases due to the higher interference levels. Quality Network coverage, capacity and quality are of course interrelated. There may be low com- patibility between the MS and network, there may be higher interference and degraded signal quality. Unavailable resources may cause problems with cell re-selection, channel unavailability, and packet data protocol activation. Another important aspect in a GPRS network is the type of packet data and the desired QoS for it. When a GPRS network is launched, quality of service is an unknown quantity because of the huge variation in data types and its required quality. QoS has to be user- specific, which is generally stated in terms of data throughput and the associated delay. The quality required for individual services needs to studied, and solutions for critical ones need to be found and implemented. Most critical are real-time data, for which high capacity and minimal delay would be the solution. Related Parameters Parameters form the basis of network design and optimisation. Parameters need to be fine- tuned. The main parameters in this regard are those related to power control (coverage), GPRS territory (capacity), MS attachment success rate (quality), and cell re-selection. 5.6.2 Transmission Network Optimisation Optimisation of the transmission network is quite similar to that for a GSM network. There are two important aspects that might come up in a heavily loaded GPRS network. First, since the GPRS part of the network (packet core) may be added to an existing GSM network, there may be no changes required during the launch phase as there is low traffic. However, once the packet data users start to increase, the capacity of the