deration. As the quality requirement for data is more stringent, a BER threshold of 10-6 is considered during link planning. Standards such as ITU-T G.828, ITU-T I.356 an ITU-T I.357 are recommended to be used, as ATM requires high-quality transmission. The recommended values of threshold are 10-3 and 10-6 for PDH and SDH respectively. Another aspect to be considered is the ATM performance itself. ATM performance is measured by using parameters such as 'cell loss ratio' (CLR). To keep this value to a minimum, availability targets are usually kept larger than 99.99%. Note, however, that this may depend on an operator's requirements. 8.5.2 Topology Transmission networks in GSM saw many types of topologies: star, chain, loop, etc. In contrast, in 3G networks this will not be the case, at least in the beginning. There are two main reasons for this: capacity requirements and delay. Traffic in 3G networks consists of voice, CS data, PS data and the common channels such as RACH, FACH, etc. If there are about seven users generating voice traffic, and three users generating CS-64 and PS-64 (both 64 kbps), then the total traffic generated by the base station (configuration 1+1+1) would be more than 50 Erl, of which common channels alone constitute more than 30 Erl, as shown in Figure 8.10. This increases the required capacity to almost four Els, whereas in a GSM transmission network a configuration of 1 + Cellular Nodes General Attributes 3G-BTS User Plane 3G-BTS NRT Services 3G-BTS Control Plane Number of generated VCC's Bts ID: Cell and Traffic editor SHO [%] Traffic Up Traffic Down All subscribers [#] Traffic [Erl] Subscribers [#] (s Voice12.2U Voice12.2D 264.258 6.60645 119.531 0.597656 NRT64 NRT64 Add new cell Duplicate cell Remove cell Add traffic to cell Remove traffic Cancel Apply Figure 8.10 Example of traffic calculations 3G TRANSMISSION NETWORK PLANNING AND OPTIMISATION 1 + 1 would need less than one E1 of capacity. Thus, longer chains/loops are not preferred in these networks. Delay in another reason for making