 can handle. The result, rounded up to the nearest integer number, will give the first estimation of the total amount of SGSN nodes needed in that given site for the calculated traffic in that concrete year. There are some important aspects to consider, however. The first aspect it is important to verify is that the throughput should be considered at the same protocol layer. It should not be a problem if the Iu-PS throughput is considered as the IP payload throughput for the ATM transport; otherwise the different overheads in Iu-PS with respect to Gn should be taken into account, since the maximum throughput of the SGSN is normally given in terms of the IP payload throughput. The second aspect refers to the type of Iu-PS interface cards and its maximum amount per SGSN node. Normally there should be STM-1 and E1/T1 interfaces available towards Iu-PS, the E1/T1 as components of IMA groups. Since the requirements of each Iu-PS interface are known, the game consists in fitting the Iu-PS traffic flows into the estimated amount of SGSN in a sensible way, trying not to exceed the first estimation of the number of SGSN nodes needed. The issue here is the high granularity of the STM-1 interfaces, which may cause more additional SGSNs to be needed than initially estimated, even though the throughput capacity limit is not reached because the limit of the maximum number of interfaces was first reached in a given SGSN node. For this reason, a careful check should be made after the first rough estimation of the amount of required SGSNs. The third aspect is actually brought up by the second. It may also be possible to have a mixed scenario, where not all the SGSNs are fully equipped at the given time, although this case should be contemplated in a much wider time perspective regarding expansion forecasts in the forthcoming years. Secondly, a new rough estimation of the maximum amount of SGSN nodes in the same given core site will be made using the limit of the maximum amount of attached subscribers. As in the previous case, thi