directly. The operators may have a tie-up such that they use common APNs and define roamers in the HLR SO that they can use the visitor GGSN. In this case, inter-PLMN traffic is reduced. Therefore it can be said that roaming traffic also depends on operator policy. The physical locations of the SGSN and GGSN also affect traffic dimensioning. If the SGSN and GGSN are co-located, the traffic will be localised. They may be connected using 100 Mbps of the Ethernet. If it is assumed that an active subscriber generates 300 bps of traffic in a busy hour, then 100 Mbps bandwidth of fast Ethernet can support around 350k subscribers. If the SGSN and GGSN are remote, then this traffic will go over the backbone. This will require IP/ATM backbone planning, which is discussed later in detail. ATM or MPLS backbone capacities, number of routers, link and topologies will have to be dimensioned accordingly. It is best as far as possible to localise the traffic coming out of the SGSNs and direct them to the GGSNs located on the same site, for minimum IP traffic across the backbone. This will save cost on building a high bandwidth backbone. However, this may not be applicable in all situations. For example, in some countries operators have to provide a service over large geographical areas and the subscriber density in each region is less and does not justify a local GGSN. Hence they opt for one centralised packet core with GGSN, DNS, BG, Internet access, etc., and remote SGSNs in each region. If roaming or DNS traffic is considerable then the BG and DNS may be co-located with the SGSN. The cost of routers, reliability of backbone transmission links and operation and maintenance issues are some other factors that should be considered while deciding upon the location of equipment. A few other aspects which need to be taken care of during dimensioning are: calculation of the required number of fast ethernet or gigabit Ethernet ports on the IP routers; calculation of the required number of STM-1 ATM interface cards required on the ATM 