switches; account of the impact of QoS requirements in the calculations; introduction of router switch modules (RSMs) on the existing catalyst switches, thereby eliminating the need for the introduction of additional IP routers. Core Network Planning and Optimisation DNS Dimensioning The SGSN caches DNS responses. This fact must be taken into account while dimensioning DNS. The time to live of a cached entry depends on how static or dynamic a network environment is. Once a day is a good option since the only time the DNS will be updated is when new APNs or routing areas (RAs) are added, removed or modified: N number of DNS queries = name of caches per SGSN X (number of APNs + number of RA updates) In each 24 hour period only the first APN and RA update name resolution request will be directed to the DNS. The minimum number of DNSs is two (primary and secondary). DHCP Dimensioning The DHCP is dimensioned according to the total number of DHCP requests during the busy hour with respect to its handling capacity in terms of requests per second. An 'n + l' redundancy scheme is recommended. Firewall Dimensioning Real-life planning scenarios and network topologies hugely impact firewall (FW) dimensioning. The firewall can be dimensioned based on the total required data traffic during the busy hour with respect to the equipment processing capacity. It is also recommended that the FWs should be implemented on an 'n + 1' configuration running the Virtual Router Redundancy Protocol (VRRP), for equipment redundancy and load sharing functionality. Firewalls are needed at the Gi interface for Internet access. The firewall may also be needed at the Ga interface for charging and the O&M interface for connectivity to the OMC (operations and maintenance centre). Separate traffic capacity dimensioning and port requirement calculations should be done for each of these. Traffic for access points on the Intranet may not go through the firewall; hence it should be excluded. Border Gateway Dimensioning The border gateway (BG) is mainly d