oth the user data and signalling data using the GTP-U and GTP-C Protocols, where U and C denote user and control respectively. The main signalling procedures are network re- quested context activation, tunnel management and path management. Network request context activation messages relate to the PDU notification requests and responses, and sending the routing information for GPRS requests and responses. Tunnel management relates to the creating, updating, deleting the PDP context requests/response, etc. Path management relates to the echo requests and responses. Core Network Planning and Optimisation Redundancy of the Gn When one active interface unit fails, the passive/spare interface unit assumes the tasks of the failed unit. The active interface unit performs this by sending gratuitous ARP messages for the failed unit. In doing so, it updates the ARP caches of the other elements in the same subnet. When the Gn interface IP addresses are not used, then the units used for tunnelling in 3G SGSN should be configured into the same routing protocols that are running on the Gn interface. Gn interfaces should be in different subnets as a router cannot have two interfaces in the same physical segment or it becomes confused when receiving the same routing protocol messages from neighbouring routers over multiple interfaces. Domain Name Server The domain name server (DNS) is required to translate the logical domain names into the IP addresses of the domains. DNS servers should be placed at accessible locations from the SGSN. These servers should be protected by introducing an additional DNS as a backup. Both the APN and the logical name of the routing area are translated to the IP addresses. An APN network identifier is mandatory though the APN operator identifier is optional. The APN network identifier is a label or set of labels that are separated by dots, e.g. network.com, which guarantees uniqueness to the domain name. The APN network identifiers should not end with gprs but operator identifiers should end in .gprs