n different PLMNs. Hence public IP addresses should be used in the GP interface. Core Network Planning and Optimisation 4.14 Packet Core Network Optimisation 4.14.1 Packet Core Optimisation Approaches The actual content of the optimisation activities for the packet core network and its motivations can be a matter of discussion. There are different possibilities, and all can be argued in different real network contexts. The next sections present a few approaches to packet core optimisation, based on different motivations and viewpoints. Concept of Optimisation as Improvement and Troubleshooting From a more traditional perspective, there are two main approaches in terms of optimisation: The first is related to troubleshooting the observed problems, a practice that in some cases is directly undertaken under the network operations common denominator. The second refers to improving the network performance by means of improving the efficiency of the mobility management and session management procedures. Both motivations are perfectly valid, although they trigger packet core optimisation projects in two very different scenarios, the first being related to identify, characterise and solve a concrete problem in the packet core network and the second concerning an attitude of constant improvement, even in the absence of problems. They can also be considered as short term and long term respectively. Bearer Optimisation and Application Optimisation There is also another way to approach the concept of packet core optimisation that has been gaining greater recognition lately, following the success of data mobile services, which is based on a layered view of mobile communications: bearer optimisation, which concentrates on activities related to the improvement of the bearer perfor- mance, i.e. the lower protocol layers on the multilayered protocol model; application optimisation, focusing on the end user perception of the actual performance of the mobile services, which are based on top of the bearer. It is being accepted as fa