es. The downside is that they need to be updated constantly, and the viruses that can be detected are supposed to be fully identified beforehand. The mutation of some viruses could eventually make the detection more difficult. The rest of the aspects of operational security are defined according to the operator's specific IT routines, in which the mobile packet core network elements should also be included. This is most probably a normal practice in many mobile networks, since many operators, or their parent companies, have ISP operations. 4.15.3 Additional Security Aspects Some additional aspects should also be considered. Physical security, for instance, also plays a role in the overall security picture, since most of the known data security attacks are carried out inside the companies, often by internal or external employees. A careful policy in premises access control and identification for the main data centres is, therefore, a normal measure. In addition to physical security issues, personal security must be considered. There is also a need for careful selection of partners and contractors dealing with packet core equipment, which nowadays is a common practice in the industry. It is also worth mentioning in this section the need for integrity and confidentiality of network configu- ration data. In normal planning and optimisation tasks, the engineers are usually dealing with confidential data regarding configuration and also internal data policies. Extreme care must be exercised when dealing with this type of sensitive data. Configuration data maintenance is one of the traditional operational rou- tines. Additionally, live network traces, commonly used in optimisation, should be regarded as extremely sensitive data, avoiding wherever possible unprotected media for their handling. Every aspect of the optimisation process should be carried out according to the relevant policies in place for the operator in question. Core Network Planning and Optimisation 4.16 Quality of Service 4.16.1 Introduction to QoS Qual