ity of service (QoS) is possibly the single most important concept in the field of optimisation for packet switched networks. The circuit switched networks also have a mechanism to manage the quality of the service, which is well known and well established a long time ago. The dimensioning of the number of available circuits or channels in a given network node or associated interface is done according to the criteria known as blocking probability, which can be used in the Erlang formula. However, this approach no longer serves the purpose of the packet switched networks, where multiple services are meant to use the same network resources while having essentially different characteristics and requirements. This fact was acknowledged a long time ago, and the basic IP protocol includes a field to mark every packet according to the type of service it is conveying, known as the differentiated services code point, which is a 6-bit field in the IP header that determines the per hop behaviour. Thus, the field can be used by the switches and routers in the network in order to prioritise traffic flows according to their tolerance to delay, jitter and packet losses. These were the known mechanisms, and the initial phase of deployment of radio access mobile networks, when GPRS appeared in the late 1990s and, soon after, UMTS, already had a more refined specification providing QoS mechanisms from its creation. 4.16.2 QoS Environment It is clearly understood that in a mobile network the major constraints can be easily located in the radio access network, which is especially true in terms of QoS, where maximising the utilisation of the limited radio resources and prioritising the different traffic flows according to their nature is a complex task. However, the QoS concept in a mobile packet core network is, in principle, easier to understand and build, at least conceptually. Nevertheless, current traffic volumes in most operators are such that the benefits of a QoS mechanism are not clearly perceived from a core network point o