ernet QoS IP packet High empty Medium empty Low empty (e.g. VoIP)? (e.g. IPTV)? (e.g. data)? Send packet Send packet Send packet from high from medium from low Figure 2.9 Priority queuing for VolP, IPTV, and Internet access service. in an uplink or downlink direction), there will be congestion, which means that QoS requirements cannot be maintained. With the aim of providing QoS, it is always necessary to provide more resources than those that can be spent (also referred to as overprovisioning) or to provide admission control (which means that the number of simultaneous connections over a given network link is limited, subject to the network design). What is admission control? It is a traffic management technique which is used to reject a flow (a call or a session) if there are insufficient resources (e.g. in the access, core, or transit network). It is different from traffic policing (which is used to reject/drop a packet). For example, in the traditional telephone networks (e.g. PSTN, Public Land Mobile Network (PLMN), it is easy to manage admission control. Why? Due to circuit-switching (allocation of a pair of channels per call, i.e. fixed resources per call). Then, teletraffic engineering provides an efficient means for planning the telephone networks (e.g. the well-known Erlang-B formula for calculation of the num- ber of channels required to serve certain traffic intensity under the given maximum acceptable new call blocking rate). Internet flows from different applications/services have different QoS requirements. Therefore, admission control is much harder for implementation in IP-based environ- ments where the same network is used for transfer of packets from many different ser- vices, each with potentially different QoS requirements and hence different admission control approaches. Let us analyze a typical example at a telecom operator when multiple services are offered though broadband access, such as VoIP service, IPTV service, and Internet access service. If an average requirement per VoIP is 100 Kb