t of the networking equipment because no additional traffic management mechanisms are deployed. However, when broadband IP access is used for both managed IP traffic (with QoS guarantees) and best-effort traffic over the Internet access service (for access to the public Internet), then there is a need for differentiation of traffic in the scheduling mech- anism, SO IP packets belonging to one traffic type (or class) will be served before packets of other traffic types (or classes). For example, delay-sensitive applications and services, which are referred to as real-time traffic, should be given priority over NRT traffic, which can tolerate longer delays (although not too long for most of the NRT services, for example Web browsing). Priority queuing is used for this purpose. Priority queuing is the simplest and most commonly technique used to differentiate between traffic types (e.g. VoIP, IPTV, and best-effort Internet traffic). It places traf- fic into different queues and serves them in priority order (e.g. high priority for VoIP, medium for IPTV, and low for best-effort packets, as shown in Figure 2.9). Overall, there is no single best scheduling mechanism. Which is best depends upon the traffic being scheduled, offered service packages to end-users, available bandwidth, traffic management techniques being used (e.g. traffic shaping for limiting the bitrate allocated to a given traffic type of a given IP flow), etc. Therefore, typically, schedul- ing mechanisms are not usually standardized; rather, it is left to the vendors to provide different scheduling options in the network equipment, and then to network designers on the side of telecom operators (i.e. network providers) to create traffic management solutions that best fit the given network and the telecom business regarding the offered services to their customers. 2.5.4 Admission Control No matter how good the scheduler is, one may still have no QoS in practice. Why? Because when traffic demand exceeds available resources (i.e. bandwidth - bitrates 2 Int