value based on priority and the an- ticipated time a given packet needs for processing. Once As we discussed in the previous section, another the packets are all given weights, they are transmitted in common QoS mechanism is to give special treatment to weight order. This gives the ability to still favor high- packets marked as voice packets. This special treatment priority traffic, but give a fairer distribution of bandwidth involves some form of prioritized queuing. to all other traffic types (see Figure 3.3-2). Prioritized queuing is a generic term for one of The biggest problem with WFQ is that it is a very a number of schemes that allow one type of traffic to complex mechanism and therefore must be imple- have priority over another when being transmitted over mented in software, SO the WFQ process can add delay to a bandwidth-limited media. The basic idea is for the the overall transmission. Classifier Input flow 1 Scheduler High priority Input flow 2 Output port Input flow 3 Low priority Input flow 4 Figure 3.3-1 Priority queuing. VolP Technology CHAPTER 3.3 Scheduler Queue 1 Output Queue 2 Queue 3 Weight Figure 3.3-2 Weighted fair queuing. Class-based queuing The presence of large data packets may result in added serialization delay for VoIP packets across WAN links. Class-Based Queuing (CBQ) is a form of queuing where This is due to the fact that smaller VoIP packets are held incoming packets are sorted into classes as in the other in queue while larger data packets are processed onto the queuing schemes, each class is queued separately and WAN link. To avoid excessive delay, there may be benefit assigned a fixed percentage of the output bandwidth. The in fragmenting the larger data packets and interleaving output of the queues is then delivered in a given time them with the smaller voice packets. One technique is to period based on its percent allocation (see Figure 3.3-3). adjust the packets by adjusting the Maximum Trans- This scheme allows all service types to have predict- mission Unit (MTU) size. Ot