forwarding nodes where packets can be labeled for preferential treatment using tech- niques such as DifServ and IntServ, which give priority to packets and flows, respectively. Network Queuing When a stream reaches a network element such as a router, it must be multiplexed with other traffic taking the same path. The network element only has a finite resource, and packets are queued prior to forwarding to their destination according to the address con- tained in the packet header. If the incoming traffic has a CBR, then, as shown in the top half of Figure 3.20 the output is predictable as NR bits/s. For VBR, the data is statistically mul- tiplexed, and therefore a burst in one stream can result in a loss of resources for another; packet loss occurs as a result of bursts in VBR traffic. Sources (each R bits/s) Total Rate = NR bits/s or FDM Multiplexer VBR Packet Sources (each with average Statistical Total Rate <= NR bits/s bitrate R bits/s) Multiplexer or Packet Network Fig. 3.20 CBR and VBR multiplexing. Chapter 3 Digital Video Transmission Different packets can have different levels of importance, and if a network permits, they can be classified and placed in different queues. These can then be scheduled to the output of the network element according to a number of queue-scheduling techniques. The tech- nique of increasing the probability of delivery of a particular stream is described generically as QoS. The techniques that are described in Woods and Siller (2006) include First-in-First- out (FIFO), Priority Queuing (PQ), Fair Queuing (FQ), Weighted Fair Queuing (WFQ), and Weighted Round-Robin (WRR), also known as Class-Based Queuing (CBQ). FIFO is the most common queue-scheduling scheme, where all packets are treated equally by placing them in a single queue; they are serviced in the order in which they arrive at the queue. PQ provides a simple way of supporting differentiated services, where packets are classified and placed into different priority queues. Packets are serviced in order of decreasing priority