QoS is only one of a broader range of protocols and ideas. The same term can refer to CoS (Class of Service), QoS, queuing, marking, policing, or even traffic shaping. For example, on Cisco routers, any serial link under 2 Mbps has weighted fair queuing (WFQ) enabled. To quote the Cisco documentation: WFQ is one of Cisco's premier queuing techniques. It is a flow-based queuing algo- rithm that does two things simultaneously: it schedules interactive traffic to the front of the queue to reduce response time, and it fairly shares the remaining bandwidth between high bandwidth flows. Sounds like QoS to me! It is, and it's probably already in use on your network. Turning off WFQ on a saturated link can have a dramatic impact. Some forms of QoS are very simple to implement, and affect only the interfaces on which they are configured. Other types can be installed on a single interface, or deployed from end to end on an enterprise network. Voice over IP is probably the biggest reason a large-scale QoS deployment might be considered. Consider Figure 29-4, which shows a simple two-building network with VoIP phones. There are PCs and IP phones in both buildings. With a simple install, all resources share the network equally. But, this is a problem in a VoIP world because voice packets must be delivered in a timely fashion, in order, and should not be dropped. Because the VoIP voice stream is UDP-based, reliable transport for these packets is not guaranteed like it is in TCP. VoIP packets must compete for bandwidth with other traffic when other protocols are in use. Let's assume that a VoIP call is active between buildings A and B, and everything is working fine. Now, let's say a PC in Building A pulls a large file from the server in building B using FTP. Suddenly, the voice call gets choppy. If you've got luck like mine, the call will be from the CEO to the payroll company about your raise-which will now be canceled because the voice quality was SO bad. At any rate, let's look at the problem, and see if we can come up with