Quality of Service (QoS) is deployed to prevent data from saturating a link to the point that other data cannot gain access to it. Remember, WAN links are serial links, which means that bits go in one end, and come out the other end, in the same order: regardless of whether the link is a 1.5 Mbps T1 or a 45 Mbps DS3, the bits go in one at a time, and they come out one at a time.
QoS allows certain types of traffic to be given a higher priority than other traffic. Once traffic is classified, traffic with the highest priority can be sent first, while lower-priority traffic is queued. The fundamental purpose of QoS is to determine what traffic should be given priority access to the link.
Figure 29-1 shows two buildings connected by a single T1. Building B has a T1 connection to the Internet. There are servers, and roughly 100 users in each building. The servers replicate their contents to each other throughout the day. The users in each building have IP phones, and inter-building communication is common. Users in both buildings are allowed to use the Internet.
Figure 29-1. Simple two-building network
The only path out of the network in Building A is the T1 to Building B. What happens when each of the users in that building decides to use that single link at once? The link is only 1.5 Mbps, and each user may have a 100 Mbps (or even 1 Gbps) Ethernet connection ...