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 con- nection 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. Internet Building A Building B Figure 29-1. Simple two-building network The only path out of the network in Building A is the T1 to Building B. What hap- pens 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 to the network. A good designer should never have built a network with these limita- tions, and the admin (if one exists) should not have let the problem get SO severe. Still, the real world is filled with networks like this just waiting for someone to fix them. Many smaller companies don't have dedicated network administrators on staff, SO problems like this can and do occur. Let's say, for example, that 30 of the 100 users start to download the free demo of GAD's Rockin' Guitar Solos, Volume VI. Let's also say that 20 of the users decide to surf the O'Reilly web site. Another 20 need to download the latest service packs from their favorite operating system vendors, and the remaining 30 want to use their VoIP phones. The problem is that the T1 isn't "big" enough for the amount of traffic that's about to be created. Imagine you had a hose with a funnel on top, as shown in Figure 29-2. The hose is capable of allowing one liter per minute to flow through it. As long as you only pour one liter of water every minute into the funnel, you'll be fine. One liter per minute input Three liters per minute input Overflow condition One liter per minute capacity One liter per minute capacit