Below is an attempt to define, in very simple terms, what QoS is and why we need it in a K12 school network environment. QoS is a VERY complex animal and is difficult to describe in simple language. We'll give it our best shot. It is important to understand the concept that network traffic (Email, H.323 Video, Web) is transmitted over your network in the form of "data packets." There are many types of packets. We'll use an Email packet and an H.323 distance education packet in our example. Let's understand our priorities.
The H.323 distance learning packets get the "top" priority in this example. This packet must get through to the folks on the other network (like the Spanish teacher). If these packets do not get through, then the class is disrupted. The Email packets also need to get to their destination but do not require the same priority as the H.323 distance education packets. If the Email packets get delayed for a few seconds, it's no big deal. So what happens if the Email packets and the H.323 packets arrive at a router or switch at the same time? Some of these packets will be "parked" for a few milliseconds. This does not affect the quality of the Email packet but certainly creates havoc with the distance education class resulting in a jerky picture, poor sound quality, or a dropped connection. QoS to the rescue!
Apply QoS technology (quality of service) to this example and the H.323 problems can effectively go away. A switch or router which supports QoS will "look" at the packets and determine who gets priority. (you assign the priorities when the switch and router is installed) In other words, the Email packets get "parked" for a few milliseconds and the H.323 distance education packets are passed through the network on to their destination without loss of quality. Where QoS really shines!
Most schools only have one (1) T-1 to the internet which serves the school's needs for Web Research, Email, Streaming Video, H.323, and many other on line resources. Think of your T-1 connection to the Internet as a "water pipe." This pipe is capable of transmitting 1,500 gallons of water per second. (1.54 megabits or 1540k)
Let's do some basic math.
~ Mrs. Jones in the computer lab is streaming some video from the Internet to computer lab workstations worth 700 gallons of water per second. (700k)
~ Mr. Johnson is sending an Email with a powerpoint attachment which is worth 1000 gallons of water per second. (1000k)
~ The library students are doing research on animals and are viewing some pretty intense graphics pictures worth around 200 gallons of water per second. (200k)
~ Now for the killer! Mrs. Smith fires up the H.323 distance learning equipment and tries to make a connection to the local University for a Spanish class which is worth 384 gallons of water per second. (384k)
~ The water pipe is full at this point. In fact, it's overloaded or saturated. If no QoS is implemented on the school network, Mrs. Smith probably will not have her Spanish class. Her distance learning packets are fighting for their lives to get out the door and some are not making it. (picture jitter, poor sound, etc.)
If QoS is implemented, and H.323 distance learning is the top priority, then the Spanish class packets will make it to their destination at the University and the class will happen. There is, of course, much more to QoS. The above is the short answer. | 
