QoS is used to prioritize network traffic and make the most efficient use of bandwidth.
To understand QoS, it is important to understand latency and jitter. Latency is a measure of delay on a network. Routers are the biggest cause of latency—each router takes a small amount of time to process a packet and forward it to the next network. While an individual router might not add an appreciable amount of latency, the combined latency of all the routers between a client and a server can be significant. In general, the busier a router is, the more latency it adds. Latency is not a problem for real-time audio and video presentations if they are one-way communications (each packet is delayed the same amount and received in appropriate intervals). However, latency presents a serious problem if the communication is two-way, as is the case with Internet telephony and video conferencing. Video conferencing across a high-latency network leads to unnatural pauses that can be frustrating to the participants.
Jitter is the measurement of change in latency. For example, if the average latency of a packet traveling between a client and server is one-half of a second, some packets might take as long as a full second to travel, while others take only a quarter of a second. Jitter is not an important consideration for file transfers, but it has a profound impact on real-time network applications such as audio and video. One of the primary causes of high jitter is a feature of IP networks: different packets in a single session can follow different paths through a network. If different paths have different latency, high jitter results. Clients often compensate for jitter by buffering network traffic, thereby increasing overall delay.