QOS (Quality Of Service

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.