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QoS
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(Quality Of Service) A defined measure of performance in a data communications system. For example, to ensure that real-time voice and video are delivered without annoying blips, a traffic contract is negotiated between the customer and network provider that guarantees a minimum bandwidth along with the maximum delay that can be tolerated in milliseconds.

     Because dedicated channels are set up between parties, the plain old telephone system (POTS) delivered the highest QoS for years. However, when data are broken into packets that travel through routers in a LAN or WAN, QoS mechanisms are used to give higher priority to real-time data, such as voice over IP (VoIP), than to non-real-time data, such as file downloads. Another option in packet switching is to overbuild the network, ensuring that it will accommodate all traffic fed to it. See packet switching.

     ATM was one of the first packet technologies to build in modes of service. Today, everything is built around IP, and there are a variety of methods competing to provide QoS in IP networks (see TOS, RSVP, Diffserv and MPLS).

QoS Vs. CoS

QoS (quality of service) refers to the mechanisms in the network software that make the actual determination of which packets have priority (see packet scheduler). CoS (class of service) refers to feature sets, or groups of services, that are assigned to users based on company policy. If a feature set includes priority transmission, then CoS winds up being implemented in QoS functions within the routers and switches in the network. See class of service.

Circuits to Packets to Circuits?

Nothing provides better quality than a dedicated channel between two parties. Some say, in time, with ever increasing wavelength division multiplexing (WDM), there will be enough optical bandwidth to have an available circuit between every telephone or videophone in every home and office in the world, and we will wind up essentially with circuit switching again, although implemented differently.

     In the meantime, the late 1990s saw a huge buildup of fiber backbones in the U.S., which resulted in a glut of capacity after the turn of the century. This fiber overbuild has enabled voice and video calls over the Internet (VoIP) to provide reasonably good quality even though there is no standard QoS mechanism built into all the Internet backbones. See infranet and QoE.
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