QoS (Quality of Service)

On the Internet and in other networks, QoS (Quality of Service) is the idea that transmission rates, error rates, and other characteristics can be measured, improved, and, to some extent, guaranteed in advance. QoS is of particular concern for the continuous transmission of high-bandwidth video and multimedia information. Transmitting this kind of content dependably is difficult in public networks using ordinary "best effort" protocols.

QoS can be improved with traffic shaping techniques such as packet prioritization, application classification and queuing at congestion points. Using the Internet's Resource Reservation Protocol (RSVP), packets passing through a gateway host can be expedited based on policy and reservation criteria arranged in advance. Using ATM, which also lets a company or user preselect a level of quality in terms of service, QoS can be measured and guaranteed in terms of the average delay at a gateway, the variation in delay in a group of cells (cells are 53-byte transmission units), cell losses, and the transmission error rate.

The Common Open Policy Service (COPS) is a protocol that allows router and layer 3 switches to get QoS policy information from the network policy server.

See also: mean opinion score (MOS), VoIP (voice over IP), CoS [class of service]

Class of Service (CoS)

Class of Service (CoS) is a way of managing traffic in a network by grouping similar types of traffic (for example, e-mail, streaming video, voice, large document file transfer) together and treating each type as a class with its own level of service priority. Unlike Quality of Service (QoS) traffic management, Class of Service technologies do not guarantee a level of service in terms of bandwidth and delivery time; they offer a "best-effort." On the other hand, CoS technology is simpler to manage and more scalable as a network grows in structure and traffic volume. One can think of CoS as "coarsely-grained" traffic control and QoS as "finely-grained" traffic control.

There are three main CoS technologies:

• 802.1p Layer 2 Tagging
• Type of Service (ToS)
• Differentiated Services (DiffServ)

802.1p Layer 2 Tagging and CoS make use of three bits in the Layer 2 packet header that can be used to specify priority. Since three bits does not allow for much sophistication in managing traffic, a new protocol, Differentiated Services (DS or DiffServ), has been developed in draft form by an IETF Working Group. Differentiated Services uses a different approach to managing packets than simple priority labeling. It uses an indication of how a given packet is to be forwarded, known as the Per Hop Behavior (PHB). The PHB describes a particular service level in terms of bandwidth, queueing theory, and dropping (discarding the packet) decisions.

The Differentiated Services protocol exists as a set of related working documents from the Internet Engineering Task Force (IETF). [http://www.ietf.org/]