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).

This was last updated in December 2008

Dig Deeper on Telecommunication networking

Start the conversation

Send me notifications when other members comment.

Please create a username to comment.