Class of Service (COS) is one type of the techniques or methods used to deliver Quality of Service (QoS) in a network. CoS is a way of classifying and prioritizing packets based on application type (voice, video, email, file transfer, transaction processing etc., the type of user (VIP or normal etc.) or other ways of classifications. A "first class" priority label is assigned to data applications - such as mission-critical data transactions, or video or voice transmissions - which require faster turnaround, while a lower-priority label is assigned to less time-sensitive traffic, such as e-mail and Web surfing.
There are three basic CoS technologies:
- 802.1p Layer 2 Tagging
- Type of Service (ToS)
- Differentiated Services (DiffServ)
802.1p Layer 2 Tagging and ToS make use of three bits in the layer 2packetheader that can be used to specify priority.
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 decisions. Based on application requirements, traffic is prioritized at the connection endpoint by setting the Differentiated Services (Diff-Serv) code in the header of an IP data packet. Letting users manage network traffic, Diff-Serv, together with other MPLS protocols, is used to prioritize data traffic in both IP and ATM networks.
Related Terms: QoS (Quality of Service), MPLS, DiffServ, Traffic Management, Service Level Agreement
Reference Links：http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/
120newft/120limit/120st/120st10/10st_cos.htm:MPLS Class of Service