Class of Service. The idea that packets on a network have a number in the header which defines a class, which then allows different classes to be treated differently as they pass through the network (for instance to be given different priorities within a switch).
CoS is usually taken to mean this being done at Layer 2 (within Ethernet). It is done using the Priority Code Point (PCP) – 3 bits within the VLAN tag, defined in IEEE 802.1Q. The use of the PCP is defined in IEEE 802.1D (but often referred to as 802.1p). Because the PCP is in the VLAN tag, it can only be done when VLAN tagging is switched on. Most clients don’t support this, but switches and some servers do.
It can be used to give video and audio over IP traffic priority over other network traffic within Ethernet switches.
The same sort of thing can be done at Layer 3 (IP) using DiffServ and the DSCP, but this is normally referred to as QoS (quality of service).
Website: www.ieee.org/standards/index.html
See QoS
