Traffic classification giving priority of some types of traffic over others. By doing so, organizations can get better performance out of the network without spending money on unnecessary bandwidth. For example, a network might prioritize voice data over informational data, in order to preserve clear voice connections. There are several Class of Service treatments for MPLS networks.

As related to network technology, CoS is a 3-bit field that is present in an Ethernet Frame header when 802.1Q VLAN tagging is present. The field specifies a priority value between 0 and 7, more commonly known as CS0 through CS7, that can be used by quality of service (QoS) disciplines to differentiate and shape/police network traffic.

CoS operates only on 802.1Q VLAN Ethernet at the data link layer (layer 2), while other QoS mechanisms (such as DiffServ, also known as DSCP) operate at the IP network layer (layer 3) or use a local QoS tagging system that does not modify the actual packet, such as Cisco’s “QoS-Group”.