Class of service

Class of service


Class of service (CoS)

As related to network technology, CoS is a 3-bit field within an Ethernet frame header when using 802.1Q tagging. The field specifies a priority value of between 0 and 7 inclusive that can be used by quality of service (QoS) disciplines to differentiate traffic.

While CoS operates only on Ethernet at the data link layer (layer 2), other QoS mechanisms (such as DiffServ) operate at the network layer (layer 3) and higher. Others operate at the physical layer.

Although 802.1Q tagging must be enabled to communicate priority information from end station to switch, or switch-to-switch, some switches use CoS to internally classify traffic for QoS purposes.

Classification of service

The term can be used generically to refer to the classification of network traffic within network equipment based on packet inspection. Cisco implements such classification through Network-Based Application Recognition (NBAR). NBAR works with the existing QoS system.[1]

Class of service (COS)

As related to legacy telephone systems, COS is often used to define the permissions an extension will have on a PBX or Centrex. The Class of Service acronym is normally written as COS vs. CoS as is often used in data networking parlance. Certain groups of users may have a need for extended voicemail message retention while another group may need the ability to forward calls to a cell phone, and still others have no need to make calls outside the office. Permissions for a group of extensions can be changed by modifying a COS variable applied to the entire group.

COS is also used on trunks to define if they are full-duplex, incoming only, or outgoing only.


  1. ^ [1] Cisco NBAR

See also