Class of service

As related to network technology, CoS is a 3-bit

Bit

A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

 field within an Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 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

Quality of service

The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...

 (QoS) disciplines to differentiate traffic.

While CoS operates only on Ethernet at the data link layer

Data link layer

The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....

 (layer 2), other QoS mechanisms (such as DiffServ) operate at the network layer

Network layer

The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...

 (layer 3) and higher. Others operate at the physical layer

Physical layer

The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....

.

Although 802.1Q tagging must be enabled to communicate priority information from end station to switch

Network switch

A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...

, 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.

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

Centrex

Centrex is a portmanteau of central exchange, a kind of telephone exchange.In the United Kingdom, British Telecom markets this service as FeatureLine .-Use of 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

Voicemail

Voicemail is a computer based system that allows users and subscribers to exchange personal voice messages; to select and deliver voice information; and to process transactions relating to individuals, organizations, products and services, using an ordinary telephone...

 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.