Class of service (COS or CoS) is a parameter used in data and voice protocols to differentiate the types of payloads contained in the packet being transmitted. The objective of such differentiation is generally associated with assigning priorities to the data payload or access levels to the telephone call.

Data services

As related to network technology, COS is a 3-

bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented a ...

field that is present in an

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in ...

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 Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

(QoS) disciplines to differentiate and shape/police network traffic. COS operates only on 802.1Q VLAN Ethernet at the

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...

(layer 2), while other QoS mechanisms (such as

DiffServ Differentiated services or DiffServ is a computer networking architecture that specifies a mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks. DiffServ can, for example, be used ...

, also known as DSCP) operate at the IP

network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of trans ...

(layer 3) or use a local QoS tagging system that does not modify the actual packet, such as Cisco's "QoS-Group". Network devices (i.e. routers,

switches In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type ...

, etc.) can be configured to use existing COS values on incoming packets from other devices (trust mode), or can rewrite the COS value to something completely different. Most Internet Service Providers do not trust incoming QoS markings from their customers, so COS is generally limited to use within an organization's intranet. Service providers offering private-line WAN services will typically offer services which can utilize COS/QoS.

Voice services

As related to legacy telephone systems, COS is often used to define the permissions an extension will have on a PBX or Centrex. Certain groups of users may have a need for extended

voicemail A voicemail system (also known as voice message or voice bank) 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 ind ...

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 A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...

, incoming only, or outgoing only.

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 either access-lists or Network-Based Application Recognition (NBAR). NBAR works with the existing QoS system.NBAR2 or Next Generation NBAR
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References

* "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann, 2007, )
Supporting differentiated classes of service in Ethernet passive optical networks, Glen Kramer, Biswanath Mukherjee, Sudhir Dixit, Yinghua Ye and Ryan Hirth
Teletraffic Network architecture Telephone exchanges {{Compu-network-stub