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class of service (CoS)

A service class (CoS) is generally a classification characteristic for the authorization level of users and for the handling and prioritization ofdata packets. For example, the different service classes can be handled and prioritized differently in queuing. For the different services and protocols, the service classes are therefore also designated differently.

  1. In IEEE 802.1p, Class of Service (CoS) stands for the classification ofdata traffic within a LAN based on different services such as e-mail, file transfer, voice or video. Each class of service has its own priority, which is taken into account and processed accordingly during queuing. In contrast to Quality of Service (QoS), CoS cannot allocate bandwidth, but sends data packets that are transmitted according to the best-effort principle. CoS operates at the data link layer and provides simple mechanisms for better resource utilization. For this purpose, Ethernet frames are marked with tags that identify the class of service.

  2. In the case of high-speed switched services ATM and Frame Relay (FR), several classes of service have been defined by the International Telecommunication Union (ITU-T) to realize the different tasks. For ATM, four classes of service have been specified for isochronous or bursty transmission, constant or variable bit rates, and connectionless and connection-oriented transmission. The respective service class is valid for the complete connection duration.

  3. In the case of the SNA protocol, the flow control, the transmission security, the bandwidth and the transmission priority are specified in the service classes.

  4. In telecommunication, CoS is understood as an authorization class for users, which refers to the use of facilities such as Tk equipment or Tk services.

  5. In the IP world, service classes are defined in the service field (ToS) in the header of the IP protocol for the transmission quality of network-related services.

  6. The Internet Engineering Task Force (IEFT) has developed several approaches for service classes in the Internet. These include the IPv6protocol in whose header the flow label field has been defined. In addition, there is the prioritization of traffic using DiffServ, IntServ and others, which are reflected in the quality of service (QoS).

  7. In the SNA architecture, there are different classes of service that are specified by the user depending on their requirements. The CoS classes specified in the SNA architecture are similar to the Type of Service (ToS) from the TCP protocol. Both methods work by prioritizing messages within the network.

  8. In Fibre Channel, the classes of service are provided at the transport layer.

    Service characteristics of the different FC service

    Service characteristics of the different FC service

    classes Fibre Channel recognizes several FC service classes, such as connectionless, connection-oriented with end-to-end flow control, datagram transmission, and sub-channel provisioning. Different service classes can be provided simultaneously in a fabric.

  9. The definition of 802.16 recognizes four classes of service that support the time-critical transmission of audio

    , video, voice, and games.
    Service classes defined in 802.16

    Service classes defined in 802.16

    The four service classes differ in delay, jitter and bit error rate and are defined for computer games, Internet connections, streaming and e-mail.

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Englisch: class of service - CoS
Updated at: 31.10.2013
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