Specifying an administrative policy for QoS requires that a specific set of service classes be defined. QoS mechanisms are uniformly applied to these individual service classes to meet the requirements of the administrative policy. Because the application of QoS mechanisms is applied to different service classes and used to differentiate between applications, users, and traffic, the service class is a key component of a successful QoS implementation.
There are many different methods in which service classes can be used to implement an administrative policy. The first step is to identify the traffic that exists in the network and the QoS requirements for each traffic type. Then, traffic can be grouped into a set of service classes for differentiated QoS treatment in the network.
One popular model for the application of QoS service classes is the customer model which is typically used by service providers when referring to customer traffic. The customer model defines the following service classes (although many variations exist):
■ Voice service class: Delivers low latency for voice services
■ Mission-critical service class: Guarantees latency and delivery for the transport of mission-critical business applications such as systems network architecture (SNA)
■ Transactional service class: Guarantees delivery and is used for more general applications that are not as sensitive to delay, such as e-commerce
■ Best-effort service class: Used to support small business, e-mail, and other best-effort applications
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