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DiffServ service classes for network QoS
Service Quality
Until a few years ago, the only critical factor on most networks was to transfer data as quickly and securely as possible between endpoints. Computers and network components on the LAN and WAN have been optimized for this purpose. However, this model is useless for Voice or Video over IP (VoIP), because both require low latency and guaranteed constant transmission rates from the network. Both packet loss and jitter can significantly affect the quality of voice transmission delivered to the end user. The Internet Engineering Task Force (IETF) has defined various quality of service (QoS) models for an IP network: best effort, IntServ (integrated services), and DiffServ (differentiated services).
QoS Models
The Internet uses the best-effort model, which offers minimal QoS assurance. For business networks, this model also applies if no QoS policies have been configured or the infrastructure does not support QoS. With the best-effort model, there is no guarantee that the packets will be delivered. Additionally, all packets have the same priority and are therefore treated identically. Packets from a VoIP stream are therefore processed with the same priority as those from an email session. Unfortunately, many companies that use VoIP still use the best-effort model because the infrastructure is not properly configured or QoS is simply not supported.
The IntServ model reserves bandwidth in the relevant network path, which guarantees the necessary bandwidth for mission-critical applications from end to end. IntServ uses signals for the QoS model: The end host signals the network's QoS requirements. Each individual communication stream has to request resources from the network. Edge routers use the Resource Reservation Protocol (RSVP) to signal that the appropriate bandwidth should be reserved for each flow on the network. One major drawback of the IntServ model is that each device in the path a
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