DiffServ was designed to overcome the limitations of both the Best-Effort and IntServ models. DiffServ can provide an "almost guaranteed" QoS, while still being cost-effective and scalable.
DiffServ is similar to a concept known as "soft QoS." With soft QoS, QoS mechanisms are used without prior signaling. In addition, QoS characteristics (bandwidth and delay, for example), are managed on a hop-by-hop basis by policies that are established independently at each intermediate device in the network. The soft QoS approach is not considered an end-to-end QoS strategy because end-to-end guarantees cannot be enforced. However, soft QoS is a more scalable approach to implementing QoS than hard QoS, because many (hundreds or potentially thousands) of applications can be mapped into a small set of classes upon which similar sets of QoS behaviors are implemented. Although QoS mechanisms in this approach are enforced and applied on a hop-by-hop basis, uniformly applying global meaning to each traffic class provides both flexibility and scalability.
With DiffServ, network traffic is divided into classes based on business requirements. Each of the classes can then be assigned a different level of service. As the packets traverse a network, each of the network devices identifies the packet class and services the packet according to that class. You can choose many levels of service with DiffServ. For example, voice traffic from IP Phones is usually given preferential treatment over all other application traffic. E-mail is generally given Best-Effort service. And nonbusiness traffic can either be given very poor service or blocked entirely.
DiffServ works like a package delivery service. You request (and pay for) a level of service when you send your package. Throughout the package network, the level of service is recognized and your package is given either preferential or normal service, depending on what you requested.
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