Computer and Communication Networks (paperback)

12.5. Summary

Methods of providing QoS are divided into two broad categories: integrated services and differentiated services . Integrated services provide QoS to individual applications and flow records. QoS protocols in this category include traffic shaping and packet scheduling. Traffic shaping regulates the spacing between incoming packets. Two traffic-shaping algorithms are leaky bucket and token bucket . In a leaky-bucket traffic shaper, traffic is regulated at a constant rate, much like the flow of water from a leaky bucket. In the token-bucket algorithm, enough tokens are assigned to each incoming packet. If the bucket becomes empty, packets stay in the buffer until the required number of tokens is generated.

Packet scheduling involves managing packets in queues. Several scheduling techniques FIFO , priority queueing , fair queueing , and weighted fair queueing (WFQ). WFQ is an improvement over fair queueing, in which each flow i is assigned a weight w _i . Another version of WFQ is deficit round- robin , in which each flow i is allocated b _i bits in each round of service.

The differentiated services (DiffServ) approach is based on providing QoS support to a broad class of applications. The traffic conditioner , one of the main features of a DiffServ node, protects the DiffServ domain. The traffic conditioner includes four major components : meter , marker , shaper , and dropper .

Resource allocation in networks can be classified by various approaches: fixed versus adaptive, router based versus host based, window based versus rate based. Finally a few methods of ATM resource control were presented.

The next chapter discusses the internal architecture of switch fabrics . This topic is the basis of discussion in several later chapters as well.