Why QoS?
In the Internet and intranets of today, bandwidth is an important subject. Moreand more people are using the Internet for private and business purposes.The amount of data that is being transmitted through the Internet isincreasing exponentially. Multimedia applications, such as IP telephony andvideo conferencing systems, need a lot more bandwidth than the applicationsthat were used in the early years of the Internet. Whereas traditional Internetapplications, such as WWW, FTP or TELNET, cannot tolerate packet loss butare less sensitive to variable delays, most real-time applications show just theopposite behavior, meaning they can compensate for a reasonable amount ofpacket loss but are usually very critical towards high variable delays.This means that without any bandwidth control, the quality of these real-timestreams depends on the bandwidth that is currently available. Low or unstablebandwidth causes bad quality in real-time transmissions by leading to, forexample, dropouts and hangs. Even the quality of a transmission using thereal-time protocol (RTP) depends on the utilization of the underlying IPdelivery service.
Therefore, certain concepts are necessary to guarantee a specific Quality ofService (QoS) for real-time applications in the Internet. A QoS can bedescribed as a set of parameters that describe the quality (for example,bandwidth, buffer usage, priority, CPU usage, and so on) of a specific streamof data. The basic IP protocol stack provides only one QoS, which is calledbest-effort. Packets are transmitted from point to point without any guaranteefor a special bandwidth or minimum time delay. With the best-effort trafficmodel, Internet requests are handled with the first come, first serve strategy.This means that all requests have the same priority and are handled one afterthe other. There is no possibility to make bandwidth reservations for specificconnections or to raise the priority for special requests. Therefore, newstrategies were developed to provide predictable services for the Internet.Today, there are two main rudiments to bring QoS to the Internet and IPbased internetworks:
- Integrated Services
- Differentiated Services.
Integrated Services
Integrated Services bring enhancements to the IP network model to supportreal-time transmissions and guaranteed bandwidth for specific flows. In thiscase, we define a flow as a distinguishable stream of related datagrams froma unique sender to a unique receiver that results from a single user activityand requires the same QoS.For example, a flow might consist of one video stream between a given hostpair. To establish the video connection in both directions, two flows arenecessary. Each application that initiates data flows can specify which QoSare required for this flow. If the video conferencing tool needs a minimumbandwidth of 128 kbps and a minimum packet delay of 100 ms to assure acontinuous video display, such a QoS can be reserved for this connection.
Differentiated Services
Differentiated Services mechanisms do not use per-flow signaling, and as aresult, do not consume per-flow state within the routing infrastructure.Different service levels can be allocated to different groups of users, whichmeans that all traffic is distributed into groups or classes with different QoSparameters. This reduces the maintenance overhead in comparison toIntegrated Services
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