QUALITY OF SERVICE (qos)

During the recent years we have witnessed a tremendous growth in the role of global networking using the system known as the Internet. This global network can be loosely modeled as a collection of end-systems that is communicated over a shared network infrastructure. This infrastructure consists of links of various transmission technologies interconnected with devices that direct traffic among their incident links enabling connectivity. We will refer to such devices as simply "routers" although their exact functionality and corresponding names can vary significantly.

A number of developments in the technology and capabilities of both end-systems and transmission technologies has fueled the phenomenal growth of this system. In particular, processing power increases allow end-systems to perform more sophisticated data processing, enabling them to handle media traffic. Increased processing power also improves the capabilities of the routers, allowing them to support faster transmission technologies and improved data handling capacity. Improvements in the integrated circuit technology allow for higher levels of integration and more capable programmable hardware support that further improves the capabilities of the routers. Finally, improved access technologies enable more users to get connected to the network.

All the above, gradually resulted in a widely used system that spans most of the world. A multitude of exciting applications attempt to use the medium in novel ways, or in ways that were traditionally delegated to different technologies. Gradually, the network starts performing the functions of the telephony network, is used for distributing video and images, and enables new communication paradigms unique to the medium such as multimedia collaboration, information broadcasting, user forums, multi-user games, information harvesting, using the shared network to build private overlay networks and shopping, to name just a few.

These new paradigms introduce unique sets of requirements from the network. While most of the conventional uses of a communication network required only connectivity between the communicating entities, this may not be sufficient for a number of new applications. In many cases, just being able to send data to a remote end is not enough; data may have to be sent in a timely fashion, at a particular rate, with only a limited amount of loss. This will be the case for example in a voice call: the data transfer delay between the communicating entities should be low enough to make bi-directional communication possible; data losses should be kept at a minimum so that the perceived quality of the sound is good. Requirements that applications may have for their traffic, are referred to as "Quality of Service" requirements. Some assurance for the levels of the quality of the service that traffic will receive is usually called QoS "guarantee". An example of a guarantee is the assurance that a piece of data will be transferred to the destination within 50 micro seconds of its transmission.

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