2.1.3 QoS Requirements

The degree to which application performance depends on low delay service varies widely and several qualitative distinctions between applications based on the degree of their dependence. One class of applications needs the data in each packet by a certain time and, if the data has not arrived by then,the data is worthless. These type of applications are Real-Time applications.Another class of applications always wait for data to arrive, these are Elastic applications.A brief description of each of them is given below:

a)Real-Time Applications:

There is an important class of real-time applications called 'playback' applications. In a playback application the source takes a signal, packetizes it and then transmits them over the network. The receiver depacketizes the data and then attempts to faithfully play back the signal. This is done by buffering the incoming data and then replaying the signal at some fixed off set delay from the original departure time.Any data that arrives before it's playback point can be used to reconstruct the signal; data arriving after the playback point is essentially useless in reconstucting the real-time signal.

The performance of a playback application is measured along two dimensions:

Based on this there are two classes or applications: Intolerant applications must use a fixed offset delay, since any variation in the offset delay will introduce some distortion in the playback. this fixed offset delay should be lager than the absolute maximum delay, in order to avoid the possibility of late packets. Such a service which is characterized by a perfectly reliable upper bound delay is called as "guaranteed service".

In contrast, tolerant  applications need not set their offset delay greater than the absolute maximum delay, since they can tolerate late packets. Also, instead of using a single fixed value for the offset delay, they can reduce their latency by varying their offset delays in response to the actual packet delays experienced in the recent past. These type of applications are called as "adaptive" playback applications and these provide a "predictive service"which supplies a fairly predictive but perfectly reliable delay bound.
 
b) Elastic Applications:

These type of applications will always wait for data to arrive. Here in this case the application uses the arriving data immediately rather than buffering it for some later time, hence these will always choose to wait for the incoming data rather than proceeding without it. Because arriving data can be used immediately, these applications do not require a characterization which defines the maximum delay the packets will experience  in order to have the application running.
Thus as cited in [RFC 1633], these applications provide "As-Soon-As-Possible"  or ASAP service.