3.7 Developing Delay Requirements

For applications that have delay requirements, we will use the terms end-to-end delay, round-trip delay, and delay variation as measures of delay in the network.

We begin by introducing some useful general thresholds and limits for delay: interaction delay, human response time, and network propagation delay. These thresholds and limits are useful in helping to distinguish low-and high-performance delay requirements for your network.

Interaction delay (INTD) is an estimate of how long a user is willing to wait for a response from the system during an interactive session. Here a session is the time period during which an application is active. The INTD will depend on user behavior, the environment, and the types of applications being used. INTDs may range from hundreds of milliseconds to a minute or more. In general, a useful range is 10 to 30 seconds.

INTD is important when building a network targeted toward interactive applications. An INTD estimate is useful in characterizing applications that are loosely interactive that is, those in which waiting to receive information is expected. This applies to transaction-processing applications, as well as Web, file transfer, and some database processing. For applications such as these, the user will notice some degree of delay, and INTD is an estimate of how long users are willing to wait. An upper limit to INTD is a measure of the tolerance level of users.

Human response time (HRT) is an estimate of the time threshold when users begin to perceive delays...