User-perceived performance of network applications is made up of a number of mainly qualitative metrics, some of which are in conflict with each other. In the case of some applications, a single metric will outweigh the others, such as responsiveness from video services or throughput for bulk transfer applications. More commonly, a combination of factors usually determines the experience of network performance for the end-user.
One of the most important user experiences in networking applications is the perception of responsiveness. If end-users feel that an application is slow, it is often the case that it is slow to respond to them, rather than being directly related to network speed. This is a particular issue for real-time applications such as audio/video conferencing systems and must be prioritised in applications such as remote medical services and off-campus teaching facilities. It can be difficult to quantitatively define an acceptable figure for response times as the requirements may vary from application to application.
However, some applications have relatively well-defined "physiological" bounds beyond which the responsiveness feeling vanishes. For example, for voice conversations, a (round-trip) delay of 150ms is practically unnoticeable, but an only slightly larger delay is typically felt as very intrusive.
Throughput per se is not directly perceived by the user, although a lack of throughput will increase waiting times and reduce the impression of responsiveness. However, "bandwidth" is widely used as a "marketing" metric to differentiate "fast" connections from "slow" ones, and many applications display throughput during long data transfers. Therefore users often have specific performance expectations in terms of "bandwidth", and are disappointed when the actual throughput figures they see is significantly lower than the advertised capacity of their network connection.
Reliability is often the most important performance criterion for a user: The application must be available when the user requires it. Note that this doesn't necessarily mean that it is always available, although there are some applications - such as the public Web presence of a global corporation - that have "24x7" availability requirements.
The impression of reliability will also be heavily influenced by what happens (or is expected to happen) in cases of unavailability: Does the user have a possibility to build a workaround? In case of provider problems: Does the user have someone competent to call - or can they even be sure that the provider will notice the problem themselves, and fix it in due time? How is the user informed during the outage, in particular concerning the estimated time to repair?
Another aspect of reliability is the predictability of performance. It can be profoundly disturbing to a user to see large performance variations over time, even if the varying performance is still within the required performance range - who can guarantee that variations won't increase beyond the tolerable during some other time when the application is needed? E.g., a 10 Mb/s throughput that remains rock-stable over time can feel more reliable than throughput figures that vary between 200 and 600 Mb/s.
-- SimonLeinen - 07 Apr 2006
-- AlessandraScicchitano - 10 Feb 2012