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Normally a LoadTest scenario runs at the highest possible rates available on the communications channel between the machine running eValid and the WebSite under test. This technique imposes load on the WebSite server under the assumption of "infinte" bandwidth between the client and the WebSite server.
In some cases a better experiment results when each eValid client is set to simulate playback as if it were running on a speed-limited channel, for example, on a 56,600 bit per second dialup modem. To accomplish this simulation when the actual available bandwidth is much greater than that speed requires that the playback engine be speed-limited to the specified rate. This is accomplished with the Simulate Modem option available from the Preferences Menu. This feature is also accessible as a command line option to LoadTest scenarios as described in the Technical Interface.
NOTE: To assure proper operation you must choose the Never use cache option on the Cache Manager menu.
The specified Simulate Modem is an integer that declares, in 1000's of bits per second, the desired modem rate. Example: A 56,600 bps modem is simulated with 56.
During playback each event involving a complete download of a file has the total playback time padded by an amount such that the effective download speed is not greater than the specified Simulated Modem Rate. These added delay times are added to the Event Log as a NOTICE type message that explains exactly how much time was added to prevent going faster than the required rate.
Assume that you have a very fast web connection, and you have a page that downloads in 0.2 sec. But, on a slower speed modem, this page would take 1.0 sec to download at the specified rate Given these settings, eValid will add a padding delay of 0.8 sec added to the playback to make sure that the download bandwidth maximum is respected. However, if for whatever reason the page actually downloads at a total time of 1.2 seconds -- a speed slower than the specified maximum -- then no padding is added.
eValid works with a specified modem speed value that you give it, expressed in 1000's of bits per second. Its calculations are very precise (1 msec resolution), so this maximum modem calculation is very accurate.
However, as experienced dialup Web users know, a 56.6 Kbps modem actually does not actually deliver transfers at 56,000 bits per second, but usually at a lower rate, typically only about 70% of the total. The rest of the available bandwidth is given over to communications protocol and other tasks. Moreover, such a modem does not provide even the lower speed at anything like a constant rate, because the built-in adaptive logic changes the actual rate based on demand and a number of other factors.
For maximum realism in the playback process it may be wise to use these settings to simulate common modem speeds.
Modem Speed Modem Type Suggested Rate Setting 150 Kbps DSL 105 56.6 Kbps Dialup 45 28.8 Kbps Dialup 20
