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Tuesday, January 4, 2011

What is the need to execute Network Sensitivity Tests?

The three principle reasons for executing network sensitivity tests are as follows:
- Determine the impact on response time of WAN link.
- Determine the capacity of a system based on a given WAN link.
- Determine the impact on the system under test that is under dirty communications load.
Execution of performance and load tests for analysis of network sensitivity require test system configuration to emulate a WAN. Once a WAN link has been configured, performance and load tests conducted will become Network Sensitivity Tests.

There are two ways of configuring such tests:
- Use a simulated WAN and inject appropriate background traffic
This can be achieved by putting back to back routers between a load generator and the system under test. The routers can be configured to allow the required level of bandwidth, and instead of connecting to a real WAN, they connect directly through to each other.
When back to back routers are configured to be part of a test, they will basically limit the bandwidth. If the test is to be realistic, then additional traffic will need to be applied to the routers. This can be achieved by a web server at one end of the link serving pages and another load generator generating
requests. It is important that the mix of traffic is realistic.

For example, a few continuous file transfers may impact response time in a different way to a large number of small transmissions. By forcing extra more traffic over the simulated WAN link, the latency will increase and some packet loss may even occur. While this is much more realistic than testing over a high speed LAN, it does not take into account many features of a congested WAN such as out of sequence packets.

- Use the WAN emulation facility within LoadRunner
The WAN emulation facility within LoadRunner supports a variety of WAN scenarios. Each load generator can be assigned a number of WAN emulation parameters, such as error rates and latency. WAN parameters can be set individually, or WAN link types can be selected from a list of pre-set configurations.

It is important to ensure that measured response times incorporate the impact of WAN effects both at an individual session, as part of a performance test, and under load as part of a load test, because a system under WAN affected load may work much harder than a system doing the same actions over a clean communications link.

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