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If you try a few different test servers, including some of the third-party ones that also use our Java test applet, then the highest speed reported is closest to your last mile speed.
Don't forget that protocol overhead will mean that you never reach the actual speed advertised by your ISP. In some cases, especially PPPoE connections, you can lose almost 15% of advertised speed in protocol overhead.
In response to this problem, certain ISPs have set the actual sync rate of consumer broadband connections higher to provide a buffer against the overhead. The idea is a 15% up in advertised speed is worth less people calling into support and complain about lack of advertised throughput.
If your ISP is prepared to locate a speed test server inside its network, then it will give you the most accurate measure for last mile speed. Having a very fast connection within your ISP's network is nice (and is what you're paying for as a customer) but that doesn't mean much in real world usage.
Either way, we have a large selection of Java and Flash based speed tests located both in ISPs and elsewhere on our speed test list. If your ISP is one of those, then you will probably be able to test last mile speed the best by using their local test server. If you are interested in real world test results, then pick another well-connected server. It is your choice.
The transfer rate expressed as kilobytes per second is based on 1024 as per data storage conventions.
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1. Physical distance. You have likely noticed that downloading from a local server location is faster than a distant one. This is because the TCP window (e.g. HTTP transfers) is not optimized for the increased latency that comes from an increase in physical distance the bits have to travel. HTTP is bound by the TCP congestion window which determines how many packets can be sent at one time before an acknowledgment. The larger the window size, the higher the TCP throughput.
2. Firewalls, wireless, Local LAN, PC and modem issues. There are many localized issues which may negatively impact speedtest results. The best (but not perfect) PC test environment is to have a high end PC capable of delivering the provisioned speeds, directly attached to the modem.
3. Capacity issues with intermediate ISPs between the server and the user. There are some speedtest servers on ISPs with poor interconnect relationships or capacity issues. The path between the user and the server factors into any results. Local, on-net, speedtest servers are best to measure your provisioned speed.
4. Tuning of the Speedtest server with parallel TCP streams. This is not well known, but does have a direct impact to higher broadband speeds. Most speedtest servers open multiple TCP streams to more accurately account for single stream TCP limitations. How many simultaneous TCP streams a server opens is important as with 4 streams a server is accurate to up to 50Mbps testing on an average PC. Some speedtest servers are configured for even fewer streams and therefor are less accurate at higher speeds.
5. Asymmetric test environments - High end commercial connections with symmetric speeds often ask why they get asymmetric results from most speedtest servers. This is because most speedtest software / servers are configured for residential broadband testing. They are normally configured for 2-4 simultaneous download streams / test, but only 1-2 upload streams. This will inaccurately give results of asymmetric bandwidth even when you have symmetric speeds. Those with higher end work connections can simulate this.