Communications equipment vendors like to think in terms of low level physical link data rates without regard to the structure or content of the data. The physical link uses a different protocol (ATM) for transferring data between two points. Internet uses IP as the protocol for communicating, in particular, TCP/IP. So your data is going over your DSL line via TCP/IP over ATM.
TCP has an overhead in transmission that can be about 5-10%, but ATM
overhead has a 15% overhead. So you can expect to lose up to 25% of your
purchased speed at when counting application data transfer rate. As a rule of thumb: Given a broadband line speed, dividing by 8 and taking off
25% is a reasonable estimate of the maximum likely data download speeds (in
bytes of data).
There are additional factors affecting the throughput results. There are
applications running that usually take up a small amount of bandwidth to communicate with their servers, therefore only the remaining bandwidth
is available for your downloads.
The Operating System TCP settings will also affect the results because it is usually not optimized. Please visit http://www.dslreports.com for further information on optimizing your TCP settings.
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Why is my throughput lower than the line speed I purchased?