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Advertised DSL speeds are your "connection rate/data rate" not "download speed". Due to transport overhead it is not possible to download end-user data at actual advertised rates. Depending on the type of service and specific protocols used you will have about 12.5% (or more) of your bandwidth allocated for overhead. So a DSL customer with advertised speeds of 1472/256 will have a real world end-user data download speeds of roughly 1250/220 at best.

9.4% 5 byte overhead within 53 byte ATM cell
0.5% 8 byte PPPoE overhead
1.3% 20 byte IP overhead
1.3% 20 byte TCP overhead
12.5% Total Communication Overhead (minimum)

Important Note: Some speed test sites measure the advertised data rate while others measure real world file transfer rates. Sites that calculate actual end user file transfer rates take into account the transport overhead and will show a lower overall speed rate when compared to sites that measure data rates.

Andy Houtz DSL

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last modified: 2009-02-19 08:47:13