Every DSL connection uses a number of different internet protocols that can reduce the bandwidth of your internet connection. 

Overhead is the combination of these protocols that influence your overall DSL connection speeds.

IP overhead:   1.30 %
TCP overhead: 1.65 %
ATM overhead: 9.4 %

Combined the total overhead for DSL is about 12.4 %.

If you had a 1.5Mb DSL connection on a "perfect" line the max speed would be a tad over 1,300 kbps.  There is also other overhead involved that gives you around 20% overhead in total.

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