DSL Network Guide Chapter 18: Overhead

When you purchase DSL the sales rep will give you options of the different speed plans that are available to you. When you purchase a speed plan you are purchasing throughput. That means how fast data can travel through your access concentrator (i.e. redback.) The lines between your redback and your homecomputer cause a slight loss of speed called overhead. On overage you will lose about 13% of your speed because of this overhead.
The majority of the overhead isn't because of TCP/IP, its because of ATM (53 octet cells - 48 octet payload, 5 octet header). The PPPoE overhead is 8 octets per MTU (normally 1500 - the size of 1 Ethernet frame) The IP overhead is 20 octets per MTU - PPPoE (normally 1492) and the TCP overhead is 24 octets per MSS (normally 1452.)

So thus:

PPPoE overhead is 0.53 %
IP overhead is 1.30 %
TCP overhead is 1.65 %
ATM overhead, on the other hand is, 9.4 %

For total overhead of about 12.9 %

So therefore on a "perfect" line at 1.5kbps running at max speed would be a tad over 1,300 kbps.
Furthermore, ATM has services on top of it like IP does with TCP and UDP, introducing more overhead, for example, Ethernet bridging over ATM. Also, many of the application-level protocols (like HTTP,FTP,telnet,IRC etc) within IP/TCP/UDP have even MORE overhead, depending on the protocol.
If none of this made any sense to you just remember that because of the way the DSL network is setup about 13% of your speed will be lost. This could be corrected by using more efficient protocols but because of the money it would cost and how few people even notice this happens it is unlikely that any ISPs will be pushing to make this change.

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