KPN – ADSL Modems #SWI1998

The Transmission Control Protocol (TCP) is a well-used protocol to send information files over the Internet. The essence of this protocol is that the total quantity of information is divided into a number of smaller packages. Sending the data error-free is made possible by detecting the packages in which an error has occurred and resending it. A user (who sends a file) will be interested mainly in a high throughput The throughput is defined as the average speed with which a file of a given size is sent (so, file size over the total time needed to send the file).

From this description of the protocol, it is clear that the throughput of the Transmission Control Protocol depends both on the capacity of a connection (the number Mb/s that is possible in a connection) and on the chance of an error in a TCP-package. The throughput will increase if the capacity increases and will decrease with the probability of errors in the TCP-package increasing.

The ADSL technique (Asymmetric Digital Subscriber Line) allows one to transport information quickly over the present copper net (which is the net between the regional centers and the consumer). At the moment ADSL is standardized at a speed of 2Mb/s and bit error 1E-7. This, however, is a rather arbitrary choice and a balance is to be made between the capacity and the bit error probability. In fact, the capacity depends on the tolerated bit errors. The higher the capacity, the more bit errors.

The specific research question is: What is the optimal speed for ADSL modems to achieve a maximal throughput?

This question is relevant to tune the ADSL modems for the implementation of ADSL in the Dutch network. Especially the rise of the Internet (and the corresponding use of the TCP protocol) is an important incentive for developing and implementing ADSL.