If data compression is used, the DTE rate should be set as high as possible. The table below compares download times for the text test page on connections with a line rate of 14.4 Kbps for DTE rates of 19.2 Kbps and 115.2 Kbps, with V.42bis compression enabled and disabled. If compression is disabled and the DTE rate is higher than the line rate it would be possible to guess your current line rate from the table below. You should only disable compression in order to perform the test, as it will reduce your perceived speed. Alternatively, the graphic test page will give an indication of the current line rate.
Most modems use LAPM error control by default.
The distance you are from the telephone exchange will affect your connection rate. One modem manufacturer recommends that you are less than 4 miles from your telephone company's equipment to achieve speeds above 33,600bps. If you hear noise, crackling or a hum on ordinary voice calls your connections won't be as good. Low throughputs, though, do not necessarily mean a dodgy telephone line. From the telephone company's point of view, they are ideal for customers who install a second line, except, it seems that they typically limit line speeds, often to around 26,400bps.