Well, yusran mashami seems to have lost interest already... :-(

But I'll just put this in for completeness, for the benefit of anyone else who might be researching the subject:

In applications suited to an 8051, one is often only transferring small amounts of data at a time. In this case, the time it takes a high-speed modem to "train" will often far exceed the time taken to actually transfer the data.
Therefore, when only small amounts of data are involved, it is often actually quicker to use a lower modem speed - as this will "train" much quicker!
The modem may also have built-in data compression - this can also help.