Transmission speed

Consider this:
If you transmit a byte (8 bits), in most cases it is always associated by "dummy" bits, such as start bit, parity bit, stop bit or bits .. so one can easilly estimate, that approximately 30% of transmission is wasted .. and on top of this, you will have "breaks" between transmisssion sequencies (blocks).

And, what about handsheaking ...?

If you want to cheat yourself, you can calculate that every clock pulse carries one bit, and therefore for 1Mbps you need 1-1.3MHz of clock frequency.

But, taking into account all other factors you can easily double or tripple this frequency, if you would like to have a real throughput of 1Mbps ..