said by BigBlarg:
It's pretty much the same as a WiFi router. Wireless G routers are marketed as "54 mbps". In practice, the maximum speed you can get is around 21 mbps. Where are the other 33 mbps? Checksum/Error correction.
The 3G/4G overhead is not that much, in labs you can achieve 18mbps on 21mbps 3G networks.