3G, edge and internet connectivity question/problem

I am jailbroken but still on 02 UK

3GS jailbroken with purplera1n

I dont fully understand the relationship between 3G and edge

I seem to have a very low 3G signal most of the time, if I switch it off the signal goes to the maximum.

I can access the internet when 3G is on - if I switch it off via sbsettings I cannot access the internet using edge only - is this normal or is something wrong?

Should I not be able to use data services using edge?

Any advice or info greatly appreciated.