Because WCDMA at 2100 MHz generally lacks the indoor coverage that’s possible with GSM 900 MHz, this leads to another use case for inter-RAT handover.

Operators have invested considerably in 2.5G and the move to 3G will at best be a transition.3G may be deployed at the level of microcells while at the macrocell level 2.5G may be used.This means that movement from urban to rural will trigger an inter-RAT handover. Going by demand, deployment of 3G cells will happen in phases whereby urban areas could be covered by 3G while rural areas continue on 2.5G.It was correctly envisaged that this coexistence will happen for the following reasons: More than competition, it was completion and cooperation. Rather, these two access standards will co-exist for a long time to come before 3G completely replaces 2.5G. The greater truth is that everyone knew that UMTS will not replace GSM/GPRS overnight. So when GPRS saw the upcoming competition from UMTS it had to invent new methods to keep itself alive longer in the cellular world.

They say necessity is the mother of invention.

Gprs class b multislot class 6