The 3G core cellular data network connects radio access networks to the public Internet.
The core network interoperates with components of the existing cellular voice network (in particular, the MSC) that we previously encountered in Figure 6.18.
Given the considerable amount of existing infrastructure (and profitable services!) in
the existing cellular voice network,the approach taken by the designers of 3G data
services is clear: leave the existing core GSM cellular voice network untouched,