23.1213GPPArchitectural requirements for Release 1999Release 1999TS
From a logical point of view, the CN encompasses two domains, a PSTN/ISDN domain and an IP domain. It shall be possible to connect the UTRAN either to both these CN domains or to one of the CN domains.
A single RRC connection (between UTRAN and UE) shall carry all user plane and signalling flows to/from a UE. This is regardless of where in the CN they originate/terminate.
UMTS shall support compatibility with GSM network from the point of view of roaming and handover. For the LM/MM functionality point of view this implies among other things the following:
a) IMSI shall be used as the common user identity in the two CN domains;
b) common MAP signalling will be applied to both GSM and UMTS. The GSM MAP mobile service operations shall be evolved and re-used as far as possible (including extensions if required). This should not stop new MAP signalling operations being developed and applied to both GSM and UMTS;
c) radio network parameters and radio resource management should be isolated in the UTRAN.
The LM/MM techniques used in UMTS should minimise radio resource usage of the UTRA.