n an evolved 2G MSC and an evolved SGSN an optionally evolved Gs interface Mobile IPv4 with Foreign Agent (FA) care-of addresses to end-users over the UMTS/GPRS network, where the FA is located in the GGSN. class A GSM mobiles. Transcoder location shall be according to the "Evolution of the GSM platform to- wards UMTS" outlined in 3G TS 23.930 UMTS/IMT 2000 Phase1 (R99) network architecture and standards shall allow the operator to choose between Integrated and Separated CNs for transmission (includ- ing L2) The UMTS standard shall allow for both separated and combined MSC/VLR and SGSN configurations. The UE shall be able to handle separated or combined MSCs and SGSNs. There can be several user planes to these CN nodes. The following general concepts should be followed: Separate the layer 3 control signalling from the layer 2 transport discussion (do not optimize layer 3 for one layer 2 technology). MSC-MSC layer 3 call control is out of scope of standardization in SMG. As future evolution may lead to the migration of some services from the CS do- main to the PS domain without changes to the associated higher-layer protocols or functions. UMTS release 99 shall provide the flexibility to do this in a way that is backwards compatible with release 99 UEs, provided this does not introduce sig- nificant new complexity or requirements in the system. 3.2.3 The R99 Core Network Synthesis In the preceding section we have already covered the evolution of the CN. However, here we are summarizing them again in the light of the R99 to provide a concise back- ground for ROO introduced in Chapter 9. The synthesis here considers the classical 2G (GSM) type CN architecture evolving to 3G CN. However, some suppliers may already The UMTS Network and Radio Access Technology use the layered architecture as illustrated in Chapter 7, to introduce R99 and directly evolve into ROO with minimum or no structural changes. The UMTS R99 CN will start with a hybrid GSM network. Thus real-time services such as voice and video will continue the 