UMTS R4 technique provides a smooth path to bridge legacy TDM-based network to an IP-based soft-switched network. R4 introduces SS technology into CS domain in which bearer function is separated from control function.  

FIGURE: UMTS R4 CN architecture

In this figure, UMTS R4 CS domain consists of three NEs: the MSCS (or MSS) or gateway mobile switchingcentre server (GMSCS); MGW or gateway media gateway(GMGW) and visitor location register (VLR) which is physically integrated in MSCS. The HLR is a commonentity that both CS domain and PS domain can access.UMTS R4 PS domain includes such NEs as serving GPRSsupport node (SGSN) and gateway GPRS support node(GGSN). 

introduced well

good introduction 

useful document, thanks