If LTE system is deployed in a network already supporting GSM and/ or WCDMA, it is possible to use a common core network for access.
In reality, it means that P-GW will provide GGSN functionality towards General Packet Radio Service (GPRS) Core network. An LTE / UMTS / GSM capable terminal will therefore not need to change the GGSN (i.e. the IP point of presence towards external networks) when it changes Radio Access Technology (RAT) between GSM, UMTS, and LTE.
Figure above shows a standardization view on how GSM, UMTS and LTE are integrated into EPC Core Network.
SGSN and MME
Also please note that SGSN and MME share a lot of common functionality . It is also required that Core network protocols, Session Management and Mobility Management used in 2G/3G are compatible with the respective protocols used in EPS meaning that SGSN and MME share a common evolution in the 3GPP standard.
In practical deployment, it is likely that 2G/3G SGSN and MME are merged together into one node as shown in the figure below.
This makes it possible to support intra SGSN/MME and inter P/S-GW /GGSN node mobility between the different accesses.