Why S-GW is connected to RNC with S12 interface ? Why not BSC?

Hi all,

I ran across this image showing S-GW interfaces and I noticed S-GW in EPC is connected to RNC in UTRAN core by S12 interface.
Screenshot 2021-08-09 061643

I am trying to understand why is there a connectivity between S-GW and RNC ? Is it because of 4G-3G handover ? If that’s so, shouldn’t it be taken over by MME?

Also, why S-GW is connected to only RNC ? Why is not connected to BSC as well for 4G-2G handover (in case 3G is not present in a network or area)?

Many thanks in advance.

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The S12 interface is a user-plane interface between the UTRAN and S-GW. It is used to transmit uplink and downlink user-plane data flows between the RNC and S-GW when the direct tunnel function is applied in UTRAN access. We have direct tunnel with RNC and with BSC there is no direct tunnel so we don’t have S12 for GERAN.

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Thank you @Saeed9769 for your response.

I am trying to understand what is the point of this direct tunnel between EPC and UTRAN ? That is, what does this “direct tunnel” do in UTRAN that is not present in GERAN?

It can help in lower delay, like 4G that in user plane the eNodeB sends the data directly to S-GW and it is not sending to mme. in 3G also in user plane we connect directly to GGSN (if direct tunnel is enabled) and SGSN is not involved in user plane.

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