How MME selects P-GW?

Question

Hi all

I find some information on internet of this question "What pros and cons of combined S-GW/P-GW?".
However it did not write the reasons.

I find this answer in http://www.cisco.com/c/dam/en/us/solutions/service-provider/mobile-internet/whitepaper_c11-577763_v1.pdf

It said that combined S-GW/P-GW have these advantages:
Reduced signaling overhead (S5 and S8 would be internal)
Fewer hops on the bearer path
Less backhaul
Reduced signaling on the S7 interface
Lower session requirement for the PGW

I cannot understand why it can reduce signaling on the S7 interface.
and although I probably know other advantages' reason, I am not sure.
Someone please tell me.

thanks

Answer 1

During the attach procedure, MME fetches the subscription profile the HSS. For this purpose it sends Update-Location-Request message to HSS and get back the "Update Location Answer" in response from HSS. Update Location Answer message carries " MIP6-Agent-Info " AVP within the APN-Configuration AVP.

MIP6-Agent-Info AVP contains PGW identity as an IP address (MIP-Home-Agent-Address) or FDQN (MIP-Home-Agent-Host).
Based on that MME builds and sends Create Session Request message to SGW and in turn SGW builds and sends Create Session Request message to the PGW.

Along with MIP6-Agent-Info AVP, PDN-GW-Allocation-Type AVP would be present which actually tells IP address presents within the MIP6-Agent-Info AVP is either statically configured or dynamically.

Hope this level of information would help you.

Comments

Thank you Harshita for your detailed explanation.

Answer 2

As additional, MME will query the PGW information to DNS core based on FQDN APN. DNS will send back the IP address of PGW to MME for create session request.