Thursday, January 14, 2010

SGSN and LTE

With LTE gaining more light I guess most of the network manufacturers are ready with basic attach, default and dedicated bearers, X2 and S1 handovers. But LTE as standalone will not make sense as it will deployed in just few places. Rest of the world will be still running on 2G/3G/CDMA networks. So there is a strong need for 3G/CDMA convergence with LTE which means inter RAT handovers. I think Europe will see 3G convergence with LTE while Verizon is working hard to converge their CDMA network into LTE in US. So, may be, this year will be about convergence and figuring out a proper solution for voice over LTE.

My interest here is SGSN. SGSN is a major signaling element in 3G packet core. It runs RANAP/NAS protocols towards RNC and GTPv1 towards GGSN. If 3G is to be converged into LTE then there has to be a software upgrade on SGSN as it needs to speak GTPv2 over S4 interface with LTE MME. This makes SGSN much more important and signaling intensive device. I am sure that both operators and equipment manufacturers are looking closely at the SGSN's now. The question here is how will the operators choose which 3G networks should support LTE and which shouldnt. Imagine this. I have a UE which has APN configured with a LTE PGW. While in roaming I may not find a LTE network so I will have to switch back to available 3G network. Now, that 3G network should be capable enough to support my UE's existence in the network. This means SGSN should go all the way to talk to LTE MME, pull the bearer contexts, map the QoS and serve the UE. This needs quite a lot of processing. Will existing SGSN be capable to do all this? This will probably need much powerful hardware on SGSN. I am very keen to see how things will go in this direction. I am hoping that Mobile World Congress this year will have some answers. Any thoughts?

9 comments:

Anonymous said...

When you are in roaming, you do not use your home SGSN/MME, but only your GGSN/PGW in most cases. You can check GSMA specs on packet data roaming - most specifically IR34.

Otherwise, yes, some SGSNs will do support MME on their existing HW platform - Starent are having MME on ST40, Ericsson have some limited MME on mkVI only for trials, but will go on for the Redback SmartEdge, I assume Huawei will also support SGSN and MME on their latest platform, but this is just a guess... The other vendors like ALU and Motorola will use newer HW, NSN also I suppose...

Santosh said...

Yes, you are right about roaming scenarios. However if there is an Inter RAT handover (3G to LTE or LTE to 3G) to be dealt with then visiting SGSN might have to talk to home MME/SGW over S3/S4 interfaces.

Thanks for the GSMA spec and vendor details. It would be interesting to see who will demonstrate first inter rat handover (including UE). :-)

Cheers, Santosh

tsabu said...

Regarding EPC vendor's platforms according to Ericsson's roadmap they will support MME on all MK harwares including MKIV and there are no indications for HW migration. Version 2010B will be a combined dual & triple access SGSN/MME.

Anonymous said...

My understanding there maybe two phases to support 2/3G and LTE interworking.
1st phase is Gn/Gp SGSN. LTE need to support legency Gn/Gp interface and interworking with SGSN.
2nd phase is S3/4 SGSN. The existing network are upgraded to support new interface with LTE and additional new functions like ISR.

3GPP coveres two cases above.

Manish said...

http://www.lightreading.com/document.asp?doc_id=186598

3G Fuels Mobile Packet Core

Some interesting relevant facts in the above article

-Manish

Saraswati said...

In regard to Gn/Gp and S3 SGSN, is a Rel 8 SGSN always S3/S4 mode and Pre-Rel 8 SGSN always Gn? Or is it dependant on ISR mode irrespective of Rel version? I mean do/can we have a rel 8 SGSN whivh operates in Gn mode i.e. non ISR mode.

Santosh said...

Yes, that is the idea. Apart from Gn interface a Rel 8 SGSN has to support S3/S4 interface too. Isn't ISR specific to Rel 8?

Anonymous said...

in 23.401, I find in annex D the handover of MME to SGSN, but for this case, with Gn/Gp. Can you put some infos, where does the procedure of attachment of UE to 3G SGSN but using s3/s4? In 23.401 v10, it still mention Gn/Gp and not s3/s4.. CMIIW

Santosh said...

Hi

For S3/S4 you should not be looking at Annex D. Refer to 5.5.2 in 23.401.