Sunday, October 10, 2010

S4 SGSN


As the LTE deployments are nearing it is becoming increasingly important to bring in smooth handover support between 3G/2G and LTE networks.  The networks are on expansion and are becoming complex. In amidst of all, different solutions for same problem is making life a bit more difficult. 

While we have a initial solution in place for migrating from 3G to LTE without changing much in the existing 3G network, it has been noticed that the service providers are now thrusting for a more smoother solution. Identifying few pockets of areas service providers are now willing to upgrade their exiting 3G network to be compliant with Release 8. I pity the SGSN for the burden it has to take to not only perform the existing functions but should also start processing LTE related calls. 

If the SGSN is migrated to Rel 8, then it has to support two new interfaces. S3 and S4. S3 is between MME and SGSN analogous to Gn interface. But S4 is totally new interface to SGSN analogous to S11 interface. The question is whether a S4 interface is needed at all. When a UE moves from 4G to 3G, SGSN requests for Context Information from MME over S3 interface. If the interface is Gn then protocol would have been GTPv1 which doest create any trouble for SGSN. As S3 is GTPv2 based, SGSN should implement the new protocol. So far so good. Anyway UE is in 3G network, SGSN has pulled the UE information from MME over S3, then what is the need for S4 interface. SGSN can as well simply go and talk to GGSN. Instead of this 3GPP has decide to route the SGSN to SGW over S4 interface. This is mainly done to remove the GGSN node as such. 

We know that LTE APNs are configured in PGW.  The idea is to reach the same APN in the PGW when UE is either connected from 4G or 3G or 2G. Which means in 3G and 2G it will become the responsibility of SGSN to talk to PGW through SGW. This will simply need no GGSN at all for LTE APNs and more over everything is Rel 8 compliant. The compliance will ensure smoother handovers and continuous data flow. So when a UE directly switches on in 3G network, SGSN will contact SGW for the establishing a session. When there is a handover involved again, SGSN will contact MME over S3 and SGW over S4. Ultimately for 4G APNs there wont be a need for GGSN and PGW to co-exist. Another important thing here will be support for IPv4IPv6 PDN types. This PDN type is not supported in 3G and is a feature of LTE. To ensure that this PDN type is supported over 3G the SGSNs should be Rel 8 compliant. 

Hence, SGSNs are overloaded. 

16 comments:

Anonymous said...

Yeah, ok, I can see 3gpp again messing between specs, versions and not much to do with real life... Poor us, we will have to somehow search for a way to put this all together...

We can not get rid of the GGSN that easily, still there will be our subs in roaming, using SGSNs that are not r8 compliant... In my experience there are still networks not supporting GTP1 :-) And add the GTP2 support in the FWs, wow, it will be a massacre again, be prepared for long night troubleshooting and fixing, hahaha.

Santosh said...

True. Software bugs are scary too :-)

Vijay said...

Hi Santosh,can you please have an article on the gngp handover from 3G to 4G.

Anonymous said...

Hi, I have an article request too. Or maybe you could just answer a question? I heard that (for LTE, maybe true for other networks) when your channel bandwidth decreases, your overhead increases. I can understand why you'd have worse performance when you have less bandwidth, but how does overhead actually increase?

Thank you!

Roman said...

What kind of overhead are you talking about? S1AP? GTP?

Santosh said...

GTPv2!

Santosh said...

Also S6d!

Dumitru said...

I must say that one of the driving force of S4 SGSN adoption will be:
1- Idle Mode Signaling Reduction
2- IPv4/v6 PDP context
The ISR will is very important especially if we consider over-congested networks(by signaling procedures) caused by ever increasing number of smartphones that tends to be always on (attached and in idle mode). Think what will happen with the spotted LTE coverage? Bingo! a lot of RAU because of cell reselection.

Dumitru said...

"I heard that (for LTE, maybe true for other networks) when your channel bandwidth decreases, your overhead increases."

It doesn't make sense, maybe they meant packet size?

john said...

-> Dumitru & Anon
"I heard that (for LTE, maybe true for other networks) when your channel bandwidth decreases, your overhead increases."

The following is probably the explanation you are looking for:- the channel size in LTE is variable, 1.4MHz, 3,5,10,15 and 20MHz. There will always be a certain amount of overhead which is common to all channel sizes, for example System Information messages. As the channel size decreases, the proportion of total resources needed by this fixed overhead increases.
Hope this helps.
John McKeague, Award Solutions

Anonymous said...

http://www.vascom.org/wireless-technology/313-s4-sgsn.html

it seems similar to yours

Santosh said...

Hmm...there are quiet a few post copied from blog there! Interesting

susilcool said...

Hi Santoshat

My query is if we are saying that GGSN will be removed with S4 then what will be happen when an 3g ue will be upgraded to LTE Ue then it will talk to Pgw or GGSN for the IpAdress

regards
susil

Anonymous said...

Dude, your blogs are just copy from wikipedia and other's blogs... Why do you mention yourself as Network "GEEK". ??? Sleeping LTE ??? You dont even know what is S4 SGSN .......

Santosh Kumar Dornal said...

Thanks for your criticism. I would be more happy to see what is copied from other blogs and wiki. Just making a vague statement is one thing and constructive critisicm is other. Next time be decent enough to leave your name at least.

Anonymous said...

Mr. Santosh Dornal Thanks for Sharing.

By
Ravi