Tuesday, March 13, 2012

Simultaneous CS & PS connections

Ever seen ATTs “Only AT&T's network lets your iPhone talk and surf at the same time” ad? If not here is the youtube link.
Ever wondered why so much emphasis on network rather than phone? Well, this is a new feature on network . Not new exactly; this feature has been there since Release 5. Good to see that it is actually rolled out to production; otherwise it seemed very stupid to own a smart phone. 
Ok! Here is how this is made possible. When a phone is switched on, it goes and attaches to the network. If phone is data capable, the phone does PS attach towards SGSN and CS attach towards MSC. Basically there are two separate radio control channels for data and voice and a phone can be hooked on to either one of it at a give time (not an expert on radio though).  So if you are browsing, and MSC is paging the phone for a voice call, phone has to disconnect the GPRS session and receive CS paging over CS signalling channel which does make a smartphone look stupid. 

For this sake 3GPP has done enhancements on network side. On the network side they introduced a new interface between MSC and SGSN, the Gs interface. This interface has been upgraded to SGs and further to Sv for CS Fallback and SRVCC features respectively. The additional Gs interface made phone to connect to both CS and PS service with one attach towards PS, which is referred to as combined attach. When a phone is powered on it looks for PS network and sends PS Attach Request with CS parameters towards SGSN. Based on the attach request, SGSN figures out the location area from routing area, finds out the MSC based on location area and sends location update towards it for CS attach. So with single attach UE is connected to both CS and PS services.  

All is good. Now assume a case where phone is using PS services and has an incoming call. MSC sees the incoming voice call and sends a CS Page request to SGSN, because that is where the location update request came from, instead of UE directly. Upon receiving the CS page, SGSN sends paging request towards MS over PS channel with CS indicator. Since the paging request has CS indication the phone prepares of a CS call. Phone responds to the paging and MSC pushes the phone call toward RNC via CS voice channels. But the best part is phone can still have its PS connection continued while still having a CS connection. The trick is to not make phone listen on both CS and PS channels instead use one signaling channel on radio side and make SGSN and MSC communicate for CS services. I tried to capture the same in above call flow. This is a brief overview, there are many more procedures inside this for feature to work flawlessly. Also the network mapping between location area and routing area needs to done correctly. It’s all easy for a green field operator, but networks like ATT does need a lot of time and money. Hence the ad :-)

For more details, refer to 3GPP TS 23.060


Anonymous said...

The Gs interface is only for 2G, for 3G you don't need this and you can make cs call and ps call at the same time with no problem.

Anonymous said...

one more thing, it is better to use Dual Transfer Mode in 2G (i'm not sure if Iphone supports it) then Gs.

Gs is risky and can overload your SGSNs.

Santosh said...

Don't we need Gs for combined attach?

Anonymous said...

Yes, you need the Gs for combined procedures and it will save you radio resource but you don't need it to make CS and PS calls simultaneously in 3G.

Santosh said...

Thanks for the comment!

Anonymous said...

Hi santosh,

Curious to know what SGs stand for..

Santosh said...


I am not sure what SGs stands for, but it is the interface between MME and MSC and is used for CS fallback.

However I think S in SGs stands for SAE, as all LTE SAE interface start with S (S1, S11, S5 etc)

trijava said...

nice info

Harish said...

Hi, It also depends on the class of the mobile whether it is class a or b or c to support this...