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

 

CS Fallback - A Study

We know that LTE doesn’t have basic voice and SMS support. To mitigate this 3GPP proposes fall back to CS network for voice and SMS. Though the industry pundits frown, I believe CS fallback has to be launched by operators deploying LTE, at-least for initial stages. Let’s look at how this works. In EPS a new interface is required to connect to the CS network. It’s called SGs, based on Gs interface, and runs SGsAP protocol. This interface is between MME and MSC Server. Next requirement is overlapping networks. It is expected to have a GERAN/UTRAN network along with EUTRAN for UE to fallback to CS. The concept is simple. UE will attach to the network through EUTRAN, MME will ask MSC to update UE location in its database, when a call is made or received UE is simply asked to fall back to CS network to answer/make the call. However note that sending and receiving SMS doesn’t need UE to fall back to CS network.

Few more details! When UE is attaching to EUTRAN, UE will perform a combined attached. That is a new IE, mobile class mark, will be sent in Attach Request asking MME to perform a combined attach. Once Attach request is received, MME will send a location update request informing MSC of UE’s location. This way UE is now known to LTE as well as CS network. When a UE has to make a call, it will simply send an extended service request to MME. MME will inform eNB that UE has to now move to GERAN/UTRAN network. If inter RAT handovers are supported, UE will be handed-over to GERAN/UTRAN network else UE’s EUTRAN connection is released and is asked to connect to GERAN/UTRAN. Because of this LTE services on UE will be disrupted.

UE attaching to EUTRAN & CS Networks

SMS is delivered over signaling channel. This means there will be no data path established to deliver or receive SMS. SMS will be sent over a signaling message. Exactly for this reason, SMS delivery or reception doesn’t need UE to fall back to CS network. Once the UE is attached to both MSC and EUTRAN and if MSC wants to deliver a SMS to UE, it will simply send a downlink unit data to MME with SMS content. MME will dump this message in NAS message and send it to UE. In the same way if UE wants to send a SMS it will dump the message in NAS message and send it to MME. MME will extract SMS content and send it to MSC over Sgs interface. This operation doesn’t need UE to fall back to CS network thus ensuring smooth delivery of SMS.

Mobile Orginated SMS delivery in Idle mode

One important thing to note here is that MME should keep a proper mapping of Tracking Area and Location Area. This is because MME needs to know to which MSC the UE should be connected to, based on the tracking area received in attach request. MME will map the tracking area to the corresponding location area and will pick the MSC based on the location area. All the details regarding CS Fallback are present in 3GPP TS 23.272. Refer to same for all the call flows.