Roaming trouble

What makes cellular communication standout from other wireless technologies is its roaming ability. No matter in which country you are just dial the number and talk. This makes mobile communication truly mobile.  But the technology behind is not that simple.

I was wondering how difficult will it get for an operator to maintain so many roaming agreements. A given operator may have to maintain like, say, 100 roaming agreements. Well considering LTE a given MME will have to talk to 100 different HSSs and SGW will have to communicate to 100 different PGWs.  Whoa!

There is a nice way to maintain the list of PGWs based on the various APNs. MME informs the SGW which PGW it has to contact for which IMSI. The S8 interface is UDP based and DNS maintains detailed records of APN to PGW mappings. So its relatively easy to fetch a PGW and establish a tunnel for the user. But for HSS its little complicated. Now that HSS runs on diameter, there are several solutions proposed to reach home hss from visiting network. Also diameter runs on tcp or sctp which means there is always a connection established. This is quite difficult as the MME has to reach several HSS across the world. Looking forward to some interesting solutions here. Diameter relays, proxies ?

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.

Mandate CS Fallback

There is so much debate going on about voice technology that should be used over LTE. I have my view here. Before a proper voice technology is figured out, I would assume that CS fallback will become a mandatory feature on all LTE phones. Though VoLGA and IMS seem to be good solutions, but they don’t solve many other issues, like roaming for e.g.

Its obvious that LTE phones will support 3G/2G technologies. Also with release 8 SGSN’s coming in the packet handovers to 3G/2G technologies from LTE and vice versa would be smooth. This means LTE phones are already equipped with 3G/2G chipsets. If so then why cant  CS fallback be made mandatory?

Yes, agreed that to access a voice call one should be cut out of LTE. But at least I am able to make a voice call even if I am not in my home network. Analyzing how the voice call can disrupt the packet service for individual users can be a separate case study. For e.g corporate users are heavily dependent on emails. So if voice call lands on their mobile the LTE service will switched off for couple of minutes which is very acceptable for an email user. Rest may not be convinced, but it does solve the problem.

One more point is SMS. I heard that SMS is mandatory in Europe, that is if a user is roaming then the roaming charges(?) should be informed to the subscriber by an SMS. If that is the case the CS fall back does help with smooth delivery of SMS over Sgs interface. However this may not be a permanent solution. As the technologies mature we can find a better way to deal with voice and data co-existence without breaking each others connection. Untill then operators please give us CS fall back support and also do not mess up your networks with putting many additional nodes.