First Voice Calls Over LTE This Week – What’s Next?

This week saw a number of interesting press announcements on first voice over LTE calls: First was the VOLGA forum who announced the first voice call between two LTE networks, one in Bonn Germany with a Kineto VOLGA Access Controller and the other one in Stuttgart, Germany where Alcatel-Lucent runs a LTE trial network on their campus. A great accomplishment given that the specifications were put together in record time and only released recently. It's not that surprising, though, as VOLGA is based on the already existing GAN (Generic Access Network) 3GPP standard that is used by a number of wireless network operators to offer Voice over Wi-Fi at home.

Next was Nokia Siemens networks who announced that they've done their first LTE voice call with the recently established One Voice Profile that describes which bits and pieces of the IP Multimedia Subsystem (IMS) should be used for voice calls over LTE. Also hardly surprising since IMS standards exist for many years now and IMS voice calls have been made with prototype devices over 3G and other IP based networks. So I guess this announcement was mainly a reaction to the VOLGA announcement.

So this was step one which now has to be followed by the most difficult step: integration. On the mobile side I am sure integrators of both systems can't wait to get an LTE smartphone into their hands. So far, only USB LTE sticks have been announced and here voice functionality is not of prime importance. But once LTE mobile devices such as smartphones become available we'll see who can integrate their solution better, quicker and, foremost, seamlessly with 2G and 3G circuit switched calling. After all, it should be absolutely transparent for the user over which air interface the mobile establishes the voice call. IMS based solutions have struggled for years to pull this off, so let's see, maybe the One Voice Profile will help to unite the supporters of IMS around a single set of features. For VOLGA, things might be a bit simpler as it re-uses much of what has already been developed for GAN and already implemented and used in a range of mobile devices today.

Perhaps even more challenging is the integration on the network side for IMS One Voice. While a standalone IMS system is quickly installed (relatively speaking), it's the interworking with the cirucit switched infrastructure to enable seamless handovers when running out of LTE coverage that makes life difficult. Also it is important that a user can be reached by the same means (i.e. same phone number) no matter whether he/she is currently connected to the IMS over 3G or 4G or just reachable via 2G and the circuit switched infrastructure. IMS Centralized Services (ICS) and Single Radio Voice Call Continuity (SR-VCC) have been specified in 3GPP for the purpose and I've reported and linked to the specs here. Specifications are one thing, complexity quite another. On the complexity scale, ICS and SR-VCC are at the upper end of the scale and require software updates in the circuit switched infrastructure.

For VOLGA, things are a bit simpler. No modifications in the existing infrastructure are necessary and only a single gateway node, the VOLGA Access Network Controller (VANC), is required for a full implementation. To the circuit switched Mobile Switching Center, the VANC just looks like a 2G Base Station Controller or a 3G Radio Network Controller. For the LTE side the VANC is just a server on an IP network. Pretty much a straight forward thing to do.

Both initiatives good luck on their way forward and I hope that once I hold the first LTE smarthphone in my hand I won't be thrown back to GSM for voice calls.