LTE and the Voice Gap

A recent report I read about the future rivalery between HSPA and LTE has made me think about an important difference between the two technologies: Voice. While UMTS / HSPA intrinsically supports voice calls in the radio and core network, LTE requires the IP Multimedia Subsystem (IMS) for voice calls. So what will happen to LTE if IMS doesn’t take off? I know, many in the industry believe even asking such a question is close to heresy but who can promisse today that IMS will be a success?

The trouble with IMS and to some extent with mobile VoIP is not that it’s a young technology, standardization has been going on for many years and books about it are going into their third edition. However, there are still no IMS systems out there today that have come out of the trial phase, and I have yet to see a mobile device with an IMS client which is nicely integrated and simply works. Also, the IMS standard is getting more complicated by the day which doesn’t make life easier. Another main issue with VoIP and consequently IMS is power consumption. I use VoIP over Wifi a lot on my Nokia N95 and can nicely observe how the phone slightly heats up during a long phone call. Also the non-IMS but SIP compliant Nokia VoIP client in the phone, which by the way is nicely integrated, sends keep alive messages to the SIP server in the network several times a minute. This is necessary mainly due to Network Address Translation (NAT). While this doesn’t require a lot of power over Wifi, power consumption skyrockets as soon as I configure VoIP for use over 3G. I can almost watch the power level of the battery drop as the network now constantly keeps a communication channel open to the device. So there are two problems here: VoIP calls cause a much higher processor load during a call, i.e. the VoIP talk time is much shorter than the 2G or 3G talk time and the standby time is significantly reduced. Add to that the missing handover capability to 2G and 3G networks (yes, I know there is VCC in theory) and you have a prefect package for a very bad user experience.

So the big question is if all of these things can be fixed, say over the next 5 years!? I have my doubts…

If not, then LTE has a big problem. Will network operators accept running GSM or HSPA alongside LTE until the problems are fixed? The choice is this and accepting that LTE is for Internet access and some niche VoIP applications on devices such as notebooks or to decide sticking to HSPA(+) until things are fixed.

In case LTE is deployed and LTE – IMS devices are not ready it’s likely that a device can’t be attached to several radio networks simultaneously. So how do you inform a device attached to LTE about an incoming voice call? It looks like the people in standards bodies are looking at different solutions:

– Send a paging message for an incoming circuit switched voice call via LTE to the device. You can do this on the IP layer or on the radio network signalling layer. The device them switches radio technologies and accepts the call.

– Some people have started thinking about extending LTE with a circuit switching emulation. This could be handled on the lower layers of the protocol stack and the software on top would not notice if the call uses GSM, UMTS or LTE. This one is easier said than done because I don’t think this concept will fly without a seamless handover to a 2G or 3G network. If such a solution ever gets into mobile phones, it would make life for IMS even harder. Who would need it then?

Are there any other initiatives I have missed so far to fix the LTE voice issue?

2 thoughts on “LTE and the Voice Gap”

  1. Hi Martin

    The question of LTE and Voice was one of the main motivations behind my VoIPo3G report that I published in November 2007.

    Put simply, there is no “vanilla VoIP” service defined for LTE at present, either with IMS or without it. In IMS, there is only IMS Multimedia Telephony that’s been standardised, and that is far from optimal and is really only advocated by Ericsson as a solution.

    Some form of circuit emulation (like the 3GPP work on CS voice over HSPA packet bearers) seems possible, but obviously would miss out on all the “cool” new services & functions that could be done with a new IP-based voice application.

    My conclusion is that operators ought to start playing around with various mVoIP services on HSPA/HSPA+ before they’re forced into it with LTE. This might involve working with SIP players & trying to sort out power management, it might involve IMS, it might be some straightforward NGN-type VoIP platform.

    Either way, some form of focus & experimentation is critical, as otherwise LTE will need to be supplemented by GSM or HSPA for circuit voice in perpetuity. (Which raises interesting questions of whether devices will have 2+ radios running simultaneously, as well)



  2. Martin,
    I would like to invite you to join the IMS Group on linkedin. It is a group of over 1,700 professionals from the industry, including service providers, vendors, system integrators, analysts, developers of 3rd party applications, marketeers and product managers. Also, if you would like, start this question to our group.
    To register follow the linked url IMS Group

    Please let me know if you have any questions

    Manuel Vexler
    Moderator,IMS Group and Chair Technical WG IMS Forum

Comments are closed.