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Tuesday, August 26, 2008

When is mobile VoIP not really mobile VoIP?

...when it's actually a circuit-switched call, that just uses an underlying packet bearer connection. So it's an IP "pipe", but the actual voice application is good old-fashioned telephony, without all the usual extra VoIP toys like presence.

I've talked before about CS voice over HSPA data bearers. I've also thrown cold water on the notion of using IMS Multimedia Telephony (MMTel) as the main long-term VoIPo3G / VoIPo4G service.

So there's an important question outstanding... if the long term trend is to move mobile towards LTE (and maybe mobile WiMAX).... how do we deal with voice? Will all phones and networks need to support GSM/UMTS in parallel, in perpetuity? It now seems clear that not all LTE networks will have an IMS core. Some will, but many will not.

My view is that there are three main options in the long term:

1) Some good (and ideally standardised) ways to implement non-IMS NGN VoIP applications over LTE. This doesn't need to be a dumb pipe approach, but could use assorted hooks for QoS, security and so on, if well-engineered.
2) Use LTE as a pipe, and put Skype's / Truphone's / Cisco's / BroadSoft's / whoever's VoIP over the top of it. Deal with issues like handover to other technologies in software in the servers and the handsets - perhaps under control of the operators, but perhaps not.
3) Develop some way to "tunnel" traditional circuit voice over LTE bearers, as we're already seeing with CS over HS.

Martin Sauter over at WirelessMoves has done some sterling work looking at this problem (he calls it the Voice Gap) recently. He's just posted on a proposal for Option 3, as well a related proposal for what's being called IMS Centralised Services.

The idea of re-using CS voice instead of a shiny new VoIP application is not new. Many fixed-NGN operators' VoIP services "look" like circuit-switched telephony to the end user. Many ordinary PSTN services have long transited "virtual circuits" in an IP-based transport network. UMA-based dual-mode VoWLAN does the same - the handset's telephony application remains unaware that it's being transported over WiFi.

Obviously, this is bad news for suppliers of VoIP application servers, whether IMS-based or not. They need to push hard to demonstrate the added benefits of "native" VoIP as an application, as otherwise they risk missing a big opportunity around LTE.


Anonymous said...

The standard solution to be used by operators in early LTE deployment will be CS-Fallback.


Why you are not considering it as an option?

Dean Bubley said...


In the original post I included the line "Will all phones and networks need to support GSM/UMTS in parallel, in perpetuity? "

As far as I can see, this is essentially what CS Fallback does. It forces voice through an LTE device's 2G or 3G radios instead. (Correct me if I'm misreading this).

Given that operators will want to start de-commissioning their GSM & UMTS networks at some point (especially if they want to refarm the spectrum for LTE), this is not a long-term solution. Short-term - yes, absolutely.

This will also impact devices that are running simultaneous voice and data applications, as either the data channel will need to be shunted back to 2G/3G as well, or it will need to be running both radios concurrently.

Or am I missing something here?


Anonymous said...

I agree it is a short-term solution, but I think we should define what is "short-term" in this scenario.

Unless we talk about green-field operators, an operator has already an existing 2G/3G network that just works. Until now, there has not been a business reason to deploy IMS for delivering voice only. In theory, it should reduce OPEX, in practice it just increases CAPEX and it needs to coexist with the existing CS world.

Also, people will own 2G/3G terminals for several years, which further postpones the migration to a full VoIP mobile network.

In my opinion, operators will start deploying LTE just to provide mobile broadband and will keep their existing CS mobile networks for voice calls.

As the operators extend the LTE coverage, then they will deploy IMS and let LTE users to perform calls using IMS, as the LTE standard says.

Eventually all users will perform voice calls using IMS, but in case of LTE, this requires full LTE coverage and full availability of LTE terminals. So, I believe the "short-term" solution will be in place for quite many years.

Dean Bubley said...


Yes, I certainly agree that 2G / 3G devices will be around for many years, and I agree that full migration to mobile VoIP will occur slowly.

Deploying LTE for mobile broadband will depend on spectrum availability as much as anything else. If operators only have 5MHz channels, they may push HSPA further for a few years instead.

I think that some (most?) LTE operators will not deploy full IMS. While I don't think that IMS is completely useless, its value is much diminished from initial expectations.

Most future value in mobile LTE networks will not come from IMS session-based services, but from other aspects such as the web, mashups, Telco2.0-style collaborative business models, portals like Ovi and iTunes and so forth.

If the LTE standard is so specific that it assumes "all users will perform voice calls using IMS" then it will need to be revised, as that is patently wrong.


Anonymous said...


On a technical point of view, CS is rather efficient when QoS and efficiency over air are concerned. On the 2 IMS infrastructures I deployed, the mobile part was plain old CS for voice and video. I guess no mobile operator would appreciate its customers complaining for not being reachable outside 3G coverage (which is still poor outside cities).

Another technical point is the support of IMS on mobile devices. Try to find RCS devices (and RCS has quite a limited feature set!).

Last point, which isn't really technical but psychological: mobile operator fear VoIP like the MPAA hates broadband networks! I guess they’ll need strong arguments to be convinced and open their networks to the “VoIP menace”… And as there isn’t any IMS killer app for the moment, the ones I know work on it as an alternative technology rather than a short term replacement.


Dean Bubley said...


I'm in complete agreement - I wrote a research report more than 2 years ago about the lack of IMS-capable devices, and there still aren't any that I would call fully-capable.

RCS is a start, but frankly I'm not impressed with that either.

One of the issues that is facing IMS and VoIP is that of battery life. Both VoIP itself, and especially always-on presence drains a huge amount of power.

If you have a 500 contacts, and expect to get real-time presence status updates from all of them(like PC IM clients), you'll essentially need the radio on (and not in "idle" mode) most of the time.