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Wednesday, May 20, 2009

LTE, mobile broadband & SMS - VoLGA vs. IMS

Just watched a presentation on the UMA-derived approach to voice over LTE, called VoLGA, which I wrote about a few months ago at launch.

As well as the discussion about voice, re-use of existing circuit cores vs. IMS and so on, there was a fascinating comment by the speaker (from T-Mobile) about the "under the hood" use of SMS in mobile broadband dongles.

It's used for a wide range of functions - most visibly in enabling texts to be sent by the user from the operator dashboard connection sooftware. But beyond that, it's used for things like roaming notifications, internal configuration of roaming lists and other settings, and assorted others. There are apparently "many more than 10" systems at T-Mobile that rely on SMS in the context of a mobile broadband computing service - and which would need to be changed if SMS was not supported easily.

This all represents a huge issue for early releases of LTE-based mobile broadband. Despite the argument by 3GPP that "IMS will eventually get deployed, and phones will take a while to develop, so voice on LTE can wait a while", it seems likely that SMS will be needed from Day 1 .

In any case, I completely disagree with the 3GPP representative that every LTE network will inevitably feature an IMS back-end. If it becomes almost mandatory (and some of the "hooks" in the radio and EPC bits of the standard are heading that way), it's a good way to ensure that it won't get deployed universally. Many operators are implacably set against IMS, as it still has huge limitations. (For example: would an MVNO on an LTE network need its own IMS, or be forced to use wholesales apps via the host operator's IMS?)

There was also a comment from 3GPP that LTE could use CS-fallback for SMS. How that would work in the context of a 3G dongle and PC I'm not sure - presumably this would mean that an incoming text (or system message) would force the connection to degrade. Fine for an occasional message - but not for a teenager sending & recieving 200 SMS's per day from their laptop. Can you imagine any other type of broadband link having to drop speed each time an email or IM arrived?

I don't think VoLGA is perfect - once again, it will need extra software in the device protocol stacks. But given the complete abdication by the vendor and standards bodies in sorting out voice/SMS over LTE in timely fashion (ie 3 years ago), it's currently the best workaround I've seen. The fact that it's not even been included in 3GPP's release 9 workplan is ludicrous.

[Sidenote for the "voice on LTE can wait" believers: pretty much every laptop user connected via LTE, especially where it's used a fixed broadband substitute, will want some sort of voice capability. Right now, the only options are Skype or various Internet SIP alternatives. Are you really happy to give them an open goal?]

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Antoine said...

All the needs fulfilled by SMS would be accomplished in a more flexible, secure and reliable way using provisionning mechanisms linked with the use of Diameter. The problem with IMS here is not the architecture itself, it is the fact that Diameter is not used to its full potential

Dean Bubley said...

That's not going to change quickly, especially for roaming scenarios.

In any case, PC-based SMS for the user (eg via the dashboard or another client) is currently about the only useful value-add that operators can provide on top of basic broadband access.

Anonymous said...

SMS can be delivered to the terminal using the Sgs interface without actually doing the CS fallback. I.e. the terminal remains in LTE access.
In a voice call the Sgs is used to page the subscriber and cause the terminal to move to a radio access technology supporting CS voice.

Anonymous said...


I would suggest to check what the guy above commented about SMS and Sgs interface.

If it is true, then it means that sending SMS over LTE is not a problem and your post should be updated.

Who is right?

Dean Bubley said...

I hadn't heard about the Sgs thing. Potentially very interesting if it works perfectly transparently.

It's notable that Stephen Hayes, the 3GPP Chair looking after LTE & who was at the event, didn't mention it. Clearly the T-Mobile speaker hadn't come across it either.

If anyone has a link to some documentation I'd be grateful - and also give me any pointers to what the other implications might happen at server or on the device.

Anonymous said...

SGs is part of the CS Fallback specs. It is only needed if CS Fallback is deployed and the Gs interface on which it was based, was specified back at the beginning of GPRS. Gs is rarely, if ever, used in the mobile networks and that may be the fate of SGs too.

The previous commenter is right that SMS would be sent without the handset actually switching off LTE. But this means CS Fallback requires changes to several other LTE control interfaces to tunnel SMS to the handset.

SGs is a new interface for CS Fallback on both the MSC and MME. SMS through SGs will require every MME to support SGs but more importantly, the MSCs in the network need to be upgraded to support SGs. I cannot imagine operators being happy with spending more money on their MSCs just to support SMS on LTE!

Anonymous said...

The previous speakers were correct. SMS over LTE can be done without IMS. The non-IMS solution is provided in Rel 8 and that is SMS over CS. The name is a misnomer as it uses the CS core network domain, but it is carried using LTE air interface signalling. There is no disruption of LTE services. There are roaming issues between SMS over IMS and SMS over CS due to operators only wanting to support infrastructure for their solution. 3GPP is looking into ways to solve this, but the basic solution exists and has existed since Rel 8.

Gareth said...

Hi Dean,

Good article, agree with the other 'posters' about SGs being an option as this is essentially the LTE evolution of Gs (i.e. enabling SMS (among other things) to be sent over the PS service to the terminal).

Also I agree that VoLGA is a good alternative to having to build an IMS core just to replicate voice and sms services. However VoLGA doesn't exclude the option of providing value add from IMS through combinational services such as GSMA's videoshare/imageshare or RCS.

I think one other very interesting point you raise is about MVNO's having to have their own IMS core. This could be viewed as a significant advantage for MVNO's who want to differentiate services, e.g. MVNO's targetting business customers, who want to provide an integrated SIP based call control in the enterprise and to the mobile device. Additionally with the significant improvements in latency provided by HSPA(+) this would enable deployments of these kinds prior to the availability of LTE devices.