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Issue with vertical handover 3G - 4G while an app is running

asked 2014-09-05 14:44:11 +0300

Laurent S. gravatar image

It looks like my Jolla device is not able to upgrade from 3G to 4G when an active connection (e.g. actual data traffic, not just paging) is ongoing.

if the application starts under 4G coverage, everything is fine and vertical handover downgrade from 4G to 3G occurs when the device leaves the 4G coverage area.

However, when the device enters 4G coverage again, the vertical handover upgrade from 3G to 4G does not occur.

To achieve it, I must stop the application, let the Jolla device discover the 4G coverage, and start the application again.

I am wondering whether this is the result of a specific setting of the cellular operator (for instance giving priority to the open 3G radio bearer over the opening of a 4G bearer) or an issue with Jolla.

Thanks for your insight, LS

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answered 2014-09-06 18:54:48 +0300

attah gravatar image

updated 2014-09-08 17:06:53 +0300


I am a telecom engineer, let me try to explain what you are experiencing.

It is likely to be the case that your network does not support LTE<->UMTS handover (not many do, if any), so it is using an older mechanism called Release With Redirect. From LTE this can happen for two reasons: due to an incoming or outgoing CS (non-voip) call (referred to as CS Fallback) or due to poor coverage. This is quite similar to establishing a new connection from disconnected in UMTS, only that some information about where to look was included in the release message. (Thus Release With Redirect).

There are several triggers for going (back) from UMTS to LTE, and the procedure is similar, you receive a Release With Redirect. But all these triggers (afaik) include a criteria of having seen a certain amount of time where the data connection has been idle. Now that is why you need to close the app, it is probably using the data connection more or less constantly. This is done in an QoS effort to keep the number of interruptions to a minimum. (Re-estabishing does take a few seconds)

Edit: Just to be extra clear: these decisions are taken by the RNC, and the phone has no say in it. (although the decisions are of course based on input from the phone, such as measurement reports and activity)

Hope this makes sense!

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Great info:)

richardski ( 2014-09-07 23:52:19 +0300 )edit

Indeed. Thx a lot for sharing this insight. LS

Laurent S. ( 2014-09-08 10:02:44 +0300 )edit

Hi @attah, thank you for the explanation. I have one additional question to this topic if I may: Is this the reason why the phone switches from 4G to 2G upon an incoming call, cutting all data connections? IMO it would make more sense to switch back to 3G as this can handle data and voice calls in parallel.

Venty ( 2014-09-08 13:46:31 +0300 )edit

Hi @Venty! Yes, that is the same mechanism. Indeed it would! Unfortunately i cannot answer why your operator has set it up like that. I did a quick check in 36.331 (LTE RRC spec.) to see if the release message could contain multiple RATs and if they are prioritized. But it seems there can only be one target RAT present in the message. So I guess we can only blame the operator's configuration. Perhaps the coverage areas map better..?

RAT = radio access technology

attah ( 2014-09-08 14:47:15 +0300 )edit

the switch from 4G to 2G for voice is motivated by the fact that the operator wants to keep 3G for data (many many devices connect to 3G by default) and use 2G for voice, making use of a network that for data is (almost) unusable but for voice quality is (basically) as good as 3G.. and 2G infrastructure has already been built and paid ;)

c.la ( 2015-04-27 08:54:42 +0300 )edit
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Asked: 2014-09-05 14:44:11 +0300

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Last updated: Sep 08 '14