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Here's a question that's been on my mind for quite a number of years know which I've finally had the time to figure out: When a 3G or LTE Voice over IP user (think IMS One Voice or VOLGA) roams in a network in another country, how can the local resources be used?

The question might seem a bit strange at first but today, GPRS and 3G connections abroad are usually connected back to the GGSN (or the LTE PDN-GW) in the home network. This has a number of advantages. For the user, such a setup is good as no configuration changes are necessary to allow packet switched roaming. Also, the additional latency incurred is often not very high when the roaming and home networks are not too far away from each other or if the servers the user accesses are in his home country anyway. For the network operator, this home routing has the advantage that the connection remains under his control and he can collect billing information and meet the traffic policy as per the contract with the subscriber.

So far, so good. But for voice over ip, the issue is that a call to someone in the visited country first goes back to the home network and then has to come back to the visited network. If one roaming subscriber calls another roaming subscriber both in the same country, the speech packets travel backwards and forwards several times. Also, its difficult to establish local emergency calls this way as the voice infrastructure of the home country is used that knows nothing about emergency numbers in foreign countries.

For this reason, the GPRS standards that govern 2G, 3G and LTE have always included a sort of "local breakout" option with which the GGSN (or LTE PDN-GW) in the visited country can be used. This is good for network operator based VoIP as this allows the use of the visited network voice infrastructure. The disadvantage is, of course, that the home network operator has little to no control over the connection and can't do the billing for it.

So much for the foreplay. How does it work in practice. There are two interesting documents that describe the details. First, there is 3GPP TS 23.060 which describes the protocol theory and then there is the GSMA IR.33 GPRS Roaming Guidelines which describes how things can work in practice which is linked at the end of this post over at LinkedIn. In short, the two documents describe the process as follows:

The subscribers record in the HLR contains a VPLMN (Visited Network) Address Allowed flag in the GPRS permissions. If set to 'no', and that's the default that is used today, all GPRS PDP context activations have to go back to a home GGSN. This is done by appending the Operator ID to the APN sent by the mobile device. A subsequent DNS query with that extended APN then results in the IP address of the GGSN in the home network to which a connection is then established.

If the VPLMN Address Allowed flag is set to 'yes', a GPRS connection can remain in the visited network, i.e. a GGSN in the visited network can be used if (!) the Access Point Name specified by the mobile device is known in the visited network. In other words, there needs to be some sort of information exchange between home and visited network on the APNs that can be used locally. In this scenario, it's the Operator ID of the visited network operator and not that of the home network operator that is attached to the APN sent by the mobile device before the DNS query for the IP address of the GGSN is made. If the APN is unknown, as far as I understand the documents (and I'm not quite sure about this), a GGSN in the home network is established in case the APN is known there.

There we go, that's the process for a "local breakout" in the visited network.

Back in 2006, it was Nokia with its N80 phone that introduced Wi-Fi in a mobile phone to me. What is so  normal now was revolutionary and daring then and I wondered at the time if we would see software for such devices that turned the phone into a Wi-Fi access point. Pretty much unthinkable only five years ago as mobile devices and operating systems were pretty much closed to the outside world and Symbian was pretty much alone at the time (perhaps acompanied by Windows Mobile?) with multitasking capabilities for third party applications and a native program runtime environment.

In 2007, first things about Android were heard and I was once again speculating about the Wi-Fi chip being used as an access point. Here, my hopes were even higher as Android is even more open so more chances for third parties to do this, at least in theory.

Fast forward to 2010 and we now have Wi-Fi AP functionality in both Android and Symbian but not quite in the way I was imagining then. On Symbian it's a third party application while on Android, the functionality is directly built in by Google. That might have something to do with apps on Android running in a virtual machine which probably makes it a bit more difficult to get to lower layer functionality.

About two years ago, I reviewed the Nokia 5000, a sub 100 euro phone to see how well Opera Mini would run on such a comparatively cheap device. I came away quite impressed at the time but since then the world has moved on. Recently I noticed that the Samsung released the Android based Galaxy 550 which is currently available for 169 euros without a contract. Albeit it costs 70 to 80 euros more, it runs Android 2.1, has a touch screen, Wi-Fi, a memory card slot, a 2 Megapixel camera, FM radio, supports 3G HSDPA and has a GPS unit built in. Incredible at this price point and according to this test report (in German, sorry), the phone including the web browser works quite well. Give it another two years and I guess Android phones with this feature set will be in a price range of the Nokia 5000 I reviewed two years ago. In other words its going to get very tough as we move forward to sell non-smartphones for more than 30 to 40 euros in the foreseeable future.

When I still had a prepaid SIM in my mobile phone and had to pay for mobile data use by the megabyte I was quite heavily using my phones Wi-Fi at home so my e-mail, web browsing, podcast downloads, etc. would run over my “free” network while being at home. Great money saver. These days, I’ve switched to a contract that includes 1GB of data a month. It’s interesting to notice that, probably unsurprisingly, I only use Wi-Fi for exceptional things like download a couple of hundred megabytes of podcasts every now and then. The rest is all 2G/3G now.

I guess it has to do with convenience as it is still a bit of a hassle to switch applications from 3G to Wi-Fi at home. Symbian makes it quite easy by having connection profiles and priorities so applications always connect to the network with the highest priority. However, the application doesn’t change the bearer once it is connected so a manual restart of applications is still required with this scheme. If you pay by the megabyte, there is an incentive to do that, but if you have a huge bucket of data available at least I’ve stopped caring.

So why do I post about it? Well, I’m thinking ahead a bit and about Wi-Fi offload scenarios. I can see network operator Wi-Fi hotspot networks and Wi-Fi networks at home as interesting offload infrastructure that is already available today. But to make it work, the switchover has to be fully automatic and fully seamless. On the usability layer that means that the phone automatically switches the bearer without the applications knowing it. On the protocol layer it means that the IP address assigned to the device must not change so the switch is transparent to the application. In other words, a form of mobile-IP is required. And there is more than one option to choose from.

When home Wi-Fi networks are part of the equation, things have to get a bit more intelligent still, as a mobile device should then also be able to access devices in the home network. Once could do that with an additional local IP address that comes and goes when the home Wi-Fi is in range or a tunnel back to the home network. This would have the additional benefit that the user could access devices and data at home even while he is not under the umbrella of his own home Wi-Fi.

I think this should be standardized to bring together manufacturers and network operators. If every operator comes up with its own solutions and manufacturers to the same thing it’ll be ugly. And since connectivity is the bread and butter service of network operators, it’s them who should take this up and get it working. In the end it has two benefits for them, macro network offload and integrating services running in the home network.

Finally, it has become more apparent than ever now that this is also a good opportunity to make sure that communication over public Wi-Fi hotspots is secure, no matter whether a notebook or mobile device is used. For details see my recent post on FireSheep. And don’t think you are secure just because you access that popular social network side from a mobile phone with an app instead of via the browser on the notebook. Are you sure the app encrypts the data before it is sent?

In some countries there's fierce competition between network operators, and the media is anually reporting about the performance of the networks. To come up with meaningful values, they perform yearly drive tests on their own in cooperation with field test companies. Reported values are then for example call setup success rates, call setup times, call drop rates, average and hotspot data throughput in the networks, etc. When recently analyzing the results of such a test in more detail, it occured to me that it's likely to be a challenge to match call setup success rates that are today beyond 98% and call setup times of less than 7 seconds (mobilt to mobile) once there is one method or the other deployed for Voice over LTE.

The challenge will be to match all of those values, be it call setup success rates, call setup times and also the drop rate when the call is handed over (or not) to a 2G or 3G network at the LTE coverage border area. CS fallback for example is unlikely to match any of those values as it is undisputed that the fallback mechanism adds 2-3 seconds to the call establishment time on each side. Also, making the fallback a success in at least 97% of the cases is a challenge as the procedure either requires the UE to blindly drop to the 2G/3G network or a well configured LTE network with perfect neighbor relationships. In addition, good interworking configuration between the 2G/3G CS core network and the LTE core for the signaling exchange of incoming calls is required. Quite a challenge. VoLTE and VoLGA would probably do a bit better in this regard but I won't go into the details in this post, no need for a fight between the two sides today.

Think about it, what will happen if a network operator deployes a Voice over LTE solution and the values do not match those of the 2G/3G network today. It's going to be a bloodbath in the press. I can already imagine the headlines…

Over at Light Reading, Michelle Donegan reports that TeliaSonera over in Sweden who were early in starting their LTE network about a year ago have now gotten competition from Telenor, who has built an LTE network together with Tele2. From a consumer point of view it's great to see a second LTE network being fired up. I haven't heard of any other country in which two overlapping LTE networks are running and open to the public yet, so that's another first.

Personally, I am a bit sceptical about the network sharing part of the announcement because that reduces competition between network operators which in the long term might not only have positive consequences for the consumer. I've done some background reading and Sweden seems to be one of those countries where this is already done for 3G as well. Interestingly, the constelation is a bit different here. While Tele2 shares the LTE network with Telenor, it's 3G network is a shared operation with TeliaSonera (click on Sweden for details). Interesting triangle. 

And on top, Telenor seems to share it's 3G network with Hutch 3G according to Wikipedia. Is your head spinning? Mine is, so here's a summary:

• TeliaSonera — LTE
• Tele2 + Telenor — LTE
• Tele2 + TeliaSonera — 3G
• Telenor + H3G — 3G

Pretty much everyone with everyone, except for Hutch 3G who doesn't yet seem to have ties with TeliaSonera.

 

Yes, app stores is where most people get their applications for their smartphones today. While this is convenient, simple and offers a way to earn money I'd argue it's not the 'natural' state of things. I'd never buy a PC, notebook, netbook or any other device where I can only download programs from a single store that is controled by a single entity with its own agenda. So why should I limit myself to that for a mobile device (think smartphone, tablet, etc.)?

This became quite apparent from a technial point of view to me when I recently tried to download something from an app store and couldn't due to some strange error that persisted for quiet a while. Well, at least I could download my most favorite apps (web browser and email client) from the web pages of the developers and thus bypass the store and the problem. A single point of failure, always a bad thing.

Recently I was in a hotel that had dedicated indoor cellular 3G coverage on the lower floors. Dedicated in this case meant that one mobile network operator installed indoor coverage with antennas distributed throughout the hotel. So I would say that most foreign visitors sooner or later landed on just that network as the other networks had only weak or no coverage at all. And despite roaming prices having come down in recent years I think calls from foreigners are still highly lucrative. So I wonder who paid whom for the indoor coverage? Did the hotel pay the network operator so guests have reception in all places where outdoor coverage is weak or is the mobile network operator paying the hotel for the opportunity to get (more or less) exclusive access to foreign visitors? I'd speculate that it's the later but that's just a guess on my side. In any case, good cooperation from both sides to make the customer happy.

One of the biggest assets mobile network operators have is the ability to handover voice calls seamlessly between different radio access technologies. If the calls starts in a UMTS network and the user leaves the coverage area of the 3G network, the call is handed over to the GSM network without the user noticing it. The reverse direction from 2G to 3G is also standardized but I have yet to observe any network actually doing it during an ongoing call. It could actually be quite useful so background data transfers such as email reception, facebook updates, etc. can continue during a voice call, something that is usually not possible unless the network supports GSM DTM (Dual Transfer Mode).

Anyway, things get a bit complicated in the LTE domain. First, there is no single Voice over LTE solution clearly dominating today, probably due to the lack of networks and devices. But let's say, for arguments sake, that we'll get there one day. How do you hand over a Voice over IP call to GSM once the user runs out of the LTE coverage area. The solution is Single Radio Voice Call Continuity, or SR-VCC for short, which has been specified some time ago in 3GPP TS 23.216. The procedure is voice technology agnostic so it can be used by IMS, VoLTE (an IMS implementation) and also VoLGA.

But once you are there, how do you get back to LTE or UTMS during the call? Well, not at all so far. However, there's a study item currently worked on in 3GPP Release 10 in TR 23.885 that gives a number of options of how reverse SR-VCC could be implemented. How interesting! I'm tempted to read it but I guess I'll just hold off until I see the first (forward) SR-VCC implementation used in a real the network. Have fun putting it into a TS until then 🙂

Have you ever tried the following: While sitting in a train, talk to someone standing outside through the window over a mobile phone.

It's interesting to experience the voice delay which isn't very big, I would say in the order of 250 milliseconds, but quite 'visible' as the lip movement of the person outside and the speech path are not quite synchronous anymore. In a mobile phone call where you don't see the other party this delay usually goes fully unnoticed unless you call someone on the other side of the planet and the delay becomes so long that there are odd silence periods and people start talking simultaneously.

The second interesting effect that can be observed is noise supression. As you know exactly how the background noise sounds outside, its interesting to hear if and how well the mobile outside surpresses the background noise and what effect that has to the talker's voice.