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While aggregating two LTE carriers in Europe exists but is still not very widespread due to the availability of 20 MHz single carriers there is talk in the industry about aggregating 3 carriers. There is still some room in the specs for the moment as the maximum number of aggregated carriers that can be accommodated so far is 5. But eventually, carriers might want to go beyond that as well so 3GPP is gearing up to work on a solution to eventually combine up to 32 carriers.

The work item description can be found in document RP-142286 presented not long ago in TSG RAN#66 in December 2014. At first I thought extending CA beyond 5 carriers might be straight forward by introducing a couple of additional information elements and extensions. But that's a bit too short sighted as the current solution puts all uplink transmissions including channel feedback on the primary cell (i.e. the primary carrier). So as more and more devices become carrier aggregation capable there's more and more uplink traffic in the PCell which increases as more and more carriers are aggregated. Therefore the model does not scale well and uplink traffic and feedback at some point needs to be distributed over several carriers if more and more of them are combined.

From an overall conceptual point if find an aggregation of up to 32 carriers quite interesting. Before the aggregation of 2 carriers made it into chipsets many people were saying that this is going to be difficult as it would increase complexity and hardware cost significantly. Fast forward to 2015 and the aggregation of 2 carriers is in the wild and built into many devices. Obviously the aggregation 32 carriers is yet again another beast. And then again who would have predicted just two years ago that we would see mass market devices that support 20 LTE bands?

One of the features I was dearly missing when I finally switched my main device from a Symbian phone to an Android device was that I had to give up the feature to put the device into silent mode but still make it ring for selected contacts. This was and still is an important feature for me, especially when traveling to other timezones and people being unaware that they are calling while it's night in my current time zone and I'm sleeping. So for the last two years I fixed this with a kludge, i.e. taking an extra phone and SIM only known to few contacts. Now I discovered that the feature was introduced on Android and the iPhone at some point between then and now.

On my Android 4.4.4 based CyanogenMod Samsung Galaxy S4 which I've been using for more than half a year now, the sound menu now contains a menu item called "Quiet Hours". When selecting the item it's not only possible to set the quiet hours but also further things such as which events shall not be indicated to the user and the "Phone Ringer" menu item which can be set to "Ring for Favorite Contacts" (only). Works like a charm!

O.k. so this is a pretty much stock Android OS, how about customized variants? I had a quick look on a current high end Samsung device with Android L and while it looks slightly different the option is there as well. Also I could find a similar option on the iPhone.

Great stuff and nobody told me… 😉 Must have been in the software for quite some time now.

Speaking of the 1 ms 5G latency myth in my previous post on the topic let's have a look at what round trip times are to servers on the Internet today over different access networks – with surprising results!

(V)DSL: When I ping a sever pretty close to my location and within the network of my access provider I get ping round trip delay times over my VDSL connection of around 24 milliseconds. Since that's what I have at home it's my reference and quite far away from the mythical 1 millisecond 5G latency. About 3-5 ms are spent on the Wi-Fi link, the rest are a result from delays in the fixed access network. Delay after that to the server is minimal, in the order of a millisecond or two.

UMTS: To the same server my round trip delay times are in the order of 100 milliseconds, so quite a bit more.

LTE: Here, I get round trip times of around 45 milliseconds to that server, so quite a bit of improvement over the UMTS network

Fiber: In Paris I have a Fiber to the Home (FTTH) GPON Link. From a server connected over Ethernet to a router which then connects to a fiber/Ethernet converter I get round trip times to a server close to the edge of that network in the order of 3-4 milliseconds. That is quite a bit closer to the mythical 1 millisecond 5G delay time already. I then pinged a server around 600 km away in Germany and got round trip times of 15 milliseconds. Out of those, 6 milliseconds is due to the physical delay of the light in the fibers, the rest is processing delay.

There we go, I was quite surprised about the phenomenal delay performance of the fiber connection, it's not far away from the physical limits. The question now is if and how this efficiency can be ported to wireless as well, when even VDSL has far more delay, even though it's a fixed line technology.

Here I am, over the clouds again and an interesting aspect of flying in the US is that they have Internet access on board on many of their flights. Here's how it worked for me while putting together this blog post:

On Delta, Internet over the clouds is provided by GoGoAir and I was getting download speeds between 1 and 3 Mbit/s with round trip times of around 90 ms without my VPN. With an OpenVPN tunnel to my gateway in Europe I got round trip delay times of around 260 ms, quite a good value as well. In the uplink direction I got around half a megabit per second out of the connection. Over the hour I used the system it was quite stable but there were temporary outages of 15-20 seconds every now and then and occasional long round trip times of several seconds while data only trickled in. Not sure why these things happen, cell edge or handover problems perhaps?

Wikipedia says that the system uses 160 ground base stations distributed over the continent and 'classic' EvDo 3G connectivity between the plane and the ground. That would be consistent with the speeds I've experienced but it could of course always be that traffic shaping is applied on a device basis and overall speeds could have been higher.

Web browsing felt snappy and just for the fun of it I dropped my VPN tunnel for a little while to see if Gogo still forges Google certificates for Youtube. It looks like the bad press around the issue has made them think about it again and I couldn't observe rogue certificates for Youtube anymore.

Today a 3G link to the ground might still be sufficient but with rising data traffic the system needs to be upgraded to a faster technology in the future. Let's see if ground based LTE will be the technology of choice for planes flying over ground rather than satellites which are the only choice over oceans for obvious reasons. Personally I'd prefer ground based communication, as using satellites in geostationary orbit results in very long round tip delay times.

In a comment to a previous blog post on new mobiles now supporting GPRS to LTE Reselection during ongoing data transfers there was a comment that this was a network and not a device feature. The answer to this is quite interesting so I decided to make a post out of the response rather than just post an answer in the comments section.

It's in the nature of 3GPP to have several options for a feature and this one is no exception. Reselection from GPRS to LTE during data transfer is optional, it can be implemented as a device only feature or the device can signal to the network that it is capable to make LTE measurements during an ongoing GPRS data transfer and let the network decide what to do. Which of these options are supported are sent to the network during the GPRS attach process: 

Message:  ATTACH REQUEST
Information Element: GERAN to E-UTRA support in GERAN packet transfer mode

Possible Values:

• 0 0 (0): None
• 0 1 (1): E-UTRAN Neighbour Cell measurements and MS autonomous cell reselection to E-UTRAN supported
• 1 0 (2): CCN towards E-UTRAN, E-UTRAN Neighbour Cell measurement reporting and Network controlled cell reselection to E-UTRAN supported in addition to capabilities indicated by '01'
• 1 1 (3): PS Handover to E-UTRAN supported in addition to capabilities indicated by '01' and '10'

I've checked a number of recent mobiles and all of them either don't support the feature at all or support the autonomous cell reselection option without involvement of the network (like it is the case for GSM to UMTS reselection today as well).

That doesn't mean there are no networks and mobiles that support a network variant but I wonder if network operators are really interested in the feature when the autonomous variant works quite well!?

For details see 3GPP TS 24.008, Table 10.5.146 and search for "GERAN to E-UTRA support in GERAN packet transfer mode".

Two years ago I estimated that my SSD would last around 30 years based on the number of re-write cycles and the theory laid out in an epic Anandtech article on the topic. That was the theory. Now we have real numbers based on a practical endurance test of several SSD drives made by TechReport.

According to the report, the Samsung SSD could take at least 100 TB before even first hints of wear could be detected. It still kept working for twice the amount of data written to it before it eventually stopped working. So based the 5 GB of data that I write to my SSD per day (have a look at the first link above of how I get to this value) my SSD would last me for 54 years based on the 100 TB value.

I should mention though, perhaps, that I already replaced the first SSD I bought after about a year and a half because it was full and I had to upgrade to a 1 TB model… 30 years, 54 years, it's all a bit academical with the amount of data I keep accumulating resulting in drive swaps to increase capacity…

Just around this time last year I wrote about 3G Roaming in the US with a Local SIM card that could be ordered from abroad before starting the trip. While it worked well, the main drawbacks were finding a mobile that would work on US frequency bands and the 'limitation' to UMTS. Also, the network kept dropping my VPN connection at random intervals. A year later, networks and offers have significantly advanced.

This time, I bought a T-Mobile US Prepaid SIM for Internet connectivity after arrival that would not only let me use their UMTS but also their LTE network. The cost for the SIM card was $15 and options that can be selected online range from $5 per day for 500 MB over $30 for 3GB for 30 days to $50 for 7 GB for 30 days. Not cheap but 'business traveler' affordable. Also, the SIM card is kept active for up to 365 days which is great if some time passes between trips to the US.

Speed wise I could easily reach data rates of 10-15 Mbit/s in downlink and 8 Mbit/s in uplink while my tethering device was camped on LTE band 4 (1700/2100 MHz) on a 10 MHz carrier at the hotel I stayed in Kansas City. I also noticed that another 5 MHz LTE carrier was on-air on band 2 (1900 MHz PCS). Reliability wise the network has also made a great step forward as I didn't notice a single VPN drop over the days.

Another thing that has significantly improved since last year is the availability of mobile devices sold in the EU that support some of the US LTE frequency bands. The iPhone supports a phenomenal 20 LTE bands and other devices, e.g. some from Sony, include support for LTE band 4 that is used by T-Mobile US and others. Here's an example from their German web presence. So if you travel to the US it's worth finding out which LTE bands are supported before you buy it.

All in all, the SIM has served me well and offers like this are another step in the right direction towards global affordable and fast Internet access.

Recently the FCC made it crystal clear that Deassociation Attacks by hotel Wi-Fi installations to force their 'guests' using the hotel's Wi-Fi instead of tethering their equipment to their smartphones and tablets is illegal. That only applies to the US, of course, and despite it being a very effective move to aggravate customers it doesn't mean nobody else will be trying to use it in the future. But it turns out, there's an effective countermeasure against this, at least in the foreseeable future.

The attack vector used by such Wi-Fi installations is to send De-association management frames to devices connected to a hotspot other than that of a local venue. Unlike data frames which are encrypted and thus can't be forged, Wi-Fi management frames are sent in the clear and can thus be sent by anyone. To mitigate rogue de-associations and other attack vectors the 802.11w amendment to the Wi-Fi standard describes a way to also protect management frames which effectively counters such attacks.

There are many amendments to the Wi-Fi standards that have never been implemented and for a long time it looked like this was yet another one. But since July 2014 the Wi-Fi Association requires implementation of the protected management frames amendment in its Wi-Fi certification scheme when a device supports 802.11ac, the latest super high speed transmission mode as reported here, here and here. That's good news as this certification is required for the Wi-Fi logo on the sales packaging and as a precondition by many companies (such as mobile network operators) to sell a Wi-Fi capable device. Also, a growing number of access points and devices such as notebooks, smartphones and tablets support 802.11ac today and even more will do so in the future.

I ran a quick trace of all access points in the neighborhood but didn't find any indication of the feature being supported in their beacon frames. As described here in detail and shown in the screenshot on the left there are two bits towards the end of the beacon frame that indicate to devices whether PMF (protected management frames) are supported or not. These indicator bits are also sent by devices during connection establishment so it's easy to find out if a device supports PMF. One source claimed that the Samsung S5 already supported it but when I traced the connection establishment both bits were set to 0. So at least my S5 does not or doesn't want to indicate this capability to an access point that itself doesn't support it. The result is perhaps a bit disappointing but not really surprising as the new rule just came into effect half a year ago. So I will have to get hold of devices certified after that date. I'll keep you posted.

The article linked above remarks that PMF counters all management frame attacks observed so far. One thing it can't protect against are attacks with Ready To Send / Clear To Send frames that include long reservation times for transmissions (up to 32 milliseconds). The good thing is, however, that such frames are not network specific and thus would not only slow down an attacked network but also the hotel Wi-Fi itself on the same channel. In other words this is no caveat for hotels that have a special treat for their customers…

It's standard practice for mobiles for many years now to implement GPRS to 3G reselection during data transfers. It might sound like a small and unimportant feature but in mobility scenarios it's very important when a subscriber losses 3G coverage during a data transfer and drops down to 2G. Without this feature the device would be stuck on the very slow 2G layer until the data transfer has finished. Quite to my surprise the same feature for LTE has only become available in mobile devices in the last year or so. My Galaxy S4 can still get stuck on 2G during data transfers or when I use it for tethering on the train and kind of 'rescues itself' to 3G rather than going back to LTE when both networks become available again. But on newer devices I have noticed that they now have the ability to also search for LTE during 2G transmission gaps and go back to LTE. That makes a real differences especially in areas where only 2G and LTE is available. In Germany that is the case in a lot of rural areas. A small feature but a big benefit to the traveling user. What's still outstanding, however, is 3G to LTE redirection/handover.

While it very rarely happens where I tend to go but still every now and then there are roads and paths I take that are not in Openstreetmap. To my pleasure I recently discovered how easy it is to add them to the OSM database with the Openstreetmap for Android (OSM) GPX plugin.

The pictures below show how it's done. First, activate "Record your trips" in the Osmand 'Plugins' settings. Afterward a "GPX" button is shown in the map on the right hand side that can be tapped-on. Once tapped-on, the path that is recorded is shown in turquoise color on the map and the GPX button is replaced by the distance to the location where the GPX recording was started. To stop the recording, a tap on the distance indicator brings up a pop-up menu from which "save" can be selected (the third option in the pop-up menu, don't mix it up with 'stop' which doesn't save anything). The resulting GPX file is put into the Osmand folder in internal storage or on the SD card.

Once the GPX file is on a PC it can simply be dropped on the Openstreetmap web site after 'editing' mode has been activated. As can be seen on the last screenshot the my GPX track is exactly on the railway track of the train I was sitting in when I took the GPX recording.