Author: Martin

Most public Wi-Fi hotspots use no encryption and hence, communication is not very secure. Using a VPN as discussed here and here solves the issue but very few people are actually aware of the problem and willing to take such measures. So far I thought there is little that can be done from the network side as the WPA Pre-Shared Key (PSK) method is ineffective if everybody uses the same key (password) as network monitoring tools can decode the encrypted traffic if the key is known and the authentication and ciphering dialogue is captured. But then I remembered that the University of Vienna offers secure Wi-Fi Internet access so I checked out how they are doing it.

It turns out that they are using individual EAP password authentication from which a Wi-Fi ciphering key (WPA2, AES)  is then calculated. The username and password used in the Wi-Fi authentication process is the student's username and password for the campus network, stored at a central place for all sorts of purposes, including Wi-Fi authentication and encryption. As each student uses individual authentication credentials, monitoring the authentication dialogue will not yield the keys to decode the ciphered traffic later-on. A very elegant solution that just requires support in the Wi-Fi access point for WPA2 enterprise authentication. On the client side, support is already built into the operating system. It's quite clumsy to set-up with Windows XP but with Windows Vista, Windows 7, Linux and Mac OS the configuration is straight forward. It even works with Symbian and Android devices and the iPhone.

The only catch of this solution: The server certificate is not provided, that would have to be done offline, i.e. it's too complicated. That means that the device can't authenticate the network and hence a rouge access point could be used for a man in the middle attack.

Yes, bandwidth requirements are rising, especially when you have a big screen and lots of GHz available for things like high resolution video telephony. I use Skype video telephony quite often these days and when the other end also has a multi-megabit per second uplink available and a good camera, the video quality is just awesome and the stream easily exceeds one megabit per second in each direction. In other words, during a 60 minute video call, over 1 GB of data is exchanged.

Let's compare that to a mobile voice (only) call that uses a 12.2 kbit/s bearer for its codec over the air interface. 2 * 12.2 kbit/s * 60 seconds * 60 minutes / 8 bit = 11 MByte per hour. There's two orders of magnitude of difference here, i.e. a single high quality Skype video call uses the same bandwidth as 100 mobile voice calls! In fixed line networks, voice calls are usually transported in 64 kbit/s channels but the difference is still 1:20. And I imagine video telephony in full-HD resolution is not too far away anymore pushing the numbers even further.

Interesting result from the Dutch 2.6 GHz spectrum auction and one I have difficulties to interpret. Three incumbents and two newcomers have bid for the 2x 70 MHz of spectrum resulting in:

• one newcomer getting 2x 25 MHz
• the second newcomer getting 2x 20 MHz
• two incumbents each getting 2x 10 MHz
• one incumbent getting 2x 5 MHz

Lightreading's Michelle Donegan is the only one on the net I've seen so far writing a meaningful report about it and calling the stunningly low result of €2.6 million paid by the five “some loose change they [the network operators] found down the back of their car seats”.

According to Lightreading, a bandwidth cap was in place to prevent incumbents from bidding for all of the spectrum but I don't quite understand how much that was in practice. In any case the resulting spectrum the incumbents now have in 2.6 GHz seems very strange to me. Having 20 MHz is something you can build a fat carrier with and get speeds far beyond what is possible with HSPA today. But 10 MHz is an awfully small carrier for LTE in this band and I completely fail to see what you do with just 5 MHz!?

Also I haven't seen a country yet where 5 network operators have really made it over time. So instead of fighting it out over an auction, are some companies speculating with a merger down the road to get some fixed line or wireless assets and further spectrum in the 2.6 GHz band? Not sure if the auction rules allow for mergers later on but at least the money lost would be negligible in case the spectrum would have to be returned.

With the spectrum auctions currently ongoing in Germany these days and LTE being the hot topic a number of people have independently asked me recently when I think UMTS will be switched-off. A refreshing variant of the question when GSM will be switched-off. I find the question quite interesting and my answer is that I personally think that UMTS won't go away anytime soon. Having reached almost nationwide coverage in many countries, offering broadband speeds and continuing development ensuring competitiveness, the only reason I can see why to switch it off at some point is to save cost. But until it can be switched-off a number of things have to happen:

• LTE must reach a similar coverage as 3G networks today.

• Most mobile devices requiring a fast mobile and wireless Internet connection have to have LTE built in.

• A voice solution for LTE must be found as falling back to GSM (which is not switched-off either…) for voice calls is from my point of view not a viable option.

So when will those things have fallen into place? I seriously doubt that this will happen within the next 5 years. And once we get there, will there still be a need to switch 3G off or will multi-mode base stations that can generate GSM, UMTS and LTE signals just make it unnecessary?

I see a coexistence of GSM, UMTS and LTE for a very long time to come. So instead of working on phasing out UMTS, it might make more sense to work on solutions to integrate the different radio systems.

As always, comments are welcome!

One of the design principles of LTE was to streamline signaling as much as possible in order to simplify the system as much as possible and to execute procedures as quickly as possible. An interesting result of this is how messages of different protocol entities can be bundled into a single message that is sent over the air interface. Take the attach process as an example where the mobile device is ultimately assigned an IP address. In addition to reducing the number of steps required compared to GSM and UMTS, a single message is used to transmit the following towards the end of the procedure:

• An RRC Reconfiguration Message to establish a data channel (a DRB) for the user data;
• An Attach Accept message to tell the mobile that the attach was successful;
• An Activate Default Bearer Request message to tell the mobile to activate a logical bearer (for which a physical air interface bearer has just been configured with the RRC message above).

And all in one message on the air interface! In UMTS, those were all separate procedures with separate message exchanges. Pretty streamlined I would say! For details see 3GPP TS 23.401.

Self observation: It's interesting how even little things can have a big impact on usability and behavior. When I am in public places and use an unencrypted public Wi-Fi hotspot I want to be as secure as using a 3G connection with proper authentication and encryption enabled. So I use a VPN. However, I manually have to activate it and even though it's only 3 clicks I don't really like to do it. So I am sure if I had a 3G card inside my netbook instead of an external 3G USB dongle I would just not bother with the Wi-Fi and VPN and just get connected over 3G, despite a Wi-Fi hotspot being available. So if Wi-Fi is to become a way to offload traffic from the 3G macro network then a piece of software needs to be available that checks which options there are to connect, selects 3G or Wi-Fi without user interaction and in case of Wi-Fi automatically establishes a secure and encrypted tunnel. Without user interaction, though, that's the important point!

Here's a thought that I recently had when I looked at how the Hybrid Automatic Retransmission Request (HARQ) functionality works in HSPA and LTE: From a conceptual point of view HARQ is quite similar to the Acknowledgement mechanism of Wi-Fi. Here, the reception of each packet has to be confirmed by the receiver by returning a MAC Ack(nowledgement) frame back to the receiver. This is done in a way that the ACK package has precedence over any other packets that are waiting in the queues of other users of the system. If the ACK is not received, the sender automatically retransmits the packet with the same or a different modulation and coding scheme.

The HARQ mechanism of HSPA and LTE is pretty similar: Each transmission has to be immediately acknowledged on the MAC layer as well. If a NAK or nothing is received the transmission is repeated. When one goes into the details, of-course, there are fewer similarities. With HARQ, the system can use incremental redundancy to send a different version of the packet with different error detection and correction bits. In addition, several HARQ processes run concurrently so a transmission failure of a single packet does not stop the overall transmission. And then, HARQ uses an 'out of band' channel for the feedback, while the Wi-Fi Ack is a normal packet on the air interface.

For about one and a half weeks or so the German spectrum auction was not very exciting for external observers as little activity could be spotted. On Wednesday morning, though, things started to heat up.

Until that time, the total amount that was bid at this point was still below 400 million euros. Then suddenly, activity spiked and within a day the proceeds were almost up to one billion euros. The main activity for the moment is focused on the 800 MHz digital dividend band. By the end of Friday, the few MHz available there accounted for around €1.2 billion of the total €1.46 billion bid for everything so far.

While in other bands there is an ample amount of spectrum for everyone, only 30 MHz is available there, and consequently not enough space for the four contenders each wanting at least a 10 MHz chunk. So while a single 2x5MHz chunk is priced at over €200 million there at the moment, the same amount of spectrum can be had for 'as little' as €7 million in the 2.6 GHz band.

Every time I attend conference calls where people dial in from all over the world, my ears are usually suffering if there's more than one speaker. It's for various reasons:

• The volume different people have on the call is widely different. So while you have to listen very closely to understand some participants, others can be heard so loudly that your ear-drums almost pop out when the call goes from one extreme to the other.

• Most of these issues are caused not only because different telephone networks seem to interconnect on different volume levels. It's also because every participant has a different phone, some use the hands-free mode and, for international calls, some countries use a different voice codec that at some point is converted into the codec used in the country of the phone bridge.

• Add to that some echo when people are not muted, background noise such as babies crying, dogs barking and cars passing by and the perfect storm approaches.

• There are always people on a conference calls who are on the move and their mobile phones often try much too hard to filter out the background noise, resulting in shriek peaks and hard to understand participants.

• Automatic announcements that people are leaving and and re-joining the conference call due to patchy network coverage every couple of minutes doesn't make things much easier, either.

And on top of all of that put non-native speakers with sometimes heavy accents and after an hour your (or at least my) head starts spinning. So what's the solution to this?

I think it's wideband audio conference calls with heavy pre-processing of the individual call legs. As you can't get that over the standard telephone network, it must be Internet based, maybe, and that's already a big compromise, with telephone dial-in for those who for one reason or another can't access the Internet (on the move, stupid company firewall, etc.). I guess anyone who've once enjoyed the difference between wideband and narrowband speech knows what I am talking about. And on top of that the conference server or the clients should do some intelligent pre- or post-processing of the signal coming from the different participants. Is it really that hard to have everybody's voice arriving at the same level?

Anyone aware of such a system?