Category: Uncategorized

A quick post today to make those of you who are interested in the finer details of LTE aware of an interesting post by Zahid Ghadialy over at the 3G and 4G Wireless blog on LTE conformance testing. Towards the end of the post, Zahid links to a couple of simulator test cases developed by 3GPP RAN5 and the GCF for which actual logs are attached. In other words, the theory you can learn from books is taken into practice there and you can see how the signaling messages look like between the mobile and the network and what their contents are. Great stuff, thanks Zahid!

23 auction days, 186 rounds and there is still no end in sight in the German Spectrum auction for 800, 1800, 2100 and 2600 GHz frequencies.

While for some time, there was hectic bidding for spectrum in the 800 MHz digital dividend band, things seem to be settled there for the moment as there hasn't been any movement there since day 16. If things remain there as they are right now, all four network operators will own spectrum for around €420 million per 2×5 MHz slot. Vodafone and T-Mobile each hold slots each while Telefonica/O2 and E-Plus
(KPN) each hold one slot. Out of the €3.2 billion currently in the pot, €2.5 billion are for the 800 MHz band. In other words, all the rest can be had for cheap.

Since bidding in the 800 MHz band has halted, things have been trickling along. Since day 16, the total sum has 'only' increased by €0.4 billion. The auction will only resume next Monday due to a public holiday in Germany on Thursday. So how much longer will it still drag out? I hope not too long anymore…

Ever since I read this book on how mobile networks have spread in Africa and discovered Eric Hersman's blog on technology and life in Africa in the process, I've been following his stories of what's going on there. In case you are planning a trip to Kenya and wonder how to best stay connected to the Internet, here's a recent post of Eric in which he describes how he keeps connected. Should be applicable to other countries in Africa as well. Makes for an interesting read and reminds me of the early days of GSM and GPRS in other parts of the world!

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.