Author: Martin

I-WLAN for IMS access over Wifi

I’ve taken a look at IMS lately and ways to access the IMS from non 3GPP networks such as Wifi hotspots and Wifi at home. Looks like 3GPP TS 23.234 and TS 33.234 contains everything required for the purpose. The first major building block of I-WLAN (Wireless Local Area Network interworking) is how the subscriber database of a 2G/3G network can be used to authenticate Wifi users that have a device with built in GSM/UMTS SIM card. For this purpose EAP/AKA or EAP/SIM is used. For EAP-SIM I’ve written a blog entry some time ago. The standard also foresees methods for the access point to deliver billing information to the 3GPP network.

What I didn’t realize at that time was that the second building block in those two documents is a method to establish an IPSec encryption tunnel between a mobile device and a gateway between an external network (e.g. the Internet) and the 3GPP core network which hosts an IMS. This gateway is called the Packet Data Gateway (PDG). The standard even says that the IPsec tunnel setup can be used without the above mentioned EAP-SIM authentication step. That’s good news as the EAP-SIM authentication requires support of the Wifi Access point while the tunnel establishment is transparent to the Wifi access point.

So let’s see maybe we’ll see 3G+/Wifi IMS devices with the ability to establish an IPSec tunnel over Wifi to the IMS of their wireless operator. Great stuff for mobile operators with DSL assets.

Nokia and Finland – The Land of the Mobile Millionaires

While driving back home from a weekend trip I listened to this very interesting BBC world service podcast on Nokia, Finland and the land of the mobile millionaires (of course on a Nokia mobile phone…). A very entertaining 22 minutes story about a BBC reporter’s trip to Finland to discover how Nokia has shaped the people, the country and how they have shaped Nokia. So if you are interested in Nokia, rubber boots, Finland, mobiles or a combination thereof, it’s worth to listen in.

UMTS pushes into the “Deep-Ruralness” of Austria

It’s been a year since I was last in what I would call a ‘deep rural’ spot in Austria in a little town in the region of Oberösterreich with around 2500 people living there and nothing but countryside and cows around. In the past months there have been press reports by a number of Austrian UMTS operators that they will now also start covering the Austrian countryside with UMTS. I didn’t give it much thought at the time as words in press statements usually have to be taken with a grain of salt.

Looks like, however, words have been followed by action. This time I was surprised when I saw UMTS networks of A1 and ‘3’ on my network selection dialog. Most excellent since A1/Mobilkom is the roaming partner of Vodafone Germany so I can use their Websessions here! A quick test revealed data rates of 1.2 MBit/s in downlink. Far below the capabilities of HSDPA but likely due to a limited backhaul. Never mind, it will do nicely.

First Signs of HSPA+ in the Standards

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It looks like 3GPP member companies are moving quickly to standardize data rate enhancements for current 3.5G UMTS / HSPA networks as promised in diverse marketing slides floating around at conferences. The two main ideas to further increase transmission speeds are to use a higher modulation scheme (64QAM) and MIMO (Multiple Input Multiple Output) to send several data streams via different spacial paths simultaneously.

3GPP TS 25.306-800 has an interesting table (table 5.1a) that specifies current and future HSPA terminal categories for downlink. Today, mobile devices typically use on of the following terminal categories:

  • Category 12: Speeds up to 1.8 MBit/s (first HSDPA cards for notebooks)
  • Category 6: Speeds up to 3.6 MBit/s (most HSDPA cards sold for notebooks in 2007 and pretty much all mobile phones supporting HSDPA)
  • Category 7/8: Speeds up to 7.2 MBit/s (the latest HSDPA cards for notebooks)

I’ve tried all three types of HSDPA cards in practice and could reach around 1.4 MBit/s with a Cat-12 device, around 2.5 MBit/s with a Cat-6 device and around 4.2 MBit/s with a Cat-7/8 device.

For HSPA+, Release 8 of the 3GPP standards now introduce terminal categories 13 to 18. In Category 18, a terminal can receive up to 27952 Bits per 2ms TTI per MIMO channel (I assume). That would result in a speed of around 28 MBit/s. Interesting to see that in 2×2 MIMO mode, only 16QAM and not 64QAM is used. Category 14 terminals can receive 42.192 bits per 2ms TTI in non-MIMO mode. That’s still an impressive 21 MBit/s, even without MIMO.

In order not to get too excited about these numbers have a close look at the physical realities of a radio channel in the real world. Even though specified on paper such speeds are only achievable under the very best of radio conditions with little to no interference from neighboring cells. Having said that I nevertheless believe that we will see nice speed enhancements in practice. I am looking forward to see just how much!

More Uses For Curbside VDSL cabinets

A couple of days ago I have rumbled a bit on the big street side cabinets that need to be put into place for VDSL every couple of hundred meters. Recently I have received an interesting eMail from one of you saying that they might just be perfect to put Wifi hotspots inside. Hm, what an interesting idea! At the size of those cabinets there should be some space left for a little access point. And backhaul is definitely not a problem with those fiber cables in the ground.

Well, why not put an HSDPA or, later on, an LTE microcell into these cabinets to increase cellular capacity in high traffic areas!? Again, no backhaul and power problem, as both are available in the cabinet. As microcells only use little transmit power and only cover small areas a little omni-directional antenna discretely put on a nearby pole or wall will do. Interesting possibilites for converged fixed/wireless network operators of the future!

Some Practical 802.11n Thoughts

A number of different sources have brought some interesting details to the light of day for me concerning 802.11n products. In the recent edition of the German computer magazine C’t (23/2007) Dusan Zivadinovic published an interesting article on the performance of 802.11n access points. Here are some interesting thoughts:

  • Some access points have a channel auto configuration option. While in general this is a good feature it will always be suboptimal because the access point can just select a channel with low noise from his point of view but can not take the situation of a client into account. For a client the selection of the access point could be less than optimal since it could receive interference from a network which is not visible at the access point.
  • 5GHz band radar avoidance: To be done once every 24 hours for 60 seconds. The access point then stops transmitting and monitors the selected frequency band for spurious signal peaks.
  • Some WLAN routers like those from LANCOM (quite expensive APs) check for general noise (e.g. from other networks) and change the channel automatically.
  • In Europe, the maximum transmission power in the 2.4 GHz band is 100mW. In the 5 GHz band up to 200 mW is allowed which can compensate for the higher absorption rate of signals in this higher frequency band. Dusan notes that in practice he didn’t observe much of a range difference between usage of the 2.4 GHz and 5 GHz band.
  • For distant devices it’s better when the access point falls back to a 20 MHz channel. Makes sense from my point of view because the signal energy can then be concentrated in a narrower band which should increase range.

  • Devices that can be operated in the 2.4 GHz band and 5 GHz band simultaneously must use different sets of antennas for each band. Interesting, I thought, so how does the simultaneous dual band capable Linksys WRT600N do this with 3 external antennas? A bit of research revealed that the WRT600N has 4 external antennas and 2 internal antennas, so 6 in total. For each band 3 antennas are used. Here’s a link to a picture that shows the cables leading to all antennas.

For more on 802.11n on this blog click on the ‘802.11n’ tag below.

Rebtel – Have The Local Numbers Follow You

Since the EU has stepped in prices for mobile phone calls to and from the home country of a subscriber while traveling have fallen to (almost) usable levels. A nice move for people like me who travel a lot. But prices for calling from one country to another are still very high, even with a local SIM card. The same is true when you want to call a friend in another country (who has a SIM card of that country) while you are mobile. This is where Rebtel comes in.

Rebtel’s idea is based on the fact that national mobile calls are usually a lot cheaper than international calls.  So instead of calling an international number you create a ‘national’ number in Rebtel which then forwards your call to the person in the other country. An example: You are in Germany, have a prepaid SIM card and want to call somebody in Austria. If you do it directly, this will cost you around 1.5-2 Euros a minute. The alternative is Rebtel. Once registered you can use their web page or their mobile site to create a local number for your friend abroad. Calling that number costs around 5-15 cents in countries where competition among mobile operators is healthy. Rebtel then charges around 2 cents to connect to a landline or around 14 cents (VAT included) to a mobile number.

The method also works the other way round. You can give your friend a local Austrian number he/she can call for little money which then connects to your mobile in the other country. Rebtel charges you for the incoming mobile call (again around 14 cents a minute, depending on the country of course).

And the best is that the local numbers follow you! Rebtel allows you to configure up to 5 different mobile phone numbers. When you go to another country, say Italy, and you have a local SIM card you can switch to that SIM card in Rebtel. All contacts you have created will then automatically get an Italian local number. In case you use the local SIM card for Internet access things are very easy since the contacts on the mobile web page which you can access from the mobile phone are automatically updated to the Italian numbers.

I’ve given it a try over the last couple of weeks and have to say that I am very happy with the service. It works most of the time (o.k. it’s still beta) and voice quality is usually excellent. The only negative thing I can really report is that a connection is established even if the other end is not available or does not pick up. Consequently you always have to pay the local mobile operator for the first minute. This aside, however, Rebtel has saved me tons of money already.

Sony-Ericsson launches Tri-Band HSDPA phone

The Sony-Ericsson press announcement for the K850i is already back from July 2007 but contains a revolutionary piece of information most haven’t yet noticed:

"[…] The Sony Ericsson K850 Cyber-shot™ phone is a Tri-Band HSDPA and Quad-Band  GPRS/EDGE phone […]"

Tri-Band HSDPA! This is great news since most current HSDPA phones are either only usable in the 2100 MHz band in Europe and Asia or in the 1900 MHz band in the U.S. Few if any 850 MHz mobile phones (e.g. for Australia and the U.S.) have yet been spotted. These two phones will change the game. One model produced for all parts of the world.

It’s not only great for economies of scale but also for users as it allows global High Speed Internet roaming, which is not possible today with single band HSDPA phones. People coming to Europe will be able to use their phones for high speed Internet access with prepaid SIM cards for Internet access which are available in many countries. Still waiting for the first brave operator in the U.S. doing the same thing.

P.S.: I know HSDPA data cards are also dual or tripple band today but I dream of a single device doing everything for me (and that’s certainly not a data card).

Devicescape Makes Use of Wifi Hotspots Easier With Mobile Devices

Jukka over at the Nokia Web Server blog made me aware of an interesting new service from Devicescape which will be bundled with Nokia Nseries and Eseries phones that makes using Wifi hotspots easier in the future.

You probably know the hassle. You sit down at the airport but before you can use the local Wifi network a user name and password has to be entered on a web page to get access to the Internet. While inconvenient with a notebook it’s still manageable since you have a keyboard available. With a mobile device however, typing in user names and passwords on a web page becomes a real pain.

Devicescape has an elegant solution for this. A little program, that has to be installed on the phone, checks all Wifi networks and automatically logs into those it knows. Usernames and passwords are stored on a centralized server so the user never has to type them in on the mobile device. When accessing the network from a Wifi hotspot they are downloaded automatically and entered into the web page by the program.

I was a bit puzzled at first. How can they store passwords remotely? Usually, access to the Internet is only possible once one the username and password have been entered on the web page. Devicescape uses an interesting trick to circumvent this. They piggyback their server interaction on DNS (Domain Name Server) queries which are required to resolve a URL (e.g. http://mobilesociety.typepad.com) into a numeric IP address. This works even before full access to the Internet is granted. The DNS server system has a treelike structure so a DNS query to a Devicescape URL will end up on the central Devicescape server. The server then checks the request and returns the username and password in the answer.

Pretty sleek!