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-shotâ„¢ 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!

IMS Applications

In my previous IMS post I’ve taken a look at the difference between SIP telephony networks and the IP Multimedia Subsystem (IMS). With that list it is quite obvious that the IMS is a centralized session management system that gives lots of control to the network operator and makes it quite difficult for third parties to integrate their services on a global scale. So while in theory the IMS is capable to be a platform for many web 2.0 services I think that in practice it will manly be used in the future for voice and instant messaging based services. Here’s a list of services which I think we will see in the IMS in the mid-term. If you have other examples, please leave a comment!

  • Voice telephony as the main application. This includes handing over voice calls between networks as the user roams out of coverage of a network. Furthermore, advanced IMS solutions will enable handovers of voice calls to a 2G network if no high speed B3G network is available anymore.
    The IMS enables video calls with the advantage over current 3G circuit switched mobile video calls that the video stream can be added or dropped at any time during the session.
    Presence and instant messaging.
  • Voice and video session conferencing with three or more parties
    Push message & video services such as sending subscribers messages when their favorite football team has scored a goal, when something exciting has happened during a Formula-1 race, and so on.
  • Calendar synchronization among all IMS devices.
  • Notification of important events (birthdays, etc.).
  • Wakeup service with auto answer and the users preferred music or news.
  • Live audio and videocasts of events. The difference to current solutions is the integrated adaptation to the capabilities of the device.
  • Peer to peer document push.
  • Unified voice and video mail because all devices used by a person are subscribed to the same IMS account.
  • One identity / telephone number for all devices of a user. A session is delivered to all or some devices based on their capability. A video call would only be delivered to registered devices of the user capable of receiving video. Sessions can also be automatically modified if devices do not support video.
  • A session can be moved from one device to another while it is ongoing. A video call for example might be accepted on a mobile device but transferred to the home entertainment system when the user arrives at home. Transferring the session also implies a modification of the session parameters. While a low resolution video stream is used for a mobile device the resolution can be increased for the big screen of the home entertainment system if this is supported by the device at the other end.
  • Use of several user identities per device. This allows only using a single device or a single set of devices to be reached friends and business partners alike. With user profiles in the network incoming session requests can be managed on a per user identity basis. This way, business calls could be automatically redirected to the voice mail system at certain times, to an announcement or to a colleague while the user is on vacation while private session requests are still connected.

Linksys Offers Dual Band Draft-N Wifi Router

Glenn Fleishman over at Wifi Networking News has posted a review on the Linksys WRT600N. What makes this device special is the support of both the 2.4 GHz and 5 GHz band simultaneously. Other products either only support the 2.4 GHz, which is too crowded in most cases to take full advantage of the 802.11n ‘double channel’ feature, or they support the 2.4 GHz and the 5 GHz band but only one at a time. Good to see these products appearing on the market!

P.S. Hopefully there is an OpenWRT or DD-WRT OS version for that device soon đŸ™‚

IMS vs. Naked SIP

Everyday we get a bit closer to all IP wireless networks in which operators are hard pressed to present a voice over IP solution. Today two approaches are on the horizon: ‘Naked SIP’, already implemented in some 3G phones such as Nokia N-Series and E-Series S60 phones. And then there is the IP Multimedia Subsystem (IMS), based on SIP but with lots of additional specification put around it. So what does IMS do that SIP doesn’t? I came up with the following list of things which are laking in naked SIP today which are dealt with in IMS:

  • General SIP implementations are network agnostic and can not signal their quality of service requirements to a wireless access network. Thus, voice over IP data packets can not be preferred by the system in times of congestion.
  • Handling of transmission errors on the air interface can not be optimized for SIP calls. While web browsing and similar applications benefit from automatic retransmissions in case of transmission errors, VoIP connections would prefer erroneous packets to be dropped rather than be repeated at a later time since such packets are likely to come too late.
  • SIP VoIP calls can not be handed over to the 2G network in case the user roams out of the coverage area of B3G networks.
  • SIP does not work in 2G networks.
  • Most SIP implementations today use the 64 kbit/s PCM codec for VoIP calls. Compared to optimized GSM and UMTS codecs, which only require about 12 kbit/s, this significantly decreases the number of VoIP calls that can be delivered via a base station. Furthermore, mobile network optimized voice codecs have built in functionality to deal with missing or erroneous data packets. While this is not required for fixed networks due to the lower error rates it is very beneficial for connections over wireless networks.
  • Emergency Calls (112, 911) can not be routed to the correct emergency center since the subscriber could be anywhere in the world.
  • No billing flexibility. Since SIP implementations are mostly used for voice sessions, billing is usually built into the SIP proxy and no standardized interfaces exist to collect billing data for online and offline charging.
  • Additional applications such video calls, presence, instant messaging, etc. are usually not integrated in SIP clients and networks.
  • It is difficult to add new features and applications since no standardized interfaces exist to add these to a SIP implementation. Thus, adding new features to User Agents and the SIP network such as a video mailbox, picture sharing, adding a video session to an ongoing voice session, push to talk functionality, transferring a session to another device with different properties, etc. is proprietary on both the terminal and the network components. This is costly and the use of these functionalities between subscribers of different SIP networks is not assured.
  • Insufficient security: Voice data is usually sent unencrypted from end to end which makes it easy to eavesdrop on a connection. Signaling can be intercepted since it is not encrypted. Man in the middle attacks are possible. No standards exist of how to securely and confidentially store user data (e.g. username/password) on a mobile device.
  • Scalability: Mobile networks today can easily have 50 million subscribers or more. This is very challenging in terms of scalability since a single SIP proxy in a network can not handle such a high number of subscribers. A SIP network handling such a high number of subscribers must be distributed over many SIP proxies/registrars.
  • There is no standardized way to store user profiles in the network today. Also, no standardized means exist to distribute user data over several databases which is required in large networks (see scalability above).

The list is quite long I have to admit. But there is one thing the list does not say: While naked SIP is available today I have yet to see an IMS capable terminal in the wild. I wonder how long it will still take?

As always, comments are welcome.

What next for mobile telephony?

… asks Moray Rumney, Lead Technologist over at Agilent in the latest edition of the Agilent Measurement Journal (3/2007). In his article, Moray takes a look at which factors contribute to the ever increasing wireless transmission speeds and explains where the limits are and why the 300+ MBit/s promised by LTE and other technologies in a 20MHz channel will remain a theoretical promise rather then becoming a practical reality. He then goes on to describe what is possible with the given physical limits and presents his thoughts about how to address capacity issues in the future. An absolute must read!

The journal is available here and the article can be found on page 32.

International MMS: A Case Study of Failure

It’s a strange situation: Most mobile operators today would like to retain control over the application layer and rollout new services themselves instead of letting Internet economics do the job. In practice however, they do not spend a lot of effort to making even the few advanced services they have universally usable. MMS is a prime example as I had to discover recently.

Situation 1: I am in France, I have a French SIM card and wanted to send an MMS to a prepaid subscriber of another French operator, Bouygues. Instead of receiving the MMS, only a text message arrives at the other end with a web link. The reason is that the other end did not have a GPRS subscription. 5 years after the introduction of MMS!? It leaves me puzzled.

Situation 2: O.k. so I can’t send my French friend an MMS but maybe I can send one to a friend in Germany. Message sent, I’ve been charged for it but the MMS never arrived. How nice.

Situation 3: Some days later I was in Spain and repeated the international MMS scenario with a Spanish SIM card. Again, the MMS to a German SIM card was not delivered.

To me it looks like even 5 years after the introduction of MMS, there are still no international agreements in place to forward MMS between operators. Could you imagine eMail not being delivered because the recipient lives in another country? No, probably not. That’s because no international agreements for applications have to be in place to forward eMail. And if there had to be, just imagine how the Internet would look like today and how many people would use it.

Some might say, the difficutlies stem from the fact that telephone numbers are used instead of eMail addresses for MMS messages. True, but international SMS messages which also use telephone numbers work just fine these days. But maybe 5 years is too short a time to make it work? One should not think so.