Author: Martin

Vacation Connectivity – Part II

In a previous post I’ve put down my thoughts and practical experiences with roaming through Europe and staying connected to the Internet with prepaid GSM/UMTS SIM cards. While it works quite well there is the disadvantage that a different SIM card is required in each country. So I was asked how people can reach me when I change my SIM card every couple of days!? There are several strategies:

1) I have a SIM card dedicated to voice calls. My friends only know this number and the SIM card is in a seperate phone.

2) If I only want to carry one phone I activate call forwarding unconditional on that SIM card to the SIM card I am currently using for Internet access in a country. There is one problem with this approach: Some prepaid SIMs by default forward calls to the voicemail and this call forwarding can not be deactivated. So I always pay for an incoming call no matter whether I pick up or not. So I ususally prefer to carry two phones.

3) I have an ISDN fixed line at home. While I travel I forward calls to the SIM card I use. There is a web interface available so I can change the call forwarding from abroad when necessary

4) Skype is also an option, though I haven’t tried this yet: Get a Skype in number and then forward the calls to a mobile phone number when you are not online.

It’s a pity one has to go through all of this just because pricing models of operators prevent people from using a single SIM card for all purposes. But who knows, one day…

If WiMAX Becomes a 3G (IMT-2000) Standard, What’s Left for 4G?

Now that 3G systems such as UMTS are under full deployment, the industry is looking forward to what comes next. While some say that WiMAX is a 4G system, the IEEE and the WiMAX forum think that 802.16e is rather a 3G technology and have asked the ITU (International Telecommunication Union) to include this standard into its IMT-2000 specification (International Mobile Telecommunications 2000). This specification is generally accepted as being the umbrella defining which standards are to be considered 3G.

This is mainly a political move since in many regions of the world, frequencies are reserved for 3G IMT-2000 systems. If WiMAX were included in IMT-2000, and it looks like it will be in the near future, some frequency bands such as the 2.5 GHz IMT-2000 extension band in Europe could be used for WiMAX without changing policies.

So what remains for IMT-Advanced, the ITU umbrella name for future 4G technologies?

Currently there is still no no clear definition by ITU of the characteristics of future 4G IMT-Advanced systems. The ITU-R M.1645 recommendation gives first hints but leaves the door wide open:

It is predicted that potential new radio interface(s) will need to support data rates of up to approximately 100 Mbit/s for high mobility such as mobile access and up to approximately 1 Gbit/s for low mobility such as nomadic/local wireless access, by around the year 2010 […]
These data rate figures and the relationship to the degree of mobility (Fig. 2) should be seen as targets for research and investigation of the basic technologies necessary to implement the framework. Future system specifications and designs will be based on the results of the research and investigations.

When WiMAX is compared to the potential requirements above it’s quite clear that the current 802.16e standard would not qualify as a 4G IMT-Advanced standard since data rates even under ideal conditions are much lower.

3GPP’s Long Term Evolution (LTE) project will also have difficulties fulfilling these requirements. Even with the recently proposed 4×4 MIMO, data rates in a 20 MHz carrier would not exceed 326 MBit/s. And that’s already a long stretch since putting 4 antennas in a small device or on a rooftop will be far from simple in practice. If WiMAX is accepted as a 3G IMT-2000 technology, how can LTE with a similar performance be accepted as a 4G IMT-Advanced technology?

Additionally, one should also not forget that IMT-2000 systems such as UMTS are still evolving. UMTS is a good example. With HSDPA and HSUPA, user speeds now exceed the 2 MBit/s which were initially foreseen for IMT-2000 systems. But development hasn’t stopped here. Recent new developments in 3GPP Release 7 and 8 called HSPA+, which will include MIMO technology and other enhancements, will bring the evolved UMTS technology to the same capacity levels as what is currently predicted for LTE on a 5 MHz carrier. HSPA+ is clearly not a 4G IMT-Advanced system since it enhances a current 3G IMT-2000 radio technology. Thus, HSPA+ categorized as a ‘enhanced IMT-2000 system’.

Maybe that’s the reason why the IEEE 802.16 working group is already looking forward and has started work on 802.16m with the stated goal of reaching top speeds of 1 GBit/s.

When looking at current research it’s clear that the transmission speed requirements described in ITU-R M.1645 can only be achieved in a frequency band of 100+ MHz. This is quite a challenge since such large bands are few. Thus, I have my doubts whether these requirements will remain in place for the final definition of 4G IMT-Advanced.

Does It Really Matter If A Technology Is 3.5G, 3.9G or 4G?

While discussions are ongoing the best one can do is to look at HSPA+, WiMAX, LTE and other future developments as "Beyond 3G" systems. After all, from a user point of view it doesn’t  matter if a technology is IMT-2000, Enhanced IMT-2000 or IMT-Advanced as long as data rate, coverage and other attributes of the network can keep up with the growing data traffic.

A whitepaper produced by 3G Americas has some further thoughts on the topic.

As always, comments are welcome!

Wireshark Now Supports WPA Decryption

Good to see that Wireshark, my favorite network analysis tool is now able to decrypt Wifi WPA protection. Starting with release 0.99.5, WPA information can be entered as shown here. It’s important that the trace also includes the authentication sequences for all mobiles in the network. This is necessary as each connection uses different session keys which are negotiated when a device enters the network.

Most of the time, it’s the beacon frames and other management information that is important when tracing Wifi. This is possible even without decrypting the conntent of the package. However, once packets are decrypted their content can be analyzed by Wireshark and frames are marked in different colors in the main window. This makes it very simple for example to detect Wifi retransmissions due to missing ACKnolwedgement frames for example. Without the different colors such retransmissions are much harder to spot.

Continuous Packet Connectivity (CPC) Is Not Sexy – Part 1

Currently, the 3GPP Standards body is giving the final touches to a set of features which are together referred to as Continuous Packet Connectivity (CPC). Several papers mention CPC but I haven’t found a single one so far who could really tell in simple words why these features are necessary and what they actually do. The reason for this is simple: While features like MIMO, spatial multiplexing, beamforming, etc. etc. are broad new concepts (and sound sexy…) CPC consists of a couple of deeply embedded features enhancing existing functionality. Twisting a couple of bits here and a couple of bits there is not very sexy and also not very understandable out of the box.

The Situation Today

With HSPA (HSDPA and HSUPA), mobile devices now have a multi megabit data bearer to both send and receive their data. As devices do not send data all the time there are the following activity states which require more or less interaction with the network:

  • Active: In this mode, the mobile uses HSDPA High Speed Downlink Shared Channels (HS-DSCHs) and an HSUPA Dedicated Uplink Channel (E-DCH).
  • During Short Periods of Inactivity (< around 10s): The network keeps the high speed channels in both uplink and downlink direction in place so the mobile can resume transferring data without delay. Keeping the high speed channels in place means that the mobile has to keep transmitting radio layer control information to the network which has a negative impact on battery life and also decreases the bandwidth for other devices in the cell. 10 seconds is certainly a compromise which is not always ideal since during a web browsing session, for example, it takes the user longer in many cases than this time to click on a new link.
  • During longer periods of inactivity (< around 30s): When no data is transfered for longer than a couple of seconds, the network puts the device on slow channels (RACH in uplink , FACH in downlink). This has the advantage that the mobile does not have to send radio layer control information back to the network anymore. This saves battery capacity to some extent. However, the mobile still has to observe the downlink channel to catch incoming data transmissions which also requires some energy. If the mobile wants to resume communication or in case data arrives for the device from the Internet, the network starts sending/receiving the data on the slow channels and starts a procedure to put the device back on the fast channels. However, this procedure takes in the order of 1 to 2 seconds so the user notices a delay when requesting a new web page for example. This delay is quite undesired.
  • Even longer periods of inactivity (> around 30 seconds): After about 30 seconds, or 60 seconds in some networks, the Radio Network Controller decides that it’s unlikely that the mobile will send or receive any more data for some time and thus puts the connection in Idle state. In this state the mobile does not have to send control information to the network and also does not have to listen to downlink transmissions except during periodic slots in which paging messages are broadcast. These paging messages are important to inform devices of incoming calls or of new data packets. For most of the time the mobile can now completely switch of the receiver and only activate it to receive paging messages and to scan for other cells of the network. If the mobile wants to transmit data again the radio layer has to request a channel again from the network. This takes even longer than the upgrade from a slow channel to a fast channel and results in an even longer delay before a web page starts loading. (Note: I won’t consider Cell-PCH and URA-PCH states for now)

The mobile keeps it’s IP address in all states, i.e. also in Idle state. Therefore, these state changes are  transparent to applications and the user except for the delay when upgrading to a faster channel once data is transfered again.

Desired Improvements

Continuous Packet Connectivity aims at reducing the shortcomings described above by introducing enhancements to keep a device on the high speed channels (i.e. in active state) as long as possible while no data transfer is ongoing by reducing the negative effects of this, i.e. reducing power consumption and reducing the bandwidth requirements for radio layer signaling during that time.

CPC Enhancements

CPC introduces the following new features to reach these improvements:

In Uplink:

  • A new UL DPCCH slot format
  • UL DPCCH gating/discontinuous transmission
  • Implicit CQI reporting reduction

In the Downlink:

  • F-DPCH gating in DL
  • Discontinuous reception (DRX) at the UE
  • A so called HS-SCCH-less operation
  • Modified HS-SCCH for retransmission(s)

Unless you regularly attend 3GPP RAN meetings, this list probably won’t tell you much. But don’t despair, I’ll publish part two of "CPC is Not Sexy" soon in which I will describe these features in understandable terms.

Prepaid Mobile Internet Access In Austria

I am in Austria for a couple of days again and finally had the chance to get hold of a 3 Prepaid SIM card for Internet access. The price 3 charges per megabyte is 80 cents. It’s a bit too expensive to be used with a notebook but o.k. to check eMails and surf the web via the mobile phone.

The SIM card is €19,90 with an already included balance of 5 euros. When buying the SIM card make sure packet data services are activated for the SIM card in the shop. The access point name (APN) for web, eMail (POP3, SMTP), etc. is "drei.at". For easy access and bookmarking I’ve put the information on the Prepaid SIM Internet Access Wiki as well.

Happy surfing!

P.S.: For Internet access with a notebook while roaming in Austria, Vodafone Germany’s Websession offer is still the best choice.

A (WiMAX) world without SIM cards

A recent blog entry of mine on WiMAX terminals with and without support of EAP-SIM and thus SIM cards for authentication has provoked a number of interesting responses. What I take away from them is that first devices will probably not have a SIM card.

So the next logical question is how authentication is done in the absence of a SIM card!? I can see two basic approaches:

1. A device comes with a built in certificate. That’s straight forward. The user goes to a shop, buys a device, it gets activated for him and he’s set. While this is all nice and well the trouble starts when the device breaks or the user wants to use the services of another operator. No way with this model.

2. Another model would be to use a username and password to be supplied by the user. It could work in a similar fashion as with Wireless LAN today. I can also imagine user installable certificates. While both  being a bit more complicated then pre-installed certificates it would preserve the flexibility the SIM card approach offers today.

I like and depend on flexibility since I travel a lot and a device locked to a single network is useless for me. While I am certainly not the average user I am sure the majority would prefer openness over being locked into a single garden.

If you have further information on this topic, please leave a comment.

The Cost Of Vacation Connectivity

You might have noticed that I am blogging a bit less at the moment than normal. Among other things it’s got something to do with that I am currently on vacation traveling through Europe. Staying connected has become much easier in recent years but still requires a fair amount of self organization, a bag full of SIM cards and willingness to spend a certain amount of money. So how much do I spend for Internet connectivity during my 3 weeks vacation?

Things are complicated since I spend my vacation in four countries: Austria, Italy, southern France and Spain. For Austria I’ve bought a prepaid SIM from ‘3’ for eMail and Web access via my Nokia N93. Works well and details will follow in a seperate blog entry. Cost: 20 Euros for the SIM card and credit which lasted me for the time I spent in Austria. During 3 days I required full Internet access so in addition I used two Vodafone Web Sessions for 15 Euros each. Total amount spent in Austria: 50 Euros.

Next stop Italy. Here, things are simple. I already have a TIM prepaid SIM and use it for notebook and phone web access. 20 Euros buy me 500 MB. That’s good enough for the 5 days I am staying in ‘Bella Italia’.

Next, the south of France is on my agenda for about 10 days. I’ll use Orange’s prepaid SIM for eMail and web access via the mobile phone. That’s 6 euros. In addition I will probably need full Internet access during 4 days. That’s four Vodafone Websessions that add up to 60 Euros.

Final Stop: Spain. Just a weekend but it’s unlikely that I want to spend them disconnected. Maybe I will find enough open Wifi Access Points in the street. An alternative is a Yoigo prepaid SIM with web access for a euro a day. The SIM will cost a couple of Euros, too. Well, we’ll see.

Altogether, that’s going to be around 150 Euros. Definitely not on the cheap side. I wished ‘3’ would be present in all countries I (live and) travel to since they don’t charge extra for data roaming in their networks.

For the details on the prepaid SIMs I use, take a look on the left side on the blog for the link to the Prepaid SIM Internet Access Wiki.

802.11 Options, Options, Options

Gone are the days when standards were pure and simple (well, probably never simple, but at least pure…). Today, it seems they are cluttered with options of which most are probably never going to be implemented. The Wireless LAN 802.11 standard seems to be no exception. Let me make two examples:

Packet Transmission:

  • Default: This is the good old "backoff period – send – ack" mechanism. Easy, works well but performance is not that great.
  • Frame Bursting: Packets are sent in the following manner: "packet – ack – packet – ack – packet – ack". Still easy, was  implemented as a proprietary enhancement in many 802.11g products and has been sort of legalized with 802.11e (WMM).
  • Block Acknowledgments: An addition to frame bursting which allows transmissions without ack’s. A whole set of frames are then acknowledged once they are all sent. To make things just a bit more complicated there’s immediate ACK and delayed ACK (which seems to have been defined for devices which can’t tell right away if all went fine).
  • Aggregation: And on top, 802.11n has now specified that several MAC frames can be put into a physical frame which can now have a size of up to 64kByte. Looks like this is mandatory so all 802.11 devices should support this.

The statistics on this one are not so bad. Even low end 802.11n devices should support the default method, frame bursting and aggregation. Haven’t seen block ack’s implemented in the devices that have come by me, however.

Power Saving:

I can see at least four possibilities here:

  • Standard Power Save (PS): This has been in the standards since the beginning. Devices tell the AP that they are going to sleep and the access point buffers incoming packets. When devices wake up and see that the access point has packets waiting for them they poll for each buffered frame.
  • U-APSD: Unscheduled Automated Power-Save Delivery: Introduced by 802.11e, optional in the WMM (Wireless Multimedia) specification. Similar to PS above but once a device sends a trigger frame, the access point forwards all frames of in the buffer that fit into the service period during which the device is active. Once the service period is over, the device automatically goes back to sleep.
  • S-APSD: Scheduled Automated Power-Save Delivery: No trigger frames. Instead, a schedule is agreed between the access point and wireless devices. The devices then wake up at predefined instants and packets are delivered automatically. This one is not included in the WMM specification, so this one probably has no chance of seeing the light of day.
  • PSMP: Power Save Multi Poll. Yet another power save scheme which was lately introduced with the 802.11n High Throughput specification. This one schedules uplink and downlink transmissions of end user devices. Outside the scheduled times, devices can enter sleep mode. It looks like this power save mode has been designed for devices and applications that have constant data streams with a static bandwidth requirement (e.g. VoIP, video streaming etc.). Nice but also optional.

Statistics on this one are bad. I haven’t seen an access point yet that supports more than the classic PS mode. Has anyone seen more than this implemented yet?

Incredible! France Telecom Has A Prepaid SIM 3G/GPRS Internet Access Offer

Sometimes I can’t believe the power of web 2.0. A couple of weeks ago I have set up a Wiki dedicated to information about prepaid SIM wireless Internet access. I’ve put all my information there which I have gathered over time on offers in different countries and invited the community to put their info into the Wiki as well. It has worked more than well for me as somebody has put information about a prepaid SIM Internet access offer in France.

Incredible, in the country that can certainly be found amongst the least competitive GSM/3G markets of all in Europe, France Telecom / Orange has silently started to offer transparent access to the Internet via their Mobicarte prepaid SIMs! 10 MB are to be had for 6 euros a month. The offer doesn’t mention what happens after the 10 MB are used up but the included data volume is enough for mobile eMail and limited mobile browsing. I wouldn’t, however, use it for connecting my PC to the Internet.

Also, one should be aware that all French operators have a minimum usage fee for prepaid cards per month. A €35.- top up card for example is valid for 3 months, i.e. the monthly minimum usage is €11,66. So be prepared to use the SIM for some phone calls as well.

So I rushed into the next Orange shop to get myself a prepaid SIM to test the offer myself. The SIM card cost 19.90 euros with a credit of 4.50 euros already on the SIM. To have more than 6 euros on the prepaid account I also bought and additional top up card for 10 euros. So my startup investment was 30 euros. All that is required to get the prepaid SIM card is a valid passport of any country and an address in France. I am sure a hotel address will do, they probably appreciate the incoming advertisement…

The card is activated by calling any number or sending an SMS. Afterwards I used my 10 euros credit to to up the prepaid account. Afterwards, I called the automatic service hotline to activate the 6 euro Internet access feature. A bit of French is required to navigate through the menu. Once confirmed the 6 euros are immediately taken from the balance and the option can be used.

Luckily, my N93 automatically created the configuration necessary when it detected the SIM card of Orange. I’ve set my mobile browser and my eMail application for manual access point selection and the list already contained an entry for Orange. Very well done, Nokia, congratulations!

So I’ve been using the option for a couple of days now and my balance has remained stable, i.e. the data traffic is taken from the option and not from the balance. Very nice. Suddenly I don’t feel completely disconnected anymore in France once I leave my Wifi cloud at home. Welcome, France, to the mobile connected world!

For the details on the offer and how to activate it take a look at this Prepaid Wireless Internet Access Wiki Page.

The Nokia N800, WiMAX And Embedded 802.16e Chips

Speculations are growing that Nokia is working on a version of the N800 Internet tablet with a WiMAX chip in addition to Bluetooth and Wifi. This report on Heise News, a reliable German tech website, links Nokia’s plans with the launch of Sprint’s WiMAX network in 2008.

They also link to an interesting data sheet of a WiMAX Module from SyChip, (which they say may or may not be related to the N800 WiMAX development) which contains quite interesting high level details of first generation embedded WiMAX chips. According to the data sheet the WiMAX module is 21.6 x 23.7 x 1.5mm in size and supports 802.16e. The block diagram shows two antenna connectors for MIMO in downlink direction and single stream transmission in uplink direction. Mobile devices communicate with the embedded module either via SDIO or USB.

One of the mysteries around WiMAX to me, still, is how users and devices will be authenticated since I haven’t seen any kind of SIM card specification for WiMAX yet. The data sheet, however, says that the following security mechanisms are supported: EAP-SIM, EAP-AKA, EAP-TLS. While EAP-TLS works with certificates, EAP-SIM was originally specified by 3GPP (UMTS standards body) for Wifi authentication using  the secret keys and subscriber ID contained on a GSM/UMTS SIM card. For details take a look here. Since EAP-SIM has been specified independently from the underlying network technology, it should integrate nicely into WiMAX as well.

This raises the question though what kind of equipment is used on the network side that supports EAP-SIM authentication!? If you have more information, please let me know!