Nortel and Cisco have decided to use some of their marketing budget for something really useful for engineers, namely to open up the IEEE standards 802 library for free public access. These include the famous Ethernet (802.1,2,3), Wifi (802.11) and WiMAX (802.16) standards. A great help for all doing research in this area. Little downside: Only approved documents are available which excludes hot documents such as the current 802.11n draft.
How Will Users Be Able To Differentiate 11n from 11n?
In the past, things were pretty much clear when it came to Wireless LAN performance. If the box in the shop said it’s an 802.11g device, users could pretty much assume the device would do 54 MBit/s on the physical layer and application layer speed would be around 20 MBit/s. Things are much less clear with the new Draft 802.11n standard, which contains a myriad of options a device may or may not implement.
The standard for example contains three different flavors of MIMO. The most popular one, MIMO spatial multiplexing will be implemented in many devices. But the standard gives devices the option to use 2, 3 or 4 transceivers/antennas. The more receivers, the higher the speed, if of course the receiver has at least as many. O.k. one might be able to sell this story in a fashion like "We do 4×4 MIMO compared to the competition which only does 2×2, so we are twice as fast".
The story doesn’t end there, however. There are two other MIMO modes, namely MIMO beamforming and MIMO STBC (Space Time Block Code) which can significantly enhance range and link stability. It just might turn out that these MIMO modes are just as important for applications such as video streaming to devices that are not close to the Wifi Access Point. Ruckus wireless for example is doing interesting things in this area.
And it certainly doesn’t end here. Draft 802.11n contains further options like multimedia power save (PSMP), shorter OFDM guard intervals, Antenna selection, Maximum Ratio combining, Modulation and Coding Scheme (MCS) feedback, etc. etc. In the end, marketing words on boxes in the electronics store are cheap. Let’s see, how about the Wifi Alliance coming up with something standardized about "enhanced 11n options"? Time will tell.
Will France Get A (Hopefuly More Agressive) Fourth 3G Operator?
Today I saw reports for example here and here that Illiad has brought forward the necessary papers to get the fourth and final batch of France’s 3G licenses in the 2.1GHz UMTS band before the deadline expired yesterday.
Illiad is the mother company of ADSL Internet provider "Free", certainly one of the most aggressive players in the French ADSL market. That gives me some hope that in case their bid is accepted and they are ready to go for it there might finally be reason for hope that competition could finally arrive in the French 3G market. Also, Free might have more of an Internet and IP angle than the local 3G incumbents so they might be more inclined to finally let customers use their network for Internet services for an attractive price rather than to let base stations sit around only producing hot air.
According to the report of Les Echos linked above the stock market was not quite happy to hear that Illiad has put their name in the hat. They see mobile as a risk rather than an opportunity. Short sighted bunch…
Need an 802.11n Beacon Frame?
Standards are well and good but usually contain a zillion options nobody ever implements. The 802.11 ‘draft-n’ standard is no exception. To find out which options different vendors have actually implemented, the best thing to do is to trace the beacon frames of ‘draft-n’ access points. A couple of months ago I described how to trace WLAN frames here.
If you are looking for beacon frames traced by other people to compare functionalities with your own ‘draft-n’ access point at home, take a look here. The beacon frame traced by ‘swordfish’ seems to be from a Linksys access point, judging from the first three bytes of the MAC address which identifies the manufacturer of the device. So here are the main ‘draft-n’ features this access point supports taken from the ‘HT Capability‘ (HT = high throughput) parameter (element ID 45):
- 20 MHz and 40 MHz channel operation support
- Greenfield mode support, i.e. protection mechanisms used to allow 802.11b and 11g devices to be part of the network can be switched off.
- the access point has two independent transmitters and supports 2 spatial MIMO streams.
- All of the other gazillion options such as short guard interval, STBC diversity, beamforming, MIMO power save, advanced coding, MCS feedback, antenna selection, etc. etc. are not supported.
The current mode of operation according to the ‘HT Information‘ parameter (element ID 61) erroneously called ‘Additional HT Capability’ in the trace):
- The access point currently operates in 20 MHz mode only (either set by user or due to other networks using the same channel)
- The access point runs in greenfield mode, i.e. only 11n devices have joined the network
Also interesting to see that the size of beacon frames has dramatically increased. Current 11g access points send beacon frames with a length of around 110 bytes. This ‘draft-n’ beacon frame has a length of 228 bytes!
Draft 802.11n Devices Must Implement 802.11e QoS
It’s good to see that both the IEEE 802.11n draft Standard and the Wifi-Alliance certification program require that future draft 802.11n Wifi devices also have to support the 802.11e QoS standard, sometimes also referred to as Wireless Multimedia (WMM).
WMM introduces Quality of Service for the Wifi Air Interface which means certain traffic flows can be prioritized over others. This is important for example when high bandwidth streaming and several phone calls are to be carried over the same wireless access point simultaneously.
By requiring QoS support from 802.11n devices I think this functionality has a real chance to become widespread in a relatively short amount of time which would probably not have happened otherwise.
Sources:
- The 802.11n draft standard which confirms this in several paragraphs, like for example in a quite simple and not very easy to understand statement in chapter 5.2.8: "An HT STA is also a QoS STA"
For further information on WMM and how applications can use the functionality take a look at this previous blog entry.
How To Top-Up A Vodafone Prepaid SIM for Websessions
A final piece of information for users of Vodafone Germany Prepaid SIMs for Websessions has so far been missing: How to top-up without scratch cards that can only be bought in Germany!?
Here’s a link to an online service here that allows to top-up German prepaid SIM cards. The pages itself are in German but pretty easy to figure out for non natives as well. To top-up a Vodafone SIM card, select "Vodafone CallNow", create an account by typing in your eMail address, specifiy the SIM card’s phone number, select the amount to top-up and enter your credit card details. After a minute the selected amount is on the SIM card and can then be used for WebSessions. All quite straight forward and works well.
Any 5 GHz 802.11n Devices Out There Except For The Airport Extreme?
Just saw an article in a German computer magazine testing a number of different pre-802.11n access points and related client adapters. Performance was around 50 MBit/s which is not a lot considering the test here and here showed performance in the range of about 100 MBit/s on the application layer.
What also struck me was that all the top 3 access points were 2.4 GHz only products. So looks like except for the Apple Airport Extreme we have to wait for a while before other 5 GHz pre-802.11n products make it to the market!?
The top three candidates where:
- the TEW-631BRP
- the SMCWBR14-N2
- the Netgear WNR834B
If anyone knows a pre-802.11n Access Point for 5 GHz except for Apple’s access point, please let me know!
Review: Wireless Communications – The Future
I have to admit I had William Webb’s latest book „Wireless Communications – The Future“ at home for a couple of months before I finally came around to read it back to back. A mistake, as it turned out, to wait that long. As title suggests, the book deals with the future of wireless telecommunications, over a period of the next 20 years.
Webb, working for Ofcom and a newly appointed fellow of the Royal Academy of Engineers draws an interesting picture of how he sees wireless technology evolving. His drastic conclusion is that fourth generation wireless will not make it off the ground. His main reasons for this are that he sees no need for systems that go beyond what’s possible in terms of data rates with 3.5G and the evolution these systems will undergo in the future. Note that he also counts LTE and similar technologies as 3G+ technologies. The second main reason at arriving at his conclusion is that today’s wireless technologies are already close to natural boundaries in terms of available frequency bands and efficiency. In short he sees continuous evolution of existing wireless systems rather than a new radio technology beyond OFDM revolutionizing wireless telecommunication in the next two decades.
Throughout the book the author discusses his own predictions such as how fixed and wireless networks will converge, industry inflection points where current vertical wireless vs. fixed network operator / content provider silos break up and new business models are required around horizontal structures with companies dealing with the network, companies dealing with the service and companies dealing with the applications on top. Also, Mr. Webb has invited a number of outstanding industry members to share their visions of the wireless future and discusses how these views integrate with his visions.
For every statement I agree with in the book there is at least another prediction which I vehemently disagree with. But I guess that’s just natural when trying to predict the future. Here are some statements which I found of particular interest and fully agree with:
- “The concept of lengthening time between generations of cellular technology being bridged through evolution of standards is now becoming increasingly accepted. This has the implications that moving to a new generation is not as urgent a need as it has been in the past […]”
- “Some [DSL] users […] are unlikely to achieve data rates above 1 MBit/s [in the future] unless there is a substantial infrastructure upgrade”
- “… a new service or product might take anything between four and ten years to reach mass adoption…”
- “Once almost all users have access almost everywhere to average data rates of several hundred megabits per second in active periods, their senses are no longer able to make use of more data and higher data rates become irrelevant” (Simon Saunders)
- “Some data measured by a large European ISP on their broadband customer base in 2005 showed that 50% of their customers were shipping less than 600 Mbytes per month” (Stephen Temple)
- “Video calls will become increasingly attractive as the ‘Metcalfe rule’ applies…”
- “The extent to which these services actually emerge will be set by industry structure and the ‘boldness’ of the strategies deployed by the key players”
- “Users will have found that the combination of 3G with its falling costs and improving data rates, and nomadic W-LAN coverage when static provides the best combination of cost and utility.”
These quotes are just the tip of the iceberg and although I do not agree with everything in the book I can say it is a real page turner and well worth a read. I learnt a great deal, I scribbled a lot of notes into the book so I can compare in 5 or 10 years from now and the book has definitely inspired a lot of new thoughts.
For those who are interested here’s a link to book’s page on the publisher’s website.
Draft 802.11n Requires Access Points To Use A Single Channel Only In Case Overlapping Networks Are Detected
I am having a good time these days browsing through the current draft D2.00 of the 802.11n standard to find out about the details of the compromise reached in the IEEE working group for the new 100+ MBit/s Wifi standard. Besides MIMO, one of the corner stones of reaching speeds beyond 100 MBit/s on the application layer is to combine two standard 20 MHz channels and transmit on them simultaneously.
This is pretty difficult to impossible in the 2.4 GHz band which only has space for 3 independent 20 MHz networks or a single 802.11n 40 MHz network together with one 20 MHz legacy network (for details see here). In my Paris flat, for example, there are already 13 networks operating in this band, many using the same channels.
In such an environment, a ‘draft n’ compliant access point has no chance to use a 40 MHz channel as according to chapter 9.20.4 of the draft standard, an access point detecting frames of another network on it’s primary or secondary 20 MHz channel has to immediately deactivate the 40 MHz channel mode. Further, it has to remain in 20 MHz channel mode for at least 30 minutes after the last frame from a different network has been detected.
I guess the standard allows the access point to switch to another channel to avoid the detected network but in the 2.4 GHz ISM band there is only one alternative. So I wonder if some vendors have put an option into their settings that allows locking the access point to a 40 MHz channel!? Not that this would be very polite, or not cause any problems to other networks and one’s own if traffic of other networks is higher than an occasional traffic burst.
So if you have a ‘draft n’ network at home, what kind of access point do you have and does it allow locking operation to 40 MHz?
Deactivating the Vodfone Websession Compression Proxy
I am quite happy with Vodafone Germany’s Web Session offer that gives me fast 3G Internet access in most European countries and in some countries overseas. I’ve reported about this extensively here. One of the things that bothered me, however, was the automatic compression of pictures in web pages. This reduces the amount of data to be transmitted but in the times of HSDPA that’s not necessary anymore. When buying a PCMCIA card and the required software from Vodafone for the service there is an option in the software to deactivate the compression. If you buy a standalone prepaid SIM card however, things are a big more tricky.
One way to get around the compression is to use a VPN software that tunnels all traffic and thus Vodafone’s transparent HTTP proxy can not touch the pictures. In some circumstances, such a solution is not practicable or not even available to all users. So I searched a bit on the web to see if there are ways to deactivate the proxy without the Vodafone software. And indeed, there is! Here and here are two links to the original German articles that describe how the proxy can be instructed not to compress the picture. In essence this is done by including extra HTTP header lines in each page request which are picked up by the proxy and tell it not to compress the images. According to one article, this works for Vodafone Germany and also for E-Plus, another German operator.
To get these extra header lines into a request, an add-on called "Modify Headers" is required for Firefox. The add-on can be installed into the browser right from the Mozilla Add-On Web Page. Once installed, a new menu entry called "Modify Headers" is available in the "Tools" menu of Firefox. In the configuration tab, select "Always On: Enable Modify Headers when this window is closed". Afterwards, two new header fields have to be added manually. In the "Headers" tab, one new header called "Cache-Control" has to be created and another one called "Pragma". Both headers have to be set to contain "no-cache". That’s it!
Restart Firefox and the nasty compression is gone. If you go to pages that have previously been loaded, they are probably still in the local cache and thus still look ugly. In that case, press "STRG" or "SHIFT" together with the reload button of Firefox and the images are refreshed to their non compressed state. Below are two screen shots of HTTP request packets traced with Wireshark that show how HTTP headers look before the tool is switched on and afterwards when they include the two additional header lines.
