Reiner Saddey’s Place

WiFi insights - Why you should avoid using channels other than 1, 6, 11 and 14 like the plague

Posted on July 8th, 2007 by Reiner.
Categories: English, at other Locations, at Home, Computers.

In short: Only use channels 1, 6, 11 (and 14 if you live in Japan) for your 2.4 GHz wireless LAN and simply ignore the other ones. The world will be a better place with no effort at all

Great! Now you have got a WiFi of your own. As you are a prudent guy you’ll carefully choose a free channel so as not to interfere with your neighbors’ WiFis. Scanning for available networks reveals that channels 1, 6, 9 and 11 are already being used, which leaves channel 3 as the way to go.  You are quite confident that everything should be in order now. The truth however is rather disconcerting and may come to you as a shock.

What you have really done is, you have set up a new jammer that will halve the throughput of at least 3 networks - including yours. How can that be true?

Only 3 out of 11  channels do not interfere: 1, 6 and 11

This is due to the channel allocation scheme that is used by 802.11 b/g standards and the behaviour of WiFi equipment: While channels are equally spaced 5 MHz apart, the bandwith consumed by a single transmission path is roughly 20 MHz, effectively occupying 5 channel slots: The center channel it is set to, 2 adjacent channels towards lower frequencies and 2 channels towards higher frequencies as well.

Thus a station set to use channel 1 will occupy channels 1-2-3. The next non-overlapping channel will be 6 occupying channel slots 4-5-6-7-8. The only channel left will be channel 11 occupying channel slots 9-10-11.

Overlapping channels substantially decrease throughput 

Assuming urban surroundings and just 3 useable non-overlapping channels, wireless networks are bound to interfere with each other. If interference cannot be avoided, why not try to minimize interference by “filling the gaps”?  Two stations using the same channel should most certainly experience much more interference than if they were using different ones, even if they were just one slot apart. Suprisingly enough, this is not true.

Wireless stations are by no means dumb passive devices. Before initiating a transmission they will listen for other ongoing transmissions and abide from starting their own transmission when they find that the channel is currently in use, thus minimizing interference (DCF, CSMA) . Sadly enough, this is an important factor that is often overlooked or set aside, even in academic documents. 

Now when you offset two stations by less then 5 channels you’ll impede their capability to properly detect ongoing transmissions - practically blindfolding them and turning them into anti-social criminals.

Furthermore, by virtue of its bandwidth, each transmission that does not adhere to the 1-6-11 rule will use up 2 instead of 1 out of the proper channel bands described. Thus stations set to channels other than 1, 6 or 11 will unknowingly and unneccessarily interfere and disrupt both each other and other adjacent channels, in turn causing frequent re-transmission which will bring throughput down to a crawl.  

What about channels 12, 13 and 14

Within Europe and Japan, there are two additional channels available: 12 and 13. But they are of little use, as they both overlap with channel 11. Still worse, devices may refuse to connect to those channels, unless they have been set up properly, e.g. a Nokia E6x will no longer connect to channels 12 and 13 when you remove your SIMM. 

Channel 14 is a very special case. It is used in Japan only and despite its number it is spaced 12 MHz apart from channel 13, so it might have been named channel 15.4. Transmissions on channel 14 will interfere only minimally with those on channel 11. Therefore, in Japan there are 4 channels that should be used in order to minimize interference: 1, 6, 11 and 14.

Links:

1. Channel Deployment Issues for 2.4-GHz 802.11 WLANs (Cisco), as PDF
2. 802.11b WiFi Channels (Moonblink)
3. DCF (Wikipedia) 
4. DCF (WiFi Planet)
5. CSMA (Wikipadia)
6. 802.11 (Wikipedia)

Comming next: Why SuperG and other 108 MBit transmissions often cause more damage than benefit. Why a 3 dB Antenna boosts more than increasing Transmit Power by 12 dB.

3 comments.

Nur für Technik-Freaks - Was Ihr Router so alles kann: DD-WRT verbindet sich mit Relakks

Posted on July 5th, 2007 by Reiner.
Categories: Deutsch, at Home, Computers.

Wie können Sie sich Ihr WLAN mit Ihren Nachbarn teilen, ohne zum Freiwild Hamburger Gerichtsbarkeit zu werden (heise: Unverschlüsseltes WLAN hat Folgen)  und ohne wegen der dem Zeitgeist geschuldeten Verdrehung der Unschuldsvermutung jeden Mausklick Ihrer Nachbarn mitschreiben zu müssen?

Hier wird keine fertige Lösung vorgestellt, sondern nur Hinweise für technisch Versierte gegeben, wie man einen DD-WRT Router (mit statischer IP)  so einrichten kann, dass er sein WLAN über Relakks tunnelt. Vielleicht findet sich ja jemand, der mehr Ahnung hat als ich, und daraus eine anklickbare Option für DD-WRT erstellen kann (more…)

12 comments.

For Techies Only - Your Router can do Tricks: DD-WRT does Relakks

Posted on July 5th, 2007 by Reiner.
Categories: English, at Home, Computers.

How can you share your WiFi with your neighbors without being forced to spy on them - just in order to proove that it wasn’t you who put the new Harry Potter into the public domain by mistake?

This post does not offer a ready-to-use solution, but instead provides some work-in-progress material on how to set up a DD-WRT device (using a static public IP) so that it tunnels its WiFi through Relakks. Maybe someone more knowledgeable than me might turn this into a clickable option (more…)

17 comments.

JBoss 4.2.0.GA + Unified Invoker + JBoss Remoting = not too much excitement for me yet

Posted on May 19th, 2007 by Reiner.
Categories: English, Computers.

I was rather excited to learn that JBoss 4.2 now uses the Unified Invoker by default, which in turn uses JBoss Remoting. Of course I assumed that JBoss Remoting’s thrilling features would be available to the Unified Invoker. I was wrong. (more…)

1 comment.

Problem solved: Microsoft Update may clog your CPU at 100% on boot starting Apr 11, 2007

Posted on May 6th, 2007 by Reiner.
Categories: English, Computers.

The problem has been fixed, but I suggest to wait until the fix will be distributed automatically starting about 22-May-07. Thereafter - in June - you should then switch back from Windows Update to Microsoft Update. Technical info from Les Connor at WSUS v2 client and high CPU fix.

0 comments.

Problem gelöst: Ab 11. April 2007 verlangsamt Microsoft Update u.U. PC nach dem Start

Posted on May 6th, 2007 by Reiner.
Categories: Deutsch, Computers.

Ein Fix zu diesem Problem ist jetzt verfügbar, aber ich empfehle, solange zu warten, bis er ab ca. 22. Mai automatisch verteilt wird, und erst danach - also so ab Juni 2007 - die automatischen Updates wieder von Windows Update auf Microsoft Update umzuschalten. Info für Spezialisten von Les Connor bei WSUS v2 client and high CPU fix (auf Englisch).

0 comments.

Ab 11. April 2007 verlangsamt Microsoft Update u.U. PC nach dem Start

Posted on April 12th, 2007 by Reiner.
Categories: Deutsch, Computers.

Es gibt plötzlich Berichte darüber, dass Microsoft Update (nicht Windows Update) den PC nach dem Starten mehrere Minuten lang sehr langsam macht.

An meinem Rechner wird eine seiner CPUs nach dem Starten von wuauclt.exe ca. 12 Minuten lang mit 100% belegt. Ältere PCs mit nur einer CPU reagieren mehrere Minuten bis zu einer Stunde lang auf keinerlei Eingaben.

Siehe z.B. Alex met SVCHOST (auf Englisch), wo ein work-around vorgestellt wird:

1. Automatische Updates ausschalten
2. Bei http://update.microsoft.com/ die Einstellungen so ändern, dass Microsoft Update nicht mehr verwendet wird, sondern nur noch Windows Update
3. Automatische Updates wieder einschalten

Aktualisierungen:

 6. Mai 2007: Ein Fix ist jetzt verfügbar, aber ich empfehle, solange zu warten, bis er ab ca. 22. Mai automatisch verteilt wird, und erst danach - also so ab Juni 2007 - die automatischen Updates wieder von Windows Update auf Microsoft Update umzuschalten. Info für Spezialisten von Les Connor bei WSUS v2 client and high CPU fix (auf Englisch).

 1. Mai 2007: Das Problem scheint immer noch nicht gelöst zu sein. Siehe SVChost performance issues — hold on just a smidge longer (auf Englisch).

12. April 2007: Ich freue mich, dass auch ein echter Guru (Bradley von The SBS Diva Blog) dies z.Zt. als Mittel der Wahl empfiehlt: SVChost and flipping back to Windows Update  (auf Englisch)

Wahrscheinlich ist dieses Problem bei Microsoft bereits bekannt und arbeitet z.Zt. an Hot-Fixes. Siehe z.B.:

• http://support.microsoft.com/kb/932494/de 
• http://support.microsoft.com/kb/927891/de

Ein m.E. gute Darstellung des Problems gibt es bei Windows Update / Microsoft Update / svchost / Autoupdate / 100% CPU - Patrick Hopp (auf Englisch)

1 comment.

Microsoft Update may clog your CPU at 100% on boot starting Apr 11, 2007

Posted on April 12th, 2007 by Reiner.
Categories: English, Computers.

There have been reports, that Microsoft Update (not Windows Update) may clog your CPU for some time after booting your PC.

On my system wuauclt.exe keeps one of its CPUs busy at 100% for about 12 minutes.

See Alex met SVCHOST, where Wim describes a work-around:

1. Disable Automatic Updates on the computer
2. Go to http://update.microsoft.com/, choose “Change Settings”, select “To stop using Microsoft Update” at the bottom of the page and then check the box “Disable Microsoft Update software and let me use Windows Update only”
3. Re-enable Automatic Updates

Updates:

06-May-07: The problem has been fixed, but I suggest to wait until the fix will be distributed automatically starting about 22-May-07. Threafter - in June - you should then switch back from Windows Update to Microsoft Update. Technical info from Les Connor at WSUS v2 client and high CPU fix.

01-May-07: The problem appears not to have been solved yet. See SVChost performance issues — hold on just a smidge longer.

12-Apr-07: Glad a real guru (Bradley from The SBS Diva Blog) backs this somewhat bleak work-around: SVChost and flipping back to Windows Update  

Microsoft may be aware of the problem and is preparing hot-fixes. See

• http://support.microsoft.com/kb/932494 
• http://support.microsoft.com/kb/927891/

There’s a post that gives some details at Windows Update / Microsoft Update / svchost / Autoupdate / 100% CPU - Patrick Hopp

1 comment.

WP-Cache 2.1 revisited: Have x-gzip and gzip compression share same cache slot

Posted on April 10th, 2007 by Reiner.
Categories: English, Computers.

While working on the 304 not modified case that causes excessive traffic for RSS feeds, I encountered a mild weakness within the compression code, that causes both gzip and x-gzip variants of a particular page to use separate cache slots (i.e. two separate files). That’s quite redundant, as x-gzip is just a lagacy term to invoke gzip processing and both x-gzip and gzip produce identical contents. 

Now, only a single cache slot is being used for both and the value of the Content-Encoding header (either gzip or x-gzip) is no longer cached, but instead derived from the current request being served. 

There’s no code here, as it will be included within the 304 not modified version.

0 comments.

How to prevent Search Everything from returning duplicate hits with WordPress 2.1

Posted on February 19th, 2007 by Reiner.
Categories: English, Computers.

After upgrading to WordPress 2.1 Search Everything often returns the same pages multiple times. 

22-Feb-07 Update: The information below has been superceeded, as Dan has just released version 3.01 of the plugin that includes code to prevent duplicate hits.

One of a my friends runs a property management agency, where WordPress is used as an internal knowledge base to document internal procedures and the like. Quite often, valuable information is not contained within the main post, but added later as a comment. Search Everything makes sure that this information can be searched and retrieved.

After upgrading to WordPress 2.1 and installing Search Everything Version 2.6, searches will quite often return the very same pages multiple times.

I had a look at both Search Everything and WordPress Code and found that WordPress 2.1 does not use the distinct option when querying the databse, but provides a filter instead that may, at its discretion, return the magic ‘DISTINCT’ to cancel duplicate entries. The distinct option may need additional resources within the database, so I believe, it’s off by default for performance reasons.

Here’s my fix (the bolded lines below), which will cause WordPress to cancel duplicate results. It should be applied to the search_everything2.php file and requires WordPress 2.1+ to work (but won’t harm older WordPress releases) :

if ("true" == get_option('SE2_use_metadata_search')) {
  add_filter('posts_where', 'SE2_search_metadata');
  add_filter('posts_join', 'SE2_search_metadata_join');
  SE2_log("searching metadata");
  }            

// http://blog.saddey.net/?p=167
add_filter(’posts_distinct’, SE2_search_distinct);              

// http://blog.saddey.net/?p=167
function SE2_search_distinct($distinct) {
  if($distinct == ”) {
    return ‘DISTINCT’;
  }
  return $distinct;
}              

//search pages
function SE2_search_pages($where) {
  global $wp_query;

The fix is very basic and may apply the distinct clause, even when it might not be required as dictated by your Search Everything options. But I don’t think, there’ll be any problem with this and would like to leave the final inclusion to the author of Search Everything. Thanks to Dan Cameron for this great plugin! 

After having implemented the fix, I found, that the concept for the fix has already been outlined at WordPress Hackers - Getting double/triple posts listed on some blogs ten days ago.

Aditya has posted a fix as well, which was taken from Google Code, but I believe that my version more closely adheres to WordPress 2.1 standards (the ‘posts_distinct’ just returns a keyword for the tasks it’s supposed to instead of modifying the whole query).

1 comment.

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