802.11 stuff

Read it again. Take note of:

: One of the major differences between 802.11b and 802.11g is : protection, which is reguired... : : [...] : : Protection is activated whenever there is a need to ensure 802.11g : stations do not interfere with 802.11b stations. : : [...] : : Protection is activated when a non-802.11g station associates with the : network, as well as when non-802.11g-capable stations are transmitting : in the area. : : [...] : : By shunning stations that are not capable of 802.11g speeds, there is : an increased probability that protection can remain disabled. Of : course, a station that is shunned may very well find a different : network that can accomodate it on the same channel, which would still : trigger the activation of protection.

When you put the AP in 802.11g mode and turn off Protection, it will run in 802.11g mode without protection until the first 802.11b station in the area starts transmitting. Even when this 802.11b station ends up associating some another AP on a different channel, the station will have scanned all available channels and triggered the activation of protection on the 802.11g-only AP, halving throughput.

And when the AP has a "manuel switch" for activating protection, protection will stay on until the AP is rebooted (or the Protection "switch" in the AP setup is "toggled"). Otherwise, there would be no reason for having the Protection switch in the first place.

You speculate. How many 802.11b/802.11g APs have you actually compared?

Quite a few--it's part of my work. How many have you tested?

Steve

Hmm! An 802.11 professional, unfamiliar with 802.11g 802.11b protection.

You've published your comparisons where?

I'm not allowed--I work for a company that produces products (I do the HW, not the SW), so the results are proprietary.

Steve

Nope. Please read:

before blundering onward. Note that there are at least 3 ways to handle 802.11b compatibility when running 802.11g, none of which resemble your terse description. The original tests were performed in

Also, you might want to look at the table of speeds in the FAQ:

or if you're a non-believer, the original Atheros article at:

Max TCP thruput with 802.11g 54Mbit/sec association is 24.4Mbits/sec. Max TCP thruput with 802.11g 54Mbit/sec association and 802.11b traffic present is 14.4Mbits/sec.

The problem is that 802.11b broadcasts beacons and preambles at

Nope. I've done benchmarks using iPerf and jPerf showing that some chipsets are much better. Early 802.11g chips were disgusting. You didn't even need to associate with the AP to get a 50% slowdown. The AP would just sit there half the time listening for possible 802.11b traffic. Needless to say, benchmarks in 802.11g modes were slothish. You can see some of those on SmallNetBuilder. I'm too lazy to dig them out right now.

The last time I ran a test on my Buffalo or Linksys Broadcom chipset router running DD-WRT, there was almost no detectable slowdown when I associated with a WAP11 wireless bridge, but generated no traffic. Benchmarks with and without the WAP11 were almost identical. However, when I started streaming some really low bandwidth music (about

I don't know any AP's that have such a hardware switch. I'll assume you mean a software or configuration switch. In all routers that I've seen, the choices are roughly: 802.11b only 802.11g only 802.11b+g mixed. Is this the switch or did I miss something between the lines?

I beg to differ. With the typical rubber ducky 2dBi ommi and -2dBi PIFA laptop antenna, you're correct. 2.4GHz will give better range than 5.7GHz, all other things being equal. However, if you replace the junk antennas with external antennas OF THE SAME PHYSICAL SIZE, the 5.7Ghz antenna will have about twice the gain (3dB). If both ends have such an antenna, the 6dB total gain will yield twice the range at

As for building penetration, that's a crap shoot. 5.7Ghz will go through holes and slots much easier and better than 2.4Ghz. If the building looks like swiss cheeze, 5.7Ghz will work better. However, if the building is a solid wall of something (doesn't matter the material, just no holes are allowed), 2.4Ghz will penetrate better.

See Table 1 and note that some materials have higher attenuation at

Compared for what? Max thruput? For *WIRED* routers see:

For *WIRELESS* routers:

There are not results for 802.11b impairment testing (yet).

Note that almost all the routers listed are 802.11n routers. The only

I'll give you that, because he wavelength of the 5GHz signal is smaller than the 2.4GHz signal, so you may have a whole wavelength in antenna size instead of half a wave. Also, vendors are turning up the transmit power on the 5GHz channels to compensate.

That's the problem--in typical residential construction, the drywall is the main barrier, and that's a problem for the shorter wavelengths.

Steve

Where does it say "hardware"?

Trendnet TEW-510APB, 802.11a/b/g, with B/G mode usualy turned off.

It doesn't. Your description was vague enough that I had to ask whether this mysterious "manual switch" is a real switch or a virtual software switch. Could I trouble you to identify this "manual switch"?

Perhaps I'm a bit dense this morning, but I don't see anything on that page that looks like an 802.11b protection on/off switch. The only item which seems involved is "Allow 54Mbs Stations Only" which seems rather vague. Does it turn off 5.7GHz? Does it disable speeds above

So, where is the disable 802.11b compatibility mode "manual switch" you previously mentioned? Did I miss something (possibly on a different web page)?

What's your question got to do with 802.11g 802.11b protection?

The switch on the fourth-, third- and second bottom line turn off

Below is another example, from the (CD-) manual for my ASUS WL-330gE multi-function pocket router:

"Wireless Mode

This field indicates the 802.11g interface mode. Selecting "Auto" allows

As I read the last part, considering that 802.11b protection on 802.1g devices is required - see earlier posts, it means that "54g Protection" will be on all the time when not checked.

Can't you read AYBABTU?

Here's another one, from my routers firewall.

"If checked, the undefined item in IP Address list is allowed to access Internet;Unchecked means reverse."

Why does asking you a question usually result in another question instead of an answer? I guess I'm doing the same thing. Do like I say, not like I do.

I was assuming that your mysterious "manual switch" was the "Allow

However, I did make a bad guess. 802.11b compatibility is only an issue at 2.4Ghz, not 5.7GHz, so the setting does kind sorta make sense if "54Mbps stations only" really means "use 802.11g only".

I don't think so. Most of that is CTS/RTS "protection", not 802.11b protection. It protects against collisions caused by hidden nodes (where multiple clients can't hear each other and therefore tend to transmit simultaneously). At least that's what I think it means.

Ok. Now I see. Your "manual switch" is really a setting which completely disables 802.11b connectivity. Different router, different names, different description, but probably the same thing.

However, the original problem was over how an access point actually handles mixed 802.11b and 802.11g clients. I don't see anything that affects this except to totally ignore 802.11b clients. (Don't assume that I know how your chipset does this. I'm a few years out of date).

This is the first time I've ever heard it refered to as "54g protection". However, I must admit that it sounds better than "Disable 802.11b mode".

In the distant past, I used to work for Standard Communication in Wilmington CA. We sold VHF/UHF marine and land mobile radios. I still have many of the manuals that were first written in Japanese, and then translated into what they considered to be English. SCC hired a full time technical interpreter to clean up the mess. For the few that I helped re-write, it was easier to start from scratch. Direct translations of idioms were the major problem. I don't think I would do any better translating my English into Japanese.

Yech, but typical. A friends son is currently doing classes in Chinese at Humboldt State. He now talks and writes like that. There seems to be a cultural thing against being specific or fortifying a statement with a redundant statement. I do it conciously by avoiding the word "it" unless it's clearly defined (or doesn't matter). That's apparently not the case in Chinese, where brevity seems to be desireable. It also reminds me of some student papers.

I'll do the jperf benchmark test for 802.11b compatibilty sometime this week, after I drag home an 802.11b only client radio (WAP11). I'm curious how different routers respond.

The number of routers involved may be interesting as well.

"There are three ways an AP will go into protection:

- An 11b device associates to the AP.

- An 11b AP on the same channel can be heard by the AP

- The AP hears an 11g AP that is in protection because of an 11b device associated.

The last statement is the hardest to explain, so here is further explanation:

Non-ERP (11b) Present: 1 = Yes Use Protection: 1 = Yes

An 11g AP that hears that will go into protection and have the following Beacon bits:

Non-ERP (11b) Present: 0 = No Use Protection: 1 = Yes

Now, lets say another 11g AP heard the one above. It will not go into protection because the one it heard does not have the Non-ERP Present bit on. "

I'm afraid you're wrong.

Hidden node protection is set by twiddling the RTS/CTS Threshold knob. You'll notice there are two such knobs on the screenshot, one for when the radio is on the 5GHz band - the top pane - and one for when the radio is on the 2.4GHz band. There's only the one radio in this model AP. And you don't see the 802.11b protection switches in the top pane, because it's not needed for the 5GHz band.

Not all 802.11g AP's have 802.11b protection switches. My D-Link DWL-G700AP and DWL-G730 pocket router do not, f.ex. It's an ekstra feature. However, they do have an RTS/CTS Threshold knobs.

Probably, but I'll never admit it. So, why is it called "CTS/RTS Protection Mode" and "CTS/RTS Protection Rate"? CTS/RTS flow control has nothing to do with "802.11b protection" (which really should be "802.11b legacy compatibility". What does the "help" button say about those two settings?

I beg to differ somewhat. Most routers have three adjustments when dealing with flow control. 1. CTS protection mode on/off 2. Fragmentation threshold 3. RTS threshold The first turns the mechanism on and off. The 2nd determines the maximum packet size. Smaller packets have a higher probability of actually getting delivers. The 3rd determines the minimum packet size that will *NOT* require flow control. That's because the CTS/RTS overhead for dealing with small packets has a huge effect on throughput. Small packets don't get or need flow control.

I can sorta see some vestige of the common terminology in the Trendnet wireless setup page. If my astute guess is correct, they've either split the CTS/RTS settings into two places, added some new settings to deal with dual band operation, or totally mislabeled what you assume is control over 802.11b compatibility. I think I could make a better guess if I knew the options available in all the settings. If my guess is correct, there's absolutely no setting on that page involving

Incidentally, on my wish list of router features is the ability to set parameters for different bands on seperate pages. Mixing them together, on one page, like most router manufacturers like to do, is confusing.

Is there an online emulator for Trendnet?

I just checked a few Linksys routers at:

for how they handle 802.11b compatibility. Most have no fine control over compatibility, giving only "off" or "auto" as choices. Those with 802.11n features add "N only" to the confusion. Not much extra in the CTS/RTS flow department.

I like simple theories. If it says CTS/RTS in the setting, it must be CTS/RTS and not something else. Check the Trendnet help button.

The number of interruptions and crisis problems that arrived yesterday may also be a problem. I have to do this at home because there are too many active wireless networks near my office. With a glass wall, I can't easily block them (tin foil curtains?). Benchmarking done at my office has been eratic. At home, I have the absorptive shielding provided by a wall of redwood and fir trees. They absorb 2.4Ghz like a sponge. However, I've been getting home at midnight lately and the last thing I need is homework. Maybe next weekend.

So, now I have to drag out the protocol sniffer (Wireshark on Linux) to verify the protection mode. Without some way to determine if it's in 802.11b compatibility mode or not, this is not going to be easy (or consistent).

I was planning to just repeat a previous test, adding a few additional cases:

1. 802.11g benchmark with no 802.11b device present. This should be the fastest.
2. 802.11g benchmark with 802.11b client device spewing beacons, but not associated or moving traffic.
3. 802.11g benchmark with 802.11b client device associated, but not moving any traffic.
4. 802.11g benchmark with 802.11b client device associated and moving low levels of traffic (i.e. streaming audio).
5. 802.11g benchmark with 802.11b client device associated and moving as much traffic as possible (i.e. file download).
6. A few of the above with CTS/RTS flow control enabled and set for fairly small packets (512 bytes). I've always been suspicious that CTS/RTS flow control is really only effective in homogenous systems and will screw up throughput on systems with a wide mix of speeds. Also, some tweaks such as CTS only, if available. (This one is a real time burner).
7. Try most of the above with the AP speed locked to 54Mbits/sec to see what breaks.

Nice find. I never thought to search there.

I got somewhat lost in the timing calculations as presented. I noted that he neglected any mention of the 1Mbit/sec beacons that are required for 802.11b compatibility. These are large airtime hogs.

I have a copy here, somewhere, maybe... He also assumes that both

Go back in the thread, to my first follow-up to Steve Fenwick's post. It has

Message-ID:

and take a look at Gast's Fig 14.1, Basic overview of protection mechanism. And the text above. That should explain it.

(Don't worry about the language in the Google stuff. For some reason it defaulted to Danish).

Methinks not so basic. I found your link for it at:

Well, I have that book but none of the 802.11b protection stuff is in there. However, I can see why. There are apparently multiple additions of the book. The version that Google books finds is at:

Chapter 14 has nothing on 802.11b protection (unless I missed something).

Customers cometh... more later.

You've got the first edition. That edition covers 802.11b and 802.11a. Not 802.11g - and hence not the bit about 802.11g 802.11b protection.

The Google Books link (the Tinyurl) is to the 2nd edition's figure 14-1 (not 14.1). I did a Google search using the string: "Basic overview of protection mechanism" - with the quotes.