B only AP & G only client.
Last Post: January 20, 2006:
I'm with Criss. I have always assumed that "g-only" meant only "no 802.11b stations can associate." I can't imagine why one would actually disable the ability for the G stations to invoke protection mechanisms. This wouldn't improve performance at all, since the protection mechanisms and their associated overhead are only invoked when necessary. In the absence of 802.11b stations, there isn't any performance overhead for allowing protection mechanisms, because they won't be used. In the presence of 802.11b stations, disabling protection mechanisms is like putting one's head in the sand.
Great discussion gentlemen.
First, I'm not sure I agree with Joshua on your last post. There are situations where, as Criss said, disabling 11b data rates altogether does provide throughput advantages. I have seen this most when your particular enterprise is not using 11b, but someone around you is. If an enterprise 11g AP even thinks that there is a 11b device around, it will invoke protection. In a lab environment when I am trying to demonstrate protection, all I have to do is insert an 11b Orinoco card into a laptop and in about 5 seconds it has gone into protection. I guess a neat thing is that it goes out of protection in about 15 seconds after disabling the card. (This test was done on a Cisco 1200 series AP). Protection was enabled just from a few probe request frames. I didn?¡é?€??t even have to set the 11b card to associate with the AP. I agree with Criss, where you have many 11b clients in your building, it may be advantageous to just let the AP go into protection.
Now, speaking about the Linksys that Joel brought up. I happen to have a few kicking around so I thought I would see how it handles "g only" mode and "mixed" mode. Here is what happens to the data rates when it is in "g only" mode:
Element ID:ID: 1 Supported RatesRates:
Supported Rate:Rate: 0x82 1 .0 (BSS Basic Rate)
Supported Rate:Rate: 0x84 2 .0 (BSS Basic Rate)
Supported Rate:Rate: 0x8B 5 .5 (BSS Basic Rate)
Supported Rate:Rate: 0x96 11 .0 (BSS Basic Rate)
Supported Rate:Rate: 0x24 18 .0 (Not BSS Basic Rate)
Supported Rate:Rate: 0xB0 24 .0 (BSS Basic Rate)
Supported Rate:Rate: 0x48 36 .0 (Not BSS Basic Rate)
Supported Rate:Rate: 0x6C 54 .0 (Not BSS Basic Rate)
Of course you will notice that all it did was make 24 a required data rate. In mixed mode, it just removed the requirement of 24MB.
Now, here is the weird thing. In mixed mode, the Linksys AP did NOT invoke protection. No matter how many scenarios I ran, I could not get it to go into protection mode. Now that I am thinking about it, I wonder if the engineers did this, hedging the fact that they know how performance can suffer (due to CTS frames) when even one 11b client is present. Thoughts?
Linksys WRT54G Version 2.02.2 released
This firmware, posted 1/28/2004, supports all versions of the WRT54G (v.1, v.1.1 and v.2) and has the following changes:
- Updated wireless driver to support all versions of WRT54G hardware
- Adds support to allow WAP54G connect as a repeater
- CTS protection mode set to disable by default to improve wireless performance in normal environments
- Resolved issue where the WLAN LED stays on even when wireless is disabled
- Resolved security vulnerability causing the web server in the router to crash
Nice find compughter! I guess I could have saved myself some time if I had RTFM. :)