1MB rate required???
Last Post: October 22, 2005:
I have a debate with a collegue about the requirement of the 1MB data rate in 802.11b. My understanding is the management beacon is broadcast at this rate and can't be disabled. If this rate is disabled your wireless clients would not operate. The ultimate question is 1MB rate required and would ever want to disable this date rate? By the way I'm using brand new Cisco access points if this helps. Thanks....
Beacons are broadcast at the whatever the lowest data rate that is required on the AP. So if you disabled the 1 and 2 Mbps data rates on the AP and 5.5 was required.... your Beacons would be Broadcast at 5.5.
For capacity reasons, it is actually a recommended practice to disable the two lowest data rates on the AP. Due to the half-duplex nature of the medium, if too many stations are transmitting on the outer edges of a 1 Mbps cell... the effective result is that it brings down the percieved throughput of all your higher data rate users. A 1500 btye frame takes 11 times longer to transfer at 1 Mbps than it does at 11 Mbps.
Dang... I posted this reply last night, but I replied to the wrong post! Anyway, this is a repeat of what Coleman said, but here it is anyway. :)
I hope you didn't bet too much money. :) You can disable the 1Mbps data rate on Ciscos and other brands as well. Here is what I did:
In my Cisco 1232, I enabled 5.5 and 11 Mbps only, and disabled all other b and g data rates. When I applied the settings, it change 5.5 (the lowest I selected) to required instead of enabled.
I then did a packet capture using Airopeek to see the results. I took a screen capture of it, but I didn't have the time to upload it anywhere right now. Anyway, what it shows is beacons at 5.5, and data at 11Mbps.
Inside of every beacon and probe response frame (both come from the AP) is "capability information", which basically tells the connection STA what the AP requires and what data rates it supports etc. That way, the STA knows at what rate it can send packets, depending on the type.
What is the advantage? I have never tried this setting in a live network, but I can speculate on the pros and cons.
Pros: You will never have any data traverse the BSS that is slower than 5.5Mbps. That could have advantages in terms of network performance.
Cons: If you require data rates on the AP that the client is not capable of, then the client will not associate with the AP. Also, by only allowing such high data rates, you limit the range of the AP, which can be a good thing if you are in a dense user environment.
I would be interested in hearing if anyone has experience with doing this and the results. And, I'm sure Criss has something insightful to say on the topic. :)
As Criss would say.... hope this helps.
Thanks, for the information.... So could I require only the 11MB data rate and disable the remaining data rates to get the best throughput and coverage only for 11MB? This obviously would keep a required data rate for beacons and narrow my clients to connect to the highest rate of 11MB. Or is it advisable to have at least two rates for good handoff. The building I'm referencing his a high number of access points.
You could do that, but since your AP's are already installed it may pose coverage problems. If the site survey and subequent installation wasn't done with only 11Mbps in mind, you will likely have dead spots where you currently have 5.5Mbps or lower coverage.
In environments where high user density is required, what you are proposing is a good solution. Your original post talks about 11b and not 11g, so do you have handheld and/or VoWLAN clients?
It would be good information if you could post any results of testing and implementation. I'm sure a lot of people on the forum would appreciate it. Take care!