How do Wireless Clients decide which band they will use?

[Craddockc]

I know this seems like an elementary wireless question and it is definitely one I should know since I am CCNA-Wireless certified, but how do wireless clients choose which band they will use? For instance: If a wireless client has a wireless NIC capable of 802.11 a/b/g/n and the dual band AP offers up these same freq ranges, how does the client choose whether it will use a/g or n? ( I dont even allow 802.11b associations on my network). Is this decided in the authentication/association exchange between AP and client? If so, how is this decision reached? Id like to know so that I may learn how to influence these decisions. I can't seem to find the answer anywhere, my Cisco books all discuss the different authentication methods at length, but make no mention of excatly how a specific band is chosen to be used. Any information you can provide would be great! Thanks so much!

Chris.

[Scott Fella]

Chris,

This have been asked for years:) bottom line, the client makes the final decision. The beacons have information that tells the client device about its utilization etc. the algorithm used on the client device decides to associate to 802.11a or 802.11b/g. My MacBook always connects I the 2.4ghz for some reason but doesn't bother me. Many would set their options on the wireless card advanced tab. I will try to find some good docs.

Thanks,

Scott Fella

Sent from my iPhone

[Craddockc]

Scott,

Thanks for the speedy response. I am glad that I am not the only one who has inquired about this. I know that (atleast in Windows) you can force the client NIC to only use a specific band by setting it in the properties, but I was really curious as to why a client would choose one band over the other given the option. In my mind, if a client is offered 802.11n (in either band) I would assume that it would always choose 802.11n, but I could be wrong. I would love to read whatever literature you could find on this subject. For some reason I cannot seem to find any. Thanks so much!

Chris.

[Scott Fella]

Don't ever assume:). Most clients will choose N, but which band is the question. I have tested many devices and each does it's own thing. I got to the point where... I don't care:) as long as the connection is solid I'm happy. Drivers are the biggest culprit but again... Let me try to find some docs.

Thanks,

Scott Fella

Sent from my iPhone

[grabonlee]

It's really hard to say as the laptop wireless cards do not have a monitor that shows which band it starts with first. My Cisco external card always scans 5GHz channel before 2.4GHz channel. In some literature, it's been mentioned that MACs scan 5GHz first.

[m.graham]

I'd be interested to know this as well.  My Macbook always seems to connect to the 2.4Ghz channel first at 144Mb/s.  If I shut the 2.4Ghz radio down, it then connects to the 5Ghz at 144Mb/s which is still not the top speed.  However if I the turn the wifi off on the Mac and back on again, it connects to the 5Ghz channel at 300Mb/s.  This would appear to be the most sub-optimal way to connect so I'm wondering if I've missed something out on the AP config or has someone really configured the driver this badly?

[Scott Fella]

Thats Apple for you.... even my Apple TV initially connects tot eh 5ghz, but later is associated to the 2.4ghz.