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Posts Tagged ‘Client Band Select

Cisco band select and client RSSI.

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I wanted to quickly touch on one more parameter pertaining to Cisco band select and that would be the acceptable RSSI level that client can register for it to be able to participate with band select.

The key thing is understand what the RSSI signifies to a WLAN device, it’s a received signal strength indicator it shows you in a numerical reading (usually dBm) of what the signal strength is of a wireless client.

The closer to 0 (zero) the value is the better the signal is and the better the link speed (of course the speed is also dependant on many other factors). The key to tweaking this is to know what client devices you have in your network, then you want to know the specifications of those client devices and see what RSSI values correlate with what link speed (You should be able to find that somewhere in the manufacturer’s website or in their documentation). Then you will need to decide what link speed is acceptable for your environment.

Here are the details from the Cisco Aironet a/b/g PCI Desktop Adapter

• -87 dBm @ 6 Mbps

• -87 dBm @ 9 Mbps

• -87 dBm @ 12 Mbps

• -87 dBm @ 18 Mbps

• -82 dBm @ 24 Mbps

• -79 dBm @ 36 Mbps

• -74 dBm @ 48 Mbps

• -72 dBm @ 54 Mbps

So judging from you could probably change the acceptable RSSI value to 81 dBM, this way any dual band clients will connect at 36 Mbps and above. (Just keep in mind different WLAN client devices register RSSI values on different scales so the above example is not going to fit well for everyone out there.) This setting may also take a bit of trial and error as well, because if set the acceptable RSSI too high not many dual band clients will connect to the 5 GHz range, set it too low and clients may just roam to the 2.4 GHz range very quickly because they may quickly be out of range of any 802.11a signal.

Written by Stephen J. Occhiogrosso

December 20, 2010 at 11:43 PM

Cisco Band Select.

with 2 comments

Thought I would shift gears to wireless for a little bit. Cisco introduced a feature some time ago called band select were the dual band clients have a better chance at joining the 5 GHz radio compared to the 2.4 GHz range. This is mainly due to the influx of dual band clients nowadays and how the 2.4 GHz range is generally over utilized.

The Cisco accomplishes this is by ignoring/delaying the first few 802.11b/g probe frames in hopes of it accepting the 802.11a probes because it will appear to have a quicker response time. I would also like to point out that this feature only works when the client first associates to the Access Point. So this feature will not kick in on the fly when the AP notices a high client count or high channel utilization. Plus this feature only goes in one direction from the 2.4 GHz range to the 5 GHz not visa-versa. So this is not a load balance mechanism.

This feature is configured very simply all from one screen in the WLC, under Wireless -> Advanced -> Band Select:

Now you’ve only got a few settings to configure here, but you still need to take care with these settings like anything on the network you are going to configure. Probe Cycle Count, tells the AP how many probe beacons/frames to ignore/delay. Scan Cycle Period Threshold tells the AP how often in milliseconds it can expect each probe from the client, this setting can be changed depending on the client Wi-Fi cards you are using in your environment and how often the send out probe requests (Check vendor documentation for this). Age Out Suppression, this is the time-out for when the clients will be declared as “new” and may have their probe frames delayed/ignored again. Age Out Dual Band is the very similar to age out suppression, however age out dual band only applies to dual band clients so it will not effect everyone. Just keep in mind something will need to happen for the client to disassociate and re-associate with access point. Acceptable Client RSSI just states the minimum RSSI value a client registers for it to be eligible for band select.

Also keep in mind this feature can be controlled per-WLAN, under the “Advanced” tab

This can also be done via the CLI of the WLC using the following commands:

config band-select cycle-count cycle_count

config band-select cycle-threshold milliseconds

config band-select expire suppression seconds

config band-select expire dual-band seconds

config band-select client-rssi client_rssi

config wlan band-select allow {enable | disable} wlan_ID

And if you want to verify the band select configuration use the following command:

show band-select

Written by Stephen J. Occhiogrosso

November 10, 2010 at 3:00 PM

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