Archive for November 2010
Wireless Networking and the 5 GHz RF Range.
As I speak with other IT professionals concerning wireless networking, one thing that seems to shock people is when I start talking about the 5 GHz RF range. Usually the first thing they say is along the lines of “You are still using that?”, most people still see the 5 GHz range associated with the 802.11a standard and nothing more, while it’s true potential is finally coming to light (and people are now seeing the limitations of the 2.4 GHz frequency).
Since this topic can get in depth, and I prefer to keep my posts to a decent length, and to the point, we will jump into the advantages of utilizing the 5 GHz range:
- Less congestion, anyone who has been administrating or implemented a wireless knows how many other devices are using the 2.4 GHz range everything from BlueTooth devices (which is found in almost every phone), microwaves (found in office lunch/break rooms), to cordless phones. More particularly microwaves and cordless phones they will congest the 2.4 GHz spectrum without regard for any other device using the RF band. The 5 GHz does not suffer from as much interference as the 2.4 GHz range does, of course proper survey’s should be done prior to rolling out a Wi-Fi network just to be sure.
- More non-overlapping channel, the 5 GHz range consists of 3 bands. These bands provide us with 21 non-overlapping channels this gives us the ability to more densely pack an area with 802.11a/n access points. Decreasing the amount of clients per AP (With proper load balancing) providing increased throughput, and making roaming a seamless process. Where as the 2.4 GHz range only gives us 3 non-overlapping channels (1, 6, 11). Detailed information on each UNII band can be found below.
- Channel Bonding. While you can perform channel bonding in the 2.4 GHz it is better suited for the 5 GHz range. Channel bonding is how you achieve speeds up to 600 Mbps in 802.11n it does this by making the channels 40 MHz wide compared to 20 MHz wide. Channel bonding at the 5 GHz range still leaves you with 12 non-overlapping channel, while channel bonding in the 2.4 GHz range gives you 1 (possibly 2) channel.
- Future use. The next wireless standard after 802.11n, is most likely going to be 802.11ac which is promising us Wi-Fi speeds in the Gbps’s it plans to accomplish this by using 40, 80, or 160 MHz wide channels this is going to rule out the 2.4 GHz range completely. (Unless it’s changed.)
- UNII-1/Lower Band (5.150 to 5.250 GHz) Non-overlapping channels 36, 40, 44, 48
- UNII-2/Middle Band (5.250 to 5.350 GHz) Non-overlapping channels 52, 56, 60, 64
- UNII-2 Extended (5.470 to 5.725 GHz) Non-overlapping channels 100, 104, 108, 112, 120, 124, 128, 136, 140
- UNII-3/Upper Band (5.725 to 5.825 GHz) on-overlapping channels 149, 153, 157, 161, 165
Cisco Band Select.
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
CCIE or Null! 