Posts Tagged ‘5 GHz’
Control roaming behavior on your Cisco wireless network.
Roaming is just another expectation from your end users. They expect to walk freely around the office to conference rooms or far off cubicles and have their laptop or handheld remain connected while downloading files or in the middle of a conversation. If the roaming process is not quick enough then you could see conversations and clients gets dropped forcing them reconnect to the WLAN, and I can guarantee you your end users will be calling.
Now, if you have done a proper site survey and have solid data to work off of, you can control the roaming behavior of your WLAN clients. The reason you need to know the details of your wireless environment is because you are going to set RSSI limits concerning when your clients should begin looking for a new AP to associate to and, how quickly they are roam between access points. Just keep in mind, making these settings will effect the entire WLAN not just individual sections.
I would also like to mention your clients should be CCXv4 or higher to take advantage of these features. To see if your clients are CCXv4 compliant go to Monitor -> Clients -> click on the client in question.
On your Cisco Wireless LAN Controller, you want to navigate to Wireless -> 802.11a/n or 802.11b/g/n (depending on which frequency you want to customize) -> Client Roaming.
The first thing you need to do when you want to customize these settings is change the mode to custom this will allow you to edit the default values for the rest of the parameters.
The next option is minimum rssi. If a clients RSSI value is below this threshold it will not associate/authenticate to the access point, instead it will continue to look for a better signal from different access points. Valid values for this field are -80 through -90. The understanding is that the signal strength/quality will be so low reliable communication will not be established.
Next we have a setting called hysteresis this value is in dB and states how much stronger the signal of another access point has to be before a client decides to roam to it. This is useful if you have multiple access points in close proximity of each other or clients are moving between the edge of coverage of different access point. The higher this value the closer a client needs to be to an access point for it to associate to the second access point. Valid ranges are from 2 through 4 dB.
Now we have the scan threshold this is another RSSI value range. When the wireless client’s RSSI drops below this threshold the client will begin actively scanning for another access point it can receive a stronger signal from. Valid values range between -70 through -77.
The last field on the page is the transition time this is the amount of a client is going to see a better signal from neighboring access, before it attempts to associate to the second access point. The client determines a better signal when its RSSI drops below the scan threshold and it sees a signal from a neighboring access point higher than the scan threshold.
So all these factors do work together and can be customized for your environment. Normal data traffic is more forgiving since it’s not as delay sensitive, but If you have voice on your WLAN you will want to fine tune these settings to avoid dropped calls.
Cisco band select and client RSSI.
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
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.
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! 