Posts Tagged ‘Wireless’
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.
My thoughts on the CWTS exam from CWNP.
Last week I went ahead and took the CWTS (Certified Wireless Technical Specialist) exam from the CWNP group, now I know they consider this almost a “sales” certification but I wanted to start with this exam just to see how the CWNP group present their exams compared to Cisco, Microsoft, RIM, and CompTIA. Well I managed to pass the exam with a 90% (70% is the minimum score required to pass) so I did fairly well on the exam. I did think a few of the questions were not worded in the best way, but it appears I understood what they were asking for.
My primary sources of study was the Official CWTS study guide from CWNP, along with their practice exams hosted off the CWNP website, not to mention a few years of supporting multiple wireless networks. Now I’m not usually one to say “yea get the official study guide you’ll be ok with that” but in this situation I have to recommend the official study guide. It does an amazing job at covering the CWTS exam objectives. Plus I found it a very easy book to read there plenty of clear concise explanations with enough images that promote the text (Images also span between some of major WLAN equipment vendors Cisco, Proxim, Motorola, etc). The only downside I found concerns the material on the CD that is included with the book. While it does come with sample tests and flash cards I found a few of the questions to be incorrect, another reason to purchase the online practice tests.
Now the material on this exam are the fundamental basics of a WLAN (pertaining to the 802.11 standards, the RF spectrum, and WLAN hardware) and because of that I really do recommend this to any type IT professional who is new to working with WLANs. Whether you are help desk/field technician or even a network administrator that needs to support/implement a new WLAN this book does deserve at least one look over. When I first started working with WLANs I would have loved to have this book it would have saved me hours of research back then. However if you are like me and you have plenty of experience with wireless already I would skip the CWTS and go straight for the CWNA certification.
Now that’s a rap for the CWTS, considering my experience with this exam and the material, I will be pursuing my CWNA certification later on this year but I want to squeeze in a lot of study time for the CCDA exam before Cisco retires the 640-863 April 30th. It’s a certification I tried pursuing a long time ago but just never dedicated that much time to, however between then and now I’ve read many Cisco design guides and both 640-863 Cisco Press books. So if you notice my upcoming entries leaning more towards network design consideration that’s why.
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.
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! 