Posts Tagged ‘Wi-Fi’
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.
Understanding a Wi-Fi connection.
Just some more details on how drastically different wireless networks differ from the traditional wired network is understanding the client connection. Surely we all understand how the wired connection works, we plug in a cable two of the four pairs carry data then speed and duplex setting are auto-negotatiated. However when you look at a wireless client you see an antenna, signal strength, data rate, RSSI, power level, and SNR values definitely a little more to think about.
I’ll start with RSSI, which is the Received Signal Strength Indicator this value is typically shown as a negative dBm value (dB and watt values are a topic for another post). RSSI is the measurement of power in an RF signal, the more power in an RF signal the better the connection quality is. So the closer this value is to 0 the stronger the signal is. So a value of -61 is stronger then a value of -74. Now different vendors do have different scales some vendors will have a max value of -100 while others go higher or lower, of course signals that weak should be avoided (and probably won’t work anyway). So it’s best to get some documentation from the vendor of your client WLAN cards to see the RSSI value range. The value of the RSSI will also play a role in the connection speed, and once again vendor documentation will provide the RSSI value to link speed ratio (and do keep in mind many other factors play a role in the connection speed as well).
SNR is the Signal to Noise ratio, this is how much stronger the wireless signal is compared to the noise floor surrounding the WLAN client. This is shown in a positive dB value. Too much RF noise around the WLAN client will cause collisions resulting in frames being retransmitted thus lowering the throughput of the connection. Try connecting a cordless phone that works in the 2.4 GHz range right next to a b/g access point, the phone can generate enough RF noise to cancel out the wireless signal completely. It’s typically best practice to have the SNR value 20 to 25 dB’s away from the RSSI value. So to go back to our previous example if our RSSI is -61 we would want our SNR value to be around -86, or if our RSSI is -74 we would want the SNR to be -99.
The data rate can be one of many values depending on which wireless standard you are connecting with. Be aware though that wireless is a shared medium so it’s half duplex it can not transmit and receive at the same time. So your actual throughput will be about half of what your client is connecting at. A WLAN device showing a connection of 54 Mbps will really have throughput of maybe 30 Mbps. Throughput can be tested using nice little utility called iperf which is available on both Windows and Linux platforms for free.
The power level is measured in mW and depicts how much power a WLAN device is using to maintain the connection. Its typically best practice to design your WLAN infrastructure so your devices operate at half their max output power. This way if an AP goes down neighboring AP’s can double their output power and maintain the availability of the WLAN.
So the overall signal strength/quality registered by client will be a mixture of all those variables.
Below is a screen shot from the Cisco Aironet Site Survey Utility
Here you will see the RSSI at -50 dBm and noise level of -96 dBm, resulting in an SNR value of 46 dB. This utility will also provide you with the BSSID (MAC Address) of the AP you are connecting to along with the RF Channel, 64 in this case utilizing 802.11a.
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
CCIE or Null! 