I was doing a kick off meeting in a power plant and explained the whole project design and was asked the question "Why use RF profiles?" The argument from the other person was that the device itself makes the decision on when to roam, so why bother?
When designing a wireless network, you set the survey AP at or below the power of the devices to be used on the network. Setting up RF profiles allows you to keep all of your access points at those power settings after deployment and then some.
An RF profile is applied to a group of access points of your choosing. This profile can have many parameters set to maintain certain controls over the access points and two of them are the power settings and the data rates.
Like I mentioned above, the power settings on the access points need to be at the level they were when you surveyed or the survey is void. You can't survey at 14dB and then allow the access points to operate at 17dB or 20dB (and if you do, there is a sign there that your survey could have flaws). The survey itself should reflect what the live deployed network is going to look like. The RF profile will help to accomplish this.
You can set the power in the profile to the 14dB limitation just as the survey was performed at, and you can also set a minimum power that you do not want the access points to drop below. Beyond that, you can also control the data rates allowed per access point.
If you surveyed correctly for a VoIP deployment and set your cell edge at -67dB or -65dB, then you do not want to allow any data rated on your network that can be achieved at a lower dB level (check your access point's specification chart to see these levels). Mostly, a -67dB cell edge would not allow for any data rates below say a 56MBs rate so you would want to turn off 48MBs and below in your RF profile.
What does this do? It does two important things worth mentioning right away....
1) it maintains the original cell size that you surveyed for.
2) it maintains excellent throughput per client device.
How??????
Maintaining the original cell size is done by holding the allowable data rates at what you surveyed for. If 48MBs or 36MBs were allowed on the access point, then the actual cell size would be bigger than the one you surveyed for at -67dB. If you do not turn off any data rates and allow for 11MBs and below, then the cell size is 4 and 5 times larger than the one you surveyed for at -67dB.
Why?????
The data rates are all based on proximity and modulation type to say the least. There are other factors such as noise floors, co-channel interference, etc... but lets look at the proximity and modulation types.
When an access point transmits through the air, there is a factor called free space path loss that plays a role in degradation of the signal. The lower the signal, the lower the dependable data rate becomes. Once a lower dependable data rate is chosen, it is more than likely because a different modulation type has come into play in order to get that dependable data rate at said distance.
Now... if you do not allow those lower data rates on your network, then all devices have to be at a closer proximity to the access point in order to connect to it, and they are forced to connect at only the higher data rates allowed on the access point. but guess what else happens........
By forcing the lower data rates out and forcing the client devices to only use higher data rates at a closer proximity to the access point, you are also forcing the client devices to roam quicker than they normally would.
Wait!!!!! Did that come out right????? An access point forcing a client device to roam?????
Yes. By controlling the data rates, you control a portion of the roaming as well. A device can no longer be sticky and hold on to its original associated access point until it rate shifts all the way out to 1MBs because you don't allow 1MBs on your network. The device can rate shift from 150MBs (if available) down to 54MBs and then it has to roam to a neighboring access point before the original access point's signal drops any lower as it moves away from it.
Ultimately.... You have controlled the health of the wireless network by having less clients per access point and they are staying connected at higher data rates.
This all results in a better customer experience, and that is great for you.
Brett Hill, CWNE #147
Can I ask why you are turning off everything 48 and below for VoIP?
ReplyDeleteSure. If you are surveying, then you are specifying what signal level and data rates you want to have inside the cell size that you have surveyed. If you specify a dB level for the edge of your cell then you are clearly wanting to control the area each access point covers. In other words, I want to have all my devices within the range of each of my access points to connect at a minimum data rate of 54mBs on my A/G radios, so I survey for that data rate at the edge of my cell. If I leave any lower data rates turned on, then my cell size is no longer the cell that I surveyed. i.e. the lower the data rate, then the further the transmission. So.... What ever data rate you want to have at the edge of your cell, survey for it and turn the others off so that devices in the area of other cells will not be connected to the wrong access point at a lower data rate than you desired in your survey. This allows you to control the rate at which devices connect, and it controls the overall health of your wireless network by keeping all devices at a minimum that they can connect at.
ReplyDeleteThis also allows your access points to force client devices to only connect at the higher data rates you specify, and it allows the access points to force client devices to roam between access points at higher data rates rather than rate shifting all the way out to 2mBs or 6mBs before it decides to roam. If you allow your access points to transmit on all allowed data rates then most of your client devices will "Stick" to their original associated access point until it rate shifts its way past several other access points it could have roamed to at higher data rates.
As for VoIP only results, the industry standard is to survey for a -67dBm cell edge with a noise floor of -92dBm. That's a SNR of 25dB. Most all access points have their minimum data rate right around 54mBs at an RSSI of -67dBm.
Bottom line for the blog post is to only allow the data rates that you have surveyed for in order to have a healthy wireless network, or else the survey didn't serve a good purpose.
Hi Brett,
ReplyDeleteGreat post BTW!
To add to Jakes comment.
I get -67dBm cell edge. However i'm very interested on the data rates you assigned to VoIP.
What type of clients are you designing for?
For example if you are planning for a Cisco 792x Wireless client / Spectra Link phone, how do you account for the aggressive roaming needed between the AP's and whether the device firmware supports it as every vendor plays different in this space.
With the Cisco Wireless phones, typically their sweet spot can work well in a range of 18~ 30Mbs with a max of 54Mbs depending on the model.
I would also assume that your AP spacing would be very "tight" so to speak?
I totally understand taking advantage of higher rates if you were designing for Factetime/Lync/Skype Client etc on a Mobile Device /tablet/notebook. Maybe we are cutting it a bit to close?
Looking forward to your thoughts as I'm really intrigued by your design method.
@WirelessStew
-Stew
Stew, Check out the reply above to the question. It will more than likely answer your question. In a nutshell, the idea of the survey is to allow the data rates you want to use for the clients you have selected and then turn off the ones below that minimum data rate to provide a healthier environment for your network.
ReplyDelete