The Right Amount of PowerBy CWNP On 10/06/2008 - 11 Comments
We've all heard, "Turn the power down, shrink the cell size, and you'll get more aggregate capacity in your Wi-Fi network." It's an incomplete and bogus statement. I know, you're thinking, "OK, Mr. Smarty Pants, where are you going with this...?" First, consider that Cisco's VoWLAN 4.1 Design Guide, Page 3-3, starting with the section entitled, "Co-channel Interference Considerations" starts on a 15-page commentary that spells out in great detail something that it sums up in the beginning by saying, "The reality is somewhat more complicated because the AP influences the WLAN RF environment around it for a much greater distance than just the bit-rate boundary." I give Cisco big kudos for the clarity of explanation and visual representation given of co-channel interference in these 15 pages.
Let's restate the problem for clarity:
Regardless of how much power you're using at the AP or client, transmissions will affect the RF environment around the device at a much greater range than the data transmission actually needs to go.
Cisco makes another statement within those same 15 pages that I very much agree with, "It is not an effective strategy to reduce the overlap in order to reduce co-channel interference. As users satisfaction can be greatly affected by poor roaming performance. In contrast, call capacity can be addressed in planning and design."
What they're saying here is that you can't spread the APs out far enough to mitigate this RF interference problem because you'll break roaming, which is true for most systems. So what is this blog post all about? It's about something totally different that needed this information as a pre-requisite. :)
Instead of turning the power down trying to change the ratio of "Connectivity Range" to "Interference Range", which will remain unchanged with an output power change, let's have an optimized amount of power for the purposes of 1) not overdriving the receiver if the receiver is too close, and 2) maximizing the SNR/RSSI and connection date rate. If the connection data rate is kept high, it will take less time for a frame to be transmitted across the wireless medium, there will be less chance of retransmissions, and overall interference is significantly reduced.
For some transmissions, high power may be needed to optimize the client/AP link, but even using higher power will minimize interference because it will maximize the data rate of the connection, minimize the likelihood of a retransmission, and busy the wireless medium for the least amount of time. For example, suppose a station is at a distance from the AP that means it connects at 24 Mbps. When the AP transmits a 1500 byte frame to the station, it will take 2X microseconds. If, by boosting signal power, it could change the SNR to make the data rate change to 48 Mbps, it would take only X microseconds to deliver the same frame to the station. Therefore, the wireless medium is busy for half the time.
Now, this isn't the entire story of course... If that transmission affects a somewhat-nearby AP on the same channel, the interference could cause retransmission of a frame between that AP and its own station. It "could" be a wash in aggregate throughput when you're talking about using all omni antennas, but what if you're using smart antenna technology like Transmit Beamforming (TxBF) that is used by Ruckus Wireless, Vivato, and even Cisco (with the Navini buyout)? Not only do these smart antennas form transmit beams in a given direction, but they also intelligently form nulls to reject inteference in other directions. When adding this type of smart antenna technology to "the right amount of power", SNR, data rates, and throughput can all be increased while decreasing co-channel interference.
It's a wicked combination to be sure. Will we see more of this in the market? Yes, TxBF is part of the 802.11n amendment, but it isn't in the current generation of chipsets, and understandably isn't on the Wi-Fi Alliance's testing roadmap just yet. That gives vendors who have implemented smart antenna technology a head start in the right direction of defeating co-channel interference.