By Jim Portaro, Principal and Founder
People like the freedom of “no wires” for mobility and aesthetics. What happens when you need to have a mixed deployment of a myriad of wireless devices operating in a myriad of different frequency ranges of the radio frequency (RF) spectrum? The short answer is you test operational frequency ranges and equipment until you break them from a capacity perspective.
We recently performed testing and analysis for a wireless device deployment with equipment in 6.1 – 6.6 GHz (super high frequency, or SHF), 5.180 – 5.825 GHz (SHF), 2.412 – 2.462 GHz (ultra high frequency, or UHF), 1.920 – 1.930 GHz (UHF) and 470 – 932 MHz (UHF). The equipment that populated that RF spectrum frequency ranges include PC laptops, smart phones (cellular and 802.11a/b/g/n/ac), tablets, wireless keyboard/mouse units, wireless headsets, wireless conference call units, wireless microphone systems, wireless environment sensors (temperature monitoring system), wireless display units, wireless printers and wireless display hubs. The wireless protocols involved included IEEE 802.11a/b/g/n/ac and Digital Enhanced Cordless Communications 6.0 (DECT 6.0), as well as many proprietary protocols specific to the OEMs of some of the wireless devices.
The summary of the approach is to slowly stress the elements through progressive traffic buildup within the bands, especially the competing bands and those affected with harmonics. Multiple video streams, uplink/downlink testing utilities, packet analysis, and network utilities carry out traffic loading and monitoring. Loading phases are planned at either a 5% or 10 % increase. Knowledge of recommended frequency and/or channel utilization of each particular component is important.
Other characteristics we are observing include jitteriness in video, voice, mouse cursor, microphone and even keyboard (exhibited as lost keystrokes). One certain takeaway is that the acceptable channel utilization level in keyboard, mice and microphone wireless devices is significantly higher than the channel utilization level, which begins to negatively impact a WLAN. Once channel utilization exceeds 40%, the WLAN begins to exhibit packet drops, and delays in video streams. Retries increase further compounding the channel utilization. Overloading access point utilization is a major cause of decrease in WLAN performance.
Two last summary points, when a keystroke rate of approximately 900 characters per minute (CPM) is reached, wireless keyboards begin to demonstrate lag. Prior to reaching an approximate CPM value of 900, no lag in keystroke response was observed. Likewise when mice movement reached a considerably high speed (much higher than would be anticipated in an office environment), the cursors began to behave erratically. The median CPM value in most office environments is 189*.
