Is leaky coax good enough to distribute Wi-Fi?
- 29 December, 2006 07:06
Many companies have successfully used leaky coax as a distributed antenna system, or DAS, for boosting cellular voice signals throughout their organisations. You might be wondering if you can also use it to propagate Wi-Fi signals for improved coverage.
Experts answer with a reluctant "yes." Some recommend it only in cases where you want to extend basic data-connectivity coverage, for example, but say "no" when you are attempting to improve coverage to support VoIP over Wi-Fi and other mission-critical WLAN applications.
Leaky-line coax technology allows a single run of coax to function as a long, winding "antenna" that repeats signal coverage, albeit with no gain. While normal coaxial cables use outer conductor shields to minimise RF leakage, the outer conductor of leaky coax contains openings to let a controlled amount of RF signal leak out into the air. Leaky coax is often used to amplify signals around curves in tunnels for cellular voice signals, for example, where traditional antennas can't bend, such as in sports stadiums.
It would seem that, given the blanket coverage required to run VoIP over Wi-Fi, leaky coax might be a good way to fill in the coverage holes in out-of-the-way places. Jeff Nelson, systems consultant at network integrator Netversant, however, warns against it.
Reducing the number of access points for Wi-Fi using leaky coax results in a given AP covering a wider scope, so the number of users each AP supports increases, he explains. "VoIP is extremely sensitive to latency and contention for the medium [the AP], so you need lots of APs for VoIP over Wi-Fi," he says. Nelson recommends one every 60 to 80 feet for optimum voice coverage. Depending on the specific equipment used, some estimates limit the number of users to a given AP to about seven when VoIP over Wi-Fi is supported.
"Leaky coax has been used for a long time in places like elevator shafts, where antennas just don't make sense," adds Craig Mathias, principal of Farpoint Group, a wireless network consultancy. "In theory, you could use it for a variety of applications. But it's not very efficient, and users should never use it for Wi-Fi if they don't have to."
Nelson adds that the use of leaky coax eliminates the possibility of running location-tracking applications across the Wi-Fi network. This application depends on measuring distances between client devices and nearby APs to pinpoint location, a model that is thrown off by reducing the number of APs in the network.