10 February 2010

The Lessons Of Wi-Fi #3: Wireless Coverage ≠ Wireless Capacity

You're excited - the two bids you were expecting for your new Wi-Fi network have just arrived. You rip open the envelopes and then stare in disbelief.

The first bid - the low bid - includes fewer than 100 access points and a note stating that the access points are specially designed to operate at full power at all times so fewer are required. The second bid includes 135 access points and a note about meeting bandwidth capacity requirements and providing resiliency in the event of failure. Both vendors had the same set of plans to review, both did a walk-through of the facility. How could their bids be so different?

Those who forget the lessons of Wi-Fi are doomed to repeat them. Lesson #3: wireless coverage ≠ wireless capacity. Designing for coverage means providing a discernible Wi-Fi signal everywhere without regard for network speed. The access points on these networks are typically run at full output power so the signal coverage is max'd out. They're also spaced with minimal or no overlapping coverage. As a result fewer access points are required.

The downsides of designing for coverage? Many. Consider these two:
  • Bit rate: There is an inverse relationship between bit rate and range. The farther away a Wi-Fi device moves from an access point, the lower the bit rate. Wi-Fi devices operating at the fringe of the coverage area will be very slow indeed. Too slow for voice, streaming video, electronic white boarding, and many other applications;
  • Failure happens - but this design can't deal with it. If an access point fails, nearby access points can't increase their output power to fill in the coverage gaps.
Designing for coverage is okay if consistent network performance and resiliency are unimportant. Otherwise it should be avoided.

In networks that are designed for capacity, the required bandwidth is available throughout the coverage area. Application performance will therefore be universally uniform.

Planning for capacity requires more access points because the distance to laptops, iPhones and other clients needs to be more limited (remember rate vs. range) for robust, high-speed operation. They're also needed to ensure adequate load balancing, a feature especially important in areas with densely packed clients such as classrooms, lecture halls, and trading floors. The benefits far, far outweigh the cost - you end up with a resilient network on which you can consistently depend for years of service.

Some vendors play on customers' lack of familiarity with the difference between coverage and capacity.
When it comes to reviewing bids and proposals, take note of differences in the number of access points and claims about "unique" features affecting coverage. If you fall for the coverage

If you'd like to get the whole picture on Wi-Fi architecture you've only to download our free white paper, WLAN RF Architecture Primer. And leave it to someone else to relearn the lessons of Wi-Fi.