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.

Who Moved My Packets, Or How I Learned To Stop Worrying And Cut The Cord

With the ratification of 802.11n just around the corner, it’s a good time to reexamine the fundamentals of Wi-Fi design and determine how this blazingly fast new technology will affect you. Who Moved My Packets is about the design considerations associated with 802.11n data, voice, and video applications.

Let’s start with a discussion about designing for coverage or capacity. For some wireless applications simple connectivity is the biggest issue with which users have to contend. Designing a network for coverage ensures that a Wi-Fi signal can be received at any location in which a Wi-Fi device is likely to be used. Connectivity is the primary objective - bit rate, packet throughout, multi-media support, quality of service, and even redundancy of coverage are secondary considerations.

Consider an indoor application in which Wi-Fi is used to communicate with a pool of bar code scanners for inventory management. The users are few in number, the amount of data transmitted is relatively small. Since the bit rate of an in-building Wi-Fi connection typically falls with distance and in the presence of interference sources, what started as a high speed connection near an access point could drop to 1Mpbs or less just a short distance away. However, even at that low throughput, a network designed for coverage should be sufficient for the application.

Any Wi-Fi network can be designed for coverage, and as a rule, designing for coverage requires far fewer access points. Just crank up the access point power to full, space the access points so that their coverage patterns overlap slightly, and the design part is done. Interference compensation, fair airtime availability, security, and network management are another matter entirely, but they’re outside the scope of this discussion.

A handful of Wi-Fi vendors have made an art of promoting their products as requiring fewer access points. Some Wi-Fi array (multiple access points in one box) and single channel vendors go so far as to tout their “unique” ability to deliver what no other Wi-Fi vendors can accomplish.

It’s all smoke and mirrors. Wi-Fi vendors all use Wi-Fi chip sets from a small pool of IC suppliers, and by regulation the power output of the radios is tightly controller by the government. The distance over which they can transmit, using comparable antennas, is the same. If you pull back the curtain, the secret of their claims is simply that they’re designing for coverage. Nothing more.

In fact, it’s really something less. Why? Because many users need a system that is designed for capacity. In a network designed for capacity, coverage is a given but bit rate, packet throughout, multi-media support, quality of service, and often fault-tolerance are primary considerations.

A capacity-based network requires that the vendor pay keen attention to internal architecture, algorithmic processing, and packet handling necessary to service deployments with a high capacity requirement: (1) large number of users; (2) users that are densely congregated; or (3) applications using voice or streaming video or business-critical telemetry data. Coverage alone is not sufficient for these scenarios – they require guaranteed bit rate, high packet throughout, and quality of service.

These scenarios are already the norm in education, healthcare, and government applications, and are fast becoming typical in enterprise, retail, and industrial deployments. With the migration of data, voice, and video applications to 802.11n from wired LANs, the need for capacity-based Wi-Fi will skyrocket. Users will expect wire-like performance with virtually unconstrained capacity on their shiny new 802.11n networks.

So the next time you’re given a pitch for a wireless LAN with one half, one quarter, one eighth the number of access points of an Aruba network, ask the vendor if they’re designing for coverage or capacity. And ask for test data to back it up. Doing so will avoid following Maj. T.J. 'King' Kong on a ride that is a mistake from the outset.