Moving from just a Coverage Design to a more Capacity Design

Wireless Coverage:

When designing a new wireless LAN, the variables to coverage are; the power settings (which are Country dependent), antenna gain, and the physical environment. The newer enterprise grade access points on the market will automatically adjust their settings for optimal coverage, but planning a wireless network has, unfortunately, never been as simple as drawing circles on a floor plan. Wireless surveys, and the documentation to understand the results, can go a long way to help creating a network that matches the initial expectations of your project. You will stand more of a chance implementing an effective WLAN when including heat maps and spectrum analysis from a professional survey, especially when VOIP is going to be a feature.

Wireless Capacity:

The location and number of access points required nowadays, depends on how many wireless clients will be using the network simultaneously, what they will be using the bandwidth for, and in where in the building (for density coverage). Looking at the design and layout of your project, you will know of the main areas that will be using wireless. For example, in Schools, there are large classrooms, assembly halls, canteens, and playgrounds where students will gather using mobiles, tablets and possibly laptops. These environments have the potential to be a high density area for WLAN clients. We can no longer install an access point in a hallway, and record the signal while walking away until the final lingering strains of signal fade. Now you design for capacity as well (or at least you should).

It’s always good to plan for the users at the edge of the coverage area as well, as depending on the access point manufacturer you have chosen, they can experience a diminished performance and the overall capacity will suffer, even if you have great coverage to start off with. If you cut lower data rates out and decrease the coverage area, you can potentially increase throughput. If you allow the lower data rates, you can increase the coverage area of each access point but users will notice the difference as they walk half way between access points placed further away than usual. This then leaves you with the difficult conundrum of whose desk to put in this black spot as a punishment.

Planning for Capacity, not just Coverage:

Providing access to a service that does not live up to expectations might be as bad as providing no service at all. In planning capacity per user, you must begin with the usable capacity of a WLAN channel and an estimate of how many users will be sharing it. One 11-Mbps 802.11b wireless LAN channel provides a real throughput of about 5.5 Mbps after we net out the impact of protocol headers, acknowledgements, retransmissions, and other network overhead. The 802.11a and g networks provide a maximum throughput of 30 Mbps, though that is reduced significantly if the 802.11g users are sharing a channel with 802.11b users. So, the estimated throughput of a WLAN channel is roughly 50% to 55% of the raw data rate. That estimate assumes all users are in fairly close proximity to the access point and so are operating at the maximum data rate.

Proper design involves providing signal coverage in all areas, but also insuring that the network delivers adequate performance for all users in a cell. It should also be noted that wireless LAN design is an ongoing process, so these capacity issues must be revisited as usage grows and more access points/cells are added to the network. Of course, adding more access points also increases the cost of the network.

Factors to Assure Adequate Capacity:

There are four basic factors in the design that we can control to ensure adequate capacity as well as adequate radio coverage:

1. Radio link interface: First we can choose the radio link interface that will be used, 802.11a, b, or g.
2. Cell layout/channel assignment: The designers then select the placement of access points, antennas, and radio repeaters to provide coverage and to limit interference.
3. Power levels: With a limited number of available channels, the same channels must be reused in different parts of the facility. When a channel is reused, we must reduce the transmit power of the other access points using that channel to limit co-channel interference (i.e. the interference created by access points in different parts of the facility that are assigned the same channel). However, reducing transmit power also reduces the range, so more access points may be required to provide the same coverage.
4. Limit association rates: One last technique to keep low-speed users from impacting the overall performance is to limit the range of rates at which users will be allowed to associate. For example, in an 802.11b installation, you can limit association rates to users whose signal strength will support data rates ¡Ý5.5 Mbps. Higher transmission rates require a stronger received signal, so supporting only the higher data rates will mean more cells have to be provided or there will be dead spots in the coverage area.