Design/Layout Challenges: Coverage and Capacity

One of the challenges in laying out a multiple access point wireless LAN installation, is in planning the layout of the access points and ensuring that adequate radio coverage is provided throughout the service area. Our experience shows that the layout must be based on measurements, not just on "rule of thumb" calculations. These measurements involve extensive testing and careful consideration of radio propagation issues when the service area is large, for example an entire campus in our installation. Even a very carefully considered access point layout may have to be modified after installation is complete in order to remedy coverage gaps.

Because the coverage area of an access point is relatively small, terrain is not a propagation issue. Rather, the layout and construction of buildings determine the coverage area of each access point. Lucent rates the WaveLAN/WavePOINT transmission range at up to 800 feet in an open environment, but this rated range may be reduced to 100 to 200 feet through walls and other partitions in some office environments. Wood, plaster, and glass are not serious barriers to the WaveLAN radio transmissions, but brick and concrete walls can be significant barriers; the greatest obstacle to radio transmissions commonly found in office environments is metal, such as in desks, filing cabinets, reinforced concrete, and elevator shafts.

One can imagine that an access point and the mobile computers within its coverage area operate something like the computers on an Ethernet segment. That is, there is only a finite amount of bandwidth available, and it must be shared by the access point and mobile computers. The IEEE 802.11 protocol, like Ethernet, is contention-oriented, i.e., it provides a mechanism which allows all units to share the bandwidth resource.

This contention-oriented protocol makes radio interference between access points a particular problem. If one access point can hear another, it will defer, just as it would defer to a mobile unit transmitting within its coverage area. Thus, interference between adjacent access points degrades performance. Similarly, if a mobile unit can be heard by more than one access point, all of these access points will defer, thus degrading performance.

In designing for coverage, one would like to space the access points as far apart as possible, minimizing cost. On the other hand, one must avoid coverage gaps, areas where no service will be available to users. In designing the Wireless Andrew network, we have found that rules of thumb are inadequate. Rather, each building design must be based on careful and exhaustive signal strength measurements. This is particularly challenging because a building is a three-dimensional space, and an access point located on one floor of the building provides signal coverage to adjacent floors of the same building. We have developed detailed procedures for conducting these measurements and for locating access points in a way that maximizes the spacing between access points but also minimizes coverage overlap between access points.

The design should also consider the issue of capacity. If many users of mobile computers are located in a small area, students in a classroom or lecture hall, for example, it may be necessary to use multiple access point s to serve the group. This implies that each access point has a smaller coverage area than might otherwise be possible. Thus, one would like to carry out a design which is both coverage-oriented (minimizing the number of access points) and capacity-oriented (assuring adequate service to all users). The coverage-oriented design minimizes the cost of access points but it may be necessary to use more access points in some areas in order to provide adequate service.

We expect that most of our campus will be a low capacity area, i.e., the user density will be low in most areas. However, there will be some areas, particularly classrooms, laboratories, and lecture halls, which will be high-capacity areas, ones with high concentrations of users, mostly students. In these high-capacity areas, more access points with smaller coverage areas may be needed.

There are two design layout techniques which are useful in high-capacity situations. They are adjusting the receiver thresholds setting and using the frequency reuse. The WaveLAN product allows one to set receiver thresholds, thus controlling the size of the coverage area of the access point. A coverage-oriented design should use the minimum receiver threshold setting, maximizing the size of the coverage area of each access point. When capacity issues are considered, however, one may wish to use higher access point receiver thresholds settings in high-capacity areas, reducing the coverage area of each access point.

Some wireless LAN products, including WaveLAN, also allow for frequency reuse. In the 2.4 GHz band, there is adequate spectrum to accommodate multiple independent channels. Thus, access points can operate on separate non-interfering channels. Further, in the WaveLAN product the network adapter can switch between channels in order to talk with the access point providing the best signal strength. Frequency reuse can be used to minimize interference among adjacent access points, which would otherwise degrade performance. Frequency reuse can also be used to allow all multiple access points to provide coverage in the same physical space. For example, one might use three access points to cover a large lecture hall with a high-density of student users.

The design of our Wireless Andrew production network uses coverage-oriented techniques in the low capacity areas, and a combination of coverage-oriented and capacity-oriented techniques in high-capacity areas. In order to help us with the design of Wireless Andrew, we are developing a computer-based simulation tool, which considers both radio propagation and traffic issues. As we continue to expand the Wireless Network to additional academic buildings across campus, the tool is helping us design for both coverage and capacity.

It is our intention that, by using the computer simulation tool and the coverage-oriented and capacity- oriented techniques of access point layout, that we can quickly and efficiently provide Wireless network access to the entire campus.