Determining the Access Points’ Initial
Locations
Now let’s look at a procedure for the initial selection of AP locations in a low-density area. In selecting locations for the APs, one should place them so that there are no coverage gaps in the target space, and the cov- erage overlaps between and among APs are minimized. While the first point is obvious, the second is more important than is immediately apparent. If too many APs are used, the cost of equipment and installa- tion will be higher than necessary, and the performance of the network may also be degraded if the final design involves a great deal of cochan- nel coverage overlap. The amount of cochannel coverage overlap is determined by both AP placement and AP frequency assignment.
The coverage area is defined in terms of a specified received signal strength. This threshold level is selected in order to provide an adequate signal-to-noise ratio (S/N) and some additional margin. If, for example, in designing an IEEE 802.11b WDLAN, one measures an ambient noise level of ⫺95 dBm and a 10-dB S/Nis needed to ensure excellent perfor- mance, one might decide to allow an extra 5 dB of margin to allow for noise levels higher than ⫺95 dBm. In this case, one would select a thresh- old of ⫺80 dBm.
When high-density spaces exist, it is suggested that the AP placement first be done for these spaces and that the remaining low-density spaces then be designed, filling in the gaps between high-density spaces.
AP Placement In this part of the chapter, an idealized notion of AP coverage is introduced. This description is offered only to provide some insight into the layout approaches that can be used in different types of buildings.
The coverage volume of the AP is idealized as three coaxial cylinders, as shown in Fig. 6-1.1The middle cylinder, representing coverage on the
floor on which the axis point is located, has radius R.The AP is located on the axis of this cylinder. The upper and lower cylinders, representing cov- erage on the floors above and below the one on which the AP is located, have radius R⬘, which is less than R.The height of each of the three cylin- ders is the height of a floor in the building. These three cylinders can be thought of as a single object, which moves about as the location of the AP moves.
The problem of locating APs within a building can be viewed as a problem of locating these shapes within the building in such a way that all spaces are filled with as little overlap as possible. While coverage vol- umes are not actually perfect cylinders, one can find the average cover- age radius inside a building and use this as the radius of an idealized cylindrical coverage volume. This can be achieved by defining an accept- able signal strength threshold (⫺80 dBm) and determining the average distance from the AP at which signals fall below the threshold.
Procedure The initial selection of AP locations begins with a complete set of signal strength measurements within the building. Signal strength measurements should be made in all areas of the building, with particular attention to the building’s construction so that the char- acteristics within each part of the building are understood. These mea- surements have two purposes: to divide the building into spaces that are relatively isolated from each other from a signal propagation perspective and to determine the typical coverage radius of an AP. Signal strength measurements should be taken to determine the same floor coverage radius Rand the adjacent floor coverage radius R⬘of an AP.
Access points can be placed within a building in an array that is either linear or rectangular. An example of a linear array is shown in Fig. 6-2, and an example of a rectangular array is shown in Fig. 6-3.1Each of these
shows how APs can be located in a single-floor building or in a building with only one floor needing WDLAN coverage. It is necessary only to locate the APs in a way that provides coverage throughout the floor and
R' R'
R Figure 6-1
Idealized access point coverage.
also minimizes as far as possible the overlap between and among AP cov- erage areas. A linear array is used when the building is narrow relative to
R,and a rectangular array when the building width is large relative to R.
On the other hand, in a building that requires coverage on more than one floor, adjacent floor coverage must be considered in locating each AP. Usually, a staggered approach is used. As one moves along the length (or width) of a building, one places APs first on one floor and then on an adja- cent floor. In this case, the coverage of an AP’s adjacent floor coverage
45° R R R D Figure 6-2
A linear array of APs in a single-floor building.
must dovetail with the coverage of the next AP’s same floor coverage. As in a single-floor building, a linear array is used when the building is narrow relative to R,and a rectangular array when the building width is large relative to R.
Let’s now illustrate by using four scenarios one will encounter when planning and designing an indoor wireless data network. Each is deter- mined by whether the building is single-story or multistory and by the width of the building relative to Rand R⬘. In each case, the appropriate layout approach is given and the figure that illustrates it is listed. Solid lines show coverage on a floor; dashed lines show adjacent floor coverage.
Scenario 1 A single-floor linear array is illustrated in Fig. 6-2.1This is a
single-story building (or a building that requires wireless data coverage on only one floor) whose width (smallest outer dimension) is not large relative to R. Ddenotes the distance between adjacent APs.
Scenario 2 A single-floor rectangular array is illustrated in Fig. 6-3. This is a single-story building (or a building that requires wireless data coverage on only one floor) whose width (smallest outer dimension) is large relative to R. Ddenotes the distance between adjacent APs.
R 45°
D Figure 6-3
A rectangular array of access points in a sin- gle-floor building.
Scenario 3 A multifloor linear array is illustrated in Fig. 6-4.1This is a
multistory building whose width (smallest outer dimension) is not large relative to R and R⬘. D⬘denotes the distance between adjacent APs on different floors.
Scenario 4 A multifloor rectangular array is illustrated in Fig. 6-5.1This
is a multistory building whose width (smallest outer dimension) is large relative to Rand R⬘. Ddenotes the distance between adjacent APs on the same floor, and D⬘denotes the distance between adjacent APs on different floors.