AP Cell Size
The basic service area (BSA) or cell that is provided by an AP can vary, depending on several factors. Obviously, the cell size determines the geographic area where wireless service will be offered. AP cell size can also affect the performance of the APs as clients move around or gather in one place.
Remember that a wireless LAN is a shared medium. Within a single AP cell, all of the clients associated with that AP must share the bandwidth and contend for access. If the cell is large, a large number of clients could potentially gather and use that AP. If the cell size is reduced, the number of simultaneous clients can also be reduced.
The signal from an AP does not simply stop at the boundary of its cell. Instead, the signal continues to expand ad infinitum, growing exponentially weaker. Devices inside the cell boundary can communicate with the AP. Devices outside the boundary cannot because the signal strength of either the client or the AP is too weak for the pair to find any usable modulation that can be used to exchange information. You can control the size of a cell by changing the parameters that are described in the following sections.
Tuning Cell Size with Transmit Power
To use a wireless LAN, devices must be located within the range of an AP’s signal and have an active association with the AP. This area is known as the BSA or cell. Consider the scenario shown in Figure 7-1. PCs 1 through 4 are within the cell’s perimeter and are associated with the AP. PC-5, however, is outside the cell and cannot form an association or participate in the basic service set (BSS).
Figure 7-1 An Example Cell That Includes All but One Client.
If the area outside a cell is a legitimate location where wireless devices might be present, the coverage area should probably be extended there. How can that be accomplished? The most straightforward approach is to increase the transmit power or signal strength leaving the AP’s antenna. A greater signal strength will overcome some of the free space path loss so that the usable signal reaches farther away from the AP.
Figure 7-2 shows the effect of changing the AP’s transmit power level. The original cell from Figure 7-1 is shown as the second concentric circle, where the transmit power level was set to 17 dBm. If the level is increased to 20 dBm, the cell grows into the area shown by the outermost circle. Notice that PC-5 now falls within the cell boundary. If the transmit power level is decreased to 10 dBm, the cell shrinks and includes only clients PC-2 and PC-3. Why would you ever want to decrease a cell’s size? That question will be answered later in this section.
Figure 7-2 The Effects of the Transmit Power Level on Cell Size.
How should you decide on a transmit power level value? Cisco APs offer eight different values for their 2.4-GHz radios and seven values for their 5-GHz radios. Most 802.11 scenarios fall within government regulations which limit the effective isotropic radiated power (EIRP) to a maximum transmit power level of 20 dBm (100 mW). You could just configure an AP to run wide open at maximum power, but that is not always appropriate or beneficial.
One thing to consider is the two-way nature of wireless communications. By increasing the AP’s transmit power, the AP might reach a distant client, but can the client’s own signal reach the AP? Notice client PC-5 in Figure 7-3. If the AP transmit power level is increased to 20 dBm (the outermost circle), PC-5 is included in the cell. However, PC-5’s wireless transmitter has a lesser power level; in its current location, PC-5 has a coverage area that falls short of including the AP. This scenario is known as the asymmetric power problem, where the two communicating devices have differing transmit power levels that might not reach each other.
Figure 7-3 The Asymmetric Power Problem.
Tuning Cell Size with Data Rates
Setting the transmit power level is a simplistic approach to defining the cell size, but that is not the only variable involved. The cell size of an AP is actually a compromise between its transmit power and the data rates that it offers.
Recall from Chapters 1 and 3 that the higher data rates or more complex modulation and coding schemes (MCS) offer the greatest throughput but require the best signal conditions—usually closer to the AP. The less complex schemes can work further away from an AP, but offer slower data rates. Therefore, at the perimeter of a cell, a client is likely to be using the least complex MCS and the lowest data rate. Figure 7-4 shows a simplified representation of the range of each data rate with concentric circles. At the outer edge of the cell, a client will probably resort to a 1-Mbps data rate.
Figure 7-4 The Relationship of Data Rates and Cell Range.
To design a wireless LAN for best performance, you would most likely need to disable some of the lower data rates. For example, you could disable the 1-, 2-, and 5.5-Mbps rates to force clients to use higher rates and better modulation and coding schemes. That would improve throughput for individual clients and would also benefit the BSS as a whole by eliminating the slower rates that use more time on a channel.
As you disable lower data rates, the respective concentric circles in Figure 7-4 become irrelevant. This effectively reduces the usable size of the AP’s cell, even though the radio frequency (RF) footprint remains the same. After all, you haven’t reduced the transmit power level which would reduce the extent of the RF energy. Be aware that as smaller usable cells are placed closer together, their available data rates are higher. At the same time, their RF footprints can remain large and overlap each other, resulting in a higher noise floor.
To provide robust wireless coverage to an ever-increasing area, you should use the following two-pronged approach:
- Tune the cell size based on data rates and performance.
- Add additional APs to build an ESS that covers more area.
Adding APs requires careful consideration for client mobility and the use of wireless channels. These topics are covered in the next section.