Wireless LAN Cells
The AP’s location must be carefully planned so that its range matches up with the coverage area that is needed. Even though you might design the AP’s location according to a floor plan or an outdoor layout, the WLAN will operate under changing conditions. Remember that although the AP’s location will remain fixed, the wireless clients will change location quite frequently. Roaming can make the AP’s coverage turn out to be much different than you expect. After all, clients can move around and behind objects in a room, walls and doorways in a building, and so on. People will also be moving about, sometimes blocking the wireless signal.
The best approach to designing an AP’s location and range or coverage area is to perform a site survey. A test AP is placed in a desirable spot while a test client moves about, taking live measurements of the signal strength and quality. The idea is to plot the AP’s range using the actual environment into which it will be placed, with the actual obstacles that might interfere with the client’s operation. An AP’s coverage area is called a cell. Clients within that cell can associate with the AP and use the wireless LAN. This concept is shown in Figure 15-4. One client is located outside the cell because it is beyond the AP’s signal range.
Suppose that a typical indoor AP cell has a radius of 100 feet covering several rooms or part of a hallway. Clients can move around within that cell area and use the WLAN from any location. However, that one cell is rather limiting because clients might need to operate in other surrounding rooms or on other floors without losing their connectivity.
To expand the overall WLAN coverage area, other cells can be placed in surrounding areas simply by distributing other APs throughout the area. The idea is to place the APs so that their cells cover every area where a client is likely to be located. In fact, their cell areas should overlap each other by a small percentage, as shown in Figure 15-5.
Tip: When AP cells overlap, adjacent APs cannot use identical frequencies. If two neighboring APs did use the same frequency, they would only interfere with each other. Instead, AP frequencies must be alternated or staggered across the whole coverage area. When a client associates with one AP, it can freely move about. As the client moves from one AP’s cell into another, the client’s association is also passed from one AP to another. Moving from one AP to another is called roaming. This movement is also shown in Figure 15-5 as the laptop PC moves along a path that passes through several AP cells. When a client moves from one AP to another, its association must be established with the new AP. As well, any data that the client was sending just prior to the roaming condition is also relayed from the old AP to the new AP. In this way, any client connects to the WLAN through only one AP at a time. This also minimizes the chance that any data being sent or received while roaming is lost.
If the client maintains its same IP address as it roams between APs, it undergoes Layer 2 roaming. If the client roams between APs located in different IP subnets, it undergoes Layer 3 roaming. When you design a wireless LAN, you might be tempted to try to cover the most area possible with each AP. You could run each AP at its maximum transmit power to make the most of its range. Doing so would also reduce the number of APs necessary to cover an area, which would in turn reduce the overall cost. However, you should consider some other factors.
When an AP is configured to provide a large coverage area, it also opens the potential for overcrowding. Remember that an AP cell is essentially a half-duplex shared medium that all clients must share. As the number of clients goes up, the amount of available bandwidth and airtime goes down. Instead, consider reducing the cell size (by reducing the transmit power) so that only clients in close proximity to the AP can associate and use bandwidth. The AP can also assist in controlling the number of clients that associate at any given time. This becomes important for time-critical or bandwidth-intensive traffic such as voice, video, and medical applications.
When cell sizes are reduced, they are often called microcells. This concept can be further extended for extremely controlled environments like stock exchanges. In those cases, the AP power and cell size are minimized, and the cells are called picocells.
