There will be pools of coverage at each data rate. If the customer wants to
provide a certain area with coverage at a specific data rate, multiple site
surveys may be necessary. Each data rate must be surveyed to find out where the
coverage pool is for each data rate.
In order to determine where to
place the APs, it is important to find out how much throughput the users will
require. Bandwidth requirements for wireless connectivity will potentially
determine the number of APs that need to be used. If a constant speed is
required and that speed is fairly high, such as 11 Mbps, then the coverage will
be fairly low and a high number of APs will be required.
In many
situations, AP coverage will be the driving factor over bandwidth and autorate
negotiation of bandwidth can be used. With autorate negotiation, the client
picks the best speed at its current distance, so as the client moves from a
close proximity to the AP, it uses a high bandwidth such as 11 Mbps. As the
client moves outward from the AP and the distance increases, the bandwidth is
reduced to allow for the best possible signal quality.
The load on an access point or the total number of potential clients should
be considered in any design. One problem with WLANs is that the number of
potential clients can be unknown, since the freedom of wireless allows any
number of people to converge within an area. The maximum number of clients as
dictated by the address table in the access point is 2048. This maximum is not
practical, because WLANs are a shared infrastructure, which is similar to hubs
in a wired network.
Each individual user has less overall bandwidth
available as more clients are added to an AP. This might be acceptable for
variable bandwidth applications. However, for many applications, especially
with modern data-intensive graphical environments, this may not be adequate.
Distributing the clients among more access points, particularly in congested
areas, easily solves this problem. This serves to distribute the load, via
overlapping coverage between APs. Make sure that each AP is communicating on
its own unique channel to prevent interference with each other. If only two APs
are going to have overlapping coverage, then any two different channels can be
used between one and 11. When using the 11 channel implementation used in the
US, the maximum number of APs that can be used concurrently is three. This is
because only three channels do not overlap with each other, which are channels
one, six, and 11 as shown in Figure
. If additional
bandwidth is required, 802.11g can be utilized. If the 2.4 GHz band is crowded
by other devices, 802.11a provides another high speed option.
In some
environments, bandwidth and AP load are strong design factors for a WLAN
implementation. One way to ensure that a small area of users access a selected
AP, is to manipulate the power settings on the AP to adjust the size of the
cell. This adjustment will create cells that cover specific areas.