Specifications

24 AP Placement for Optimal Performance
Handset a/b/g
If the handset detects an energy level that is stronger then -70 dBm or confirmed 802.11 traffic it
will consider the air as occupied and not transmit. For example, if it hears an AP with -80 dBm and
can identify it as 802.11 traffic, it will not transmit. A non 802.11 disturbance at -72 dBm will, how
-
ever, not stop the handset from transmitting.
Hidden Node Problem
The “Listen before Talk” mechanism, mentioned in Section , “Clear Channel Assessment, CCA”,
on page -23, works as long as all STAs in a cell can hear each other. However, when STAs are
positioned at the cell boundaries on opposite sides of the AP, they can not hear each others trans-
missions. Therefore if they transmit at the same time, collision is likely to occur at the AP which
will not be able to receive an error free frame from any of the two STAs.
Figure 6.
Figure 5 2 STAs and an AP showing simultaneous transmission and collision
The hidden node problem is accentuated when adjacent cells use the same channel. One com-
mon solution to this problem is to use Request-To-Send/Clear-To-Send (RTS/CTS). However, the
use of RTS/CTS introduces overhead for all clients in the cell and is not recommended.
AP Placement for Optimal Performance
There is a contradiction between the two essential requirements for optimal AP placement. Good
performance requires good coverage, but “over-coverage” will reduce the performance.
As described in <Blue>Chapter , “Basic Cell Planning”, enough overlap between adjacent cells is
needed in order to have sufficient radio signal strength at all places and enough margin when
roaming between cells. However, the co-channel interference problem, described in
<Blue>Chap-
ter , “Co-Channel Interference”, is reduced by increasing the distance between APs working on
the same channel.
This means that for every unique combination in the cell planning, these two requirements must
be proved against each other to obtain the optimal placement.