Specifications

Co-Channel Interference 23
There are 19 channels available in total in Europe and 24 in the USA (FCC channels), whereof
there are four non-DFS in Europe and nine non-DFS in the USA. Data traffic only can use DFS
channels, but it is not recommended for voice, since handsets can not use active scanning due to
DFS regulations.
NOTE: The handset can use the DFS channels, but the Voice quality may be distorted.
How closely these channels are reused is dependent on the geometrical prerequisites of the site
that shall be covered. If it is a one-floor hallway only, there will be enough distance separation be
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fore re-use of the same channel is needed. For a multi-story building with a large floor area, it will
be impossible to have coverage at all places without having adjacent cells that use the same chan
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nel to some extent.
Installing two adjacent cells working on the same channel introduces the following problems:
1. Capacity reduction. All STAs in the two cells will share the RF channel as if they were present
in one cell.
2. Error introduction. The STAs will introduce transmission errors due to the “hidden node
problem” described in
Section , “Hidden Node Problem”, on page -24.
Clear Channel Assessment, CCA
a/b/g
802.11 specifies a distributed channel access function that basically can be summarized as “listen
before talk”. The “listen” procedure is called clear channel assessment and reports if the media
(air) is busy or idle. If a STA wants to transmit a packet, it must first determine if the media is idle,
then it can transmit the packet. If the media is busy, the STA has to wait for the media to be idle.
The same channel access rules apply for an AP.
CCA is affected also by non-802.11 RF signals in the 2.4 GHz band.
Even if APs that use the same channel are placed far away, there can be STAs present in the cells
that are closer and thereby causing transmission interruptions, see
<Blue>Figure 4.
Figure 5.
Figure 4 CCA might cause problems even for far away STAs