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RF and channel management



Channel Management
RF management in a multi-channel environment is, in 5 GHz, easy. You have multiple non-overlapping channels, but what about 2.4 GHz? You need a better channel plan in your environment since there are only 3 non-overlapping channels. You can discuss the 4-channel plan (1,5,9,13) that some companies use in Europe. With a single channel plan, you can add a new channel blanket. For example, one floor on channel 1, second floor on channel 6, and third floor on channel 11. As well, it is possible to use channel 11 for voice and channel 6 for data traffic.

Radio Resource Management
Radio Resource Management is dynamic channel assignment and dynamic RF output power control. RRM is a very complex mechanism that is beyond the scope of the exam, but it is based on neighbor communication. Access points provide information about their neighbor access points and pass this to the controller. All the information is gathered and used for making decisions about channel and power levels.
- Access points send out neighbor communication
- Access points listen to all the channels for neighbor communication
- The information is sent to the controller
- Controller adjusts the access point configuration based on the information
There are extra features, as well, like coverage hole detection and interference detection. Coverage hole detection can increase the power level of an access point and with interference detection the controller can decide to change a channel with no or less interference.

Co Channel Interference is when two cells are too close to each other. The solution for this is cell sizing, so lowering the power level. This can be done by RRM. You can create smaller cells by disabling the lower data rates, however there are two boundaries. You have the association boundary, that can become smaller by disabling the date rates, and you have the CCI boundary. The CCI boundary is bigger since the CSMA/CA protocol still needs to listen to the lower data rates, even if they are disabled. The signal will not be processed, but still detect. It becomes more complicating if we add a client in this. If the boundary from the client reaches both access points, both access points will be quiet and listen to the client when the client transmits information. When the other client is transmitting, the access point, where the client is not associated with, cannot transmit with other clients at that moment. Keep in mind that this is only when they operate in the same channel.

Adjacent Channel Interference should also be mitigated by RRM. Adjacent Channel Interference is in 2.4 GHz when one access point is on channel 1 and the other on channel 2. In RRM you can tell which channels the controller can assign. So, for 2.4 GHz only channels 1, 6, and 11 should be picked. However, with high power it is possible that the side lobes of channel 6 can interfere with channel 1 or 11. Also in 5 GHz, where all the channels are non-overlapping, it is still possible that the side lobes of a channel can interfere with another channel. In 5 GHz, it is better not put two access points in channels close to each other, like channel 36 and channel 40.

As said earlier, if you are doing multi-channel plans, you need to pick non-overlapping channels. For the 2.4 GHz, you have channel 1 to 14, but channels 12-13 only in Europe and channel 14 is only in Japan. So, it depends on the location which non-overlapping 2.4 GHz channels you can pick. In 5 GHz, all channels are non-overlapping, but there are other reasons that you cannot pick the channel. You have DFS channels and Terminal Doppler Weather Radar (TDWR) channels that are not in all regulatory domains allowed. Channel 144 is only for 802.11ac and not in 802.11n. When you select manually the channel, it is also possible to select manually the output power. You can pick the output power in the range from 5mW to 1 W, but it depends on the type of access points, the restrictions of the regulatory domain, and even whether it’s indoor or outdoor. With designing you need to pick an output power for when you do a survey with access point on a stick, even when you are planning to use RRM.

When the power and channel is selected, you need to pick the channel widths. With 802.11n you can pick 20 MHz or 40 MHz channels, in both 2.4 GHz and 5 GHz. However, in 2.4 GHz it is not smart to pick 40 MHz channels, and there are too few channels anyway. In 5 GHz, with 802.11ac, it is possible to pick 20, 40, 80 and even 160 MHz channels overlap. With 160 MHz channels, you have only 2 possibilities. There are also 80 + 80 MHz channels, and this is not the same as 160 MHz channels. 160 MHz channels need to be adjacent; 80 + 80 MHz channels don’t need to be adjacent.