How does Dynamic Channel Selection work?
Features > Dynamic Channel Selection (Smart Manager)
Smart manager tells the MM6108 chip which channels to scan, when to scan them, collate the results (Measure), decide on the best channel (Selection) and trigger the move to the new channel (Channel Switch).
DCS is accomplished by periodically performing Off Channel Scans (OCS). This process consists of the AP configuring a RAW window with an unused AID, which causes all associated stations to stop transmitting during this window. During this time, the AP will temporarily switch to the off channel in question, and perform an assessment using the PHY channel metric. The PHY will then start sampling the medium, and compute a metric to be reported back to Smart Manager. This metric takes into account noise floor, interference, and WLAN traffic. It returns an opaque number between 0 and 100, with 100 being an ‘ideal’ channel.
These channel metrics are then fed into an algorithm which will average them over time, and decide if/when the channel needs to be changed.
If a decision is made to change channels, the AP will signal this using an ECSA (extended channel switch announcement). ECSA is an 802.11 feature which adds an information element into the beacon which contains the parameters of the new channel, and a count-down which is decremented each successive beacon. When this countdown reaches zero, the AP and all stations will seamlessly move to the new channel and continue service as normal.
Note: Smart Manager is released in binary form only
DCS Algorithm Flow Chart
Measurement
Smart Manager will utilize an Off Channel Scan (OCS) to make periodic measurements of all channels. The available channels will be queried from hostapd/mac80211 and stored prior to scanning.
Once Smart Manager triggers a scan, it will send a command with the requested channel to scan to the MM6108 chip. After the scan completes, the driver emits a MORSE_VENDOR_EVENT_OCS_DONE event via netlink, containing the scan results, which will be captured and stored by Smart Manager. This operation will iterate on all candidate channels to get an accurate view of the channel conditions. Multiple measurements will be taken over time, as traffic and interference may be sporadic.
The scan results will need to be analysed by Smart Manager and a metric established to gauge the quality of the channel.
Selection
Once sufficient channel data has been obtained, Smart Manager will make a decision on whether it should trigger an ECSA based on the measurement (e.g. each channel’s quality, adjacent channels, channel's historic readings etc.).
Currently, Smart Manager will only try to switch within the channels which have the same bandwidth.
If considering changing operating BW, it is also important to take into consideration the maximum throughput of the network, which will be dependent on RSSI of stations and the traffic profile of the BSS.
Channel Switch
Once the “best” new channel has been selected, Smart Manager will trigger an Extended Channel Switch Announcement (ECSA). This will inform all currently associated stations to switch to the new channel at the coordinated time.
After the channel switch, Smart Manager will continue to evaluate alternative channels, thus ensuring the optimal channel is used for BSS operation.