1. Support for Dynamic Channel Selection (DCS) in 802.11v
January 2003
IEEE /100r1
May 2005
doc: IEEE /xxx1r0
Support for Dynamic Channel Selection (DCS) in v
Joe Kwak, Marian Rudolf (InterDigital)
Submission
Kwak, Rudolf

2. Introduction Proposed goal of Dynamic Channel Selection for 802.11v
“Change the operating channel (and/or band) for the entire BSS during live system operation”
Channel change should be done seamlessly with no loss of connectivity both when idle and in a session
The channel change should be done in a bandwidth efficient manner
Channel change should not require manual user intervention
Benefits of Dynamic Channel Selection
Dynamic Channel Selection can be used for:
Channel escape in the presence of external interference
Channel selection optimization purposes
Right setting of an AP’s operating channel is one key tool to increase effective throughput for a BSS (> factor 2-3)
Cooperation between APs is not required for DCS to be efficient
APs can independently observe interference levels and choose best channel
Kwak, Rudolf
Submission

3. DCS illustration
Throughput
Kwak, Rudolf
Submission

4. DCS illustration
Interference
Throughput
Kwak, Rudolf
Submission

5. DCS illustration
Interference
Throughput
Kwak, Rudolf
Submission

6. DCS illustration
Interference
Throughput
Kwak, Rudolf
Submission

7. What we can and cannot do today
Today, any AP can change its channel
AP will simply stop Tx on old channel and switch to new channel
Associated STAs in the BSS will re-select a “new” BSS, i.e. they may or may not choose the old AP on the new channel or choose another BSS altogether
Cons: Service interruption for all STAs currently in a session
Cons: AP has no mean of knowing where the STA will end-up
Cons: STA cannot request a channel change or indicate their preferred channel to the AP
802.11h DFS service (see annex)
Primarily motivated by regulatory requirements in the 5GHz bands
Goal is to stop Tx on interfered channel, not to ensure a smooth transition of the BSS (AP and all associated STAs) to the new frequency
Cons: AP has no means of knowing if STA can/will follow or not
Cons: After channel change, AP does not know who did / did not follow
Cons: STA cannot request/suggest a channel change or indicate their preferred channel to the AP
Kwak, Rudolf
Submission

8. Shortcomings summarized
AP has no mean of knowing if STA can/will follow or not
AP does not even know if STA received the channel change notification
Will result in service interruption / loss of connectivity for some STA
Would the AP really change channel had he known its only STA would not follow?
After channel change, AP does not immediately know who did follow and who did not
This forces AP to try sending packets to STA that are no longer there
Exponential backoff
poor spectrum efficiency
AP cannot release resources (802.11e) allocated to STA who did not follow
STA cannot request / suggest a channel change or preferred channel to AP
Interference experienced by STA is different than interference experienced by AP
Measurement reporting is not enough as interference can arise suddenly
Relying solely on AP to detect interference problems at STA would imply huge signaling overhead due to need for high frequency of measurement reports
DCS needs to ensure that
All STAs are aware of the intention of the AP to change the channel
The AP knows, prior to making the channel change, which STA(s) will follow on new channel
Had the AP decided to go forward with the channel change, AP knows which STA have followed on the new channel
STA can suggest a channel change
Kwak, Rudolf
Submission

9. 802.11v DCS Procedure (AP initiated)
AP sends channel change intention message indicating its intention to change channel to channel X at time Y. Reason and measurements can be appended
STAs send Channel Change Response message indicating whether or not it can/will follow the AP on its new channel
If AP decides to change channel, it sends channel change notification, which confirms channel change
STA X sends the AP a channel change executed message after it has changed channel
Kwak, Rudolf
Submission

10. 802.11v DCS Procedure (STA initiated)
The station sends change channel suggestion to its AP
Upon reception of this message,
If AP has multiple STAs, it will send Channel Change Intention message to other STAs indicating its intention to change channel to channel X at time Y
If AP decides to go forward with channel change, AP sends Channel Change notification message
Kwak, Rudolf
Submission

11. 802.11h DFS Background
Kwak, Rudolf
Submission

12. 802.11h DFS Services
Association of STAs with an AP in a BSS based on the STAs’ supported channels.
Quieting the current channel so it can be tested for the presence of radar with less interference from other STAs.
Testing channels for radar before using a channel and while operating in a channel.
Discontinuing operations after detecting radar in the current channel to avoid interference with radar.
Detecting radar in the current and other channels based on regulatory requirements.
Requesting and reporting of measurements in the current and other channels.
Selecting and advertising a new channel to assist the migration of a BSS or independent BSS (IBSS) after radar is detected.
Source: IEEE Std h-2003
Kwak, Rudolf
Submission

13. 802.11h DFS Signaling (1/2) Supported Channel capabilities
The Supported Channels element contains a list of channel in which a STA is capable of operating.
“Supported channel” IE is added to Association/Re-association Request frame
“Channel Switch Announcement” IE
Used by an AP in a BSS to advertise when it is changing to a new channel and the channel number of the new channel, optionally, enforce stop of Tx on the current channel until switching time
“Channel Switch Announcement” IE can be attached to the Beacon or Probe response frame or sent by itself as Channel Switch Announcement action frame (particularly IBSS case)
Kwak, Rudolf
Submission

14. 802.11h DFS Signaling (2/2) Measurements Request/Report action frames
Measurement Request contains measurement type and configuration (i.e. periodic, once), like 11k
Measurement Report can be used to alert other STAs about presence of radar
Quieting the current channel
The Quiet element defines an interval during which no transmission shall occur in the current channel
This interval may be used to assist in making channel measurements without interference from other STAs in the BSS or IBSS.
Quiet IE attached to the beacon or probe response frames
IBSS DFS element
It contains information for DFS operation in an IBSS.
Kwak, Rudolf
Submission

15. BSS Operation
AP can configure the quiet interval for measurement purposes
AP detects radar signals on its own or via a measurement report from the STA
AP sends channel switch announcement in the beacon or probe response frame
Can be sent few times to make sure PS STAs received it
Cannel switching may take up to several hundred of ms’s
Kwak, Rudolf
Submission

16. IBSS Operation
DFS owner STA detects radar signals on its own or via a measurement report from other STA
DFS owner STA sends channel switch announcement frame to all STA in the IBSS
Other STAs are not mandated to switch channel
However, they are mandated to cease transmission on current channel
If other STA detects radar and did not receive channel switch announcement frame for DFS recovery interval
STA starts the DFS owner recovery procedure to determine a new DFS owner for the IBSS
Kwak, Rudolf
Submission