802.11k Measurement Frame Proposal

1. 802.11k Measurement Frame Proposal Steve Emeott, Tim Wilson, Floyd Simpson, Stephen Wang and Walter Johnson Motorola Steve Emeott, et. al.

2. Mobile Stations and Signal Quality Measurements • Mobile stations will employ several functions that require measurements of AP signal quality as input • Discovery of 802.11 service areas • Identification and ranking BSS transition (handover) candidates • Initial transmit rate selection • Flexible techniques for measuring signal quality are required to accommodate an increasing range of applications • Mobile stations roaming into an 802.11 coverage area must be able to rapidly scan, preferably passively, the available channels prior to association • Handheld stations camped on an 802.11 system must be able to take measurements while still offering long standby times • Complexity of mechanisms used to take measurements must be suitable for wide range of wireless networks Steve Emeott, et. al.

3. Measurement Functions • Both the base standard and 802.11k provide tools the station may use to take measurements • Beacon frames generated by the access point • Probe response and request functions • Neighbor report currently provides optional fields providing timing information that may be used to predict beacon transmission times • Several use cases will be provided to illustrate why these capabilities are suboptimal from the perspective of a handheld device Steve Emeott, et. al.

4. A voice station associated with a foreign network scans for 802.11a coverage Challenge: The station (possibly with a call in progress) has to find an access point that is located on one of the 19 channels in the 5 GHz band.  AP1 AP2 Use Case 1: Serving Channel Acquisition • Neighbor reports are generally unavailable to stations in areas where there is no 802.11 coverage • Because of radar avoidance regulations, the station must passive scan (in the 5 GHz band) until it finds a channel with an active AP.  • The channel acquisition time is a function of the beacon interval, unless something new is done.  Steve Emeott, et. al.

5. A voice station roams into 802.11a coverage   Challenge: The station (possibly with a call in progress) has to scan for neighbors in the 5 GHz band.  AP1 AP2 Use Case 2: Neighbor AP Scanning • After a station is associated, the neighbor reports provided may not include the optional TBTT offset fields • Without the TBTT offset information, the station must passively scan in the 5 GHz band until TBTT information is collected   • This scanning time is a function of the beacon interval, unless something new is done. Steve Emeott, et. al.

6. A voice station with a call in progress roams within an 802.11a wireless network   Challenge: Due to the beacon interval, beacon frames may provide sparse measurement opportunities. AP1 AP2 Use Case 3: Handover Trigger • When measurements are missed, a STA operating at 5 GHz is unable to issue a probe request (radar avoidance regulations).  • Measurement accuracy is compromised if too much time elapses between measurements • Ill advised handover decisions may result if a STA is unable to update its signal quality estimate in a timely fashion (resulting in call dragging or handover flurries) Steve Emeott, et. al.

7. Proposal: Measurement Frame • Definition: A measurement frame is a new management frame (for example, using frame type 6) that contains only the fields needed by a station to take a link margin measurement plus those required by a station to formulate a valid probe request frame • Necessary Fields include: • Timestamp (8 octets) • Measurement Frame Interval (2 octets) • Capability info (2 octets) • DS Parameter set (3 octets) • Country info (3 octets) • Max regulatory power (1 octet) • AP Max Power (1 octet) • AP Power Used (1 octet) • AP Noise Floor (1 octet) • Total Frame Body Length = 22 Octets Steve Emeott, et. al.

8. New Link Margin Calculation • Setup • Assume the STA knows STA_Noise_Floor and STA_Max_Power • New measurement frame provides AP_Noise_Floor and AP_Max_Power, AP_Power_Used • Step 1: Calculate RCPIc, • RCPIc = RCPI + (AP_Max_Power - AP_Power_Used) • Defined as the RCPI that would have been received had the message been sent at AP_Max_Power • Step 2: Calculate Link Margin • Downlink LM = RCPIc – STA_Noise_Floor • Uplink LM = RCPIc - (AP_Max_Power - STA_Max_Power) - AP_Noise_Floor. Steve Emeott, et. al.

9. New AP Max Power Field • Why do we need AP Max Power when we have Max Regulatory Power? • A particular AP may not choose to transmit at the regulatory max (it might be "dialed back"), but the SU wants to probe at the regulatory max, because probing is used to survey all APs; • The AP max power has to be referenced to the same point as the AP Power Used and AP Noise Floor, which may not be the same point as the regulatory max Steve Emeott, et. al.

10. New AP Power Used Field • Why do we need AP Power Used when 11h defines TPC report, which contains Transmit Power field? • Because TPC report is variable-length and some fields not used (which unnecessarily lengthens the frame); • Because the value of the Transmit Power field is defined as EIRP, when what is needed isa value referenced to the same point as AP Max Power and AP Noise Floor Steve Emeott, et. al.

11. New Max Regulatory Power Field • Why do we need "max regulatory power" when 11d defines "Max Transmit Power" in Country and 11h defines "Power Constraint" IE? • Because the Country IE describes the whole band, which wastes space since we only need to know about the one channel; • Because the Power Constraint is defined as a variable IE, which is a waste of bytes; • Because the Power Constraint is defined as a delta off the Country max tx power, and why give a number and a delta when a single number will do. Steve Emeott, et. al.

12. New AP Noise Floor Field • AP Noise Floor is not provided in any standard IE Steve Emeott, et. al.

13. Measurement Frame Generation • A measurement frame generation function shall transmit the Measurement frame when the value of the TSF Timer (in us) modulo the Measurement Frame interval equals 0. • The Measurement frame shall be addressed to the broadcast destination address. • The Measurement frame shall not be buffered for power save reasons. Steve Emeott, et. al.

14. Beacon versus Measurement Frame Steve Emeott, et. al.

15. Reducing Power Consumption • Several factors influence power consumed while taking measurements • The accuracy with which the measurement start time can be estimated • The length of the frame being measured • The number of steps involved in taking measurements on multiple channels • Consideration 1: Once a station knows the measurement frame interval, it may calculate the expected arrival time of future measurement frames • Since measurement frame are generated when the value of the TSF Timer (in us) modulo the Measurement Frame interval equals 0 • Consideration 2: Measurement frames contains only the fields needed by a station to estimate link margin and to formulate a valid probe request frame • Standby mode: the station has longer battery life because it need not stay awake to receive beacons when taking the measurement • Voice call: the station may return to its serving channel and resume transmitting voice frames in a shorter period of time   • Consideration 3: Since measurement frame intervals are shorter than beacon intervals, it is possible for a station to complete multiple measurements without cycling between sleep and awake modes each time.  Steve Emeott, et. al.

16. Pros and Cons • Pros • Substantially reduces need to use active scan, without reducing measurement opportunities or increasing current drain • Provides measurement opportunity taking significantly less time to receive than a beacon frame • Includes fields not found in beacon and probe response that are required to accurately estimate link margin • Provides frame that station roaming into 802.11 coverage can take measurement on using passive scan • Offers station a standard way to rapidly collect measurements using passive scan • Cons • Consumes capacity in addition to bandwidth used for beacon frames, but saves capacity otherwise consumed by active probing. • When deploying a virtual AP, it is not advisable to transmit a different set of measurement frames for each BSSID Steve Emeott, et. al.

17. Straw Poll 1 • Should 802.11k include a means to reduce the amount of time and effort required by a station to detect the presence of active AP, measure signal quality and estimate link margin. • Yes/No/Abstain Steve Emeott, et. al.

18. Straw Poll 2 • Should 802.11k include a means to permit AP to provide stations with measurement opportunities in addition to those afforded by beacon frames and probe responses since the length of the beacon interval may be too large to provide adequate measurement opportunities? • Yes/No/Abstain Steve Emeott, et. al.