1. 802.11k Measurement Frame Proposal
November 2004
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
November 2004
Mobile Stations and Signal Quality Measurements
Mobile stations will employ several functions that require measurements of AP signal quality as input
Discovery of 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 coverage area must be able to rapidly scan, preferably passively, the available channels prior to association
Handheld stations camped on an 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
November 2004
Measurement Functions
Both the base standard and k 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. Use Case 1: Serving Channel Acquisition
November 2004
Use Case 1: Serving Channel Acquisition
Neighbor reports are generally unavailable to stations in areas where there is no 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. 
AP2
A voice station associated with a foreign network scans for a 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
Steve Emeott, et. al.

5. Use Case 2: Neighbor AP Scanning
November 2004
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.
AP2
A voice station roams into a coverage  
Challenge:
The station (possibly with a call in progress) has to scan for neighbors in the 5 GHz band. 
AP1
Steve Emeott, et. al.

6. Use Case 3: Handover Trigger
November 2004
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)
AP2
A voice station with a call in progress roams within an a wireless network  
Challenge:
Due to the beacon interval, beacon frames may provide sparse measurement opportunities.
AP1
Steve Emeott, et. al.

7. Proposal: Measurement Frame
November 2004
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
November 2004
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. November 2004
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. November 2004
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 is a value referenced to the same point as AP Max Power and AP Noise Floor
Steve Emeott, et. al.

11. New Max Regulatory Power Field
November 2004
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
November 2004
New AP Noise Floor Field
AP Noise Floor is not provided in any standard IE
Steve Emeott, et. al.

13. Measurement Frame Generation
November 2004
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
November 2004
Beacon versus Measurement Frame
Attribute
Beacon Frames
Measurement Frames
Frame Length
Around 137 bytes plus MAC and PHY headers w/ null SSID, zero length TIM and no optional fields
22 bytes plus MAC and PHY headers
Interval
Typically around 100ms, possible that shortening interval could negatively impact legacy equip.
Shorter than beacon interval to provide additional measurement opportunities
Link Margin
Frame does not include fields necessary to accurately estimate link margin
Frame type includes fields required to calculate link margin in uplink/downlink
Interworking w/ Legacy
Format of frame may not be shorted without risking problems with legacy equipment
New mgmt frame sub-type that may be defined to have any length
Steve Emeott, et. al.

15. Reducing Power Consumption
November 2004
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. November 2004
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 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. November 2004
Straw Poll 1
Should k 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. November 2004
Straw Poll 2
Should k 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.