March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 1

Measurement Pilot Frame

Steve Emeott, Walter Johnson, Floyd Simpson, Stephen Wang, Tim Wilson

Motorola

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 2

Channel Acquisition 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 detect a transmittable channel, preferably passively and in compliance with DFS regulations (5 GHz band), among the available channels prior to active probe and/or 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

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 3

Use Case 1 – Serving Channel Acquisition

• When a station is approaching a WLAN from a foreign network (e.g. WAN), it has to quickly locate the transmitting channel among 11 (2.4GHz band) or 19 (5 GHz band) possible channels.

• Because of DFS regulations for the 5GHz band, a station must first passive scan up to 19 channels before it can detect the active channel and transmit a probe request. 

• Fast detection of a transmittable channel leads to shorter channel acquisition time and lower power consumption. 

AP2

A voice station associated with a foreign network scans for WLAN coverage

Challenge: The station (possibly with a call in progress) has to locate the transmitting channel by conforming to DFS regulations as quick as possible.

AP1

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 4

Use Case 2 – Neighbor AP Scanning

• Neighbor report may not always include the optional TSF Offset field (e.g. due to accuracy requirements).

• Information carried in Neighbor report may be stale, making it less useful for accurate prediction of neighbor AP TBTT.

• Without precise information about neighbor TBTT, the station must passively scan neighbor beacons or transmit probe request (provided DFS requirement is met in the 5 GHz band).

• This scanning time is a function of the beacon interval, unless something new is done.

AP2

A voice station roams into 802.11a coverage  

Challenge:The station (possibly with a call in progress) has to scan for neighbor beacons. 

AP1

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 5

Use Case 3 – Handover Trigger

• Stations must make timely handover decision based on real-time link quality measurements.

• Measurement accuracy is compromised if too much time elapses between consecutive 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 a WLAN wireless network  

Challenge: For a typical 100ms beacon interval, beacon frames may be too sparse to provide real-time link quality measurements.

AP1

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 6

Use Case 4 – Fast Neighbor Report

• In most cases, neighbor report is generated based on information gathered via beacon reports

• Measuring STAs must suspend current tasks in order to actively probe or passive sniff up to a full beacon interval on each neighbor channel. This is costly to the measuring STA in terms of resource and battery life.

• Active probe may not always be possible in the 5GHz band.

• The stretched beacon reporting time results in longer neighbor report turnaround time, affecting STA’s ability in making timely transition decisions.

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 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)– Beacon Interval (2 octets)– Capability info (2 octets)– RSN Capabilities (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 = 26 Octets

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 8

Measurement Pilot Bandwidth Consumption

• Total bandwidth consumption is less than 1% in an 11a network

– Each measurement pilot MMPDU is 54-octet long (24-octet MAC Header + 26-octet Frame Body + 4-octet FCS)

– Total length of PPDU is 624 bits (144-bit PLCP Preamble + 48-bit PLCP Header + 432-bit MMPDU)

– 100 Measurement Pilot frames transmitted per second (10ms interval)

– AP transmits at a fixed rate of 6 Mbps

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 9

Bandwidth Consumption Breakeven Point– Measurement Pilot vs. Active Probe –

Assumptions:

Probe Request (min size): 72 octets

Probe Response (min size): 106 octets

Measurement Pilot: 54 octets

Measurement Pilot Interval: 10ms

STA Probe Interval: 250ms

Time Span: 10 secs0

10000

20000

30000

40000

50000

60000

70000

80000

1 2 3 4 5 6 7 8 9 10

No. of STAs Making Active Probes

No.

of T

rans

mitt

ed B

ytes

Measurement Pilot Probe Request+Response

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 10

Impact of Sniffing Time on Battery Life

WLAN Recv Power: 900 mW

GSM Standby Power: 14 mW

Battery Energy: 2400 mW-Hr

Sniff Interval: 30 sec

No. of Channels: 19

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70 80 90 100

Sniff time per channel (msec)

Sta

ndby

tim

e (h

ours

)

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

Red

uctio

n (p

erce

nt)

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 11

Beacon versus Measurement Pilot

Attribute Beacon Measurement Pilot

Frame LengthAround 137 bytes plus MAC and PHY headers w/ null SSID, zero length TIM and no optional fields

26 bytes plus MAC and PHY headers

IntervalTypically around 100ms, possible that shortening interval could negatively impact legacy equip.

Shorter than beacon interval to provide additional measurement opportunities

Link MarginFrame 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

March 2005

Stephen Wang, et. al.

doc.: IEEE 802.11-05/0176r0

Submission Slide 12

Measurement Pilot Generation• Measurement Pilot shall be optional and may be

configured by MIB variable.

• A measurement pilot generation function shall transmit the Measurement Pilot frame when the value of the TSF Timer (in us) modulo the Measurement Pilot interval equals 0.

• The Measurement Pilot frames shall be addressed to the broadcast destination address.

• The Measurement Pilot frames shall not be buffered for power save reasons.

• Beacon transmission takes precedence over Measurement Pilot transmission.