1. Mechanism for Link Quality Measurement
May 2007
doc.: IEEE /0691r0
May 2007
Mechanism for Link Quality Measurement
Date:
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Michael Souryal, NIST
Michael Souryal, NIST

2. May 2007
doc.: IEEE /0691r0
May 2007
Abstract
This presentation discusses the need for a standard mechanism to obtain link quality measurements, in particular to obtain timely bi-directional link measurements in mobile mesh networks. It compares alternative mechanisms for link quality measurement and recommends adoption of the Link Measurement Request/Report capability specified by TGk.
(This presentation is in response to the strawpoll taken at the March 2007 meeting in Orlando: “Should IEEE s TG consider reviewing specifications about certain aspects of a multi-hop breadcrumb communication system, such as a real-time channel assessment and breadcrumb deployment strategy, based on IEEE 802 PHY/MAC protocols?”)
Michael Souryal, NIST
Michael Souryal, NIST

3. Link Quality Measurement: Uses
May 2007
Link Quality Measurement: Uses
Computation of link metric, and ultimately path metric, for path selection
Link state defined by two parameters
r : current bit rate in use
ef : frame error rate at bit rate r
Airtime cost metric
where Oca, Op, and Bt are constants defined in clause 11A.5
Maintaining connectivity during mesh network deployment (e.g., Public Safety and Military usage models)
Michael Souryal, NIST

4. Excerpts from IEEE 802.11 TGs Usage Models (11-04/662r16)
May 2007
Excerpts from IEEE TGs Usage Models (11-04/662r16)
From Description of Public Safety Usage Model
“Communications for public safety networks are mostly outdoors, but may include communicating with first responders inside buildings (potentially deep inside with contact only by multi-hop relaying).... Node mobility, dynamic variations in radio propagation, equipment/power failures, etc. make network self-configuration and self-management essential in these scenarios.”
From Usage Model Characteristics Table
Usage Model Category
Mesh AP/ Point Mobility
Public Safety
Mix of fixed radios (e.g. on poll tops) and mobile nodes deployed in emergency vehicles and carried by emergency workers.
Military
May have a few stationary radios … and mobile nodes deployed in combat vehicles, ships or carried by dismounted troops.
Michael Souryal, NIST

5. Link Quality Measurement: Requirements
May 2007
Link Quality Measurement: Requirements
Timely measurements for mobile mesh links
Channel coherence time: rough measure of time duration over which channel impulse response is invariant
Example: 2.4 GHz, 1.2 m/s (pedestrian speed)
Measurement procedure that works across vendors
Measurement mechanism not specified in P802.11s/D1.03 (April 2007)
In particular, no mechanism is defined for bi-directional link quality measurement
Important for detecting asymmetric links
Michael Souryal, NIST

6. Possible Mechanisms for Link Measurement
May 2007
Possible Mechanisms for Link Measurement
Need to receive a frame in order to make a measurement.
Possible means for obtaining link measurements include:
Passive listening to
Beacons
RA-OLSR HELLO messages
Other traffic (e.g., data transmissions)
802.11k Measurement Pilot frames
Active probing with
Probe Request/Response
802.11k Link Measurement Request/Report
Michael Souryal, NIST

7. Mechanism
Pros
Cons
Passive Listening
Beacons
No additional messaging needed
Unidirectional measurement
Not all MPs continuously beacon
RA-OLSR HELLO messages
RA-OLSR is optional
Other traffic (e.g., data messages)
Measurement availability not guaranteed
802.11k Measurement Pilot frames
More efficient and frequent than beacons
Overhead for entire network
Active Probing
Probe Request/ Response
Application can control measure-ment interval
On-demand measurement, only when and where needed
Not all MPs respond to Probe Requests
802.11k Link Measurement Request/Report
Bi-directional measure-ment
Additional message overhead
Passive Listening
Active Probing
Unidirectional measurement

8. Link Measurement Request/Report
May 2007
Link Measurement Request/Report
based on P802.11k/D7.0 (January 2007)
Link Measurement Request Frame
Tx Power
Max Tx Power MP MP
Link Measurement Report Frame
TPC Report Element,
…, RCPI, RSNI
RCPI: Received channel power indicator
RSNI: Received signal to noise indicator
Michael Souryal, NIST

9. Sample Data: Bi-directional Filtered SNR Measurements of a Mobile Link
May 2007
Sample Data: Bi-directional Filtered SNR Measurements of a Mobile Link
Fixed STA “A”, mobile STA “B”
“B” carried down 110 m corridor and back at ~ 1.2 m/s
Sampling period: 100 ms
Filter: Uniform moving average of last 20 samples (2 s filter)
Data rate: 2 Mbps
Michael Souryal, NIST

10. May 2007
Summary
Mobile mesh networks (e.g., public safety and military usage models) require the ability to measure link quality on-demand
For up-to-date link metric evaluation in time-varying channels
To facilitate maintaining connectivity during rapid deployment of MPs
Standard mechanism for link quality measurement ensures operation across vendors
Link Measurement Request/Report capability specified by TGk satisfies these requirements
Permits on-demand, bi-directional link measurement
Michael Souryal, NIST

11. May 2007
Appendix
Michael Souryal, NIST

12. Current Text in P802.11s/D1.03 (April 2007)
May 2007
Current Text in P802.11s/D1.03 (April 2007)
In 11A.5.2, “Local link state maintenance procedures”:
“The peer MP with the larger MAC address determines the link quality.” “An MP shall update the values in its neighbor MP table whenever a local link state announcement message is received.”
Michael Souryal, NIST

13. 802.11k, “Radio Resource Measurement”
May 2007
802.11k, “Radio Resource Measurement”
Provides standard measurements across vendors of radio link performance and the radio environment
Addresses mobility requirements by speeding up handoffs through collection of pre-handoff information
Request/response measurements:
Beacon
Measurement Pilot
Frame
Channel Load
Noise Histogram
STA Statisitics
Location Configuration Information
Neighbor Report
Link Measurement
Transmit Stream Measurement
Michael Souryal, NIST

14. Raw SNR Measurements of Sample Data
May 2007
Raw SNR Measurements of Sample Data
Sampling period: 20 ms
Michael Souryal, NIST

15. May 2007
References
Michael Souryal, NIST