March 7 2006 Rice University Slide 1 doc.: IEEE 802.11-06/0466r0 Submission Modeling Imperfect Clock Synchronization in CSMA Wireless Networks Date: 2006-03-07 Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures < http:// ieee802.org/guides/bylaws/sb-bylaws.pdf >, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <[email protected]> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If Authors: Name Address Company Phone Email Theodoros Salonidis 6100 Main St. #366 Houston, TX 77005 Rice Universi ty [email protected]Jingpu Shi 6100 Main St. #366 Houston, TX 77005 Rice Universi ty [email protected]Edward Knightly 6100 Main St. #380 Houston, TX 77005 Rice Universi ty [email protected]Joseph Camp 6100 Main St. #366 Houston, TX 77005 Rice Universi ty [email protected]
Transcript
March 7 2006
Rice UniversitySlide 1
doc.: IEEE 802.11-06/0466r0
Submission
Modeling Imperfect Clock Synchronization in CSMA Wireless Networks
Date: 2006-03-07
Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11.
Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <[email protected]> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at <[email protected]>.
• Flows 32 mini-slots late from earliest flow starve
• Guard Time less fair
March 7 2006
Rice UniversitySlide 9
doc.: IEEE 802.11-06/0466r0
Submission
Example Analysis (GT system)
p12= p22 = P(x1 > + x2) p11=1 - p12*where x is chosen backoff and is drift p21=1 - p22
• Guard Times exacerbate the unfairness• Earlier Flow 1 always has the advantage
March 7 2006
Rice UniversitySlide 10
doc.: IEEE 802.11-06/0466r0
Submission
Single Cell Analysis Results (Random drifts)
• 10 flows, W=32 mini-slots• Clock drifts drawn from
uniform distribution [0 to max clock drift]
• Measure lowest throughput as a function of max clock drift / W
• Even if clock drift less than contention window, exponential decrease of lowest throughput
March 7 2006
Rice UniversitySlide 11
doc.: IEEE 802.11-06/0466r0
Submission
Multi-hop Topologies
• More issues in addition to clock drift– Carrier sense (not all nodes sense the same thing)
– High collision due to hidden terminals from transmitter (gets worse as REQ packet size increases).
• Two simple scenarios to isolate and study in detail using our model– Flow in the Middle
– Information Asymmetry
March 7 2006
Rice UniversitySlide 12
doc.: IEEE 802.11-06/0466r0
Submission
Flow in the Middle Scenario
• Middle flow senses both outer flows but outer flows sense only the middle flow
• Middle flow can starve sensing misaligned transmissions of outer flows
• Model applies: use one state for outer flows and another state for middle flow
March 7 2006
Rice UniversitySlide 13
doc.: IEEE 802.11-06/0466r0
Submission
Flow in the Middle scenario
• Early outer flow and middle flow drifts = 0, W=32
• Throughputs as drift of late outer flow increases
No GT: Middle flow starves if outer flows relative drift > W
GT: Throughput of middle flow increases
No GTGT
March 7 2006
Rice UniversitySlide 14
doc.: IEEE 802.11-06/0466r0
Submission
Flow in the Middle scenario
• Relative drift of outer flows =16, W=32 mini-slots
• Throughputs as drift of middle flow increases
No GT
No GT: Middle flow: Steep decrease with a stable region C
A B C D
16
16a
GT
GT: Middle flow: Smoother decrease but no stable region
March 7 2006
Rice UniversitySlide 15
doc.: IEEE 802.11-06/0466r0
Submission
Information Asymmetry Scenario
• tx1 unable to sense flow 2• tx2 senses flow 1 (through GNT of rx1)• Flow 1:disadvantage even under perfect synchronization
– Can succeed only if its backoff + REQ packet < backoff of flow 2
• Disadvantage of flow 1 aggravated as REQ packet size increases
March 7 2006
Rice UniversitySlide 16
doc.: IEEE 802.11-06/0466r0
Submission
Information Asymmetry Scenario
• Fix drift of flow 1 to 0, REQ packet size = 16 mini-slots• Throughputs as drift of advantaged flow 2 increases
GBNo GB
No GT and GT: perform similarly (carrier sense no effect)
Cross-over point: when drift difference = REQ packet size
March 7 2006
Rice UniversitySlide 17
doc.: IEEE 802.11-06/0466r0
Submission
To GT or not to GT?
• Pros– Better fairness in single-hop
systems– Protection regions for clock
phase jitter
• Cons (multi-hop systems)– Flows may starve irrespective of
their own contention windows due to high clock drift difference of their outer flows
– Hard to exploit protection regions
– Two-hop max drift bound
No GT• Pros
– Easier to extend MC model to arbitrary topologies
– Allow for simpler solutions• Only address clock drift differences
and REQ packet size.
– One-hop max drift bound
• Cons– Guard Time overhead– Less fairness in single-hop
networks– No inherent protection against
clock phase jitter
GT
March 7 2006
Rice UniversitySlide 18
doc.: IEEE 802.11-06/0466r0
Submission
Counter-starvation algorithm for GT systems
• Goal– Achieve per-flow throughputs above a set of reference
slotted aloha rates
• Operation– Utilize a model for arbitrary topologies for GT systems to
adjust contention windows.– Rule: ask neighbors with higher throughput to increase their
contention windows by a certain factor
• Outcome– Proportionally fair minimum throughput guarantees.– Throughputs greater than the reference rates.
March 7 2006
Rice UniversitySlide 19
doc.: IEEE 802.11-06/0466r0
Submission
Counter-starvation Algorithm
March 7 2006
Rice UniversitySlide 20
doc.: IEEE 802.11-06/0466r0
Submission
Conclusions
• Single-hop networks– Starvation: if W < max clock drift– Min throughput: exponential decrease with max clock drift– GT system less fair than no GT system
• Multi-hop networks– No GT systems
• Offer protection regions against clock phase jitter, yet solutions harder to implement
• Starvation sensitive to clock drifts of two-hop neighborhood
– GT systems allow for simpler solutions with predictable performance at the expense of GT overhead.
• Distributed CW adjustment mechanism to counter starvation in a GT system
