1. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 1 On Hidden and Exposed Terminal Problems 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, 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 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.http:// ieee802.org/guides/bylaws/sb-bylaws.pdfstuart.kerry@philips.compatcom@ieee.org Date: 2005-01-19 Authors:

2. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 2 Abstract This presentation revisits the hidden and exposed terminal problems and reviews the existing solutions for these problems.

3. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 3 Outline Carrier Sensing Radio Ranges Hidden Terminal Problem Exposed Terminal Problem Dynamic carrier sensing Other approaches Conclusions

4. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 4 Carrier Sensing Shared Media Physical Carrier Sensing: Sense the channel before transmitting Not enough. Virtual Carrier Sensing: RTS/CTS handshake Not enough. c d b a

5. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 5 Transmission Range The range within which a packet is successfully received if no interference Dependent on transmission power and attenuation transmitter

6. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 6 Carrier Sensing Range The range within which a transmitter triggers carrier sense detection Dependent on antenna sensitivity transmitter

7. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 7 Interference Range The range within which receivers will be interfered with and suffer a loss Interference range bigger than transmission range if d >.5 Rtx [Ref] receiver

8. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 8 Hidden Nodes Any node within interference range of a receiver can potentially be a hidden node Hidden when not captured by physical carrier sensing (at transmitter) c in Interference range of b AND out of carrier sense range of a b a c

9. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 9 Hidden Nodes Any node within interference range of a receiver can potentially be a hidden node Hidden when not captured by virtual carrier sensing (RTS/CTS transmission range) b a c c in Interference range of b AND out of carrier sense range of a AND out of transmission range of b

10. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 10 Exposed Nodes Any node within carrier sense range of transmitter and out of interference range of receiver Prevents simultaneous transmissions Reduction in Spatial Reuse c in carrier sense range of a AND out of interference range of b b a c d

11. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 11 System Throughput Carrier sense range affects hidden and exposed node problems Hidden and exposed node problems have opposing effects on system throughput Smaller Larger High number of hidden nodes Low number of exposed nodes High collision probability High spatial reuse probability Low number of hidden nodes High number of exposed nodes Low collision probability Low spatial reuse probability carrier sense range

12. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 12 Physical Carrier Sensing - revisited Physical carrier sensing Assume channel idle if carrier less than threshold Current implementations: Fixed threshold  arbitrary point of performance What if we make it dynamic  Optimal operation point to improve the performance of the system

13. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 13 Dynamic Physical Carrier Sensing Adaptive threshold as a function of transmitter-receiver distance and receive power Two solutions: [Jing et. al. ’04] –Unique carrier sensing threshold throughout the network Requires info exchange [Veeravali et. al. ’04] –Heterogeneous carrier sensing thresholds in the network Local decisions –Fairness could be an issue Practical issues: –Limits on threshold – antenna sensitivity –Estimation and measurements

14. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 14 Other Approaches to Address Hidden/Exposed node problems Handshake (virtual carrier sensing) RTS/CTS RRTS Conservative CTS reply Multi-channel – Multi-radio Busy Tone Others Other mechanisms that mitigate the effects Rate control/fairness (scheduling) solutions

15. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 15 RRTS Receiver initiates RTS Not always helpful B A C D B A C D Helps! Does not Help! RTS from A is not heard at B

16. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 16 Handshake - Conservative CTS Conservative CTS reply Reply to RTS only if the received power indicates that the distance between S and R is less than.56txRange (no hidden terminal) Reduce transmission range to prevent interference. Dual of carrier sensing Tx power and estimation of distance

17. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 17 Multi-Channel Multi-Radio Multi-channel –Busy tone –Contention Plane separation High complexity with single-radio Color graphing solutions Challenge: Distributed and dynamic

18. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 18 Fairness solutions Ordering / scheduling / rate control algorithms Mitigate the hidden node problem by reducing probability of collision Cause of hidden/exposed node: Lack of knowledge Information exchange helps

19. doc.: IEEE 802.11-05/0065r0 Submission January 2005 Bahareh Sadeghi, Intel CorpSlide 19 Conclusions Hidden/Exposed terminal problems –Improvement of one could worsen the other Virtual carrier sensing –Not a complete solution –High overhead Multi-channel –Recommended for high performance in mesh –Too complex as a solution for hidden/exposed node problems –Helps mitigate the effect Scheduling schemes –Do help, not stand-alone solutions Adaptive carrier sensing –Potential solution