On Hidden and Exposed Terminal Problems January 2005 doc.: IEEE 802.11-05/0065r0 January 2005 On Hidden and Exposed Terminal Problems Date: 2005-01-24 Authors: 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 <stuart.kerry@philips.com> 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 <patcom@ieee.org>. Bahareh Sadeghi, Intel Corp Bahareh Sadeghi, Intel Corp
January 2005 doc.: IEEE 802.11-05/0065r0 January 2005 Abstract This presentation revisits the hidden and exposed terminal problems and reviews the existing solutions for these problems. Bahareh Sadeghi, Intel Corp Bahareh Sadeghi, Intel Corp
Outline Carrier Sensing Radio Ranges Hidden Terminal Problem January 2005 Outline Carrier Sensing Radio Ranges Hidden Terminal Problem Exposed Terminal Problem Dynamic carrier sensing Other approaches Conclusions Bahareh Sadeghi, Intel Corp
Carrier Sensing Shared Media January 2005 Carrier Sensing Shared Media Physical Carrier Sensing: Sense the channel before transmitting Not enough. Virtual Carrier Sensing: RTS/CTS handshake d a c b Bahareh Sadeghi, Intel Corp
January 2005 Transmission Range The range within which a packet is successfully received if no interference Dependent on transmission power and attenuation transmitter Bahareh Sadeghi, Intel Corp
January 2005 Carrier Sensing Range The range within which a transmitter triggers carrier sense detection Dependent on antenna sensitivity transmitter Bahareh Sadeghi, Intel Corp
January 2005 Interference Range The range within which receivers will be interfered with and suffer a loss Interference range bigger than transmission range if distance between transmitter and receiver > .5 transmission range [Ref. 1] receiver Bahareh Sadeghi, Intel Corp
c in Interference range of b out of carrier sense range of a Hidden Nodes January 2005 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 c a b Bahareh Sadeghi, Intel Corp
Hidden Nodes January 2005 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) c in Interference range of b AND out of carrier sense range of a out of transmission range of b c a b Bahareh Sadeghi, Intel Corp
c in carrier sense range of a out of interference range of b Exposed Nodes January 2005 Any node within carrier sense range of transmitter and out of interference range of receiver Prevents simultaneous transmissions Reduction in Spatial Reuse d c a b c in carrier sense range of a AND out of interference range of b Bahareh Sadeghi, Intel Corp
January 2005 System Throughput Carrier sense range affects hidden and exposed node problems Hidden and exposed node problems have opposing effects on system throughput Smaller Larger carrier sense range High number of hidden nodes Low number of exposed nodes Low number of hidden nodes High number of exposed nodes High collision probability High spatial reuse probability Low collision probability Low spatial reuse probability Bahareh Sadeghi, Intel Corp
Physical Carrier Sensing - revisited January 2005 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 Bahareh Sadeghi, Intel Corp
Dynamic Physical Carrier Sensing January 2005 Dynamic Physical Carrier Sensing Adaptive threshold as a function of transmitter-receiver distance and receive power Two solutions: [Ref. 2] Unique carrier sensing threshold throughout the network Requires info exchange [Ref. 3] Heterogeneous carrier sensing thresholds in the network Local decisions Fairness could be an issue Practical issues: Limits on threshold – antenna sensitivity Estimation and measurements Bahareh Sadeghi, Intel Corp
Other Approaches to Address Hidden/Exposed node problems January 2005 Other Approaches to Address Hidden/Exposed node problems Handshake (virtual carrier sensing) RTS/CTS RRTS [Ref. 4] Conservative CTS reply [Ref. 1] Multi-channel – Multi-radio Busy Tone [Ref. 5] Others Other mechanisms that mitigate the effects Rate control/fairness (scheduling) solutions [Ref. 6 and references therein] Bahareh Sadeghi, Intel Corp
RTS from A is not heard at B January 2005 RRTS Receiver initiates RTS Not always helpful Helps! A B C D Does not Help! A B C D RTS from A is not heard at B Bahareh Sadeghi, Intel Corp
Handshake - Conservative CTS January 2005 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 Bahareh Sadeghi, Intel Corp
Multi-Channel Multi-Radio January 2005 Multi-Channel Multi-Radio Multi-channel Busy tone Contention Plane separation High complexity with single-radio Color graphing solutions Challenge: Distributed and dynamic Bahareh Sadeghi, Intel Corp
Fairness solutions Ordering / scheduling / rate control algorithms January 2005 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 Bahareh Sadeghi, Intel Corp
Conclusions Hidden/Exposed terminal problems Virtual carrier sensing January 2005 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 Bahareh Sadeghi, Intel Corp
January 2005 References [Ref. 1] Kaixin Xu, Mario Gerla and Sang Bae, "HOw Effective is the IEEE802.11 RTS/CTS Handshake in Ad Hoc Networks?", In Proc. IEEE Globecom 2002, Taipei, Taiwan, R.O.C., November 17-21, 2002 [Ref. 2] Jing Zhu, Xingang Guo, L. Lily Yang, W. Steven Conner, Sumit Roy, Mousumi M. Hazra, “Adapting physical carrier sensing to maximize spatial reuse in 802.11 mesh networks,” Wireless Communications and Mobile Computing, Volume 4, Issue 8 , Pages 933 – 946. [Ref. 3] Xue Yang and Nitin H. Vaidya, "On Physical Carrier Sensing in Wireless Ad Hoc Networks", to appear in IEEE INFOCOM 2005 [Ref. 4] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, “ MACAW: A Media Access Protocol for Wireless LAN’s,” in Proc. ACM SIGCOMM’94, pp. 212-25, London, UK, Aug. 31-Sept.2, 1994. [Ref. 5] F. A.Tobagi and L. Klienrock, "Packet Switching in Radio Channels: Part II The Hidden Terminal Problem in Carrier Sense Multiple Access and the Busy Tone Solution," IEEE Trans. Commun., COM-23, 1975, pp. 141733 [Ref. 6] V. Gambiroza, B. Sadeghi, and E. Knightly, “End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks,” in Proceedings of ACM MobiCom’04, Philadelphia, PA, September 2004 C. L. Fullmer and J.J. Garcia-Luna-Aceves, "Solutions to Hidden Terminal Problems in Wireless Networks", Proc. ACM SIGCOMM 97, Cannes, France, September 14-18, 1997. Bahareh Sadeghi, Intel Corp