1. On Hidden and Exposed Terminal Problems
January 2005
doc.: IEEE /0065r0
January 2005
On Hidden and Exposed Terminal Problems
Date:
Authors:
Notice: This document has been prepared to assist IEEE 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
Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures < 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 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 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at
Bahareh Sadeghi, Intel Corp
Bahareh Sadeghi, Intel Corp

2. January 2005
doc.: IEEE /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

3. 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

4. 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

5. 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

6. January 2005
Carrier Sensing Range
The range within which a transmitter triggers carrier sense detection
Dependent on antenna sensitivity
transmitter
Bahareh Sadeghi, Intel Corp

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. January 2005
References
[Ref. 1] Kaixin Xu, Mario Gerla and Sang Bae, "HOw Effective is the IEEE 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 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 , 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