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Click to edit Master title style 3 © 2009 Wichorus Inc. All rights reserved. Asymmetry, Time-Variation, and Non-Transitivity Terminology: let’s use “A can hear B” Suppose node A can hear node B. Then it is possible that B cannot hear A. Suppose A can hear B at time t 0. Then at any later time t 1 it is possible that A can no longer hear B. Suppose A can hear B and B can hear node C. Then it is possible that C cannot hear A and A cannot hear C. BATRANSCRIPT
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© 2009 Wichorus Inc. All rights reserved.© 2009 Wichorus Inc. All rights reserved.
Multi-hop Ad Hoc Wireless Communication
Autoconf WGNovember 9, 2009
2 © 2009 Wichorus Inc. All rights reserved.
Click to edit Master title styledraft-baccelli-multi-hop-wireless-communication-03
• Informational document• Intended to motivate design decisions about address
architecture and guide exploration of solution space• Wireless media are not at all new• What assumptions should guide protocol design?• Asymmetry, Time-Variation, and Non-Transitivity
What’s a “neighbor”?Synonyms?: is reachable from, “can hear”, “has a link”, …
• Radio Range and Wireless Irregularities
3 © 2009 Wichorus Inc. All rights reserved.
Click to edit Master title styleAsymmetry, Time-Variation, and Non-Transitivity
Terminology: let’s use “A can hear B”
• Suppose node A can hear node B. Then it is possible that B cannot hear A.
• Suppose A can hear B at time t0. Then at any later time t1 it is possible that A can no longer hear B.
• Suppose A can hear B and B can hear node C. Then it is possible that C cannot hear A and A cannot hear C.
B
A
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Click to edit Master title styleHidden Terminals
• It is possible for a node B to detect carrier quiescence around node A even though A is receiving a packet from another neighbor C. Then node C is, from the standpoint of node B, a hidden terminal. Node A may suffer data loss if B transmits.
A
CB
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Click to edit Master title styleExposed Terminals
• Node A might be unable to send a packet to node B because of an unrelated packet transmission between two other nodes C and D. Node A is then an “exposed terminal”. This can happen even if B and D could both safely receive transmissions from A and C respectively.
A
CB
D
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Click to edit Master title styleMAC layer support
• For 802.11, these problems can be almost (but not quite) completely eliminated by RTS/CTS. Too bad those signals are often administratively disabled.Bandwidth, bandwidth, bandwidth
• For other wireless media, scheduling is imposed possibly along with statutory access controls
• IP typically does not assume responsibility for preventing hidden terminals / exposed terminals “Best effort” retransmissions “O.K.” [??]
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Click to edit Master title styleTime variations
• MAC layer may not help here at all (might even make things worse)
• IP has to be resilient against unpredictable invalidation of link connectivity statusBeacons, path breakage updates, keepalives
• Probably best to minimize global state, use distributed algorithms mostly relying on local state
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Click to edit Master title styleConclusions
• Wireless media are not at all new - but they are not very similar to Ethernet
• Hidden terminals, exposed terminals, time variations, asymmetries are quite well understood
• IP achieved worldwide proliferation based on designs suitable for Ethernet
• But this means it’s required to “think different” to enable IP over wireless to be successful
• Should this draft become WG Informational doc?