Multi-hop Ad Hoc Wireless Communication

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© 2009 Wichorus Inc. All rights reserved.

© 2009 Wichorus Inc. All rights reserved.

Multi
-
hop Ad Hoc Wireless Communication



Autoconf WG

November 9, 2009

charliep@wichorus.com

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2

© 2009 Wichorus Inc. All rights reserved.

draft
-
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

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© 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.

B

A

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© 2009 Wichorus Inc. All rights reserved.

Hidden 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

C

B

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© 2009 Wichorus Inc. All rights reserved.

Exposed 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

C

B

D



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© 2009 Wichorus Inc. All rights reserved.

MAC 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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© 2009 Wichorus Inc. All rights reserved.

Time 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 status


Beacons, path breakage updates, keepalives


Probably best to minimize global state, use
distributed algorithms mostly relying on local state

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© 2009 Wichorus Inc. All rights reserved.

Conclusions


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?