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MANET Autoconfiguration (Autoconf) E. Baccelli
Internet-Draft INRIA
Intended status: Informational C. Perkins
Expires: August 27, 2009 WiChorus
February 23, 2009
Multi-hop Ad Hoc Wireless Communication
draft-baccelli-multi-hop-wireless-communication-01
Status of This Memo
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Abstract
This document describes some important aspects, experienced over the
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past decade, of multi-hop ad hoc wireless communication between
routers.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Communication on Multi-hop Ad Hoc Wireless Networks . . . . . . 3
3. Asymmetry, Time-Variation, and Non-Transitivity . . . . . . . . 3
4. Radio Range, Exposed Nodes and Hidden Terminals . . . . . . . . 4
5. Alternative Terminology . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Informative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The goal of this document is to describe some important aspects of
multi-hop ad hoc wireless communication between routers, observed
over the years. Experience gathered with multi-hop ad hoc wireless
communication [RFC2501] [RFC3626] [RFC3561] [RFC3684] [RFC4728]
[DoD01] shows that this type of communication presents specific
challenges. This document briefly describes some of these
challenges.
2. Communication on Multi-hop Ad Hoc Wireless Networks
In this document, we consider a multi-hop ad hoc wireless network to
be a collection of devices that all have radio transceivers using the
same physical and medium access protocols. All are configured to
provide store-and-forward functionality on top of these protocols, as
needed to enable communications; consequently, they can be classified
as routers in the resulting wireless network. In the following, we
will refer to these devices equivalently as nodes, or routers.
Let A and B be two nodes in a multi-hop ad hoc wireless network N.
Suppose that, when node A transmits a packet through its interface on
network N, that packet is detectable by node B without requiring
storage and/or forwarding by any other router. In this circumstance,
we will say that B can receive packets directly from A.
Alternatively, we may also say that B "hears" packets from A. Note
that therefore, when B can hear IP packets from A, the TTL of the IP
packet heard by B will be precisely the same as it was when A
transmitted that packet.
Let S be the set of nodes that can hear packets transmitted by node A
through its interface on network N. We will now describe some
fundamental characteristics of multi-hop ad hoc wireless
communication. Because of these characteristics, some assumptions
about packet transmission that are typically made in wired networks,
are often untrue in multi-hop ad hoc wireless networks.
3. Asymmetry, Time-Variation, and Non-Transitivity
First, there is no guarantee that a router C within S can,
symmetrically, send IP packets directly to router A. In other words,
even though C can "hear" packets from node A (since it is a member of
set S), there is no guarantee that A can "hear" packets from node C.
Thus, multi-hop ad hoc wireless communications may be "asymmetric".
Such asymmetry is often experienced on multi-hop ad hoc wireless
networks, due to well-known properties of wireless communication.
Second, there is no guarantee that, as a set, S is at all stable.
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The membership of set S may in fact change at any rate, any time.
Thus, multi-hop ad hoc wireless communications may be "time-variant".
Such variations are often experienced on multi-hop ad hoc wireless
networks due to variability of the wireless medium, and to router
mobility.
Now, conversely, let V be the set of routers from which node A can
directly receive packets -- in other words, A can "hear" packets from
any node in set V. Suppose that router A is communicating at time t0
through its interface on network N. As a consequence of time
variation and assymetry, we observe that A:
1. cannot assume that S = V,
2. cannot assume that S and/or V are unchanged at time t1 later than
t0.
Furthermore, transitivity is not guaranteed over multi-hop ad hoc
wireless networks. Indeed, let's assume that, through their
respective interfaces within network N:
1. node B and node A can hear each other (i.e. node B is a member of
sets S and V), and,
2. node A and node C can also hear each other (i.e. node C is a also
a member of sets S and V).
This neither implies that node B can hear node C, nor that node C can
hear node B (through their interface on network N). Such non-
transitivity is often observed on multi-hop ad hoc wireless networks.
In a nutshell: multi-hop ad hoc wireless communications often prove
to be asymmetric, non-transitive, and time-varying in character.
4. Radio Range, Exposed Nodes and Hidden Terminals
Wireless communication links are often subject to significant
limitations to the distance across which they may be established. In
the extreme cases, some radio links are measured in centimeters, not
meters, although such short-range radio links are not typically
considered to support multi-hop ad hoc networks. More often, radio
links are encountered with range limited to several tens or hundreds
of meters.
The range-limited characteristic of wireless communications creates
new problems that are often observed in multi-hop ad hoc wireless
networks. One such problem is shown in Figure 1. Observe that, even
though the nodes are shown as all having equal communication ranges,
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they are not at all equally accessible to each other. In the figure,
two wireless communications are shown to be in progress; one from
node D to node C, and the other one from node A to node B. As shown,
this figure illustrates that while router D can hear router C without
interference, router C is prevented from hearing router D because
router A (in C's radio range), is already communicating with another
node. This case is known as the "exposed node" problem, and is often
observed on multi-hop ad hoc wireless networks.
Radio Ranges for Routers A, B, C, D
<~~~~~~~~~~~~+~~~~~~~~~~~~> <~~~~~~~~~~~~~~+~~~~~~~~~>
|<~~~~~~~~~~~~+~~~~~~~~~~~~>|<~~~~~~~~~~~~+~~~~~~~~~~~~>
+--|-+ +--|-+ +--|-+ +--|-+
|RtrD|=======>|RtrC| |RtrA|------->|RtrB|
+----+ +----+ +----+ +----+
Router C becomes an Exposed Node
Figure 1: The exposed node problem. Router C is prevented
from hearing router D while router A is
communicating with router B.
Another case which is caused by the range-limited characteristic of
wireless communications and is often observed in multi-hop ad hoc
wireless networks, is shown in Figure 2. In this example routers B
and C cannot hear each other. On the other hand, routers A and B can
hear each other and furthermore A and C can also hear each other.
When routers B and C try to communicate with router A at the same
time, their radio signals collide. Router A will only be able to
detect noisy interference, and may even be unable to determine the
source of the issue. This case is known as the "hidden terminal"
problem, and is often observed on multi-hop ad hoc wireless networks.
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Radio Ranges for Routers A, B, C
<~~~~~~~~~~~~~+~~~~~~~~~~~~~> <~~~~~~~~~~~~~+~~~~~~~~~~~~>
|<~~~~~~~~~~~~~+~~~~~~~~~~~~~>|
+--|-+ +--|-+ +--|-+
|RtrB|========>|RtrA|<========|RtrC|
+----+ +----+ +----+
Hidden Terminals at Rtr A
Figure 2: The hidden terminal problem. Router C and Router B
try to communicate with router A at the same time,
and their radio signals collide.
5. Alternative Terminology
Many terms have been used in the past to describe the relationship of
nodes in a multi-hop ad hoc wireless network based on their ability
to send or receive packets to/from each other. The terms used in
this document have been selected because the authors believe (or at
least hope) they are relatively unambiguous, with respect to the goal
of this document (see Section 1).
Nevertheless, here are a few other phrasings, describing the same
relationship between wireless nodes. In the following, let network N
be, again, a multi-hop ad hoc wireless network. Let the set S be, as
before, the set of routers that can directly receive packets
transmitted by router A through its interface on network N. In other
words, any router B belonging to S can "hear" packets transmitted by
router A. Then, due to the asymmetry characteristic of wireless
links:
- We may say that router B is reachable from router A. In this
terminology, there is no guarantee that router A is reachable from
node B, even if router B is reachable from router A.
- We may say that router A has a link to router B. In this
terminology, there is no guarantee that router B has a link to
router A, even if router A has a link to router B.
- We may say that router B is adjacent to router A. In this
terminology, there is no guarantee that router A is adjacent to
router B, even if router B is adjacent to router A.
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- We may say that router B is a neighbor of router A. In this
terminology, there is no guarantee that router A is a neighbor of
router B, even if router B a neighbor of router A.
- We may say that router B is downstream from router A. In this
terminology, there is no guarantee that router A is downstream
from router B, even if router B is downstream from router A.
This list of alternative terminologies is given here for illustrative
purposes only, and is not suggested to be complete or even
representative of the breadth of terminologies that have been used in
various ways to explain the properties mentioned in Section 2.
6. Security Considerations
This document does not have any security considerations.
7. IANA Considerations
This document does not have any IANA actions.
8. Informative References
[RFC2501] Corson, S. and J. Macker, "Mobile Ad hoc Networking
(MANET): Routing Protocol Performance Issues and
Evaluation Considerations", RFC 2501, 1999.
[RFC3626] Clausen, T. and P. Jacquet, "The Optimized Link State
Routing Protocol", RFC 3626, October 2003.
[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
Demand Distance Vector (AODV) Routing", RFC 3561,
July 2003.
[RFC3684] Ogier, R., Templin, f., and M. Lewis, "Topology
Dissemination Based on Reverse-Path Forwarding", RFC 3684,
February 2004.
[RFC4728] Johnson, D., Hu, Y., and D. Maltz, "The Dynamic Source
Routing Protocol (DSR) for Mobile Ad Hoc Networks for
IPv4", RFC 4728, February 2007.
[RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903, 2007.
[IPev] Thaler, D., "Evolution of the IP Model",
draft-thaler-ip-model-evolution-01.txt (work in progress),
2008.
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[DoD01] Freebersyser, J. and B. Leiner, "A DoD perspective on
mobile ad hoc networks", Addison Wesley C. E. Perkin,
Ed., 2001, pp. 29--51, 2001.
[MC03] Corson, S. and J. Macker, "Mobile Ad hoc Networking:
Routing Technology for Dynamic, Wireless Networks", IEEE
Press, Mobile Ad hoc Networking, Chapter 9, 2003.
Appendix A. Acknowledgements
This document stems from discussions with the following people, in no
particular order: Thomas Clausen, Erik Nordmark, Teco Boot, Seung Yi,
Stan Ratliff, Fred Templin, Thomas Narten, Ronald Velt in't,
Christopher Dearlove, Shubhranshu Singh, Carlos Jesus Bernardos Cano,
Kenichi Mase, Paul Lambert, Ralph Droms, Ulrich Herberg, Zach Shelby,
Alexandru Petrescu, Ian Chakeres, Dave Thaler, Jari Arkko, and Mark
Townsley.
Authors' Addresses
Emmanuel Baccelli
INRIA
Phone: +33-169-335-511
EMail: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
Charles E. Perkins
WiChorus
Phone: +1-408-435-0777 x337
EMail: charliep@wichorus.com
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