Internet DRAFT - draft-baccelli-intarea-adhoc-wireless-com
draft-baccelli-intarea-adhoc-wireless-com
Mobile Ad-hoc Networks (MANET) E. Baccelli
Internet-Draft INRIA
Intended status: Informational C. Perkins
Expires: January 7, 2016 Futurewei
July 6, 2015
Multi-hop Ad Hoc Wireless Communication
draft-baccelli-intarea-adhoc-wireless-com-01
Abstract
This document describes characteristics of communication between
interfaces in a multi-hop ad hoc wireless network, that protocol
engineers and system analysts should be aware of when designing
solutions for ad hoc networks at the IP layer.
Status of This Memo
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Multi-hop Ad Hoc Wireless Networks . . . . . . . . . . . . . 2
3. Common Packet Transmission Characteristics in Multi-hop Ad
Hoc Wireless Networks . . . . . . . . . . . . . . . . . . . . 3
3.1. Asymmetry, Time-Variation, and Non-Transitivity . . . . . 3
3.2. Radio Range and Wireless Irregularities . . . . . . . . . 4
4. Alternative Terminology . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Informative References . . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Experience gathered with ad hoc routing protocol development,
deployment and operation, shows that wireless communication presents
specific challenges [RFC2501] [DoD01], which Internet protocol
designers should be aware of, when designing solutions for ad hoc
networks at the IP layer. This document does not prescribe
solutions, but instead briefly describes these challenges in hopes of
increasing that awareness. For example, even though a wireless link
may experience high variability as a communications channel, such
variation does not mean that the link is "broken"; indeed many
layer-2 technologies serve to reduce error rates by various means.
Nevertheless, such errors as noted in this document may still become
visible above layer-2 and so become relevant to the operation of
higher layer protocols.
2. Multi-hop Ad Hoc Wireless Networks
For the purposes of this document, a multi-hop ad hoc wireless
network will be considered to be a collection of devices that each
have a radio transceiver (i.e., wireless network interface), and that
are moreover configured to self-organize and provide store-and-
forward functionality as needed to enable communications. This
document focuses on the characteristics of communications through
such a network interface.
Although the characteristics of packet transmission over multi-hop ad
hoc wireless networks, described below, are not the typical
characteristics expected by IP [RFC6250], it is desirable and
possible to run IP over such networks, as demonstrated in certain
deployments currently in operation, such as Freifunk [FREIFUNK], and
Funkfeuer [FUNKFEUER]. These deployments use routers running IP
protocols e.g., OLSR (Optimized Link State Routing [RFC3626]) on top
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of IEEE 802.11 in ad hoc mode with the same ESSID (Extended Service
Set Identification) at the link layer. Multi-hop ad hoc wireless
networks may also run on link layers other than IEEE 802.11, and may
use routing protocols other than OLSR (for instance, AODV [RFC3561],
TBRPF [RFC3684], DSR [RFC4728], or OSPF-MPR [RFC5449]).
Note that in contrast, devices communicating via an IEEE 802.11
access point in infrastructure mode do not form a multi-hop ad hoc
wireless network, since the central role of the access point is
predetermined, and devices other than the access point do not
generally provide store-and-forward functionality.
3. Common Packet Transmission Characteristics in Multi-hop Ad Hoc
Wireless Networks
In the following, we will consider several devices in a multi-hop ad
hoc wireless network N. Each device will be considered only through
its own wireless interface to network N. For conciseness and
readability, this document uses the expressions "device A" (or simply
"A") as a synonym for "the wireless interface of device A to network
N".
Let A and B be two devices in network N. Suppose that, when device A
transmits an IP packet through its interface on network N, that
packet is correctly and directly received by device B without
requiring storage and/or forwarding by any other device. We will
then say that B can "detect" A. Note that therefore, when B detects
A, an IP packet transmitted by A will be rigorously identical to the
corresponding IP packet received by B.
Let S be the set of devices that detect device A through its wireless
interface on network N. The following section gathers common
characteristics concerning packet transmission over such networks,
which were observed through experience with MANET routing protocol
development (for instance, OLSR[RFC3626], AODV[RFC3561],
TBRPF[RFC3684], DSR[RFC4728], and OSPF-MPR[RFC5449]), as well as
deployment and operation (Freifunk[FREIFUNK], Funkfeuer[FUNKFEUER]).
3.1. Asymmetry, Time-Variation, and Non-Transitivity
First, even though a device C in set S can (by definition) detect
device A, there is no guarantee that C can, conversely, send IP
packets directly to A. In other words, even though C can detect A
(since it is a member of set S), there is no guarantee that A can
detect C. Thus, multi-hop ad hoc wireless communications may be
"asymmetric". Such cases are common.
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Second, there is no guarantee that, as a set, S is at all stable,
i.e. the membership of set S may in fact change at any rate, at any
time. Thus, multi-hop ad hoc wireless communications may be "time-
variant". Time variation is often observed in multi-hop ad hoc
wireless networks due to variability of the wireless medium, and to
device mobility.
Now, conversely, let V be the set of devices which A detects.
Suppose that A is communicating at time t0 through its interface on
network N. As a consequence of time variation and asymmetry, 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. device B and device A can detect one another (i.e. B is a member
of sets S and V), and,
2. device A and device C can also detect one another (i.e. C is a
also a member of sets S and V).
These assumptions do not imply that B can detect C, nor that C can
detect B (through their interface on network N). Such "non-
transitivity" is common on multi-hop ad hoc wireless networks.
In a nutshell: multi-hop ad hoc wireless communications can be
asymmetric, non-transitive, and time-varying.
3.2. Radio Range and Wireless Irregularities
Section 3.1 presents an abstract description of some common
characteristics concerning packet transmission over multi-hop ad hoc
wireless networks. This section describes practical examples, which
illustrate the characteristics listed in Section 3.1 as well as other
common effects.
Wireless communications are subject to limitations to the distance
across which they may be established. The range-limitation factor
creates specific problems on multi-hop ad hoc wireless networks. In
this context, the radio ranges of several devices often partially
overlap. Such partial overlap causes communication to be non-
transitive and/or asymmetric, as described in Section 3.1. Moreover,
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the range may vary from one device to another, depending on location
and environmental factors. This is in addition to the time variation
of range and signal strength caused by variability in the local
environment.
For example, as depicted in Figure 1, it may happen that a device B
detects a device A which transmits at high power, whereas B transmits
at lower power. In such cases, B detects A, but A cannot detect B.
This examplifies the asymmetry in multi-hop ad hoc wireless
communications as defined in Section 3.1.
Radio Ranges for Devices A and B
<~~~~~~~~~~~~~+~~~~~~~~~~~~~>
| <~~~~~~+~~~~~~>
+--|--+ +--|--+
| A |======>| B |
+-----+ +-----+
Figure 1: Asymmetric wireless communication example. Device A can communicate with device B, but B cannot communicate with A.
Another example, depicted in Figure 2, is known as the "Hidden
Terminal" problem. Even though the devices all have equal power for
their radio transmissions, they cannot all detect one another. In
the figure, devices A and B can detect one another, and devices A and
C can also detect one another. On the other hand, B and C cannot
detect one another. When B and C simultaneously try to communicate
with A, their radio signals may collide. Device A may receive
incoherent noise, and may even be unable to determine the source of
the noise. The hidden terminal problem illustrates the property of
non-transitivity in multi-hop ad hoc wireless communications as
described in Section 3.1.
Radio Ranges for Devices A, B, C
<~~~~~~~~~~~~~+~~~~~~~~~~~~~> <~~~~~~~~~~~~~+~~~~~~~~~~~~~>
|<~~~~~~~~~~~~~+~~~~~~~~~~~~~>|
+--|--+ +--|--+ +--|--+
| B |=======>| A |<=======| C |
+-----+ +-----+ +-----+
Figure 2: The hidden terminal problem. Devices C and B
try to communicate with device A at the same time,
and their radio signals collide.
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Another situation, shown in Figure 3, is known as the "Exposed
Terminal" problem. In the figure, device A and device B can detect
each other, and A is transmitting packets to B, thus A cannot detect
device C -- but C can detect A. As shown in Figure 3, during the on-
going transmission of A, device C cannot reliably communicate with
device D because of interference within C's radio range due to A's
transmissions. Device C is then said to be "exposed", because it is
exposed to co-channel interference from A and is thereby prevented
from reliably exchanging protocol messages with D -- even though
these transmissions would not interfere with the reception of data
sent from A destined to B.
Radio Ranges for Devices A, B, C, D
<~~~~~~~~~~~~+~~~~~~~~~~~~> <~~~~~~~~~~+~~~~~~~~~~~>
|<~~~~~~~~~~~~+~~~~~~~~~~~~>|<~~~~~~~~~~~~+~~~~~~~~~>
+--|--+ +--|--+ +--|--+ +--|--+
| B |<======| A | | C |======>| D |
+-----+ +-----+ +-----+ +-----+
Figure 3: The exposed terminal problem. When device A is communicating
with device B, and device C is "exposed".
Hidden and exposed terminal situations are often observed in multi-
hop ad hoc wireless networks. Asymmetry issues with wireless
communication may also arise for reasons other than power inequality
(e.g., multipath interference). Such problems are often resolved by
specific mechanisms below the IP layer, for example, CSMA/CA, which
ensures transmission in periods perceived to be unoccupied by other
transmissions. However, depending on the link layer technology in
use and the position of the devices, such problems may affect the IP
layer due to range-limitation and partial overlap .
Besides radio range limitations, wireless communications are affected
by irregularities in the shape of the geographical area over which
devices may effectively communicate (see for instance [MC03],
[MI03]). For example, even omnidirectional wireless transmission is
typically non-isotropic (i.e. non-circular). Signal strength often
suffers frequent and significant variations, which are not a simple
function of distance. Instead, it is a complex function of the
environment including obstacles, weather conditions, interference,
and other factors that change over time. Because wireless
communications have to encounter different terrain, path,
obstructions, atmospheric conditions and other phenomena, analytical
formulation of signal strength is considered intractable [VTC99], and
the radio engineering community has thus developed numerous radio
propagation models, relying on median values observed in specific
environments [SAR03].
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The above irregularities also cause communications on multi-hop ad
hoc wireless networks to be non-transitive, asymmetric, or time-
varying, as described in Section 3.1, and may impact protocols at the
IP layer and above. There may be no indication to the IP layer when
a previously established communication channel becomes unusable;
"link down" triggers are generally absent in multi-hop ad hoc
wireless networks, since the absence of detectable radio energy
(e.g., in carrier waves) may simply indicate that neighboring devices
are not currently transmitting. Such an absence of detectable radio
energy does not therefore indicate whether or not transmissions have
failed to reach the intended destination.
4. Alternative Terminology
Many terms have been used in the past to describe the relationship of
devices 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
previous sections of this document have been selected because the
authors believe they are unambiguous, with respect to the goal of
this document (see Section 1).
In this section, we exhibit some other terms that describe the same
relationship between devices in multi-hop ad hoc wireless networks.
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 devices
that can directly receive packets transmitted by device A through its
interface on network N. In other words, any device B belonging to S
can detect packets transmitted by A. Then, due to the asymmetric
nature of wireless communications:
- We may say that device A "reaches" device B. In this
terminology, there is no guarantee that B reaches A, even if A
reaches B.
- We may say that device B "hears" device A. In this terminology,
there is no guarantee that A hears B, even if B hears A.
- We may say that device A "has a link" to device B. In this
terminology, there is no guarantee that B has a link to A, even if
A has a link to B.
- We may say that device B "is adjacent to" device A. In this
terminology, there is no guarantee that A is adjacent to B, even
if B is adjacent to A.
- We may say that device B "is downstream from" device A. In this
terminology, there is no guarantee that A is downstream from B,
even if B is downstream from A.
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- We may say that device B "is a neighbor of" device A. In this
terminology, there is no guarantee that A is a neighbor of B, even
if B a neighbor of A. As it happens, terminology based on
"neighborhood" is quite confusing for multi-hop wireless
communications. For example, when B can detect A, but A cannot
detect B, it is not clear whether B should be considered a
neighbor of A at all, since A would not necessarily be aware that
B was a neighbor, as it cannot detect B. It is thus best to avoid
the "neighbor" terminology, except for when some level of symmetry
has been verified.
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 3. We do
not discuss bidirectionality, but as a final observation it is
worthwhile to note that bidirectionality is not synonymous with
symmetry. For example, the error statistics in either direction are
often different for a link that is otherwise considered
bidirectional.
5. Security Considerations
This document does not make any detailed description about the
security implications of wireless communications. Notably,
eavesdropping on a wireless link is much easier than for wired media
(although significant progress has been made in the field of wireless
monitoring of wired transmissions). Nevertheless, the need for
securing high-level (layer-3 and above) protocols for wireless media
is a priori independent from the need to secure the layer-2 and
layer-1 protocols for such media.
6. IANA Considerations
This document does not have any IANA actions.
7. Informative References
[RFC2501] Corson, M. and J. Macker, "Mobile Ad hoc Networking
(MANET): Routing Protocol Performance Issues and
Evaluation Considerations", RFC 2501, January 1999.
[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
Demand Distance Vector (AODV) Routing", RFC 3561, July
2003.
[RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol (OLSR)", RFC 3626, October 2003.
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[RFC3684] Ogier, R., Templin, F., and M. Lewis, "Topology
Dissemination Based on Reverse-Path Forwarding (TBRPF)",
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, June
2007.
[RFC5449] Baccelli, E., Jacquet, P., Nguyen, D., and T. Clausen,
"OSPF Multipoint Relay (MPR) Extension for Ad Hoc
Networks", RFC 5449, February 2009.
[RFC5889] Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
Hoc Networks", RFC 5889, September 2010.
[RFC6250] Thaler, D., "Evolution of the IP Model", RFC 6250, May
2011.
[DoD01] Freebersyser, J. and B. Leiner, "A DoD perspective on
mobile ad hoc networks", Addison Wesley C. E. Perkins,
Ed., 2001, pp. 29--51, 2001.
[FUNKFEUER]
"Austria Wireless Community Network,
http://www.funkfeuer.at", 2013.
[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.
[SAR03] Sarkar, T., Ji, Z., Kim, K., Medour, A., and M. Salazar-
Palma, "A Survey of Various Propagation Models for Mobile
Communication", IEEE Press Antennas and Propagation
Magazine, Vol. 45, No. 3, 2003.
[VTC99] Kim, D., Chang, Y., and J. Lee, "Pilot power control and
service coverage support in CDMA mobile systems", IEEE
Press Proceedings of the IEEE Vehicular Technology
Conference (VTC), pp.1464-1468, 1999.
[MI03] Kotz, D., Newport, C., and C. Elliott, "The Mistaken
Axioms of Wireless-Network Research", Dartmouth College
Computer Science Technical Report TR2003-467, 2003.
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[FREIFUNK]
"Freifunk Wireless Community Networks,
http://www.freifunk.net", 2013.
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Appendix A. Acknowledgements
This document stems from discussions with the following people, in
alphabetical order: Jari Arkko, Teco Boot, Carlos Jesus Bernardos
Cano, Ian Chakeres, Thomas Clausen, Robert Cragie, Christopher
Dearlove, Ralph Droms, Brian Haberman, Ulrich Herberg, Paul Lambert,
Kenichi Mase, Thomas Narten, Erik Nordmark, Alexandru Petrescu, Stan
Ratliff, Zach Shelby, Shubhranshu Singh, Fred Templin, Dave Thaler,
Mark Townsley, Ronald Velt in't, and Seung Yi.
Authors' Addresses
Emmanuel Baccelli
INRIA
EMail: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
Charles E. Perkins
Futurewei
Phone: +1-408-330-4586
EMail: charlie.perkins@huawei.com
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