One document matched: draft-ietf-intarea-adhoc-wireless-com-00.xml
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<rfc ipr="trust200902" category="info" docName="draft-ietf-intarea-adhoc-wireless-com-00">
<front>
<title abbrev="Multi-hop Ad Hoc Wireless Communication">
Multi-hop Ad Hoc Wireless Communication</title>
<author initials="E.B." surname="Baccelli" fullname="Emmanuel Baccelli">
<organization>INRIA</organization>
<address>
<email>Emmanuel.Baccelli@inria.fr</email>
<uri>http://www.emmanuelbaccelli.org/</uri>
</address>
</author>
<author initials="C.P." surname="Perkins" fullname="Charles E. Perkins">
<organization>Futurewei</organization>
<address>
<phone>+1-408-330-4586</phone>
<email>charlie.perkins@huawei.com</email>
</address>
</author>
<date /> <!-- day='23' month='November' year='2012'/ -->
<area>Internet</area>
<workgroup>Internet Area</workgroup>
<keyword>I-D</keyword>
<keyword>Internet Draft</keyword>
<!-- <t><vspace blankLines="6" /></t> -->
<abstract>
<t>
This document describes <!-- key --> 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.
</t>
</abstract>
</front>
<middle>
<section anchor='introduction' title='Introduction'>
<t>
<!-- The goal of this document is to
describe key aspects of multi-hop ad hoc wireless communication. -->
Experience gathered with ad hoc routing protocol development,
deployment and operation, shows that wireless communication presents
specific challenges <xref target="RFC2501"/>
<xref target="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.
</t>
</section>
<section anchor='manets' title='Multi-hop Ad Hoc Wireless Networks'>
<t>
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.
</t>
<t>
Although the characteristics of packet transmission over multi-hop
ad hoc wireless networks, described below, are not the typical
characteristics expected by IP <xref target="RFC6250"/>, it is
desirable and possible to run IP over such
networks, as demonstrated in certain deployments currently in
operation, such as Freifunk <xref target="FREIFUNK"/>, and
Funkfeuer <xref target="FUNKFEUER"/>. These deployments use
routers running IP protocols e.g., OLSR (Optimized Link State Routing
<xref target="RFC3626"/>) on top 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 <xref target="RFC3561"/>, TBRPF <xref target="RFC3684"/>,
DSR <xref target="RFC4728"/>, or OSPF-MPR <xref target="RFC5449"/>).
</t>
<t>
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.
</t>
</section>
<section anchor="links"
title="Common Packet Transmission Characteristics in
Multi-hop Ad Hoc Wireless Networks">
<t>
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".
</t>
<t>
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.
</t>
<t>
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<xref target="RFC3626"/>, AODV<xref target="RFC3561"/>,
TBRPF<xref target="RFC3684"/>, DSR<xref target="RFC4728"/>,
and OSPF-MPR<xref target="RFC5449"/>), as well as deployment and
operation (Freifunk<xref target="FREIFUNK"/>,
Funkfeuer<xref target="FUNKFEUER"/>).
</t>
<section anchor="graphs"
title="Asymmetry, Time-Variation, and Non-Transitivity">
<t>
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.
</t>
<t>
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.
</t>
<t>
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:
</t>
<t>
<list style="numbers">
<t>
cannot assume that S = V,
</t>
<t>
cannot assume that S and/or V are unchanged at time t1 later than t0.
</t>
</list>
</t>
<t>
Furthermore, transitivity is not guaranteed over multi-hop ad hoc
wireless networks. Indeed, let's assume that, through their
respective interfaces within network N:
</t>
<t>
<list style="numbers">
<t>
device B and device A can detect one another (i.e. B is a member of
sets S and V), and,
</t>
<t>
device A and device C can also detect one another (i.e. C is a
also a member of sets S and V).
</t>
</list>
</t>
<t>
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.
</t>
<t>
In a nutshell: multi-hop ad hoc wireless communications can be
asymmetric, non-transitive, and time-varying.
</t>
</section>
<section anchor="reality"
title="Radio Range and Wireless Irregularities">
<t>
<xref target="graphs"/> 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 <xref target="graphs"/>
as well as other common effects.
</t>
<t>
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 <xref target="graphs"/>. Moreover, 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.
</t>
<t>
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 <xref target="graphs"/>.
</t>
<t>
<figure>
<artwork>
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.
</artwork>
</figure>
</t>
<t>
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 <xref target="graphs"/>.
</t>
<t>
<figure>
<artwork>
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.
</artwork>
</figure>
</t>
<t><vspace blankLines="6"/></t>
<t>
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.
</t>
<t>
<figure>
<artwork>
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".
</artwork>
</figure>
</t>
<t>
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 .
</t>
<t>
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
<xref target="MC03"/>, <xref target="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
<xref target="VTC99"/>, and the radio engineering community has thus
developed numerous radio propagation models, relying on median values
observed in specific environments <xref target="SAR03"/>.
</t>
<t>
The above irregularities also cause communications on multi-hop ad hoc
wireless networks to be non-transitive, asymmetric, or time-varying,
as described in <xref target="graphs"/>, 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.
</t>
</section>
</section>
<section anchor="moreterms" title="Alternative Terminology">
<t>
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 <xref target="introduction"/>).
</t>
<t>
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:
</t>
<t>
<list style="hanging">
<t>
- 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.
</t>
<t>
- 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.
</t>
<t>
- 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.
</t>
<t>
- 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.
</t>
<t>
- 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.
</t>
<t>
- 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.
</t>
</list>
</t>
<t>
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
<xref target="links"/>. 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.
</t>
</section>
<section anchor="security" title="Security Considerations">
<t>
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.
</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>
This document does not have any IANA actions.
</t>
</section>
</middle>
<back>
<references title="Informative References">
<?rfc include='reference.RFC.2501.xml'?>
<?rfc include='reference.RFC.3561.xml'?>
<?rfc include='reference.RFC.3626.xml'?>
<?rfc include='reference.RFC.3684.xml'?>
<?rfc include='reference.RFC.4728.xml'?>
<?rfc include='reference.RFC.4903.xml'?>
<?rfc include='reference.RFC.5449.xml'?>
<?rfc include='reference.RFC.5889.xml'?>
<?rfc include='reference.RFC.6250.xml'?>
<reference anchor="DoD01">
<front>
<title>A DoD perspective on mobile ad hoc networks</title>
<author initials="J." surname="Freebersyser"
fullname="J. Freebersyser">
<address>
<uri>http://www.funkfeuer.at</uri>
</address>
</author>
<author initials="B." surname="Leiner" fullname="B. Leiner">
</author>
<date year="2001" />
</front>
<seriesInfo name="Addison Wesley "
value="C. E. Perkins, Ed., 2001, pp. 29--51" />
</reference>
<reference anchor="FUNKFEUER">
<front>
<title>Austria Wireless Community Network,
http://www.funkfeuer.at</title>
<author>
<address>
<uri>https://map.funkfeuer.at/wien/</uri>
</address>
</author>
<date year="2013" />
</front>
</reference>
<reference anchor="MC03">
<front>
<title>Mobile Ad hoc Networking: Routing Technology
for Dynamic, Wireless Networks</title>
<author initials="S." surname="Corson" fullname="S. Corson">
</author>
<author initials="J." surname="Macker" fullname="J. Macker">
</author>
<date year="2003"/>
</front>
<seriesInfo name="IEEE Press"
value="Mobile Ad hoc Networking, Chapter 9" />
</reference>
<reference anchor="SAR03">
<front>
<title>A Survey of Various Propagation Models for Mobile Communication</title>
<author initials="T.K." surname="Sarkar" fullname="T.K. Sarkar">
</author>
<author initials="Z." surname="Ji" fullname="Z. Ji">
</author>
<author initials="K." surname="Kim" fullname="K. Kim">
</author>
<author initials="A." surname="Medour" fullname="A. Medour">
</author>
<author initials="M." surname="Salazar-Palma" fullname="M. Salazar-Palma">
</author>
<date year="2003"/>
</front>
<seriesInfo name="IEEE Press"
value="Antennas and Propagation Magazine, Vol. 45, No. 3" />
</reference>
<reference anchor="VTC99">
<front>
<title>Pilot power control and service coverage support in CDMA mobile systems</title>
<author initials="D." surname="Kim" fullname="D. Kim">
</author>
<author initials="Y." surname="Chang" fullname="Y. Chang">
</author>
<author initials="J.W." surname="Lee" fullname="J.W. Lee">
</author>
<date year="1999"/>
</front>
<seriesInfo name="IEEE Press"
value="Proceedings of the IEEE Vehicular Technology Conference (VTC), pp.1464-1468" />
</reference>
<reference anchor="MI03">
<front>
<title>The Mistaken Axioms of Wireless-Network Research</title>
<author initials="D." surname="Kotz" fullname="D. Kotz">
</author>
<author initials="C." surname="Newport" fullname="C. Newport">
</author>
<author initials="C." surname="Elliott" fullname="C. Elliott">
</author>
<date year="2003" />
</front>
<seriesInfo name="Dartmouth College Computer Science "
value="Technical Report TR2003-467" />
</reference>
<reference anchor="FREIFUNK">
<front>
<title>Freifunk Wireless Community Networks, http://www.freifunk.net</title>
<author>
<address>
<uri>http://www.freifunk.net</uri>
</address>
</author>
<date year="2013" />
</front>
</reference>
</references>
<section anchor="acknowledgements" title="Acknowledgements">
<t>
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.
</t>
</section>
</back>
</rfc>
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