Download Technology Background

Ad hoc networking –
Technology and applications
Helsinki University of Technology,
T-109.551, Telecommunications Business II
Seminar Report
Henrik Petander, 44300N
Olli Savolainen, 48034C
Helsinki University of Technology,
T-109.551 Telecommunications Business Seminar II seminar report.
6.5.2017
Executive Summary
Ad hoc networks are self organizing wireless networks, in which also end nodes act as
routers. Ad hoc networking allows nodes to communicate over multiple wireless hops and
form self-organizing networks. Ad hoc networking improves the efficiency and range of
fixed and mobile Internet access and enables totally new applications such as sensor
networks.
Advances in radio technology and ad hoc routing protocols are needed for wide spread
use of Ad hoc networking. New radio and link control technologies need to be designed
to meet the requirements of Ad hoc networking, which differ from those set by cellular
radio technologies. These technologies also need to be standardized to offer a sustainable
basis for interoperating products and a wider market.
The challenges are being met by several start ups trying to provide a new wireless access
architrecture to the fixed and mobile Internet access market curently dominated by
operators struggling under large amounts of debt and by entrenched equipment vendors,
such as Nokia, Ericsson and Motorola.
In this report we will survey the protocols used for ad hoc networking and their
standardization status. We will also cover potential applications of ad hoc networking
and analyze the suitability of the technology to those fields. Furthermore we will estimate
the timing of the deployment of ad hoc networking.
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Executive Summary ......................................................................................... 2
1 Introduction ............................................................................................... 4
2 Technology background ........................................................................... 5
2.1
Routing .............................................................................................. 6
2.2
Radio ............................................................................................... 12
2.3
Security............................................................................................ 16
2.4
Quality of Service ............................................................................ 19
2.5
Billing and incentives ....................................................................... 22
2.6
Internet connectivity ......................................................................... 23
2.7
Standardization and patents ............................................................ 24
2.8
Technical problems and limitations .................................................. 25
3 Applications ............................................................................................ 27
3.1
Military and emergency networks .................................................... 29
3.2
Fixed broadband wireless ................................................................ 31
3.3
Extending cellular mobile access networks ..................................... 35
3.4
Sensor networks .............................................................................. 37
3.5
Vehicle networks ............................................................................. 38
3.6
Home and personal networking ....................................................... 39
4 Technology barriers and timing............................................................... 40
5 Conclusions ............................................................................................ 41
Terms and Abbreviations ............................................................................... 42
References..................................................................................................... 43
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1 Introduction
Computer networking has been a modern way of communication and an enabler for
various new applications for decades. Recently wire-based networks have got a
competitor from wireless networking technologies. The wireless networks have
various advantages over wired networks, and therefore have been the subject for
enormous amount of research.
First of all, wireless networking means that there is no need for a physical connection
to the socket in the wall, because data is transferred through air in form of radio
waves. Second, wireless networking has more and more enabled user mobility.
Basically, the chance to move freely anywhere, anytime, and still have a change to be
connected to the network, is a natural source of interest. Third, the costs of building a
wireless infrastructure are virtually nothing compared to the costs of wire-based
network installation.
All that is needed for wireless communication between two computers is devices for
transmitting and receiving radio signals and the transmission medium for the signals,
which in this case is air. There would not be a need for established network
infrastructure, whatsoever. Therefore, a network could actually be formed anywhere
and anytime. Anyone could join the network while passing through, or otherwise
coming to the radio transmission range of the other computers. This kind of
networking is called ad hoc networking. Ad hoc networks are self organizing wireless
networks, in which also end nodes act as routers. Ad hoc networking allows nodes to
communicate over multiple wireless hops and form self-organizing networks.
Ad hoc networking is an attractive concept and has various possibilities for different
kinds of applications. However, ad hoc networking is not quite as simple and easy as
it may sound. The enabling technologies have their own limitations and some
technologies and models of operation have been subject to reconsidering to suit ad
hoc networks.
In this report the underlying technologies are introduced and some applications of ad
hoc networks are presented. The technology background part of this document
describes the existing technologies needed in ad hoc networks. It points out some
issues related to the ad hoc networking model, and also advantages and disadvantages
of different technologies are discussed. After the technological background is given,
some applications and their possible value in the market are discussed.
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2 Technology background
An ad-hoc wireless network is a multi-hop network without any base stations, or
infrastructure. Ad hoc networks support anytime and anywhere computing, allowing
the spontaneous formation and deformation of mobile networks. A Mobile Ad hoc
Network (MANET) consists of a set of mobile host within communication range.
These hosts are called nodes in ad hoc networks.
MANETs are wireless, which means that data transmission is based on radio
technologies. Multi-hop networking means that data from source to destination has to
travel through multiple hop routes and each node within the network participates
cooperatively in forwarding data packets between other nodes. In practice, each node
acts as a router.
MANETs have several characteristics that have to be identified when discussing the
underlying technology of ad hoc networks. (Corson, Macker, 1999)
 Topologies are dynamic. Nodes are free to move arbitrarily, which means that
topology may change randomly and rapidly.
 Network is bandwidth-constrained and capacity of the links may vary.
Wireless links have significantly lower capacity than wired links. Also effects
of multiple access, fading, noise, and interference, etc. often decrease the
performance from maximum transmission rate.
 Operations are energy-constrained. Most nodes in a MANET are usually
running on batteries or on other exhaustible means. For these nodes, the most
important system design criteria for optimization may be energy conservation.
 The physical security is limited. Ad hoc networks are generally more prone to
physical security threats than fixed-cable networks. Wireless links and lack of
infrastructure support make ad hoc networks more vulnerable. On contrast,
some of the applications of ad hoc networks are security sensitive.
The physical properties and the special characteristics of MANETs more or less
indicate the required topics for technology background analysis. In the following
sections routing, radio technologies, security, and other interesting issues as quality of
services, Internet connectivity, incentives, and standardization are discussed.
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2.1 Routing
Each node in an ad hoc network participates in forming the network topology. As
there are no dedicated routers, each node is on its own part responsible for routing
packets between other nodes, too. Basically the routing infrastructure is yet similar to
the one of Internet. There are many different routing protocols that provide
information to forward packets to the next hop. In ad hoc network it would be
necessary to manage topology changes, as all the nodes are required to run routing
protocols. The routing protocols used in Internet are typically not applicable to ad hoc
networks as such.
In general, mobility, dynamic topologies, and the constraints of power and bandwidth
in ad hoc wireless networks have given the guidelines for routing protocol
development. As nodes in a MANET usually have to deal with limited power
resources, it is suitable to develop such protocols that need minimum amount of
information exchanges, thus minimizing radio communication and also power
consumption.
The Internet routing protocols are based on network broadcast, as is the case with
common Open Shortest Path First (OSPF) protocol. OSPF is a link-state protocol,
which means that the routing tables are sent to everyone. These traditional link-state
protocols are not applicable for dynamic networks, because a considerable amount of
bandwidth is needed to maintain network state. Instead of being link-state protocols,
most of the routing protocols use distance vector algorithms, which send their routing
tables only to neighbors.
The routing algorithms can be further divided into proactive and reactive algorithms.
Proactive ones keep route tables all the times, which creates extra overhead due to
routing updates. Reactive ones find route on demand, which in turn adds some latency
to route acquisition. Proactive protocols also maintain routes to each node in network,
whereas reactive protocols maintain only active rows. A categorization of ad hoc
routing protocols is presented in Figure 1.
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AD-HOC MOBILE
ROUTING PROTOCOLS
TABLE-DRIVEN /
PROACTIVE
ON-DEMANDDRIVEN / REACTIVE
HYBRID
DSDV
CGSR
OLSR
WRP
STAR
TBRPF
ZRP
DSR
AODV
TORA
RDMAR
Figure 1. Categorization of ad hoc routing protocols (Modified from Toh, 2002)
DSDV
AODV
DSR
TORA
ZRP
LAR
OLSR
TBRPF
CGSR
WRP
STAR
RDMAR
Destination-Sequenced Distance Vector
Ad hoc On-demand Distance Vector
Dynamic Source Routing
Temporally Ordered Routing Algorithm
Zone Routing Protocol
Location-Aided Routing
Optimized Link State Routing
Topology Dissemination Based on Reverse-Path Forwarding
Cluster Switch Gateway Routing
Wireless Routing Protocol
Source Tree Adaptive Routing
Relative Distance Microdiversity Routing
Some of the routing protocols in ad hoc networks are shortly presented in next
paragraphs. AODV and DSR, which are the two most important ones, are introduced
a bit more thoroughly.
2.1.1 DSDV
Destination-Sequenced Distance Vector (DSDV) protocol is a proactive routing
protocol. Important feature of DSDV is its use of a sequence number for each node.
These sequence numbers are used for loop freedom. In DSDV each node maintains
routing table with entry for each node in the network. The nodes transmit updates
periodically and in cases of important link changes. DSDV uses incremental dumps
and settling time to control overhead. In general, proactive protocols perform well in
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networks where the mobility is low or moderate and the number of nodes is relatively
small. (Toh, 2002)
2.1.2 AODV
Ad hoc On-demand Distance Vector (AODV) routing protocol is a bettered
modification of DSDV and it is one of the most important protocols for routing
between nodes in an ad hoc network. The algorithm is reactive and it enables
dynamic, self-starting and multi-hop routing. AODV allows mobile nodes to obtain
routes to new destinations quickly and it does not require the nodes to maintain the
routes to inactive nodes. AODV makes it possible to mobile nodes to react to link
breakages and changes in network topology. AODV avoids also loops in routes.
One important feature inherited from DSDV is the use of a destination sequence
number for each route entry. It is an increasing number maintained by each
originating node. When used in control messages, other nodes use it to determine the
freshness of the information contained from the originating node. Given the choice
between two routes to a destination, a requesting node always selects the one with the
greatest sequence number. With destination sequence numbers loops are avoided, by a
somewhat easily implemented manner.
Route discovery in AODV is based on route request (RREQ) and route reply (RREP)
messages. If a mobile node needs a route to another node, it creates a RREQ, which
contains destination’s IP address and sequence number, its own IP address and
sequence number, and initial hop count, which is zero. The node broadcasts RREQ to
its neighbors. A neighbor node makes reverse route entry for the broadcaster to its
routing table and if it does not have a route to the requested destination it further
broadcasts RREQ to its neighbors. If it has route to the requested destination and the
sequence number for route to destination is bigger than destination’s sequence number
in RREQ (which is the greatest sequence number received in the past by the originator
for any route towards the destination), it unicasts a RREP to previous requester. This
information is unicasted all the way to the original requester and all the nodes along
the way back update their routing tables with the information.
Route maintenance in AODV needs an additional route error (RERR) message. If a
link between a node and its neighbor breaks, this node creates a RERR message
listing all the destinations that become unreachable, because of the link break, and
sends the message to its other neighbors. The neighbors delete the routes to the listed
destinations if the sender was the next hop along the route and forward the message
onwards.
AODV has relatively low memory requirements and reasonable network load. Also
route convergence is quick because of triggered updates. AODV is designed for
networks with over 500 mobile nodes. The reasonability of the size depends on the
level of mobility, though. (Perkins, Belding-Royer, Das, 2003)
2.1.3 DSR
The Dynamic Source Routing (DSR) protocol is a simple and efficient, highly
reactive, routing protocol, which is designed specifically for use in multi-hop wireless
ad hoc networks. DSR allows mobile nodes to dynamically discover a source route to
any destination in the ad hoc network across multiple hops. Each data packet sent
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contains the complete, ordered list of nodes along the route in its header. This makes
routing trivially loop-free and does not require up-to-date routing information in the
intermediate nodes through which the packet is forwarded. This way also the other
nodes along the route can easily cache the routing information for future use.
Basic DSR route discovery goes as described here. When a node needs to send a
packet to another node it places a complete list of hops to follow in the packet’s
header. The source node may have this list in its route cache. If the route is not found
in the cache, the node initiates the Route Discovery protocol to obtain such list. To
initiate the route discovery, the source node transmits a route request as a single local
broadcast packet. Each route request identifies the initiator and the target of the route
discovery and also contains unique request identification. Each route request also
contains a record listing the address of each intermediate node that has forwarded this
particular copy of the route request. If a node that receives the route request is the
target, it returns a route reply that contains the accumulated route record to the
initiator. The initiator caches this route and is able to send packets to this destination.
If the receiving node is not the target, it appends its address to the route record and
broadcasts it further. A node discards the request if has recently seen another route
request from the same initiator or the route record already contains its own address.
This prevents looping.
The route maintenance in DSR is based on acknowledgements. Each node forwarding
a packet along a certain route is responsible for the next hop from itself to another
node. If a node does not receive an acknowledgement after forwarding a packet, it
sends an acknowledgement request a certain number of times. If no acknowledgement
is still received, the node generates a route error message and sends it to the sender of
packet that was to be forwarded and to any other source that have tried to use this
broken link since the last acknowledgement received from the unreachable node.
In DSR, route discovery and route maintenance both operate entirely on demand.
DSR, unlike other protocols, does not require sending any periodic packets, such as
route advertising, over the network. While having very low overhead, DSR is still
able to react rather quickly to changes in the network. (Johnson, Maltz, Hu, 2003)
2.1.4 ZRP
Zone Routing Protocol (ZRP) is a hybrid protocol combining the merits of on-demand
and proactive protocols. In ZRP, a routing zone consists of a few nodes within one,
two, or a couple of hops away from each other. Within this zone a table-driven-based
routing protocol is used. This implies that route updates are performed for nodes
within the zone. Therefore, each node has a route to all other nodes within its zone. If
the destination node resides outside the source zone, an on-demand search-query
routing method is used. (Toh, 2002)
2.1.5 LAR
Location-Aided Routing (LAR) uses, unlike other ad hoc routing schemes, location
information, via GPS, for example, to improve the performance of ad hoc networks.
LAR limits the search for a new route to a smaller request zone, thus reducing
signaling traffic. LAR defines two concepts: expected zone and request zone.
Expected zone is based on the location of the destination and its velocity, and thus
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requires the source to have advanced knowledge about the destination. The request
zone is a rectangle including the source and the expected zone. (Toh, 2002)
2.1.6 OLSR
Optimized Link State Routing (OLSR) is a proactive, link-state protocol optimized for
mobile ad hoc networks. Classical link-state protocols exchange routing information
with the other nodes regularly, creating significant overhead. OLSR reduces the
amount of overhead by introducing the concept of MultiPoint Relays (MPR). MPRs
are selected nodes in the network that are responsible for forwarding broadcast
messages during the flooding process. The overhead is significantly lower compared
to a classical flooding mechanism, where every node retransmits each message after
receiving the first copy of the message. In practice, other nodes in the network do the
selection of MPRs; each node selects a set of its neighbor nodes to be multipoint
relays. In OLSR, only nodes selected as MPRs create link state information. This
minimizes the number of control messages flooded in the network. An MPR node can
also choose to report only links between itself and its MPR selectors. This is also an
optimization of classical link state protocols, as only partial link state information is
distributed in the network. (Adjih et al., 2003)
Reduced overhead is important in ad hoc networks. OLSR protocol is also particularly
suitable for large and dense networks as the technique of MPRs works well in this
context.
2.1.7 TBRPF
Topology Dissemination Based Reverse-Path forwarding (TBRPF) is a proactive
routing protocol for MANETs. Each node running TBRPF computes a source tree,
which contains shortest paths to all reachable nodes, based on partial topology
information stored in its topology table. A modification of Dijkstra’s algorithm is
used to determine the shortest paths. Each node reports only part of its source tree to
neighbors and thus overhead is minimized. A combination of periodic and differential
updates is used to keep all neighbors informed of the reportable part of the source
trees. Neighbor discovery is done with messages that report only changes in the status
of neighbors. TBRPF consists of two modules: the neighbor discovery module and the
routing module. TBRPF can support networks with up to a few hundred nodes, and
can be combined with hierarchical routing techniques to support much larger
networks. (Ogier et al., 2003)
2.1.8 Clustering and hierarchical routing
In large ad hoc networks routing tables get larger and the amount of exchanged
information increases. The network overhead increases because routing tables to be
exchanged are larger and also because in a large network the number of topology
changes, that trigger routing information updates, increases, too. One possibility to
reduce the transmission overhead is clustering. OLSR protocol introduced multipoint
relays to offer a solution to similar problem, but the concept of clustering is broader.
In the general cluster-based schemes for ad hoc networks, all nodes are divided into
clusters, and usually a clusterhead (CH) is elected for each cluster. Routing algorithm
may now consist of routing from source to its local CH, form this CH to the CH of
destination node, and finally from to the destination itself. Special clustering
algorithms are needed for doing the clustering. The efficiency of such algorithm is
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measured by the delivery rate and the average hop count from source to destination. In
ad hoc networks clustering also has to adapt to user mobility by rearranging cluster in
case of nodes moving from the transmission range of one CH to another one’s
transmission range. Because of additional cluster management, it is not recommended
to use clustering when networks are small and a flat routing scheme works well.
Clustering enables hierarchical routing, which is an important feature when MANETs
get large. A multi-level hierarchy has nodes organized in a tree-like fashion with
several levels of clusterheads. A three level hierarchy employs ordinary nodes,
clusterheads and superclusterheads, and is suitable for networks with few thousand
nodes.
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2.2 Radio
A packet radio network consists of a number of packet radio stations that
communicate with each other. A packet radio network carries messages in packets
like a wired packet network, but uses radio signals instead of wires to carry the
packets between stations. Basically a station in a packet radio network includes radio
transmitting and receiving equipment and a computer to perform packet routing and
forwarding functions. In a multi-hop packet radio network each station participates
cooperatively in forwarding traffic between other stations. Compared to wired
networks, radio networks are much more cheaper to install and provide a user a
chance of mobility. With multiple short hops link quality improves and stations can
use less power or achieve better data rates. On the other hand, there are several
differences in the access medium, as with radio signals propagation, interference,
frequency band choices, and such things have to be considered more carefully.
(Shepard, 1995) Also real support of mobility and power consumption are important
issues, when considering radio transmission.
In theory multi hop wireless radio networks, also known as ad hoc networks provide
more capacity than traditional radio networks. Figure 2 compares the theoretical
scalability of multi hop wireless radio to the scalability of a point-to-multipoint radio.
The capacity refers to the total capacity of the network. Multi hop wireless networks
can in theory scale linearly with the amount of nodes, when multiple antennas are
used in each node. This means in practice that the transmission speed available to
each node remains constant. Even with single antennas the capacity grows in
proportion to N , where N is the number of nodes, leaving a capacity of 1 / N to
each node (Shepard 1995). With IEEE 802.11b WLAN the total capacity actually
decreases according to Gupta, Gray and Kumar (2001), due to sub optimal design of
the MAC layer.
Capacity
Ad Hoc, in theory using multiple
Antennas, BLAST (linear scaling)
Ad Hoc, in theory using single
Antennas ( N )
PTM radio, no scaling
Ad Hoc, IEEE 802.11b, measured
Network size
Figure 2 Comparison of theoretical and real-life performance of ad hoc networks.
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Nodes in a MANET can wirelessly communicate with each other with various radio
technologies. Some of these radio technologies are discussed shortly in this section.
The properties are pointed out, but no larger comparison is made between these
technologies. The role of these different radio technologies from MANET's point of
view becomes clearer in the part of this paper discussing different applications of ad
hoc networks.
2.2.1 IEEE 802.11 Wireless LAN
There are two different ways to configure an 802.11 Wireless Local Area Network: ad
hoc and infrastructure. It is obvious that interest in this paper is on the ad hoc mode,
but the focus here is on technical radio properties and the distinction between these
two configurations is more or less left aside.
Actually, IEEE 802.11 Wireless LAN is a protocol family, which offers data
transmission at different speeds, costs, and power consumption levels. There are
currently four (major) specifications in the family: 802.11, 802.11a, 802.11b, and
802.11g. All four use the Ethernet protocol and CSMA/CA (Carrier Sense Multiple
Access with Collision Avoidance) for path sharing. There are also other members
(802.11c, 802.11d, 802.11e, 802.11f) in this family, but these four are of greater
interest here.
802.11 applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4
GHz band. It uses either Frequency Hopping Spread Spectrum (FHSS) or Direct
Sequence Spread Spectrum (DSSS) for radio frequency methods.
802.11a is an extension to 802.11 that applies to wireless LANs, wireless ATM
systems, and is used in access hubs. It provides up to 54 Mbps in the 5 GHz band, but
most commonly communication takes place at 6 Mbps, 12 Mbps, or 24 Mbps.
802.11a uses an Orthogonal Frequency Division Multiplexing (OFDM) encoding
scheme rather than FHSS or DSSS.
802.11b, also referred to as 802.11 High Rate or increasingly popularly WiFi,
provides 11 Mbps transmission, with a fallback to 5.5, 2 and 1 Mbps. It uses only
DSSS. 802.11b was 1999 ratification to the original 802.11 standard, allowing
wireless functionality comparable to Ethernet. It was designed to be faster and to have
lower costs than 802.11. However, it has very little immunity for either self-induced
or externally generated interference. The changes also made 802.11b unusable in wide
area mobile applications, but very usable in short distance communication.
802.11g is the most recently approved standard. It provides 20+ Mbps in the 2.4 GHz
band. Transmission over relatively short distances can be up to 54 Mbps. Like
802.11b, and unlike 802.11a, 802.11g operates in the 2.4 GHz band. So, 802.11b and
802.11g are compatible.
So far, from ad hoc networks point of view 802.11b has been the protocol in widest
usage. In short words, its key properties are: It provides relatively high-speed wireless
transmission for short-distances. It is basically intended that 802.11b would be used
indoors and that users are assumed to relatively immobile. Compared to the other
available radio technologies, also others than members of 802.11 family, 802.11b is
most often chosen alternative, because of its high transmission rate, rather low costs,
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and importantly, its wide usage base. Still, 802.11-protocol family does not provide
optimal technology to ad hoc networking. As an experimental study of scaling laws in
802.11 based ad hoc networks (Gupta, Gray, Kumar, 2001) indicates, the throughput
per node declines surprisingly fast when the number of nodes increases. By saying
surprisingly fast, it is meant that theoretically it would be possible to attain better
scalability, if hardware and protocols were further improved. Another problem with
802.11b, in addition to scalability, is its assumption of relative user immobility. In
other words, if the mobility of nodes in the network is high, 802.11b, or any of other
802.11-family protocols, has problems in form of handover technology.
2.2.2 Bluetooth
Bluetooth is a radio technology specifically designed to have very low power
consumption and to be very cheap. It is therefore suitable for communication between
portable devices within short distances. Link speed in Bluetooth is less than 1 Mbps
and range is less than 10 meters. Bluetooth uses the 2.4 GHz ISM (Industrial,
Scientific, Medical) band and has frequency hopping spread spectrum (FHSS) of
1,600 hops/second. Bluetooth also defines its own protocol stack and thus does not
use OSI protocol model.
Bluetooth devices form so-called piconets, where there can be 8 devices and 3 voice
channels per piconet. A piconet contains a master and several slave devices. The
master controls all channel access, so slaves can only talk to the master and not to
other slaves directly. A piconet can have number of active members (up to 8), and the
other devices become parked members. Parked slaves can remain synchronized to the
master, but they are not active on a channel. Each piconet has its own hopping
channel.
Scatternet is a set of two or more interconnected piconets. A master in one piconet can
be a slave in another piconet. To switch between piconets, time multiplexing is used.
In practice, Bluetooth’s applicability in ad hoc networking is very limited. It has
concrete weaknesses compared to 802.11b, for example. The radio transmission range
is rather small and also transmission rate is low. Maybe the most important issue is,
although, the inefficiency of handovers. The handover is very slow, which limits
actual mobility dramatically. Bluetooth’s main application is in Personal Area
Networks (PANs), where it is suitable because of its very low power consumption and
low costs.
2.2.3 HiperLAN/2
HiperLAN/2 is a radio technology mainly for wireless local area networking. It is a
standard developed by ETSI. The transmission rate with HiperLAN/2 is up to 54
Mbps in the 5 GHz frequency band. Similarly to 802.11a, HiperLAN/2 uses
Orthogonal Frequency Digital Multiplexing (OFDM) as a modularization method.
HiperLAN/2 has signaling functions on the control plane of its protocol stack. This
enables connection-oriented properties for transmission and also support for QoS
negotiation. (Jonsson, 1999)
HiperLAN/2 suits rather well for ad hoc networking. Properties that make it
applicable are high-speed transmission, mobility support in form of flexible and
efficient handover capability, and power saving mechanism. In addition to these, also
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QoS support with connection-oriented links and security through authentication and
encryption support are favorable in ad hoc networking. Its main problem is high costs.
2.2.4 UWB
Ultra Wideband (UWB) is a technology for wireless communication, precision
location, and portable radars. An UWB device works by emitting a series of short,
low-powered electrical pulses across the entire radio spectrum at once. UWB emits its
pulses at a pre-determined rate, which can only be picked up by receiver tuned in to
that exact pulse sequence. So, unless the receiver knows exactly what to listen, it
won’t hear UWB transmission. This way UWB does not jam other wireless devices,
mobile phones, or radios. Transmission rate is 60 Mbps, and UWB is also cheap and
low power consuming technology. Its one application is, for example, communication
between multimedia applications at home. (Economist: Tech. Quarterly, 2002)
2.2.5 QDMA
Quadarature Division Multiple Access (QDMA) is radio technology optimized for
wide area mobile applications. QDMA radio, for example, adapts well to rapidly
varying signal strength generated in mobile environment. It also has increased error
correction capability compared to 802.11b. Transmission speed with QDMA is 6
Mbps and range up to almost 5 kilometers.
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2.3 Security
On one hand, the security-sensitive applications of ad hoc networks require high
degree of security and on the other hand, ad hoc networks are vulnerable to security
attacks.
Ad hoc wireless networks do not have any predefined infrastructure and all network
services are generated on the fly. It is obvious that lack of support of underlying
infrastructure and possibility of attacks on wireless links make security an issue in ad
hoc networks. It is challenging to achieve security within ad hoc networks basically
due to following reasons: dynamic topologies and membership, vulnerable wireless
links, and roaming in dangerous environment. (Vinayakray-Jani, 2002)
Topology of an ad hoc network is very dynamic as mobility and membership of nodes
are random and rapid. For example, nodes in ad hoc networks may, unlike in Mobile
IP architecture, dynamically become affiliated with administrative domains. This
implies that there is a need for secure solutions to be more dynamic, as well. The
point is that traditional security schemes with support of infrastructure do not apply in
ah doc networks as such.
Passive and active link attacks like eavesdropping, spoofing, denial of service,
masquerading, and impersonation, are possible on insecure wireless link.
Eavesdropping might give an adversary access to secret information and this way
violates confidentiality. Active attacks might allow the adversary to delete messages,
to inject erroneous messages, to modify messages, and to impersonate a node, thus
violating availability, integrity, authentication, and non-repudiation.
Roaming in different environments, where setting up an ad hoc network and
participating an existing one is easy, brings difficulties, because any malicious or
misbehaving node can create a hostile attack or deprive all other nodes from providing
any service. In ad hoc networks not only attacks from outside the network are
possible, but also attacks from compromised nodes within the network form a serious
threat. If a centralized entity was introduced to and ad hoc network and if this entity
was compromised, the entire network would be subverted.
2.3.1 Link-level security
The wireless communication between nodes is generally insecure. As data is
transmitted by radio impulses through air, it is easy to eavesdrop. Practically anyone
within the range of the radio signals can pick up the data transmitted. Out of radio
technologies 802.11b Wireless LAN, the most common radio technology in ad hoc
networks, has well-known security risks. Main risks are: insertion attacks, interception
and monitoring wireless traffic, misconfiguration, jamming, and client-to-client
attacks. Insertion attacks are based on placing unauthorized devices on the wireless
network without going through a security process and review. Interception and
monitoring are popular on wireless networks, as well as on broadcast wired networks
like Ethernet. Misconfiguration means that there is basically a security configuration
available, but it is configured properly. Jamming is a common denial of service attack,
which is relatively easy to implement in a broadcast network. Client-to-client attacks
especially are more common wireless ad hoc networks than in infrastructure
networks, because clients can communicate directly to each other and thus by-pass a
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base station that might provide security support. Therefore, nodes in ad hoc networks
have to protect themselves from all the other nodes.
There are schemes to provide solutions to security issues in wireless communication,
but most of them are based on architecture design and infrastructure support. In ad
hoc networking these are not really an option. Link-level security can basically be
provided only by encryption and authentication mechanisms.
2.3.2 Routing security
As each node in an ad hoc network acts as a relay, the routing protocols are more
vulnerable to attacks than routing protocols in Internet, for example. There are two
sources of threats to routing protocols. The first one comes from external attackers.
Attackers can disturb the network by distorting routing information, injecting
erroneous routing information, or replaying old routing information. They can also
create excessive traffic load to network by causing retransmission and inefficient
routing. The second threat comes from compromised nodes, which might advertise
incorrect routing information. The difference between outsiders and compromised
insiders is not in the threats themselves, because both can basically create similar
kinds of malicious attacks. The difference is that compromised insiders can operate in
spite of possible securing mechanisms, because they have valid private keys.
Therefore, detecting false information is difficult because compromised nodes can
also generate valid signatures using their private keys.
Proposed routing protocols for ad hoc networks can dynamically adapt to changes in
topology, but they provide only partial or no solution at all to security issues in ad hoc
networks. False routing information can basically be detected and its effect minimized
with existing features or routing protocols and route redundancy. As long as there are
sufficiently many correct nodes, redundancy with multiple routes can be used in going
around the compromised nodes. Nodes could switch to an alternative route when the
primary route appears to be failed. One possibility to implement redundancy is
diversity coding (Zhou, Haas, 1999). It takes advantage of multiple paths without
message transmission. The basic idea is to transmit redundant information through
additional routes for error detection and correction.
2.3.3 Key management
Using cryptographic schemes, such as digital signatures, to protect routing
information and data traffic, usually requires a key management service. A common
way for doing this is adopting a public key infrastructure, which in turn requires a
trusted entity, Certification Authority (CA), to the network for key management.
Establishing a key management service with a single CA is problematic in ad hoc
networks. If this CA is unavailable, nodes cannot get the current public keys of the
other nodes, or establish a secure connection. Furthermore, if the CA is compromised
and leaks its private key to an adversary, the adversary can then sign any erroneous
certificate using this private key to impersonate any node or to revoke any certificate.
Replication of the CA service, or services in common, is a way to around the
problems described above. But a naïve replication can make the service even more
vulnerable. Any compromised single replica possessing the service private key, could
lead to collapse of the entire system. Better scheme would be distributing the trust to a
set of nodes by letting them share the key management responsibility.
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Distribution of trust in key management service can be accomplished using threshold
cryptography (Zhou, Haas, 1999). An (n, t+1) threshold cryptography allows n parties
to share the ability to perform cryptographic operations, so that any t+1 parties can
perform this operation jointly. For example in case of creating digital signature, t+1
parties create partial signatures and a combiner constructs a complete signature out of
these partial ones. If this signature does not match the service public key, and is thus
invalid, a reason for this may be an invalid partial signature made by a compromised
party within the t+1 parties. In case of invalid signature, the combiner can request a
new one from another group of t+1 trusted parties.
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2.4 Quality of Service
Quality of Services (QoS) generally means the guarantee by the network to satisfy a
set of pre-determined service performance constraints for the user in terms of the endto-end delay statistics, available bandwidth, probability of packet loss, and so on. The
concept of QoS has advanced in wire-based networks, although, it is not entirely
problem-free yet. In MANETs, there are also additional constraints to QoS coming
from the bandwidth constraints, dynamic topology changes, and limited storing and
processing capabilities of mobile nodes acting as routers. The first two constraints are
obvious and the variability of bandwidth and unreliable links affect the capabilities to
provide QoS in a straightforward manner. The limited processing and storing
capabilities make providing QoS difficult, because finding the path from source to
destination with requested properties and dynamically finding a new one with similar
properties in case of topology changes, require the routers to have capacity to store a
large amount information about the state of the network and significant resources to
compute routes filling various combinations of QoS parameters.
2.4.1 QoS Models
There are two major QoS models defined for Internet environment: IntServ/RSVP
using end-to-end reservation-based engineering, and DiffServ using traffic classes to
provide prioritization of data packets on each router. These models are not applicable
to ad hoc networks as such. IntServ/RSVP model is not suitable for MANETs due to
the resource limitations. Each node would have to maintain the information of the
state of network and that causes a huge storage and processing overhead. The RSVP
reservation and maintenance process is also network consuming and RSVP, being an
out-of-band signaling protocol would eat the resources of data traffic. Classification
and scheduling required by a complete QoS model mechanism would also require a
lot of network resources.
Differentiated Services DiffServ, in turn, is lighter than IntServ, because the interior
routers have to handle parameters for set of flows instead of individual flows. The
DiffServ definition of core, ingress, and egress routers is problematic in ad hoc
networks, because of the dynamic topology. Also the concept of Service Level
Agreement (SLA) is not applicable in ad hoc networks, because basically paying for
certain set of QoS parameters would need an entity assuring the parameters. These
more or less central entities are always problematic in totally ad hoc networks.
2.4.1.1 FQMM
Xiao et al. have presented a Flexible QoS Model for MANETs (FQMM) in 2000.
(Zeinalipour-Yatzi, 2001) The idea is to combine the per-flow state property of
IntServ and the service differentiation of DiffServ. Generally, this model proposes
that highest priority is assigned per flow provisioning and other priority classes are
given per-class provisioning. It is based on the assumption that not all packets are
seeking for highest priority, because otherwise it would be similar to IntServ. In
practice, FQMM defines three types of nodes, exactly as DiffServ: ingress, core, and
egress. The difference is that the type of a node has nothing to do with its physical
location in the network. A node is characterized as ingress if it is transmitting data,
core if it is forwarding data and egress if it is receiving data.
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2.4.2 QoS signaling
Signaling in QoS networks is used to reserve and release resources. It can be divided
into out-of-band signaling and in-band signaling. In out-of-band signaling, such as
RSVP, explicit control packets are used and given priority over data packets. In inband signaling the control information is encapsulated into the data packets. In
practice, in-band signaling is much more lightweight from networks point of view.
Especially in MANETs, bandwidth and power constraints make in-band signaling
favorable. MANETs also cannot tolerate complex signaling protocols.
2.4.2.1 INSIGNIA
INSIGNIA is the first signaling protocol designed especially for MANETs in 1998 by
Ahn et al. (Zeinalipour-Yatzi, 2001) It encapsulates control signals in the IP option of
every IP data packet. INSIGNIA also maintains flow state information for the realtime flows on end-to-end basis, informing the source nodes for the status of their
flow. INSIGNIA is an effective signaling protocol for ad hoc networks, but it also has
drawbacks. The flow state information should be kept in the mobile hosts, which can
lead to a scalability problem as the number of flow states increases. INSIGNIA also
enables only two classes of services: real time (RT) and best effort (BE).
Having a signaling protocol is just a small part of really implementing quality of
services scheme. Also a routing protocol, which tracks the changes in topology and
updates routing tables in each node, is needed. Admission control module allocating
bandwidth resources, packet forwarding module, packet scheduling module and
medium access module would furthermore be needed. QoS routing, out of these, is
discussed in more detail below.
2.4.3 QoS routing
QoS Routing in MANETs is an essential component in realizing a complete QoS
MANET Architecture. The routing procedure can inform a source node of the
bandwidth and QoS availability to destination node in the network. Two basic matters
have to be discussed: finding and obtaining a route with certain QoS parameters, and
detecting a broken route and finding another. Both finding a route and rerouting are
natural issues in general routing and routing protocols designed for MANETs have
taken the constraints of ad hoc networking into account when considering these
basics. What QoS adds to this is an extension to routing information enabling
discussing these parameters, and also some kind of guarantee to preserve the level of
quality also in case of changes in topology.
Requirements of enabling QoS in routing information are discussed a little bit later
along with an example of QoS version of AODV routing protocol. The case of
detecting broken routes and then either repairing the broken route or rerouting the
flow on an alternate route with the desired QoS is shortly discussed here. As said
before finding a route after dynamic changes in topology should be a trivial task for
routing protocols in ad hoc networks. Some agreed upon QoS parameters, although,
give stringent requirements for delay caused by finding new routes, and also reducing
the likelihood of violating any QoS agreements needs extra attention.
In practice, redundant routes of various kinds are used for avoiding QoS violations.
First, detecting route unavailability is generally handled with a kind of end-to-end
acknowledgement mechanism from destination to source. If the source does not
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receive these “refresher packets” in a given time window, the established QoS route is
declared unavailable and the associated resources are released. In case of using route
redundancy to avoiding possibly time consuming process of repairing routes or
finding alternate ones, there are basically three levels of redundancy to be considered.
At the highest level, multiple alternate routes with the same QoS guarantee are
established for the flow and used simultaneously. At the next lower level, the routes
and associated resources are reserved and rank ordered, but not used unless the
primary route fails. At the lowest level of redundancy, the alternate routes are only
identified, but no resources are reserved. When the primary path fails, the next
alternative has to be checked, whether the resources are still available. It is obvious,
that at the higher levels, more network resources are consumed and in the lower level
the redundancy is decreased. (Mishra, Chakrabarti, 2001)
Perkins et al proposed a QoS version of Ad hoc On-Demand Distance Vector
(AODV) protocol in 2000. It is based on extensions in routing table structure and in
route request (RREQ) and route response (RREP) messages. The routing table
information for each entry is extended with four fields: maximum delay, minimum
available bandwidth, list of sources requesting delay guarantees, and list of sources
requesting bandwidth guarantees. In AODV route discovery, a node, which receives a
RREQ with quality of service extension, must be able to meet the service requirement
to act as nodes do in AODV route discovery: rebroadcast the RREQ or unicast a
RREP to the requester, if it has a suitable route. If the node does not meet the
requirements, the RREQ should be discarded. After route establishment, if a node
detects that requested QoS parameters could no longer be maintained, an ICMP
QOS_LOST message is sent to source that originally requested QoS. (ICMP
messaging is part of IP protocol family and is not further discussed here.) (Mishra,
Chakrabarti, 2001)
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2.5 Billing and incentives
Because ad hoc networks do not have an established infrastructure, there is no entity
to collect payment for infrastructure services. However, as all the nodes in a MANET
work as routers, there may arise a question: why should the traffic between other
nodes be forwarded? Of what use it is for a node to forward packets that do not have
anything to do with it? In fact, forwarding data between other nodes has two
disincentives for an autonomous node: energy expenditure and possible delays for its
own data. Not forwarding the data would, in turn, result in impossibility of forming ad
hoc networks.
There may be need for incentives in ad hoc networking, depending on their
application, though. If, for example, each node in the network is owned by one
organization, it is obvious that it is of everybody’s interest to actually do the
forwarding. If there are selfish nodes, which do not want to forward data from nodes
owned by another company, for example, or if there are compromised nodes, there is
no clear incentive to participate. An incentive system would give benefit to the nodes
that route packets for others. This incentive should also be fair, stable, independent of
traffic patterns, and obtainable only through routing. It might also be beneficial to
punish nodes that misbehave.
One possible solution for handling incentives in ad hoc networks is called Ad hoc
Participation Economy (APE). In APE nodes receive virtual currency for routing.
Money received could be used to paying other nodes to route your packet. In long
term, the ability to send packets depends on having routed for others. APE is based on
routing information with prices charged by intermediate nodes. Inter-node payments
are handled by sending special payment packets. There are also so called Banker
nodes that serve as accountants and dispute resolution authority of a group of
participating nodes. Regular nodes have periodically their accounting records
approved by banker nodes. (Fratkin, Liu, Vijayaraghavan, 2002)
Ad hoc networks can be formed for many different reasons, and there are very
different views and needs for actual billing or incentive considerations. As incentives
and billing in ad hoc networks, in general, is a rather difficult subject, it would
perhaps be more fruitful to consider possible money flows in case of each application
of ad hoc networks separately. Of course, it is still necessary to consider general
schemes for incentives, like APE, to provide the ways of implementing these things
within different applications.
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Internet connectivity
If a node in ad hoc network has connectivity to the global Internet, it can also offer
connectivity to the other nodes. There are several ways to organize this from the
network infrastructure point of view. An Internet gateway can be considered
advertising itself as a default router. This will work if the other nodes in the ad hoc
network can adapt to the thought that they are connected to their default router by a
multi-hop path through other nodes. This idea contrasts with the traditional model of a
default router. (Perkins, 2001)
Another idea is to consider the entire ad hoc as a single hop from the point of view of
Internet routing. (Belding-Royer, 2001) This view is analogous to the way BGP
(Border Gateway Protocol) characterizes an entire administrative system (AS) as a
single hop in its route advertisements. However, given that multi-hop wireless
connectivity incurs significant overhead, it is more important to minimize unnecessary
hops in an ad hoc network than in high-speed wired AS.
One solution to finding the gateway is based on Ipv6 anycast. All gateways could
have an assigned anycast for MANET gateway in their interface. Nodes could try to
find a route to the anycast address with normal route discovery.
Another problem is related to addressing. In addition to gateway, nodes need a
topologically correct address. (Topologically correct from Internet infrastructures
point of view, of course.) Address also has to be globally unique, when connecting to
Internet. One possible solution to this is Mobile IP. So, after a gateway is found, one
way or another, the next step would be to make the default router a foreign agent for
mobile IP. Then every node in the ad hoc network can appear to be accessible as if it
were still located on its home network. Finding foreign agents would be based on
agent advertisements and route requests to an assigned multicast address for “all
mobility agents”.
From an operator’s point of view, access to Internet could be provided through default
gateway settings. An operator could offer devices where a company-owned Internet
gateway is set as the default gateway. Furthermore, these settings could be hard-coded
so that it would not be possible to modify them. This way an operator would provide
Internet access, but also make sure that it would be the only way for subscriber to
access Internet.
Connecting to Internet is a difficult question and answers to that have been few. No
single agreed upon model has come up. However, the problems related to the Internet
connectivity can be defined. As a summary, they are gateway discovery and
addressing problems.
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2.7 Standardization and patents
Standardization in the ad hoc networking field is done simultaneously on separate
fronts, because there are many different technical fields concerned with ad hoc
networking. Radio technologies and routing can be considered the most important
areas of standardization in ad hoc networking. Different radio technologies have been
standardized and are standardized all time. At the same time routing protocols
specifically for MANETs are introduced and also standardized.
Generally all the radio technologies introduced earlier in this paper are standards as
such. IEEE is the standardization organization behind the 802.11 protocol family,
whereas HiperLAN/2, for example, is standardized by ETSI. The situation with
routing protocols at the moment is, that IETF MANET Working Group has most of
the routing protocols as drafts and DSR, OLSR, and TBRPF are going to be published
as RFCs. AODV has recently been published as experimental RFC.
Of areas presented in previous sections, security and Quality of Service solutions are
still far from standardization at IETF.
All in all, the amount of different standards for similar tasks is large, and is likely to
be large also with future development. This may lead to incompatibilities in the future
or at least be confusing people and companies working in ad hoc networking field.
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2.8 Technical problems and limitations
Ad hoc networking comes with a number of limitations, as discussed in previous
sections. A limitation here can be considered as a boundary of some kind, that the
applications designed for ad hoc networks have to take into account. Technical
problems, on the other hand, can be seen as issues that require solving as soon as
possible, because they are hindering development or deployment of the technology.
Furthermore, if an application is designed without taking some limitations into
account, the limitations may become technical problems from the applications point
of view. Most of the problems and limitations were discussed in the sections giving
the technology background for ad hoc networking. For example, problems related to
security were, to some extent, discussed in the Security section. Therefore, this
section is intended to be wrapping the most important issues together.
MANETs are wireless networks. This affects security of ad hoc networks
significantly, because wireless links are easy to eavesdrop. The poorer performance of
wireless transmission technologies, compared to wired transmission, also has an effect
on routing and QoS issues. Bandwidth constraints of wireless transmission require the
routing protocols to minimize the network overhead in route discovery and
maintenance. Also QoS parameters themselves have limited boundaries due to
capacity of wireless links. Furthermore, QoS signaling has to adapt to these
bandwidth constraints. Also concrete limitations for different radio technologies and
for their applicability in MANETs exist. It is problematic to develop a radio
technology that would have low power consumption, high transfer rate, relatively
large range, low costs, good scalability, and flexible support for rapid user mobility in
the same package. Also a wide range of competing standards may be confusing and
thus slow the development of ad hoc networking down.
Dynamic topology adds a new twist into routing, because routing protocols have to
adapt to changes in topology caused by link losses and user mobility. Topology issues
and more specifically the lack of an established infrastructure makes also maintaining
security and guaranteeing QoS a lot harder. These both have traditionally been based
on centralized authorities or entities, or dedicated router configurations. In ad hoc
networks central entities are problematic because of the dynamic nature of the
network. Compromised nodes may get involved with tasks of central entities.
Both routing and radio technologies are limited with energy-constrained. Routing
protocols have to minimize the number of routing information updates, which
consume energy in form of radio transmission overhead. Also radio technologies have
to have limited power consumption to be applicable in MANETs. The power issue has
also an effect on participation incentives. The threshold for forwarding data between
other nodes gets higher, because radio traffic consumes own power “in vain”.
Scalability is one issue that was not very broadly discussed in previous sections. The
scalability from routing perspective can be improved with clustering and hierarchical
routing. Still, scalability of current routing protocols as such is fairly poor. Also
scalability of IEEE 802.11 Wireless LAN protocol was shortly discussed and the
conclusion with it is that the scalability could, at least theoretically, be much better.
Scalability is also problematic with security support and QoS architecture. Clustering
on some level might be helpful with these issues, too.
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At present, different organizations, consortiums and workgroups consider all the
issues presented in the previous sections. This means that technology related to
MANETs does not stand still, but is all time evolving. Scalability and Internet
connectivity in terms of technology and business are perhaps the most important
subjects of future work.
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3 Applications
Ad hoc networking protocols allow building of self-configuring multi hop
wireless networks. The concept in itself is generic and can be used in several
application areas. Ad hoc networking is a critical enabling technology to some of
the applications, such as sensor networks, where as others, e.g., fixed wireless
broadband access networks can operate more efficiently using ad hoc networking
protocols.
In general the use of ad hoc also known as mesh networking or multi hop wireless
networking increases the spectral efficiency of communications, thus increasing
the communications capacity of the network and allowing higher speeds for an
individual user. At the same time use of multiple wireless hops decreases the
power consumption required for sending data when compared to sending the data
directly between communication end points, i.e., a mobile terminal and a wireless
access point. The use of ad hoc protocols allows the networks to be selfconfigurable, decreasing the amount of configuration needed to set up a network.
These theoretical characteristics make ad hoc networking a disruptive technology.
However, in practice these advantages cannot be fully exploited due to limitations
in radio technologies and routing protocols, but offer still notable benefits in
certain application areas.
Also the requirements on networking differ depending on the application area. In
mobile networking the computers, ad hoc network nodes, have limited
computational and storage capabilities, and battery life is also limited. Spectral
efficiency and communications overhead should be minimized for scalable and
efficient operation. With fixed access, battery life is not an issue, but spectral
efficiency and minimal communications overhead are critical for large scale
deployment. With vehicle networks the protocols need to be able to deal with very
fast moving mobile nodes. The most challenging applications are in the military,
where in addition to the above-mentioned challenges the adversary will try to
disrupt and eavesdrop communications.
In this section we will investigate promising application areas and the
requirements these applications set on the networking technologies. The
application areas are overlapping, but have clearly separate markets. For example
fixed broadband access technologies and mobile data services overlap to some
degree, especially for portable access to the Internet. Still the two applications
have at least for now separate markets, e.g., home and corporate Internet access
for desktop computers using fiber, DSL and cable modems and on the other hand
GPRS and the emerging 3G WAN and IEEE 802.11 LAN mobile data services.
As speed for mobile data access increases, these two markets may merge to some
degree at least for the corporate segment, in which the cost is not as big an issue,
as in the residential segment.
Mobile data services can be seen to include also vehicular networks where computers
in vehicles communicate with other vehicles and also with computers located in the
Internet. However, the vehicle-to-vehicle communication distinguishes this
application from normal mobile Internet access.
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Figure 3 Applications for ad hoc networking.
Figure 3 classifies potential applications of ad hoc networking based on the mobility
of the nodes in the network and the size of the network. The technological challenges
become more demanding as these parameters increase.
In this section we will describe the most promising application areas for ad hoc
networking briefly and take a look at the existing products and services. We will
focus more on the more generic areas of fixed and mobile wireless access services
and cover other more specific areas only briefly. Furthermore, we will analyze
how well ad hoc networking is suited to the area currently, what are the biggest
challenges or limitations to applying it. We will also try to grasp the prospects for
applying ad hoc networking to the application area in the near future.
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3.1 Military and emergency networks
Military units need to be able to communicate while moving about freely, which makes
use of wired communication media impractical. The communication systems need to be
robust to be survivable in battlefield conditions. Moreover, the military users cannot rely
on the existence of an infrastructure network, since all locations do not have one and they
are likely to be damaged or destroyed in the course of battle.
The use of multi hop wireless networks provides robust, survivable communications in a
highly dynamic battlefield environment. Most of the research on ad hoc networks was
initially funded by US department of defense (DoD). In a military setting ad hoc nodes
may be highly mobile, the security of the networking is critical, and the ad hoc networks
can cover tens of thousands of nodes, which need to be able to execute communications
of varying priority. The computational, storage and battery capabilities of the nodes also
differ from personal communications devices to vehicle-mounted nodes. These
requirements make design of the protocols and network architectures very challenging for
military scenarios. However, these challenges have lead to the design of robust protocols,
which can be applied also to commercial applications.
3.1.1 Requirements
Ad hoc networks have additional characteristics, which make them useful for military and
emergency use: They can be deployed rapidly without extensive pre-configuration and
provide connectivity beyond line-of-sight for higher radio frequencies (>100MHz)
(Perkins 2001).
The networking protocols need to be able to cope with nodes moving at speeds varying
from walking speed to several hundreds of kilometers per hour to work in a battlefield
environment. Fast moving nodes use long range radios, whereas slow moving nodes can
use short range technologies for higher throughput and smaller electro magnetic
emissions. The network layer binds these together allowing end-to-end communications
between nodes using different radio technologies. Ad hoc routing protocols are used for
maintaining routes between the nodes. To limit the amount of routing protocol signaling
clustering can be used to localize the effects of node mobility as shown in Figure 4.
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Figure 4. A clustered military mobile ad hoc network.
Military and emergency use sets strict requirements on the security of the network. Nodes
may be compromised and the enemy may try to disrupt and eavesdrop communications
and send false information. To increase the robustness of the security architecture, public
key infrastructure based mechanisms can be deployed in a distributed manner. In a
military setting trust relationships already exist in the command hierarchy, which can be
used for creation of the certificate hierarchy.
Use of public key certificates allows building of more flexible trust relationships without
extensive pre-configuration and also the blacklisting of compromised nodes. Distributing
trust with threshold cryptography can also be used for mitigating the effects of node
compromise or destruction of key management servers, as it can be assumed that only few
nodes higher in the command and trust hierarchy will be compromised simultaneously
(Zhou 1999).
Quality-of-service is also important as messages have different priorities and important
messages should be delivered even in the event of highly congested links.
Emergency and law enforcement have also similar requirements for security and qualityof-service. The solutions developed for military use can be retrofitted for use in other
government network
3.1.2
Current deployment
Currently there are two deployed MANETs in the U.S military (Perkins 2001):
U.S. Army's Tactical Internet, TF XXI AWE, is a large-scale MANET, comprising
thousands of nodes. The nodes are both vehicular and man-packed radios and they form a
multihop packet radio network. The network runs Internet protocols, such as the Open
Shortest Path First, OSPF, protocol modified for military requirements over a narrow
band direct-sequence spread-spectrum physical layer.
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U.S. Marine Corps have built a WLAN based MANET of a smaller scale, the ELB
ATCD, which uses aerial relays for forming a network backbone. Land and sea based
units connect to the backbone using Lucent Wavelan cards modified for a longer range.
The aerial backbone uses a direct-sequence spread-spectrum physical layer technology,
VRC-99A.
Both networks have limited throughput and are based on non-IETF MANET protocols
and based on the publicly available information do not support quality of service services.
The security characteristics of the systems are not publicly available. However, the view
of the authors is that the security solutions are likely to be based on shared secret keys
protecting the link layer, especially for ELB ATCD based partially on Lucent's
commercial IEEE 802.11 WLAN.
3.1.3 Future directions
The currently deployed MANET architectures provide either low bandwidth for large
scale networks or relatively fast connections for smaller networks. For multimedia
applications the military needs a MANET providing both high speed and scalability.
The aim of the U.S department of defense, DoD, is to use commercial products running
IETF MANET protocols as building blocks for building the next generation tactical
internet. In the pursuit of this goal DoD funds several research projects through DARPA
and participates in the standardization process of the MANET IETF working group.
3.2
Fixed broadband wireless
Fixed broadband wireless networking can be used for providing Internet access for
residential and corporate customers. Fixed wireless broadband solutions have some
inherent strengths when compared to wired access technologies (Insight 2002):
oLow deployment costs;
oRapid deployment intervals;
oEasily re-deployable network infrastructure;
oEasily deployable in harsh environments;
oLow maintenance and troubleshooting expenses;
oHigh reliability;
oDemand-based deployment (pay as you grow);
oOwnership and control over the network; and
oExcellent back-up or temporary network solution.
Fixed wireless networks can be divided into three broad categories. Traditional
deployments typically use point-to-point or point-to-multipoint topologies. Multipoint-tomultipoint network topologies also known as mesh networks have some advantages over
the older technologies and are gaining popularity.
Point-to-point (PTP) networks have dedicated wireless connections between two fixed
locations, with one end providing access to the wired network. The two locations must be
within a clear line-of-sight and range of transceiver power. PTP networks are quick to
install, provide reliable service and are effective for high-bandwidth, performance-driven
applications. Unfortunately, PTP networks have limited scalability by design. The need
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for fail-safe redundancy in PTP networks often requires expensive duplication of access
systems.
Point-to-multipoint (PTM) networks rely on a single transceiver or base station connected
to the wired network that services multiple customers simultaneously. The base stations
can form a cellular network for increasing the coverage and capacity. PTM networks
scale better than PTP networks, since a single base station can serve multiple customers.
With higher frequencies (>100 MHz) PTM networks require clear line-of-sight and have
limited range to the transceiver. The capacity available to a single customer depends on
the amount of customers served by a base station.
Multipoint-to-multipoint (MTM) topologies, often called mesh networks or ad hoc
networks, provide multiple redundant paths through the network. As with other network
topologies, access points provide connectivity to the wired network. But in mesh
architectures, each customer's radio acts as a router or repeater of wireless signal. The
routers form a multi hop wireless network.
A wireless user need not be within direct range of an access point to connect to the wired
network. The user just needs to be within the range of another connected unit, which then
forward the other nodes packets as shown in Figure 5. Thus, additional users increase the
effective coverage area of an access point, making the entire network more robust.
Internet access
router
Mesh node
Mesh node
LAN
LAN
Mesh node
Mesh node
LAN
LAN
Figure 5. Ad hoc fixed wireless access network
A wireless access network that utilizes multi hop wireless networking, a so called mesh
topology, has several advantages, as described below (Shorecliff Communications 2002):
o A multi hop wireless network requires less power than single hop network topologies.
The radio transmission between two nodes at a range of R increases in proportion to
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the square of R. Thus, use of multiple hops between the end points decreases the
power usage significantly;
o Shorter transmission ranges improve signal strength and increases data rates;
o Packets can be routed around obstacles blocking line-of-sight;
o Traffic can be routed around congestions using load balancing techniques;
o Ad hoc routing protocols can adjust to network changes and failures;
o It requires less centralized infrastructure than other topologies;
o Multi hop wireless networking increases the effectiveness of frequency use, while
minimizing interference. The spectral efficiency of a multi hop network allows a
larger number of subscribers per area without compromising the bandwidth
available to a customer.
However, the use of ad hoc networks for fixed broadband access presents some
challenges:
o Multiple wireless hops increase the end-to-end latency, which is a limiting factor
in real-time interactive services, e.g., voice. The latency depends much on the
specifics of the radio technology used, though. For non-interactive data services
this is not an issue.
o Integration with back-end systems for billing and authentication requires new
protocols, as customers are not connected directly to ISP equipment.
3.2.1
Existing products and services
Nokia RoofTop Wireless Router
Nokia has the RoofTop® Wireless Routing product, which is a wireless access router.
The routers form a multihop wireless network using ad hoc routing protocols, which
allows the network to deliver packets around line-of-sight obstructions, such as trees tall
building, etc. The routers act as wireless LAN access points for users within their range.
The router models R240 and R240A use a modified MAC layer on top of a proprietary
MAC protocol on the unlicensed 2.4 Ghz radio band. The router provides a 12Mbps
aggregate speed per cell and a speed of 1 - 2Mbps to a single customer. The routers use
an ad hoc routing protocol for providing connectivity to the network mesh. Other features
of the router include authentication and access control, confidentiality of the traffic and
centralized management of the routers.
Nokia claims to have over 60 customers for its product, mainly U.S. ISPs.
Radiant Networks Meshworks
The U.K. based Radiant Networks product Meshworks® is another fixed wireless
broadband product based on ad hoc networking. However, unlike Nokia's product
Meshworks uses licensed radio bands to achieve higher data rates, ranging up to 25 Mbps
duplex or 50 Mbps one-way.
Radiant Networks uses the spectral efficiency of their solution as a central selling point to
ISPs. They analyze the efficiency of their multihop wireless system and compare it to a
PTM wireless broadband system and show that their product is several orders of
magnitude more efficient. The radio part of the system can tune the transmission power
level on 1db intervals allowing the system to balance link quality with interference to
achieve small cell sizes and therefore better frequency reuse. The theoretical models on
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the scalability of multi hop wireless networks developed by Gupta and Kumar and
Shepard support the analysis. However, with practical radio systems it is not possible to
achieve the linear scaling presented in Gupta’s and Kumar's paper Gupta, at least without
use of multiple antennas.
The high spectral efficiency allows a larger amount of subscribers per area per frequency
band. Meshworks supports a subscriber density of 600 -1000 users per square kilometer
with a sustainable duplex data rate of 5Mbps per second to 30% of the users
simultaneously. The spectral efficiency of Meshworks is 9Mbps / MHz / km^2.
Meshworks is based on ATM and uses point-to-point links between the router nodes. The
maximum length of a link is 2.8 km. The Meshworks network connects to the wired
infrastructure through Trunk Connection points, i.e., nodes with both wired and wireless
interfaces. As the amount of customers increases more trunk connection points are
needed. The actual routing nodes need to be within the range of other nodes and for this
purpose the ISP needs to set up seed nodes to fill in the connectivity gaps between
customers nodes.
Radiant Networks is doing trials with British Telecom Wholesales unit in U.K. and Star
21 Networks A.G. in Frankfurt, Germany to connect customers outside Star 21's ADSL
coverage area or requiring more bandwidth than ADSL provides. Other customers include
Jazztel (Spain), Mitsubishi (Japan), Nsight (USA) and Tradewinds (USA).
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3.3 Extending cellular mobile access networks
Mobile ad hoc networking can be used for extending the coverage of a cellular mobile
network. This allows mobile users to access the network even when they are outside the
range of any base station. The use of multiple hops between a mobile node and a base
station improves the signal quality, which either increases the data rate or decreases the
required transmission power.
The cellular network in question can be, e.g., a CDMA or an IEEE 802.11 WLAN
network. For example a user surfing the web in an Internet cafe with his laptop computer
and a WLAN card could also provide access to other users outside the range of the
cafeteria's WLAN access point.
The products and protocols developed for fixed broadband wireless access could possibly
also be employed for mobile or at least portable Internet access. However, mobile use
leads to a frequently changing network topology. Changes in the network topology pose
challenges to any routing protocol used in the network. The amount of routing protocol
related signaling makes large-scale flat mobile ad hoc networks impractical. To overcome
this limitation, a clustered or hierarchical approach presented in section 2.1.8 is needed.
The fixed wireless network would then form the backbone network to which mobile nodes
could attach through a small number of hops via other mobile nodes as shown in Figure 6.
3.3.1
Existing products and services
Mesh networks - Mesh Enabled Architecture
Mesh Networks, a company based in Maitland, Florida has transferred ad hoc networking
technology originally developed for the U.S. Military to commercial products. The Mesh
Enabled Architecture®, MEA, provides mobile broadband Internet access to users and
also geo-location services.
MEA uses Mesh Networks proprietary QDMA modulation technology on top of the
unlicensed 2.4GHz radio band. MEA consists of access points connected to the wired
infra and wireless nodes providing multi hop routing services to mobile nodes. The main
selling points of MEA are easy deployment and scalability as new users also increase the
capacity and the coverage of the network.
The end user equipment required to access the MEA is a PCMCIA card, which
contains the whole protocol stack required for using the MEA. The QDMA radio
technology (according to Mesh Networks) is better suited to mobile WAN use than
802.11b and has some common characteristics with cellular radio technologies, such
as multi-tap rake receiver and a real-time equalization algorithm to compensate for
rapidly varying signal strength generated in a mobile environment. The end users
mobile nodes connect to the Internet through fixed wireless routers and intelligent
access points, which have both wired and wireless interfaces. The mobile nodes also
forward packets for other nodes as shown in Figure 6.
The ad hoc routing protocol run in the network is invisible to the mobile nodes as it is run
below the TCP/IP protocol stack. This allows the use of unmodified TCP/IP stacks in the
mobile nodes and makes the use of an ad hoc network for Internet access straightforward,
when compared to MANET protocols operating on the IP layer.
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The use of QDMA radio technology makes the Mesh Networks product expensive and
they offer an 802.11b-based system MeshLAN for 802.11b as a lower cost alternative for
non-mobile Internet access. The MeshLAN uses a modified version of the IEEE 802.11b,
to make it work better in multi hop wireless networks.
Mesh Networks also offers an ASIC chip for adding MeshNetworks Multi-Hop routing
technology, high-speed data rates, Quality of Service (QoS) management, and precision
geo-location to mobile computing, telematics and entertainment platforms. The chip is
radio-technology agnostic and can be used with IEEE 802.11b, QDMA and UWB radio
technologies.
Customers of Mesh Networks include Viasys Corporation, which deploys Intelligent
Transportation Systems (ITS) for Departments of Transportation and unnamed major
corporations in the car, consumer electronics, defense and public mobile data service
industries. Mesh Networks also has an FCC experimental license to build a 4000-node
nation-wide test network in the US.
Figure 6. Mesh Enabled Architecture. Groups can form ad hoc peer-to-peer networks
anytime, anywhere. No network infrastructure is needed. MeshNetworks users can hop
through each other to become part of the group. Peer-to-peer networking is supported for
both fixed and mobile subscribers
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3.4 Sensor networks
Distributed self-configuring sensor networks are an emerging area of computing, that
shows great promise. Distributing sensors provides more accurate and reliable
information of measured phenomena. Typically sensors are small low power units with
minimal computing capabilities equipped with a short range wireless radio. The sensors
are able to do some processing on the data they collect and send it to an observer node,
which can then create “the large picture”, based on the preprocessed data.
ad hoc networking is a key enabler for sensor networks allowing the sensors to selforganize into a network and deal with device failures and even with mobility of the
sensors. Once organized into a network they can deliver the measurement information to
an observer, as shown in Figure 7. Using multiple short wireless hops decreases the
battery drain significantly when compared to each sensor sending the information directly
to a centralized collector node. The ad hoc routing algorithms used in sensor network aim
to minimize the power consumption to maximize the battery life of the sensor network.
Sensor networks can be used for a wide range of application, although the primary focus
has so far been on military applications, as most research on the area has been funded by
the U.S. Department of Defense. Especially the “smart dust” idea has received lots of
attention in DoD funded research. The “smart dust” term refers to a large amount of very
small sensors delivered by e.g. airplane to a battlefield location to survey the area. The
sensors could then measure and partially process phenomena such as noise, existence of
biological or chemical agents, pressure, vibration, etc., Pister (2001). Sensor networks
can be also applied to civilian purposes, e.g. traffic measurements.
Figure 7. Sensor Network.
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3.5 Vehicle networks
Automobile networks allow vehicles to get weather and hazard alerts, provide access
to online navigation data and allow online gathering and analysis of maintenance data.
Currently automobile networks use cellular or satellite based mobile data services,
such as GPRS for connecting the automobile to the Internet.
Automobile networks could employ ad hoc networking for vehicle-to-vehicle
communications and Internet access. The vehicles would form an ad hoc network,
which would be connected to the Internet through roadside access points. Ad hoc
networking would shorten the transmission distances and increase data rates. Use of
ad hoc networking could also be more cost efficient than using cellular access
technology. However, short range radio technologies and MANET routing protocols
need adjusting to work with fast moving vehicles. Current and near future automotive
networks are based on cellular technology and the development of MANET protocols
and radio technologies for automotive networks is still mostly confined to research
labs.
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3.6 Home and personal networking
Mobile ad hoc networks are a generic technology, which can be applied to a wide range
of communications scenarios. Personal area networking, home networking and ad hoc
collaboration could all benefit from multi hop wireless networking. Use of selfconfiguring networks together with service location mechanisms offers clear benefits to
ad hoc collaboration often needed in meetings and conferences. Self-configuring short
range wireless networks enable ubiquitous networking of home electronics and
appliances. The networks can be either multi hop or single hop wireless networks and
can be implemented using Bluetooth, IEEE 802.11 WLAN or similar technologies.
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4 Technology barriers and timing
There exist a number of technology barriers, which slow the adoption of ad hoc
networking. The requirements on the networking increase with the size of the network
and the frequency of topology changes. Small static networks, such as home networks,
pose no problems to the ad hoc routing and radio technologies of today. However, when
the network size, and frequency of node mobility grow the overhead created by routing
protocol consumes a growing part of the network capacity. With currently deployed radio
technologies, especially IEEE802.11b the situation is even worse as shown in Figure 2.
Although alternative radio technologies, such as HiperLAN 2 offer far better
performance, they have not been widely deployed.
The limitations of radio technologies and routing protocols are likely to slow down the
adoption of ad hoc networking in fields where its other advantages are not essential.
Figure 3 illustrates network size and node mobility for different applications and it also
helps in estimating the timing of the applications. The applications in the lower left corner
can apply ad hoc networking first and the ones in the upper right corner last. This
development is estimated in Figure 8.
Figure 8. Timing of ad hoc applications.
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5 Conclusions
Ad hoc networking allows nodes to communicate over multiple wireless hops and form
self-organizing networks. These two features make it an important technology, which
may have a large effect on wireless communications in the near future. Ad hoc
networking improves the efficiency and range of fixed and mobile Internet access and
enables totally new applications such as sensor networks. Use of ad hoc networking will
also make wireless networking more flexible, as users do not need to be within the direct
reach of wireless access points to access the Internet.
More efficient use of radio spectrum through ad hoc networking will allow users to enjoy
faster connection speeds at lower rates. Operators will be able to provide larger coverage
and faster data rates at lower prices by extending their networks with user terminals
acting as wireless routers.
Advances in radio technology and ad hoc routing protocols are needed for wide spread
use of Ad hoc networking. New radio and link control technologies need to be designed
to meet the requirements of Ad hoc networking, which differ from those set by cellular
radio technologies. These technologies also need to be standardized to offer a sustainable
basis for interoperating products and a wider market.
The challenges are being met by several start ups trying to provide a new wireless access
architrecture to the fixed and mobile Internet access market curently dominated by
operators struggling under large amounts of debt and by entrenched equipment vendors,
such as Nokia, Ericsson and Motorola. Only the near term future will show us, whether
Ad hoc networking is ready for more than connecting near by devices for Ad hoc
communications.
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Terms and Abbreviations
3G
Bluetooth
CDMA
DSL
GPRS
HiperLAN/2
IEEE 802.11
IETF
LAN
MANET
QoS
RFC
WLAN
Next generation of cellular radio
technology.
A wireless technology designed for
personal area networking.
Code Division Multiple Access, a cellular
radio technology.
Digital Subscriber Line, a broadband
access technology.
General Packet Radio Service, an
extension to GSM cellular networks for
packet data services.
A wireless LAN technology standardized
by ETSI.
A wireless local area networking
standard.
Internet Engineering Taskforce, the
standardization body for Internet
protocols.
Local area network.
Mobile Ad Hoc Network.
Quality of Service.
Request for Comments, an Internet
standard.
Wireless LAN.
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