To deliver on the promise of ubiquitous, anytime, anywhere wireless communications to mobile users, mobile elements must become part of the existing infrastructure itself [Kle00][Kle01]. A strikingly economical means of achieving this is to use a wireless mesh network that is a network that handles many-to-many connections and is capable of dynamically updating and optimizing these connections. A mesh network allows nodes or access points to communicate with other nodes without being routed through a central switch point, eliminating centralized failure, and providing self-healing and selforganization. Some terminals of the network can be mobile units that change position over time. Mesh networks are an example of "spontaneous ad hoc networking", the type of networking that occurs in an impromptu fashion in a board meeting, in a classroom, on a plane, at sport events etc among individuals interested to interwork as a group, to exchange files, play computer games or agree on a market strategy.
Spontaneous mesh networks differ from the highly organized ad hoc networks one finds in military, civilian defense or emergency applications (e.g., nuclear disaster cleanup, rescue of avalanche victims etc.). The main characteristic of mesh networks is that they are "individual" centric rather that "mission" centric. As such, they cater to the need of the individual and his/her interactions with environment, internet and other peers. From a conceptual standpoint, we are interested in the way these mesh networks form, and they intercommunicate among peers, with the surrounding environment and the Internet.
The user centric MANET we are dealing with here is
often referred as a PAN (Personal Area Network),
consisting of a mobile, networked individual (e.g., a
professional on the move, an Internet-wise tourist, a
student attending virtual classes, an avid Internet game
player, etc) and the collection of devices he/she
carries. The devices include any subset of the following:
laptop, palm pilot, beeper, mobile phone, portable
scanner, CD reader, head set, digital camera, video
camera, heart monitor, etc. The connectivity within the
PAN is wireless using for example a low transmit power
wireless LAN such as Bluetooth.
The PAN can expand
and contract dynamically depending on needs. In
particular, the PAN may opportunistically include wall
repeaters (access points) when available in order to
access to the Internet, to allow the mobile professional
to access his/her e-mail from a coffee shop, movie
theater, airplane, etc.
The PAN can be dynamically
stretched to a MANET that connects multiple individual
PANs. For example, in a meeting room, dining hall, or
convention center it is conceivable that some users may
want to network with each other to exchange opinions about
a speaker, or agree on the negotiating tactic to be
adopted during the meeting etc.
The PAN can also
access sensors and actuators. Such access is critical when
the nomad walks into a new environment and wants to
quickly become aware of what is going on, or wants to
control temperature, adjust the lighting, select a
particular background music etc. In some cases, the nomad
himself carries sensors as part of his PAN: for example, a
patient may walk around the hospital or nursing home with
several monitors that transmit to a base station on the
walls, allowing customized 24 hours monitoring.
In a society like ours based on the car, the most
prominent example and instantiation of a PAN is the
automobile itself. The automobile-PAN interconnection
scenario then becomes a network of automobiles in which
each car is equipped with some basic information
processing and networking capability. The automobile is no
longer simply an end point in the network, but rather it
becomes part of the infrastructure, providing an
opportunity to dramatically improve the range, throughput
and performance of the latter.
The coupling of the
car based ad hoc network with the existing mesh
infrastructure results in cost savings, performance
improvements and an exceptional resilience to
failures. This new paradigm of opportunistic networking
where ad hoc meets the mesh infrastructure changes how
networks will operate and will be optimized as it will
focus mostly on resource allocation. Opportunistic
networking will combine ad hoc networks and infrastructure
based networks, matching the resources to the
communication connectivity needs. It will lead to a
ubiquitous, extremely flexible and robust Urban Vehicular
Grid that will become the nerve system of urban
communications.
The concept of opportunistic networking in mobile networks has a basis in information theory. The seminal work by Gupta and Kumar [GuK00] showed that ad hoc sensor networks have an information rate per node that goes to zero as the number of nodes gets large. This work temporarily put a damper on the promise of large scale wireless networking until people started to consider mobility. Grossglauser and Tse [GrT02] showed that adding mobility counters this characteristic at least for delay insensitive transactions. The key idea is that a node waits to communicate until mobility brings source closer to destination and provides a good channel with high data rates. This way the nodes do not transmit often and produce little interference to other transmissions but wait until they can transmit efficiently. In an integrated wired/wireless infrastructure such as the urban grid these information theoretic results motivate the idea of setting up and executing "information showers or kiosks". Coordinated use of information showers will require long distance communication and control, and the knowledge/prediction of current and future link geometries. These tasks can be easily accomplished in the Urban Grid since GPS will be available in all vehicles and most transportation is usually planned in advance and executed on well defined pathways.
Wireless communications will eventually become a
dominant part of vehicle electronic subsystems. Wireless
communications provides two important functions that have
made it a technology of choice for consumers and for
transportation applications: mobility and inexpensive
infrastructure. The mobility support is a key in any
vehicle based communication system. A vehicle-based
communication system must support anytimeanywhere
connections to accomplish the vision of the future driving
experience that has evolved in the transportation
community. Vehicle-based wireless communication can
support navigation, driver's information, electronic
communication, entertainment, and improved driver
safety. At the same time, vehicle communications must be
affordable.
The car safety subsystem is one important
example where current wireless communications and network
technology must be revisited to satisfy unique
requirements. Safety will have extreme requirements in
reliability (false alarms or missed detection in automated
braking applications will be highly unacceptable). In a
similar way, delay in a network supporting safety
applications cannot be tolerated. Secondly, the network
for safety must work also without infrastructure. A truly
universal solution must be as useful to the consumer in
rural Wyoming as it is in the freeways of Los Angeles. An
ad hoc network that can adapt to the environment and the
available resources will be a critical component of
vehicle safety systems. The final important characteristic
is that vehicle safety systems must support high mobility
at the physical layer (Doppler spread) and network layer
(rapidly evolving network connectivity). No current
telecommunication system can support the extreme
requirements in reliability, delay sensitivity, ad hoc
network capability, and mobility needed for safety
applications. Hence new network and radio technology must
be developed or evolved.
This and other innovative
applications can be achieved only by making the vehicle an
information processing and communications hub. Giving the
vehicle this capability matches well with the relatively
large size of the vehicle and the readily available
resources (power, processing, storage, etc). The vehicle
hub is the nexus where the extensive fixed infrastructure
radio communications resources and the highly mobile ad
hoc radio capabilities meet to provide the necessary
services in the future Urban Vehicle Grid. New networking
and radio technologies are needed when operations occur in
the extremes (extreme here expresses the notion of
emergency, non routine type functionalities, such as
disaster recovery, and autonomous operations). The
extremes of this opportunistic networking paradigm are
extreme mobility (radio and networking), extreme quality
of service attributes for safety applications (networking
and radio), extreme resource management flexibility and
reliability (adaptive networks), and extreme throughputs
(radios). Consequently the technologies that are needed
for such a paradigm are network protocols that prioritize
and optimize network resource management and allow
connectivity to occur either in the ad hoc modes or within
an infrastructure based system. Additionally extremely
flexible physical layer implementations are needed to
realize this paradigm. Moreover, cross layer adaptation is
necessary to explore the tradeoffs between transmission
rate, reliability, and error control in these environments
and to allow the network to gradually adapt as the channel
and the application behaviors are better appraised through
measurements