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