Traditional wireless services such as cell phones, radio and Wi-Fi are designed using a hub-and-spoke model where the hubs are deployed and interconnected as part of a well planned structure. In contrast, urban grids based on mesh networking emerge opportunistically from existing wireless hardware, without much prior planning. Packets skip from device to device, seeking one that is connected to a wireless access point from which to enter the Internet. Thus, in the urban grid the user devices become the network. This radically new model of topology growth/evolution and of network operation poses unique challenges at every level. In this project we plan to address these challenges.
In order to better focus our study, we first consider some representative applications of the urban mesh network. From these applications we then derive the research themes and goals that will drive this study.
- Urban mesh networks. Mesh Networks have the potential to allow users to bypass broadband providers and to create ubiquitous, low cost wireless networks. Mesh networks become de facto the last mile solution where one house could connect to the Internet and then extend access to the entire neighborhood via ad hoc radio hops. Mesh networking offers huge possibilities for community networks to provide access to homes and businesses. In addition, mesh networks could be used to connect public access sites throughout a community.
- Car to Car networks. By virtue of enhanced wireless communications the car has recently been elevated from a simple means of transport to an office on the move, as well as an information hub and entertainment center. Several important applications are now emerging which will leverage the communications and computer resources of the car. Firstly, the car represents the alternate home/office for most people commuting to work. As such, it is used to access the Internet for documents, to engage in teleconferencing, news, etc. Related to the concept of alternate home is the entertainment center, e.g. downloading music from the web or playing distributed videogames with passengers in other cars.Secondly, a broad range of navigation applications are motivated by the fact that the car moves, and in fact must reach a predefined destination. Here, wireless communications support geo positioning (GPS), path finding, parking, hotel reservations, tourist attraction promotions, and a host of other location based services. A third critical application is safety on the road. This includes a large spectrum of services, from traffic congestion avoidance, to collision avoidance, to braking coordination and even remote diagnostics of engine trouble. In this case, the cars can be viewed as sensors in a moving fabric, ready to pick up environment data and alarms on demand. The fourth and most visionary application is probably the establishment of an opportunistic multihop backbone network among cars which spans the urban road grid and which extends to intercity highways. This backbone network can alleviate the overload of fixed wireless infrastructures (3G and hotspot networks) and does offer an emergency backup in case of massive fixed infrastructure failure (e.g., terrorist attack, act of war, natural or industrial disaster, etc), as further elaborated in the next paragraph.
- The intelligent transportation system is another interesting application of mesh networks. Using devices such as video cameras or traffic sensors wirelessly connected, the intelligent transportation system can solve traffic congestion problems, public transportation synchronization and can offer position services. For example using a PDA we can find the closest bus station, the bus schedule and the current bus position. A driver can learn the frequency of traffic light changes on a boulevard at a particular time of day (i.e., green wave) and plan his approach to the city center accordingly.
- Urban emergency recovery. Natural causes, chemical disasters and terrorist attacks can disrupt communications and bring urban traffic to a standstill. In the past few years we have witnessed several such events. The urban mesh network approach offers an effective remedy to the situation. In fact, the ad hoc network represented by cars will survive even if the power and communications infrastructure collapses. The cars will become the nerve system of the urban network. Via car to car multihopping, disaster information will be propagated; police, fireman and ambulance operations will be coordinated; and car evacuation instructions will be dispatched to all cars so as to avoid major road congestion and allow room for the emergency vehicles to intervene and operate.
Figure 1: Urban Grid and Intelligent
Transport System.
In summary, all the above applications and
operational modes require advanced radios and novel
network protocols. Current protocols for the most part
enable Internet access (e.g. music download, path
finding, etc.). The critical link is the wireless link
from the station to an Internet resource and the main
challenge is the selection of the best alternative for
the last hop (e.g. IEEE 802.11, 3G, satellite,
etc.). This problem, also known as the vertical
handoff problem, has been already studied extensively
in the literature [StK98].
However, an
increasing number of emerging scenarios involve mesh
networks and a multihop ad hoc wireless network is
much more cost effective than traditional cellular
type systems if the nodes are in proximity to each
other. For example, for car safety and more precisely
accident prevention it behooves a car to broadcast in
the ad hoc network its position/velocity/acceleration
(and possibly other parameters and environment
information) so that two cars on a collision course
can take evasive action. Passengers in cars going in
the same direction may engage in distributed video
games, chats and impromptu videoconferences to review
notes before the meeting, etc. To this mobile mesh
network pedestrian cellular phone users may also
connect using IEEE 802.11, for example. More
generally, one will notice that the last hop and
multihop model must coexist in most of these
applications. For example, if a boulder has obstructed
the road, this information must be propagated into the
mesh network as well as via 3G or via long range radio
links, so that the information quickly reaches nodes
many hops away. Mesh networks have a number of other
benefits. First of all, they are highly adaptable and
scalable. Mesh networks can have redundancy built-in
so "each device has two or more paths for sending
data". This redundancy makes mesh networks very
reliable. For example, in case of a collapse of the
fixed infrastructure, the back up would be the mesh
network supported by the thousands of cars on the road
(driven or even parked).
In this project the focus will be on the opportunistic coexistence and balance between mesh networking and fixed infrastructure communications. We will design and test transparent, adaptive MAC, routing and transport protocols. The lessons learned here will be transferable to many other systems consisting of large numbers of units in motion. For instance, the Campus environment where students equipped with PDAs can establish opportunistic mesh networks to extend wireless LAN reach or to support group communications. Or, the automated battlefield where manned units are interconnected with unmanned UAVs. UCLA is engaged in both Campus and battlefield networking studies. Our study will leverage the results of those projects. The urban mesh environment however is unique for the richness and breadth of the applications and the challenge in designing the radios (eg, radios equipped with MIMO antennas) which must be powerful, flexible, reconfigurable, robust and yet economical. The breadth of applications and the flexibility and richness of the radio options translate in turn into a set of demanding requirements for the network protocols, from MAC to middleware.