To better and practically point out the advantages of enabling MMHC, let us rapidly sketch an example of a possible scenario. Consider the realistic case of a group of tourists moving together and sharing pictures via Wi-Fi/Bluetooth single-hop links. Due to their limited coverage range, there could be the need for multi-hop paths to reach target friends who are currently lingering in a shop; that is enabled by collaborating tourist devices that, for instance, can transparently exploit IEEE 802.11 in ad-hoc mode to receive packets and Bluetooth to forward them along the right direction, e.g., node C in Figure 1. In addition, some tourists may be willing to periodically publish their pictures on their Web blogs even if they have no direct UMTS connectivity, e.g., they do not want to subscribe to a local UMTS provider while visiting Italy. These tourists can benefit from Bluetooth multi-hop ad-hoc connectivity toward the devices of friends with flat-rate UMTS subscription who offer them free Internet connectivity, e.g., node A in the figure below. Note that tourists' mobility may relevantly reduce the reliability of these self-organizing MMHC opportunities; depending on application-specific requirements, there is the need to favor the selection of MMHC opportunities with compatible durability, which should be estimated based on practical, lightweight, and effective mobility awareness.
Similarly, when moving from city to city by train, tourists should be able to exploit MMHC opportunities offered by other passengers, possibly in other wagons, reachable via multi-hop heterogeneous paths, and connected to the Internet via Wi-Fi/Wi-MAX APs, such as node B. In this case the nodes tend to move together (joint mobility) and MMHC opportunities have similar expected durability. Therefore, MMHC selection should not only be mobility-aware, but also consider application-specific quality requirements, e.g., expected throughput. Thus, there is also the need for practical, lightweight, and effective ways for coarse-grained estimation of the quality of available MMHC opportunities. Note that the synergic management of multiple MMHC opportunities can also improve handoff effectiveness in the case of abrupt path unavailability, e.g., by seamlessly re-routing traffic to a different collaborating tourist with Bluetooth/UMTS when the one with Wi-Fi/WiMAX leaves the train at a station.
An example of MMHC scenario.