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SUMMARYThe communications environment is currently well organized. Still, it is not easy to handle requirements for urgent communications in a disaster. In a large-scale disaster, it is highly probable that communications functions will be hampered, making access to communication links itself difficult. There is a need for studies directed at assuring robustness and reliability in the emergency communications system. This paper proposes an access protocol for wireless networks in which the centralized control hierarchical network and the horizontal connection ad hoc network are combined. The routing method in which a terminal that cannot directly communicate with the base station (BS) sends urgent data to the BS through neighboring terminals, and an access control protocol for multihopping, are described. In the proposed scheme, the terminal monitors communications in neighboring nodes and dynamically generates a route based on the number of hops to the BS. Evaluations show that the percentage of terminals that can reach the BS by multihopping involving a few hops is significantly improved. It is also shown that the terminal knows the number of hops to the BS in the proposed method, which makes it possible to predict the throughput and the delay time, which is a property suited to urgent communications.

SUMMARYIn a multihoming communications environment in which multiple communications interfaces are connected to a single terminal to use multiple paths, if different paths are used for the going-out path and return path, more efficient information communications may be possible. To enable this to occur, one-way delay must be taken into account to select the appropriate interface. However, it is generally difficult to directly obtain an accurate one-way delay because the communications path load varies according to such factors as the time period and amount of communications, and the going-out path delay and return path delay cannot be easily separated. In this paper, the authors simply obtain the one-way approximate differential delays from multiple patterns of round-trip delay combinations and propose a method of selecting the path based on these differentials. This method has such features as control packets for measurement and synchronization with the communication destination which are unnecessary and it is extendable since it does not depend on the network layer. The authors also implemented the proposed method on a computer by using ICMP and evaluated it experimentally in various types of multihoming communications environments. The results showed that one-way path selection is possible and that the proposed method is effective.