A network that allows two or more mobile devices to link up spontaneously to conduct mutual communication is called an ad-hoc wireless network, a multi-hop wireless network, or a wireless mesh network. Such a network does not require a specific control station, and the devices define a self-organized local network by themselves. In an ad hoc wireless network, terminal devices that cannot communicate directly with each other can transmit and receive packets via a third terminal device located between them, while keeping the transmit power levels low. The communication range can be expanded with this arrangement. In fact, several routing protocols for controlling packet transmission routes are proposed as the Internet standard of ad hoc wireless networks. (See, for example, Non-patent Publication 1 listed below.)
With the communication scheme disclosed in this publication, all the terminal devices participating in the network need to have a packet transfer function. In other words, those devices with inferior functions cannot participate in the network, and a cost-increase problem due to furnishing additional functions arises.
It is also proposed to provide one or more terminal devices that serve as location management agents to manage the locations of mobile communication devices. (See, for example, Non-patent Publication 2 listed below). When a mobile communication device has moved, the agent device of the original location and the agent device of the new location encapsulate the communication route between them to make the route transparent for the network. A communication node that wants to communicate with the mobile communication device can access this mobile communication device via the location management agent. Therefore, the mobile communication device can participate in the network without a packet transfer function.
It is also proposed to arrange the location management agents in a hierarchy to allow a mobile communication device to communicate only with a lower-layer agent when the device is moving in a short range in order to reduce overhead when location management agents are not nearby. (See, for example, Non-patent Publication 3 listed below.)
On the other hand, in a wired network, packets are transmitted between different network segments using bridges, and a packet transmission technique based on a spanning tree protocol is known. (See, for example, Non-patent Publication 4 listed below.) With this technique, a specific bridge or base station is selected as a root bridge, and a transmission tree extending from the root bridge is created using the spanning tree protocol to prevent a loop and realize a fault-tolerant system.
An example of the transmission tree in a wired network is illustrated in FIG. 1. Bridge 1 is selected as a root bridge, and a nonloop transmission tree is created from the root bridge. The nodes to which packets are transmitted are recorded in a learning table, in association with the radio interfaces.
In order to apply the above-described packet transmission technique using wired bridges to a wireless network, multiple wireless interfaces have to be provided to each of the nodes through which packets are transmitted. This attempt further requires an antenna and a modulation and demodulation circuit to be added to each of the interfaces, and is disadvantageous on the cost front.
To overcome this problem, a technique for virtually providing multiple wireless interfaces by regarding the address of the correspondent node as a virtual interface is proposed. (See, for example, Patent-related Publication 1 listed below.) This method allows the concept of wired network spanning tree to be applied to packet transmission in a wireless network, using substantially a single wireless interface. Each of the wireless base stations is furnished with an address table, in which table the address of the correspondent node is recorded in association with the next node to which the packet is to be transmitted.
Another technique is also proposed to reduce redundancy of packet transmission routes, making use of the feature of wireless communications, that is, the fact that packets can reach a wireless station as long as that wireless station is located within the communication range, even if the station is not the correspondent node. (See, for example, Patent-related Publication 2 listed below.) With this technique, each wireless station monitors packets approaching that station along a tree-like transmission route. A table is created to record the source device indicated by the source address contained in the packet, in association with the previous wireless node (or base station) indicated by the address of the transmitting station. Using the table, a shorter route can be selected in packet transmission.
Still another technique is proposed to determine whether the power level of the signal received from a base station is at or above a threshold level when creating a transmission tree, and not to establish a link if the detected power level is below the threshold in order to improve the communication quality. (See, for example, Patent-related Publication 3 listed below.)
By the way, it is necessary for an ad hoc wireless network to select the optimum route in a short time when transmitting packets or creating a transmission tree if such a tree is used. However, in wireless communication channels, the network environment and conditions are likely to change, compared with wired communication channels. Accordingly, it is proposed to introduce a weighting value reflecting the bit error rate and the transmission rate between adjacent wireless devices (or wireless base stations) in determining the optimum route. (See, for example, Patent-related Publication 4 listed below.) The weighting values set in the links between adjacent devices are successively added up to determine the optimum route.    Non-patent Publication 1: S. Corson and J. Macker, “Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations”, Internet standard RFC 2501, January 1999    Non-patent Publication 2: C. Perkins, “IP Mobility Support”, Internet Standard RFC 2002, October 1996    Non-patent Publication 3: K. Malki and H. Soliman, “Hierarchical Mobile IPv4/v6 and Fast Handoffs” INTERNET DRAFT, March 2000    Non-patent Publication 4: ISO/IEC 10038, ANSI/IEEE Std802.1D, “Information Technology, Telecommunications and Information Exchange between Systems, Local Area Networks, Media Access Control (MAC) Bridges”, 1993.    Patent-related Publication 1: JP 2000-69046A    Patent-related Publication 2: JP 2000-78147A    Patent-related Publication 3: JP 2003-188811A    Patent-related Publication 4: JP 2003-152786A