As a form of network including a plurality of nodes, an ad hoc network has been put into practical use. In the ad hoc network, path selection is performed autonomously in each node. That is, each node equipment (or, a communication equipment) of the ad hoc network has a function to operate as a router or a switch. Here, each node equipment is capable of recognizing the network configuration in the surrounding by transmitting and receiving a message and the like with adjacent node equipments. Therefore, by adopting the ad hoc (especially the wireless ad hoc) system, only by implementing node equipments in the environment in which establishment of the network is desired, the desired network may be established, without providing a management equipment to manage the entire network. Furthermore, in the ad hoc network, addition or deletion of a node is easy, making it possible to easily change the network configuration.
As an embodiment of the ad hoc network, a wireless multi-hip network having a tree structure topology has been known, where data is transmitted from each node to a certain node (for example, a gateway node or a root node). For example, a sensor network has been put into practical use where a gateway node equipment collects information detected at each node. As an access scheme of the wireless link, an autonomous distributed access scheme is adopted in many cases. For example, in wireless LAN and ZigBee, CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is adopted. In this scheme, transmission opportunities are equally assigned to the respective nodes, and the respective node equipments can regularly transmit a packet.
As a related art, a configuration and method for improving the throughput of the entire system by preventing the buffer overflow due to data packet congestion in the wireless communication equipment that is a relay node in the wireless ad hoc network system. (For example, Japanese Laid-open Patent Publication No. 2006-101477)
In the wireless multi-hop network, when each node equipment autonomously transmits a packet, congestion may occur in a relay node for example. Especially in the wireless multi-hop network where data is transmitted from each node to the gateway node, congestion may occur in the vicinity of the gateway (hereinafter, gateway may be referred to as GW).
For example, in the wireless multi-hop network illustrated in FIG. 1A, it is assumed that each of node equipments A-G regularly transmits a packet to the GW node. In the example illustrated in FIG. 1A, in a transmission cycle, the node equipments A-G respectively generate and transmit packets A-G. Meanwhile, it is assumed that each node equipment transmits a packet to an adjacent node on the path towards the final destination node (that is, the GW node). For example, the node equipment C transmits the packet C to the node A in a certain transmission cycle. Then, the node equipment A stores the packet C received from the node C in the transmission buffer, and forwards the packet C to the node E in another transmission cycle. Further, the node equipment E stores the packet C received from the node A in the transmission buffer, and forwards the packet C to yet another transmission cycle. In this way, the packet transmitted from each node is forwarded to the final destination node.
However, in the configuration where each node autonomously transmits a packet, there is a possibility that congestion occurs in one or a plurality of relay nodes. In the example illustrated in FIG. 1A, in a certain transmission cycle, the node equipments B, C, D respectively transmit the packets B, C, D to the node A. Then, as illustrated in FIG. 1B, the packets B, C, D are stored in the transmission buffer of the node A. In a similar manner, in the transmission buffer of the node equipment E, the packets A, F transmitted respectively from the node A, F are stored. In addition, in the transmission buffer of the node equipment F, the packet G transmitted from the node G is stored. After that, if each node equipment respectively generate and transmit a new packet regularly, there is a risk that overflow occurs in the transmission buffers of the nodes A, E, F (especially the node A). This is because, while the transmission opportunity obtained in CSMA/CA is equal to each node, the number of packets to be transmitted (that is, the number of received packets) is larger in anode where more paths concentrate. That is, when the nodes A-D respectively obtains one transmission opportunity in a transmission cycle, while the node A is capable of transmitting one packet (packet A), the node A receives packets B-D at the same time. A similar operation is repeated in the subsequent cycles. For this reason, in the transmission buffer of the node A, three packets are added in each transmission cycle.
This problem may be solved, for example, by the node equipment where congestion has occurred sending a notification of a message to make a request to the transmission source node for a reduction in the transmission rate. However, in this method, there is a need to transmit a packet to send a notification of the message, and this packet is also stored in the transmission buffer, which may deteriorate the congestion condition.
In addition, when the network topology is known, the problem above may be solved by setting the transmission rate or the transmission bandwidth of the node equipment that is susceptible to the occurrence of congestion higher compared with other node equipments. However, in the wireless multi-hop network, the radio wave condition is not constant, and there may be a change in the wireless link between nodes. In this case, the network topology changes, and accordingly, the node that is susceptible to congestion also changed. In addition, even in a case in which the topology (forwarding path) does not change, the number of packets that each node forwards may change in some cases. In this case, assuming that a transmission rate is fixedly assigned to a node where the paths concentrate, when there are no or a small number of packets transmitted from the node and nodes under the node, unnecessary resources are assigned. Then, the transmission opportunity of other nodes is reduced, and may decrease the throughput of the entire network.