1. Field of the Invention
The present invention relates to a ring communication network wherein packets are erased at destination nodes, and more particularly to a node apparatus of such a ring communication network including buffers capable of improving performance of the ring communication network while maintaining a global fairness in use of ring communication network bandwidth among nodes, and a method for controlling the same.
2. Description of the Prior Art
In a ring communication network adapted to erase packets transmitted from originating nodes, the propagation delay of the packets may be relatively longer than the delivery time of packet data itself as the ring has a more rapid processing rate and a larger size. As a result, the ring communication network is degraded in performance.
In order to solve such a problem, there has been proposed a system for removing each of the transmitted packets at a corresponding destination node. In a ring communication network employing such a system, spatial reuse of bandwidth is possible. In other words, a waste bandwidth to be generated between each destination node and each corresponding originating node can be reused by erasing a packet and transmitting a new packet at the destination node. Accordingly, an increase in effective communication network bandwidth may be obtained even by the same ring construction. Where destination nodes respectively corresponding to packets are uniformly distributed, the mean packet transportation distance corresponds to 1/2 of the total ring length. Accordingly, it is possible to obtain a transportation processing capability doubling that of a ring communication network employing a system for erasing packets at originating nodes. In the case of the ring communication network employing the system for erasing packets at destination nodes, however, downstream ones among overload nodes are difficult to access by the communication network. In other words, a bandwidth starvation phenomenon occurs. In the case illustrated in FIG. 1, for example, when messages from a node A are continuously transmitted to a destination node C under a condition that nodes C and D have no message to be transmitted, a node B can not transmit any message to be sent.
For this reason, it is necessary to implement a global fairness control (GFC) enabling all nodes to use the communication network bandwidth fairly. This GFC is performed in response to a hardware GFC signal circulating the ring or a reset packet generated when all nodes have completely used the bandwidth allocated thereto or have no packet to be transmitted, and adapted to allow all nodes to use a bandwidth re-allocated. By the GFC, all nodes are ensured to use the ring communication network bandwidth fairly. For example, in a case illustrated in FIG. 2, each node is ensured to use an allocation amount of Di for a GFC interval.
Due to such a GFC, however, a loss of ring communication network bandwidth is generated. Such a loss of ring communication network bandwidth is caused by a bandwidth used by the reset packet itself or the GFC signal or by a self transmission control feature of nodes where the nodes do not transmit any packets so as to ensure the GFC even under a situation where they are allowed to use the communication network. Since the loss of bandwidth caused by the GFC signal or the reset packet itself is very small and required to achieve the GFC, it is unavoidable. However, the loss of usable bandwidth caused by the limited packet transmission of nodes may be eliminated.
Conventionally, there has also been a proposal wherein the GFC interval is lengthened to reduce the relative loss of communication network bandwidth. However, this method has a limitation on the reduction because it causes another problem of increasing a packet delivery delay time. Also, there has been a proposal that the packet allocation amount transmissible within the GFC signal interval ranges between a minimum value and a maximum value, thereby obtaining a slight increase in processing capability. However, this method also has the problem of increasing the packet delivery delay time. Recently, there has been also a proposal for using a waste ring bandwidth in order to reduce the loss of bandwidth caused by the self transmission control feature of nodes. This method enables a packet transmission when there is a packet to be transmitted even after the allocated bandwidth is completely used in a double ring having control packet circulating in a direction opposite a packet transmission direction in so far as an empty ring path is defined between the node to transmit the packet and a destination node for the packet, that is, there is no node to transmit the packet. However, the waste ring bandwidth can not be used when the ring path is empty only between the node to transmit the packet and an optional midway. Furthermore, this method involves a slight increase in GFC interval, thereby causing the packet delivery delay time to increase. Also, the method can not be applied to a single ring.
As apparent from the above description, conventional methods include the method of erasing packets at destination nodes, the method of ensuring a global fairness among nodes by use of a satisfaction control signal and the method of increasing the communication processing amount by an allocation of the minimum number of packets and the maximum number of packets transmissible between successive GFC signals. In accordance with these conventional methods, however, a node which has completely transmitted a packet allocation amount for a current GFC signal interval is not permitted to transmit any packet until a next GFC signal interval is begun. As a result, a loss of ring communication processing capability is generated.