1. Field of the Invention
The present invention relates to a packet exchange network and a method of controlling discard of packets in a packet exchange network in packet communication, asynchronous transfer mode (ATM) communication, line communication and so on, and more particularly, to a method and a system for controlling discard of packets in a packet exchange network capable of realizing a constant packet discard rate throughout an entire network independently of the number of packet exchange nodes through which the packets pass when they are transmitted.
2. Description of the Related Art
In packet communication, data are transmitted through a packet exchange network in the form of packets. In the packet exchange network, packets are asynchronously sent out and arrive at their target terminals through various routes. Accordingly, there may occur a so-called traffic congestion. In the "traffic congestion", packets are converged into a particular packet exchange node or trunk line at a specific time.
In order to avoid the traffic congestion, in a conventional network, a buffer having a proper capacity is provided upstream of a packet switch in a packet exchange node or upstream of a trunk line to which a packet is sent from the packet exchange node.
FIG. 1 shows a block diagram of a packet discard control unit for packet exchange nodes in a packet exchange network provided in a conventional packet discard control system for the purpose of avoiding the traffic congestion.
In FIG. 1, reference numeral 1 denotes a terminal, 5 a trunk line, 55 a packet switch 55 provided within a packet exchange node, 56 a buffer capacity detector, 58 a buffer and 60 a packet discard processor 60. In operation, a packet arrived at the buffer 58 through the packet switch 55 in the packet exchange node from the terminal 1 is temporarily stored in the buffer 58 to be sent later to the trunk line 5.
When the buffer 58 is filled up with packets, the buffer capacity detector 56 detects the full state of the buffer and informs the packet discard processor 60 of the full state. The discard processor 60, when receiving the information indicative of the full state, discards some of the packets, usually the last-arrived packet.
In conventional packet discard control systems, a buffer such the buffer 58 described above is provided and in the traffic congestion, the buffer discards a packet or packets overflowed therefrom with no conditions (although there is a condition in a conventional system that all data packets should be passed even some of them are passed with delay while some voice packets may be discarded but others should be passed faster). As a result, in conventional systems, packet discard rate varies depending on the number of packet exchange nodes passed in each communication.
Accordingly, when a fixed packet discard rate which ensures desirable communication quality is to be determined in view of statistics of the past traffic conditions, it is necessary to adopt the worst discard rate, i.e., a discard rate corresponding to a communication requiring to pass the maximum number of packet exchange nodes in the packet exchange network. This allows to give most communications which require a smaller number of packet passage exchange nodes a communication quality beyond their requirements, which is a waste of the transmission capacity of trunk lines.
Thus, the conventional packet discard control system may possibly result in the reduction of the throughput in the entire network.