1. Field of the Invention
This invention relates to a communication apparatus used in transmission systems wherein bursty information (or messages) such as LAN frames and packet frames are segmented into packets of a single fixed length or multiple fixed lengths, and a plurality of packets are multiplexed and transmitted to a transmission line. This invention particularly relates to the apparatus structure for reassembling received packets into the original transmitted information and to the reassembling method.
2. Description of the Related Art A slotted ring LAN such as described in IEEE, Journal on Selected Areas, Vol. SAC-3, No. 6, 1985, pp. 815 to 824 essentially requires the technique that information or message from a terminal station is segmented at a LAN node into data blocks of a single fixed length or multiple fixed lengths, and a fixed length packet (hereinafter called a cell where applicable) including the data block is sent to a transmission line, whereas cells received from the LAN transmission line are reassembled to the original information or message. The slotted ring LAN allows each node connected thereto to freely use a vacent slot, thereby realizing communication between a plurality of nodes at the same time. Each LAN node receives its own cells from the LAN transmission line, and temporarily buffers them correspondingly with source addresses added to cells. When the last cell of a message is received, the buffered last cell and other cells from the same source node are reassembled into the original message which is in turn sent to a terminal station or sub-LAN accessible by the node.
In order to ensure reliable and correct communications even under the condition that cells sent from different source nodes are concentrated to a single LAN node, it is necessary for each node to have a buffer memory whose capacity is sufficient for the temporary storage of all received cells until the reassemble operation is completed. For example, assuming that each node in a LAN having n nodes transmits messages received from its terminal station one message at a time to LAN, the maximum number of messages received at a node at a time is (n-1). Assuming further that the maximum length of a message sent from a terminal station is M, then the memory capacity of at least (n-1).times.M becomes necessary for receiving all the packets of maximum length messages sent from all the other nodes at the same time. In such a case, consider the case where there is used a scheme that each time the first packet of each message is received, a buffer memory area of length M is reserved, and that succeeding packets constituting the same message are stored in the buffer memory area reserved for that message. This buffer storage memory area cannot be used as the area for storing packets of the succeeding messages even after the last packet of the preceding message is received, until the read processing (reassemble) of all the received packets from the buffer memory area is completed. Accordingly, with the scheme for allocating the maximum length buffer memory area correspondingly for each message, it is necessary to provide a memory capacity near (n-1).times.M.times.2 in order to store all packets to be received while the read operation for the previously received packets is executed.
In the Drafts B-486 for a Spring national meeting of the Institute of Electronics, Information, and Communications Engineers of Japan, 1989, there has been proposed a data communications system for a slotted ring LAN, which uses an access method by a token. According to this data communications system, each frame among multiple frames on the LAN transmission line is assigned a token and data transfer area, and only the node which seized a token is allowed to transfer the data to the data transfer area. Each node is assigned beforehand a unique token number, and a source node is not allowed to communicate unless it seizes a specific token corresponding to a destination node. Accordingly, a plurality of messages will not arrive at each node at the same time, thereby making small the capacity of a buffer memory for use in reassembling a message. With this method, however, each node is not allowed to transmit data unless it seizes a token corresponding to a destination node. Therefore, as a long message is transmitted from a node, other nodes are required to wait for a long time for data transmission. It is therefore considered that the above method is not most suited for multi-media communications where general data and other data requiring real time transmission such as voices and images are transmitted to the same LAN.