1. Field of the Invention
The present invention relates to a communication network, and more particularly, to a communication network capable of establishing asynchronous data communication not requiring real time properties and isochronous data communication requiring the real time properties.
2. Description of the Background Art
Examples of a conventional loop communication network include one disclosed in "JP-A-9-289518". FIG. 29 is a block diagram showing the structure of the loop communication network. In FIG. 29, a plurality of stations (four stations 29.sub.1 to 29.sub.4 are illustrated) are connected so as to be capable of communicating with each other by a loop-shaped transmission path 295. Identifiers which are not overlapped with each other (hereinafter referred to as actual IDs) are fixedly assigned to the stations 29.sub.1 to 29.sub.4, respectively. The station 29.sub.1 of the four stations 29.sub.1 to 29.sub.4 functions as a master station, and the other stations 29.sub.2 to 29.sub.4 function as slave stations (which may, in some cases, be hereinafter referred to as the master station 29.sub.1 and the slave stations 29.sub.2 to 29.sub.4).
The procedure for communication in the communication network will be described.
The master station 29.sub.1 holds a table 296 for managing a transmission bandwidth. The master station 29.sub.1 generates a token packet in which a station to be a source of transmission data (hereinafter referred to as a sending station) and a station to be a destination of transmission of the data (hereinafter referred to as a receiving station) are designated using actual IDs (hereinafter referred to as a sending station ID and a receiving station ID) in accordance with contents described in the table 296, and sends out the token packet to the transmission path 295. It is assumed that the station 29.sub.4 and the station 29.sub.3 are designated as the sending station and the receiving station, respectively. The token place returns to the master station 29.sub.1 upon circulating around the loop-shaped transmission path 295. The master station 29.sub.1 deletes the returned token packet. When the token packet circulating around the transmission path 295 arrives, each of the slave stations 29.sub.2 to 29.sub.4 branches the token packet into two token packets, to receive one branched token packet, while sending out the other branched token packet to the transmission path 295 on the downstream side. Each of the slave stations 29.sub.2 to 29.sub.4 judges whether or not the sending station ID or the receiving station ID of the accepted token packet identifies with the actual ID of the slave station. The slave station 29.sub.4 judges that data transmission is enabled because the sending station ID and the actual ID of the slave station identify with each other. Further, the slave station 29.sub.3 judges that data receiving is enabled because the receiving station ID and the actual ID of the slave station identify with each other.
The slave station 29.sub.4 sends out, when it holds a data packet to be transmitted immediately after judging that data transmission is enabled, the data packet to the transmission path 295. The data packet is transmitted so as to succeed the token packet when it is sent out to the loop-shaped transmission path 295, and returns to the sending station (the slave station 29.sub.4) upon circulating around the transmission path 295. The slave station 29.sub.4 deletes the returned data packet.
The master station 29.sub.1 controls a switch (not shown) when the token packet and the data packet arrive from the transmission path 295 on the upstream side, to send out only the data packet of the two types of arrived packets to the transmission path 295 on the downstream side. FIG. 29 illustrates the data packet immediately after the master station 29.sub.1 controls the switch.
The slave station 29.sub.3 receives, when it judges that data receiving is enabled, the data packet arriving within a predetermined time period elapsed since it received the token packet.
In the communication network, point to point communication can be established between the sending station and the receiving station which are designated by the token packet. Further, the master station 29.sub.1 sends out the token packet at predetermined time intervals in accordance with contents described in the table 296 for managing the transmission bandwidth in the communication network. When isochronous data is communicated between the sending station and the receiving station, therefore, the real time transfer of the isochronous data is reserved if the transmission bandwidth is ensured on the table 296.
In the communication network shown in FIG. 29, a first method in which the master station 29.sub.1 sets a plurality of receiving station IDs in the token packet in order to establish communication between one station and a plurality of stations (which is referred to as multicast communication, and broadcast communication particularly in the case of communication between one station and all stations) is considered. Since the token packet generally has a fixed length, the number of receiving station IDs which can be set has a limit. If the number of stations which are accommodated in the communication network is increased, it is possible to anticipate the problem that multicast communication cannot be ensured. Next, a second method in which special IDs for multicast communication are provided in addition to actual IDs is considered. That is, it is assumed that a special ID (for example, #1) is assigned to a combination of the slave stations 29.sub.2 and 29.sub.3, for example, in FIG. 29. If the special ID; #1 is set to the receiving station ID of the token packet, therefore, the master station 29.sub.1 can designate the plurality of stations (the slave stations 29.sub.2 and 29.sub.3) as receiving stations. However, an attempt to apply the second method to ensure multicast communication requires the number of special IDs corresponding to combinations of all the stations. If the number of stations accommodated in the communication network is increased, the data length of the IDs is increased. Therefore, it is possible to anticipate a first problem that the special ID cannot, in some cases, be set in the token packet having a fixed length. Further, the master station 29.sub.1 must manage not only the actual ID but also the special ID, whose management becomes difficult.
In the communication network shown in FIG. 29, the master station 29.sub.1 sends out the token packet in which the sending station and the receiving station are designated to the transmission path 295. However, the station 29 designated as a sending station by the token packet does not necessarily hold transmission data. That is, each of the stations 29 sends out nothing to the transmission path 295 when it does not hold a data packet to be transmitted immediately after judging that data can be transmitted. As a result, there is a second problem that an unnecessary token packet may be transmitted in the communication network.
In not only the communication network shown in FIG. 29, but also in a general communication network, actual IDs are managed by a network administrator and should not identify the actual ID of the other station 29. In the communication network, however, a new station 29 may be added, and the existing station 29 may be withdrawn. Therefore, there is a third problem in that it is difficult for the network administrator to manage actual IDs. Such a problem has been conventionally pointed out. A method of automatically setting actual IDs is disclosed in "JP-A-4-88737". However, a communication network disclosed in the publication differs from a communication network disclosed in the present application in that it does not consider communication relating to isochronous data.