(1) Field of the Invention
The present invention relates to a communication method in a point-to-multipoint communication system and a subsidiary transmission apparatus for the same system. The invention relates to a communication method and a subsidiary transmission apparatus suitable for use in SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) transmission systems.
(2) Description of the Related Art
At present, as a user network interface for realizing the B-ISDN (Broadband Aspects of Integrated Services Digital Network), the SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) is standardized as international standards for the purpose of standardization of existing digital hierarchies of countries world-wide.
As a mode of communication network using the SONET/SDH, there is a point-to-multipoint communication system as shown in FIG. 9.
As shown in FIG. 9, the previous point-to-multipoint communication system 600 includes: one main transmission apparatus (hereinafter will be also called the “main station”) 100; and N-number (N is an integer greater than 1) (in the present example, three) of subsidiary transmission apparatuses (hereinafter will be also called the “subsidiary station”) 200A, 200B, and 200C (simply called the “subsidiary station 200” when individual apparatuses are not distinguished).
In the point-to-multipoint communication system 600, when one-to-N inter-transmission apparatus communication (that is, communication from the main station 100 to the subsidiary station 200) is performed, the main station 100 broadcasts (or multicasts) the same signal to be transmitted to the subsidiary stations 200 through a sending line (downstream line) 120, whereby the signal reaches the signal terminal unit 400. As a result, the main station 100 is capable of communicating (transmitting) with all the subsidiary stations 200 without any trouble.
On the other hand, in the point-to-multipoint communication system 600, when N-to-1 inter-transmission apparatus communication (that is, communication from the subsidiary stations 200 to the main station 100) is performed, exclusive control needs to be carried out for communication (transmission) because the subsidiary stations 200 share a returning line (upstream line) 210. To realize such exclusive control, at present, a maintenance engineer needs to manually perform a switch operation of the switch unit 500 of each of the subsidiary stations 200, thereby controlling the line connection state between the main station 100 and the subsidiary stations 200, so that a signal to be sent from the subsidiary stations 200 can reach the signal terminal unit 300 of the main station 100.
With the above-described method, for manually controlling a switch state of the switch unit 500 of each of the subsidiary stations 200, the maintenance engineer must spend a lot of time and effort. Thus, when a number of subsidiary stations 200 need to be managed, the manual management is not a down-to-earth control method. In addition, in the above-described method, when a transmission request to the main station 100 is generated in any of the subsidiary stations 200, it is impossible to perform lines witch control (switch control of the switch unit 500) immediately, so that the usability of the lines can be affected.
Thus, to resolve the above problem, an automatic line control method performed in cases where N-to-1 inter-transmission apparatus communication is carried out is considered.
For example, the following patent document 1 discloses a communication method in which a signal from each subsidiary station is super imposed using an OR circuit to be sent to the main station. If only one subsidiary station is sending significant information, transmission information of the subsidiary station is inevitably received by the main station.
[Patent Document 1] Japanese Patent Application Laid-open No. SHO 61-161843
According to the above-described previous art, when exclusive control is performed among the subsidiary transmission apparatuses, dedicated lines, which are necessary for the exclusive control, need to be installed. Hence, if the scale of the communication system is increased due to an increase in the number of subsidiary stations, not only the subsidiary stations and the main signal transmission path but also such dedicated lines need to be installed, so costs are increased. In addition, the number of lines needed is also increased, and the reliability of the above communication system and the convenience of maintenance may deteriorate.