1.  Field of the Invention
The present invention relates to a ring transmission system of a communication network, and more particularly relates to network configurations for flexibly incorporating and integrating conventional communication networks into a broadband ISDN network (Integrated Services Digital Network). 
Recent public telephone networks and narrowband ISDN networks have tended to be changed to a new broadband ISDN network which involves these conventional networks and can provide various communication services.  In the transfer to the new broadband ISDN network, a problem arises in how to harmonize, integrate, and develop the conventional communication networks into the new broadband ISDN network without suddenly changing the conventional communication networks to the new ISDN network. 
2.  Description of the Related Art
As prior art of the present invention, there are the following documents:
.circle.1 Japanese Examined Patent Publication No.  54-29052, published on Sept. 20, 1979;
.circle.2 Japanese Unexamined Patent Publication No.  58-210743, published on Dec. 8, 1983;
.circle.3 Japanese Unexamined Patent Publication No.  60-65643, published on Apr. 15, 1985; and
.circle.4 "A Strategy of Evolution for the Broadband ISDN", by Takao Takeuchi et al, GLOBECOM '87, IEEE pp1881-1886. 
The above reference .circle.1 discloses a time-division data exchange system accommodating a plurality of terminal equipment having different communication speeds.  This reference does not directly relate to the present invention, but the relation between an exchange and remote equipment which relation itself is a prior art of the present invention is disclosed. 
The above reference .circle.2 discloses a conventional concept of a loop network in which a plurality of local networks are connected to a large loop network. 
The above reference .circle.3 discloses exchange networks having different communication speeds by utilizing loop transmission lines. 
A conventional public telephone network or a narrowband ISDN network is constructed as a star network configuration in accordance with optimum network operation and to meet various service requirements, as disclosed in the above-reference .circle.1 .  The construction of these conventional networks provide various functions, but is not able to provide a convenient service including broadband communications required by current media, such as video, image, data, sound etc. 
In narrowband communications, a channel of 64 Kbps, 384 Kbps, 1.5 Mbps, etc.,  is used for transmission.  In broadband communications, a channel of 30 to 35 Mbps, 60 to 70 Mbps, 132 to 138 Mbps is used. 
A communication configuration in a single broadband ISDN network has been developed in which the various services described above can be provided, but such a broadband ISDN network is usually independent of an existing communication network consisting of a public telephone network and a narrowband ISDN network, and a workable connection between these networks cannot be established.  Namely, the various communication networks are not integrated as a desired communication configuration, and therefore, a useful communication network is not achieved. 
Accordingly, further research into the techniques of incorporating and integrating the existing communication network into the broadband ISDN network, which is considered to be a flexible communication configuration, to develop to a unitary communication configuration, has not been carried out.