An interconnection between fully synchronous networks and next-generation frame communications networks is disclosed. A means of bidirectional frame format conversion between a synchronous multiplexing system and a logical multiplexing system is provided, along with a method of transmitting data between different networks on a path as if it were being transmitted in the same network. Further, when converting network control information in an STM network into data suitable for a packet network, even across the boundary of a synchronous multiplexing system and a logical multiplexing system, a unified communication management means is provided over the whole path.