A system for use in access to ISDN (Integrated Service Digital Network) from a packet data equipment for producing a packet data signal to be transmitted, said ISDN comprising an ISDN switch including a line switch and a packet switch, a plurality of subscriber's lines extending from said ISDN switch, each of said subscriber's lines transmitting a TDM (Time Division Multiplex) signal which comprises first and second communication information channels for conveying communication information and a control channel for conveying a control signal for data link establishment and call control, said system comprising:
- switching means having an input port and first and second output ports and coupled to said packet data equipment at said input port for switching said input port to one of said first and second output ports by a switching signal, said switching means receiving said packet data signal through said input port from said packet data equipment without interruption and delivering said packet data signal as a first switched signal to said first output port when said input port is switched to said first output port, said switching means delivering said packet data signal as a second switched signal to said second output port when said input port is switched to said second output port;
- first gateway means coupled to said first output port for performing protocol conversion of said first switched signal to produce a converted signal;
- control channel processing means responsive to an enabling signal for producing a control channel signal as a call connection requires, said control channel signal having a destination address of a called party, a channel indicator representative of a particular channel, said particular channel being one of said first and second communication information channels to be connected, and a switch indicator representative of a particular one of said packet switch and said line switch which is predetermined corresponding to said particular channel, said control channel processing means producing a connection acknowledge signal after said call connection is completed in said ISDN switch;
- generating means for generating a channel change signal;
- control means coupled to said generating means and responsive to said channel change signal for producing said enabling signal, said control means responsive to said connection acknowledge signal for producing switching signal;
- and
- interface means coupled to said first gateway means, said second output port of said switching means, said control channel processing means, and a specific line, said specific line being one of said subscriber's lines, said interface means delivering said converted signal into said first communication information channel when said switching means switches said input port to said first output port, and delivering, said second switched signal into said second communication information channel when said switching means switches said input port to said second output port and delivering said control channel signal into said control channel, respectively, which is transmitted to said ISDN switch through said specific line.