hard disk unit 110 to the path selection unit 113 under control of the data storage control unit 111 or the controller 112 the data storage control unit 111 controls the operations of storing data in the hard disk unit 110 and reading data from the hard disk unit 110 the controller 112 monitors and controls the entire sonet transmission equipment and flows of data in the sonet transmission equipment the path selection unit 113 selects paths of data under control of the data storage control unit 111 or the controller 112 the poh insertion units 114 to 117 insert pohs in virtual tributaries containing the data portions output from the path selection unit 113 respectively under control of the control information insertion unit 118 the control information insertion unit 118 controls the poh insertion units 114 to 117 so that control information is included in the pohs inserted in the virtual tributaries the framers 119 to 122 produce sonet frames containing the virtual tributaries output from the poh insertion units 114 to 117 respectively the e o conversion units 123 to 126 generate optical signals conveying the sonet frames produced by the framers 119 to 122 by electrical to optical conversion referring back to fig2 each of the clients 77 to 79 sends to the contents server 70 a request for.