| 摘要 |
PROBLEM TO BE SOLVED: To attain audio data communication with a stable quality by utilizing a public switched telephone network as a backup line, when the communication quality of an IP(Internet protocol) network is deteriorated. SOLUTION: When the communication quality of an IP network 10 deteriorates, while audio data communications is made between extension telephone sets 1 and 12 respectively housed in private branch exchanges 5 and 11 by utilizing the IP network 10, IP network audio communications equipment switches the communication route to a public switched telephone network which is utilized as a backup network. Upon discriminating that the communication quality of the IP network 10 is deteriorated, the exchange 5 sends an automatic incoming call to the exchange 11 by using a modem signal so as to make speech possible through the telephone network 9, and at the same time, makes the switching of the communication route smoother by controlling the switching timing at the exchanges 5, 11 and transferring control data on the switching timing to the telephone network 9.
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