| 摘要 |
<p>Forward/Backward Call Indicators and the Progress Indicator (PI) of ISUP messages are used in call control signals, such as for setting up a mobile multimedia call, to indicate the involvement of a Public Switched Telephone Network (PSTN) terminal. Either terminal of an end-to-end connection can initiate the call and likewise either can respond to the call control signals initiated by the other terminal to immediately recognize the unambiguous fact that a PSTN terminal is involved in the call and to immediately respond by correctly setting up an appropriate and compatible communication format.</p> |
