| 摘要 |
In an integrated communication system (BIGFON) which allows narrow-band and wide-band links (video telephone), a video telephone link may only be through-connected if the appropriate telephone link exists. For this purpose, the wide-band exchanges (18A, 18B) are connected together through common channels (19A, 19B), through which the signal information required for establishing video telephone links in long-distance traffic are exchanged. One calling tone circuit is allocated to each of the user terminals (1A, 1B), by means of which a test signal within the voice frequency band is transmitted to provide a check as to whether the telephone link has been established, before the video telephone link is through-switched. <IMAGE> |
