| 摘要 |
Modems including line verification circuits for automatically advising that a direct distance dialed network has been connected between the communicating modems with echo suppressors in the network disabled, is disclosed. The line verification circuitry is either identical for all modems or may be master/slave equipment. The line verification circuitry includes a first tone generator for emitting a unique tone for disabling echo suppressors in the telephone line. A tone generator at a communicating modem, upon receiving the unique tone, emits a different tone which is not capable of disabling the echo suppressors but is capable of transmission through the telephone line when the echo suppressors are disabled. In one preferred embodiment, the second tone is near the limit of the telephone line bandwidth, and thus its reception is an indication that an acceptable line for data has been established between the modems. Circuitry at both modems is responsive to the first and second tones for automatically verifying the existence of the desired communication link. Upon completion of data transmission, the modem itself will automatically disconnect the existing telephone line if the data terminal equipment associated with the modems does not have such capability. |
