| 摘要 |
This invention relates to a method and apparatus providing a common handshaking protocol for the 201, 202 and 208 modems in long distance data transmission.
<??>According to the present invention a novel handshaking sequence trains up any echo canceller 74 in the circuit, detects the presence of any echo suppressor 82 and uses a conventional signal preamble in the presence of echo cancellers 74 and a sacrificial carrier sequence in the presence of echo suppressors 82. As in the prior art, a ringing signal is applied to an answer modem 40 in response to a signal from an originate modem 30. The answer modem 40 is connected to the communication line after receipt of the ringing signal, and an answerback tone is transmitted from the answer modem to the originate modem after a period of silence following connection of the answer modem to the line. After receipt of the answerback tone, the originate modem 30 is connected to the communication line. In accordance with the invention, this is followed by a period of silence that is sufficiently long to reactivate any echo suppressors 82 in the communication line. Thereafter at least two training sequences are transmitted, one from the originate modem 30 to the answer modem 40 and the other from the answer modem 40 to the originate modem 30, each training sequence being sufficiently long to train up any echo canceller 74 in the line. The lengths of the training sequences as received are compared with their lengths as transmitted to determine if the length of the sequence was shortened by an echo suppressor 82. If the length of the training sequences as received are significantly shorter than their lengths as transmitted, a sacrificial carrier is transmitted at the beginning of each transmission of data in a new direction. In the event the training sequences as received are not significantly shorter, each transmission of data in a new direction begins without a sacrificial carrier. As a result, greater data throughput can be achieved in the absence of echo suppressors 82 and errors can be avoided while echo cancellers 74 are being trained. |
