| 摘要 |
The present invention provides a method and apparatus for performing a multipoint polling protocol which employs silence intervals for controlling operation of the multipoint circuit. When a tributary station which has no data to send is polled by a control device, the tributary station does not send any signal in response to the poll. The control device measures the time which has lapsed since the last poll was sent and compares it to a "no-data" silent threshold timing interval. When the elapsed silent time exceeds the threshold interval, the control device assumes that the tributary has no data to send and immediately sends the next message. If the beginning of a non-silent response is detected before the threshold interval has passed, the control device receives the response and processes the response in the normal manner, e.g., in the manner in which typical multipoint polling protocols process responses. Before issuing a poll to a low-speed device, the control device allows a predetermined delay period of silence to elapse between the end of the last transmission on the line and the start of the poll transmission. This delay period is longer than the no-data threshold interval. All tributary stations measure this silent delay interval and, when it occurs, they prepare to receive a low-speed modulation transmission from the control device. When the period of silence does not exceed the predetermined silent delay interval, the low-speed devices know they will not be able to demodulate the received high-speed signal and, therefore, disable their receivers until energy is no longer detected on the line. When the tributary stations are connected to the subscriber line via a shared bus, the tributary stations determine whether a transmission is from another tributary station or from the control device based on a silent delay interval which occurs after the last transmission on the subscriber line.
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