| 单词 | telco return |
| 释义 | telco return A hybrid communications technique for videoconferencing in lecture mode, telco return combines satellite communications in one direction and the public switched telephone network (PSTN) in the other.This technique involves satellite transmission of the downstream video and audio signal from the lecturer’s location to the multiple locations where the audience members are located. Audience participation is via the PSTN, either on a dial-up basis or over dedicated circuits.This hybrid approach provides sufficient bandwidth at reasonable cost for the point-to-multipoint presentation. Although the downstream satellite transmission involves propagation delay of approximately 0.32 seconds, the lecture is all one-way, so the delay is not an issue, as long as it is consistent, i.e., there is no jitter. (Even at roughly the speed of light, it takes approximately .25 seconds for a radio signal to travel from the transmitting Earth station to the satellite at an altitude of 22,300 miles, and back to a receiving Earth station. Processing time at the Earth stations and onboard the satellite takes another 0.07 seconds or so.) If, however, the interactive question-and-answer portion of the presentation were to take place over the satellite, the propagation time would triple (0.32 seconds for the presentation point, 0.32 seconds for the question, and 0.32 seconds for the response), and the flow of the presentation would suffer irreparably. Telco return overcomes this issue, as the audience interaction takes place over the PSTN, which offers negligible delay.Telco return also eliminates the requirement for two-way satellite antennas and bandwidth, which considerably lowers the costs of the satellite links. Some providers of direct broadcast satellite (DBS) services use this same hybrid approach in support of interactive Internet access. See also jitter, propagation delay, satellite, and speed of light. |
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