Tham khảo tài liệu 'anatomy of a robot part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 246 CHAPTER NINE Be aware that certain communication protocols cannot use retransmission as a tool to decrease errors. Video and audio links for example cannot use retransmission. Video and audio streams cannot pause while the data is retransmitted because the screen will go blank. These data streams must be continuously available at the transmitter and rely entirely on unidirectional data transmission which we ll discuss shortly . CHANNEL TUNING A bidirectional communications link can be optimized in real time by sending control information in both directions. Channels can change over time and sometimes need tuning to work properly. Some communication protocols have built-in control signals and specified tuning algorithms that keep the communication link healthy and robust. The following methods can be used to tune a system Power A data communication link will often work better if more power is used to transmit each bit. The Eb No ratio is directly affected. The receiver can measure the signal strength it is receiving from the transmitter. If it determines the signal is too weak the receiver can send a request to the transmitter to boost its power when transmitting. In the same manner the transmitter can request the receiver to boost its transmitting power. This technique can be used in all bidirectional communication links as long as the power stays within limits. What can be done with power control however is limited. Too much power can pollute the spectrum and make it impossible for any communication link to function properly. A properly constructed power control protocol for a communication link often includes a limit on the power that is received. If a receiver senses too much signal strength coming in from a transmitter it can request the transmitter to decrease the signal strength to an acceptable level. After all the signal for one receiver may just be the noise for another receiver. Some cooperation is therefore required. The Code Division Multiple Access
