Page 274 - Anatomy of a Robot
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                                                                                  COMMUNICATIONS 259
                            fit at once, there may not be enough time to determine the proper solution. The net result
                            is that full bandwidth is rarely achieved in these circumstances. For more info, access
                            the following sites:
                                http://eng.murdoch.edu.au/EngModules/m108demo/Section01/
                                 Section0102c.html
                                www.nist.gov/dads/HTML/binpacking.html
                                http://mathworld.wolfram.com/Bin-PackingProblem.html

                              It should also be noted that some receivers have receiving buffers that must not over-
                            flow or underflow. This is true of MPEG transmission, so be sure to read the following
                            section. It’s possible for the channel bandwidth to vary because of errors. This presents
                            much the same problem as the varying requirements. Sometimes errors must just be
                            accepted.


                            Guaranteed Bandwidth

                            Some communication links require a guaranteed bandwidth. MPEG video data streams
                            coming back from the robot would, in general, require constant bandwidth. Such band-
                            width would have to be reserved in advance, or at least not be subject to repeated rene-
                            gotiation.


                            Reverse Channel Bandwidth
                            Bandwidth is often thought of as a one-way parameter. The truth is, if the channel is
                            bidirectional, then the bandwidth must be sufficient in both directions. This can greatly
                            complicate systems where bandwidth is arranged at the spur of the moment.


                            DELAYS
                            Several types of delays can disrupt a communications link. All communications links
                            have delay. Even at the speed of light, data can take microseconds to cross a county.
                            Most electrical signals move far slower than that. Electronic boards for communication
                            have significant processing time, which will delay data. If real-time control loops
                            depend on a communication path, then these delays must be calculated into the design
                            of the system.
                              In some systems, bandwidth is relinquished when it is not needed. Further, when
                            bandwidth is needed, it must be requested and granted. The delay in regaining the rights
                            to the communication channel must be added to the communication delay to determine
                            the worst-case delay.
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