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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.