Page 120 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Communications and Control

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program like a record jumping a track . Thus, it would begin the program over even
though it might now be 50 feet from where that program was supposed to start. If it had
crossed through a node before the second message arrived, the robot would throw an
error. If it had not reached a node, then the second transmission would not cause a
problem because the program pointer was still at the first step anyway.

The result was that the problem only occurred in areas of poor communications where
the distance from the first node to the second node was relatively short. The reason it
was so hard to detect was because it occurred so rarely and because of the assumption that
the message did not go through if a reply was not received. This was a classic assumption
bug. 11

Other issues
There are other issues that arise in some situations. One of these occurs when com-
munication is routed over a network that has excessive message persistence. If the
protocol handler or post office has given up on a message due to a time-out, but one
of the networks along the way is still repeating the message, then a return message
may eventually arrive that is badly out-of-date. Worse yet, we may believe that it is
the reply to a new message, causing great confusion. For this reason, if possible, it is
best to set the message persistence of such systems to a minimum value.

Another safeguard involves tagging messages with message numbers so that it can be
determined for sure whether a reply belongs to a particular message. If a messaging
system is full duplex, then messages are being sent without waiting for replies, and
this type of tagging is essential.

Books have been written about single aspects of the issues discussed here. The im-
portant thing is to recognize the value of a good communications system, and to
anticipate as many problem scenarios as possible in structuring it. Once the structure
has been selected, you can research specific areas to the depth required. It is also
crucial not to fall into the trap of sacrificing bandwidth for development conve-
nience. It may seem that a system has more than enough bandwidth to allow such
waste, but as it grows this will almost universally prove to be untrue.

10 For the reader who is not eligible for senior discounts, I should explain that records were an
ancient means of storing and playing back music. They were plastic disks imprinted with grooves
whose bumpy sides crudely represented a series of sounds.
11 See Chapter 17.

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