Page 117 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 6

Multipath interference occurs when the robot is at a point where radio waves are not
being predominantly received directly from the transmitter, but rather as the result of
reflections along several paths. If one or more paths are longer than the others by one-
half wavelength, then the signals can cancel at the antenna even though the radio should
be within easy range. When the patrol car approached, it added a new reflective path and
communications was restored. 8

Data integrity
The most obvious problem in communications is data integrity. A system can be made
tolerant of communications outages, but bad messages that get through are much
more likely to cause serious problems. There are many forms of error checking and
error correction, ranging from simple checksums to complex self-correcting proto-
cols.
It is useful to remember that if an error has a one in a million chance of going un-
detected, then a simple low-speed serial link could easily produce more than one
undetected error per day! Luckily, our application protocol is likely to be carried by
protocols that have excellent error detection. Even so, it is useful to discuss error
checking briefly.

The two most popular error checking techniques are the checksum and the CRC
(cyclical redundancy check). Checksums are calculated by merely adding or (more
commonly) subtracting the data from an accumulator, and then sending the low byte
or word of the result at the end of the data stream. The problem occurs when two
errors in a message cancel each other in the sum, leaving both errors undetected.

A CRC check is calculated by presetting a bit pattern into a shift register. An
exclusive-or is then performed between incoming data and data from logic con-
nected to taps of the shift register. The result of this exclusive-or is then fed into the
input of the shift register. The term cyclical comes from this feedback of data around
the shift register. The function may be calculated by hardware or software.
When all the data has been fed into the CRC loop, the value of the shift register is
sent as the check. Various standards use different register lengths, presets, and taps.

8 The multipath problem disappeared in data communications with the advent of spread-spectrum
radios, because these systems operate over many wavelengths. The problem is still experienced
with analog video transmission systems, causing video dropout and flashing as the robot moves.
All of these problems go away when communications is combined with video on 802.11 spread-
spectrum Ethernet systems as shown in Figure 6.5.

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