Page 256 - Anatomy of a Robot

09_200256_CH09/Bergren 4/17/03 11:24 AM Page 241
COMMUNICATIONS 241
BIDIRECTIONAL COMMUNICATION CHANNELS
A bidirectional communication channel enables the receiver to send the transmitter
information about the state of the channel and the integrity of the received data.
Several tools are used in a bidirectional communication channel to help send duplicate
data. These tools are not confined to use in a bidirectional channel, but they can be used
to take maximum advantage of the reverse communications link. In fact, all the tools
used in a unidirectional communication channel will also work in a bidirectional
channel.

BLOCK CHECKSUMS

When the receiver receives data, it must determine, to the extent possible, whether the
channel has changed the data. It does not matter where in the channel the data was
changed. Noise from lightening storms or sunspots may have changed the data en route
or the receiver might have had a temporary power glitch. The only thing that counts is
whether the receiver’s data buffer got the same data that was transmitted. Much like
aspirin bottles that come with a safety seal that ensures protection, data can be wrapped
in a checksum that will guarantee the integrity of the data.
A checksum is a series of data bits that serve to summarize a block of data. The
sender can chop the data stream into a series of blocks that may be many bytes long.
The checksum is computed and appended to the data block before transmission. We’ll
discuss just how checksums are computed later. The receiver knows, by prior arrange-
ment, how the checksum will be computed. The receiver, upon receiving the data block
(and checksum), independently computes the checksum again and compares it to the
received checksum. If the results are different, then a problem exists. If the checksums
are the same, then the data is accepted and the receiver moves on to the next block. But
suppose a problem exists. In this case, several different actions are possible.

Single Error Detection

If the transmitted checksum information has relatively few bytes, it’s possible that an
error can only be detected. There may not be enough information to either correct the
error or to even detect more than one error in the data block. If an error is detected, the
receiver can ask the transmitter to retransmit the block of information. One protocol
used in the retransmission of data is discussed later.