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246 CHAPTER NINE
Be aware that certain communication protocols cannot use retransmission as a tool
to decrease errors. Video and audio links, for example, cannot use retransmission. Video
and audio streams cannot pause while the data is retransmitted because the screen will
go blank. These data streams must be continuously available at the transmitter and rely
entirely on unidirectional data transmission (which we’ll discuss shortly).
CHANNEL TUNING

A bidirectional communications link can be optimized in real time by sending control
information in both directions. Channels can change over time and sometimes need tun-
ing to work properly. Some communication protocols have built-in control signals and
specified tuning algorithms that keep the communication link healthy and robust. The
following methods can be used to tune a system:

Power A data communication link will often work better if more power is used
to transmit each bit. The Eb/No ratio is directly affected. The receiver can meas-
ure the signal strength it is receiving from the transmitter. If it determines the sig-
nal is too weak, the receiver can send a request to the transmitter to boost its power
when transmitting. In the same manner, the transmitter can request the receiver to
boost its transmitting power. This technique can be used in all bidirectional com-
munication links as long as the power stays within limits.
What can be done with power control, however, is limited. Too much power can
pollute the spectrum and make it impossible for any communication link to func-
tion properly. A properly constructed power control protocol for a communication
link often includes a limit on the power that is received. If a receiver senses too
much signal strength coming in from a transmitter, it can request the transmitter
to decrease the signal strength to an acceptable level. After all, the signal for one
receiver may just be the noise for another receiver. Some cooperation is therefore
required.
The Code Division Multiple Access (CDMA) protocol uses just such a power con-
trol protocol to optimize the communication link. This technique is especially use-
ful in situations where a cellular phone is moving from one area to another in a
car. The cellular base stations used by the phone change as the phone moves. To
make sure the phone is well behaved and doesn’t disturb the neighboring phones,
power control is used. Here are some web sites and PDF files describing the tech-
nique further:
www.comsoc.org/livepubs/surveys/public/2000/dec/dukic.html
www.commsdesign.com/main/2000/09 /0009feat3.htm
http://vig.pearsoned.com/samplechapter/0130871125.pdf