Biomedical Engineering Reference
In-Depth Information
general-purpose electronic analog computer was built with funds from the National Defense Research
Committee. This trend of government and military funding of leading-edge computer and
communications technologies continues to this day.
With the declassification of information about the analog computer after World War II, several
commercial ventures to develop computers were launched. Around the same time, Claude Shannon
published his seminal paper on communications theory, "A Mathematical Theory of Communication."
In it, he presented his initial concept for a unifying theory of transmitting and processing information.
Shannon's work forms the basis for our understanding of modern communications networks, and
provides one model for communications in biological systems.
As illustrated in
Figure 1-4
, Shannon's model of Information Theory describes a communication
system with five major parts: the information source, the transmitter, the medium, the receiver, and
the destination. In this model, the information source, which can be a CD-ROM containing the
sequence information of the entire human genome or a human chromosome, contains the message
that is transmitted as a signal of some type through a medium. The signal can be a nucleotide
sequence in a DNA molecule or the dark and light patches on a metal film sandwiched between the
two clear plastic plates of a CD-ROM. The medium can be the intracellular matrix where DNA is
concerned, or the clear plastic and air that the laser must pass through in order to read a CD-ROM.
Regardless of the medium, in the propagation of the desired signal through the medium, it is affected
to some degree by noise. In a cell, this noise can be due to heat, light, ionizing radiation, or a sudden
change in the chemistry of the intracellular environment causing thermal agitation at the molecular
or nucleotide level. In the case of a CD-ROM, the noise can be from scratches on the surface of the
disc, dirt on the receiver lens, or vibration from the user or the environment.
Figure 1-4. Information Theory. Shannon's model of a communications
system includes five components: an information source, a transmitter, the
medium, a receiver, and a destination. The amount of information that can
be transferred from information source to destination is a function of the
strength of the signal relative to that of the noise generated by the noise
source.
When the signal is intercepted, the receiver extracts the message or information from the signal,
which is delivered to the destination. In Shannon's model, information is separate from the signal.
For example, the reflected laser light shining on a CD-ROM is the signal, which has to be processed to
glean the underlying message—whether it's the description of a nucleotide sequence or a track of
classical music. Similarly, a strand of RNA near the endoplasmic reticulum is the signal that is carried
from the nucleus to the cytoplasm, but the message is the specific instruction for protein synthesis.
Information theory specifies the amount of information that can be transferred from the transmitter
to the receiver as a function of the noise level and other characteristics of the medium. The greater
the strength of the desired signal compared to that of the noise—that is, the higher the signal-to-
noise ratio—the greater the amount of information that can be propagated from the information
source through the medium to the destination. Shannon's model also provides the theoretical basis
for data compression, which is a way to squeeze more information into a message by eliminating
