Week 4:Data communication
-Data Communications is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. The actual generation of the information is not part of Data Communications nor is the resulting action of the information at the receiver. Data Communication is interested in the transfer of data, the method of transfer and the preservation of the data during the transfer process.
Modem ?
-Modem, device that converts between analog and digital signals. Digital signals, which are used by computers, are made up of separate units, usually represented by a series of 1's and 0's. Analog signals vary continuously; an example of an analog signal is a sound wave.
Modems are often used to
enable computers to communicate with each other across telephone lines. A
modem converts the digital signals of the sending computer to analog
signals that can be transmitted through telephone lines. When the signal
reaches its destination, another modem reconstructs the original
digital signal, which is processed by the receiving computer.
To convert a
digital signal to an analog one, the modem generates a carrier wave and
modulates it according to the digital signal. The process of receiving
the analog signal and converting it back to a digital signal is called
demodulation. The word "modem" is a contraction of its two basic
functions: modulation and demodulation.
-Amplitude (A): how high
the peaks are or how low the troughs are, in meters.
The displacement is how far the wave vibrates /
oscillates about its equilibrium (center) position.
Amplitude is correlated with the total energy of the
system in periodic motion. Larger amplitude = greater energy.
Period (T): the time
it takes for one cycle, in seconds.
Frequency (f): the rate,
or how many cycles per second, in Hertz (cycles per second)
Coaxial Cable
-It consists of 2 conductors. The
inner conductor is held inside an insulator with the other conductor
woven around it providing a shield. An insulating protective coating
called a jacket covers the outer conductor.
The outer shield protects the inner conductor from
outside electrical signals. The distance between the outer conductor
(shield) and inner conductor plus the type of material used for
insulating the inner conductor determine the cable properties or
impedance. Typical impedances for coaxial cables are 75 ohms for Cable
TV, 50 ohms for Ethernet Thinnet and Thicknet. The excellent control of
the impedance characteristics of the cable allow higher data rates to be
transferred than Twisted Pair cable.
Optical Fibre
- consists of thin glass fibres that can carry
information at frequencies in the visible light spectrum and beyond. The
typical optical fibre consists of a very narrow strand of glass called
the Core. Around the Core is a concentric layer of glass called the
Cladding. A typical Core diameter is 62.5 microns (1 micron = 10-6
meters). Typically Cladding has a diameter of 125 microns. Coating the
cladding is a protective coating consisting of plastic, it is called the
Jacket.
Data Transmission
-The physical connection determines
how many bits (1's or 0's) can be transmitted at a single instance of
time. If only 1 bit of information can be transmitted over the data
transmission medium at a time then it is considered a Serial
Communication.
If more than 1 bit of information is transmitted
over the data transmission medium at a time then it is considered a
Parallel Communication.
Data Flow
Data flow is the flow of data between 2 points. The
direction of the data flow can be described as:
Simplex: data flows in
only one direction on the data communication line (medium). Examples are
Radio and Television broadcasts. They go from the TV station to your
home television.
Half-Duplex: data
flows in both directions but only one direction at a time on the data
communication line. Ex. Conversation on walkie-talkies is a half-duplex
data flow. Each person takes turns talking. If both talk at once -
nothing occurs!
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