Data Communications
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 (types of signal)
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, period, frequency
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).
Data
transmissions
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!
Full-Duplex: data flows in both directions simultaneously. Modems are
configured to flow data in both directions.
Modes of Transmitting Data
- Asynchronous Transmission sends only 1 character at a time. A character being a
letter of the alphabet or number or control character. Preceding each
character is a Start bit and ending each character is 1 or more Stop bits.
For example: for every byte of data, add 1 Start Bit and 2 Stop Bits. 11
bits are required to send 8 bits! Asynchronous is used in slow transfer
rates typically up to 56 kbps.
- Synchronous Transmission sends packets of characters at a time. Each packet is
preceded by a Start Frame which is used to tell the receiving station that
a new packet of characters is arriving and to synchronize the receiving
station's internal clock. The packets also have End Frames to indicate the
end of the packet. The packet can contain up to 64,000 bits. Both Start
and End Frames have a special bit sequence that the receiving station
recognizes to indicate the start and end of a packet. The Start and End
frames may be only 2 bytes each
Multiplexing
Multiplexing is the transmission of
multiple data communication sessions over a common wire or medium. Multiplexing
reduces the number of wires or cable required to connect multiple sessions. A
session is considered to be data communication between two devices: computer to
computer, terminal to computer,
