What is coding (Morse Code, ASCII,
Explain Digital and Analogue definitions
Advantages of digital vs. analogue
Explain frequency, band, bandwidth, baud
rate, bit rate and codec theory
Explain “Why we need a modem”?
time taken by the signal to travel from
the source to the destination
One way = 22,280 miles (GEO satellite
distance about 36000 kms)
Time=Distance/speed of signal
in the satellite receive the
incoming signal and transmit them back to
decrease in magnitude of power of the
signal. (weakening of the signal)
What is coding?
character is a symbol that has a common,
constant meaning. A character could be a
“A” or “B” or “1” or “8”
US-ASCII (7 bit code = 128 valid characters
and 8 bit code = 256 valid characters),e.g.
A=1000001 (7 bit ASCII code)
EBCDIC (IBM’s standard Mainframe code, 8
bits, giving 256 valid character combinations)
More info on link :
A digital signal is discontinuous (gaps) and varies
instantaneously, e.g. zero or 1.
data is in a continuous form.
It varies smoothly.
Electronically, an analogue signal is a voltage
that is continuously going up and down
Analogue signals preserve all the information
with no loss. It is, however, difficult and
expensive to get a computer to work with
analogue data, as well as re create origninal
data once affected by random noise
Advantages of Digital
transmission vs. Analogue
error because of only two distinct values
more efficient transmission (switching,
higher maximum transmission rate
more secure (easier to encrypt)
integration (of voice, data and video) easier
with digital transmission
frequency is the number of cycles
(waves) in an analogue signal that occur per
It is measured in hertz (Hz) - 1HZ is one cycle
or wave per second.
band is a range of frequencies.
For example, the FM radio band covers the
range of frequencies from 80MHz to 108
bandwidth is the width of the band.
The difference between the highest and
lowest frequencies in a band
In digital networks, bandwidth is used to
refer to the amount of data that can be sent
per second (bit rate or bandwidth).
example, each FM station has a
bandwidth of 25kHz, so an FM station
broadcasting on 99MHz (99,000kHz) can
broadcast a signal that varies between
98,987.5kHz (LSB) and 99,012.5kHz (USB)
Humans can hear 20Hz - 14KHz (varies upto 20
voice grade telephone circuits have a
bandwidth of 300HZ to 4 KHz for voice
The baud rate is the number of times that a signal
changes in one second.
It is measured in bauds.
For example, 1,200 baud means that the voltage in
an electrical cable changes 1,200 times every second.
A baud rate of 2,400 baud means that the voltage
changes 2,400 times every second.
o how fast a modem can transmit data. Technically,
the baud is the number of voltage or frequency
changes that can be made in one second. When a
modem is working at 300 baud, this means that the
basic carrier frequency has 300 cycles per second.
bit rate is the number of bits of data sent
It is measured in bits per second (bps)
A bit rate of 33,600 BPS means that 33,600
bits are sent every second. A bit rate of
9.6kbps (9,600bps) means that 9,600 bits are
sent every second.
Bit rate = baud rate * number of bits per
for example… QAM has 4 bits per
so…bit rate = baud rate * 4
convert analogue voice into digital form
for transmission over a digital link (need them
at both ends). Needed for switching over
change computer digital signal to
analogue for transmission over telephone
line (local loop) which is analogue.
the technique that modifies the form of
an electric signal so the signal can carry
information on a communication link. e.g.
change digital to analogue signal
can adjust the capacity (by changing
bits per signal element) depending on quality
of the line
Chapter 3 (96-106) relevant parts
How does analogue data differ from digital signal?
Why do we need a Modem?
What is coding, how many characters 7-bit ASCII provides?
Describe the advantages of digital over analogue.
Explain the following terms:
Explain the following terms: