Transcript W3-1Rev

W3-1:Learning Outcomes
 Transmission
Time/Propagation time
 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”?
Propagation Delay
 The
time taken by the signal to travel from
the source to the destination
 One way = 22,280 miles (GEO satellite
distance about 36000 kms)
 Prop.
Time=Distance/speed of signal
Satellite Transponders
 Transponders
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”
 Morse Code
 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.
 Analogue
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
 Less
error because of only two distinct values
(0, 1)
 more efficient transmission (switching,
 higher maximum transmission rate
 more secure (easier to encrypt)
 integration (of voice, data and video) easier
with digital transmission
 The
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
 The
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).
Bandwidth cont..
 For
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
 Most
voice grade telephone circuits have a
bandwidth of 300HZ to 4 KHz for voice
Baud rate
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
 The
bit rate is the number of bits of data sent
every second.
 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
symbol/interval/voltage level
 for example… QAM has 4 bits per
symbol/interval :
so…bit rate = baud rate * 4
Codec (Coder/Decoder)
 To
convert analogue voice into digital form
for transmission over a digital link (need them
at both ends). Needed for switching over
telephone networks
 To
change computer digital signal to
analogue for transmission over telephone
line (local loop) which is analogue.
 is
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
 Modems
can adjust the capacity (by changing
bits per signal element) depending on quality
of the line
 Read
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:
Propagation delay,
Digital, Analogue,
Explain the following terms:
Baud rate,
Bit rate,