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Lecture 4 - Multiplexing & Spreading (Multiplexing - the placing two…
Lecture 4 - Multiplexing & Spreading
Multiplexing
- the placing two or more simultaneous transmissions on a single communications circuit
Multiplexer (Mux)
- combines data from
n
input lines and transmits over a higher-capacity data link
Demultiplexer (Demux)
- accepts multiplexed data stream, separates the data according to the channel, and delivers them to the appropriate output lines
Primary benefit is more
efficient use of bandwidth, also saving money on thing like cabling
Analog
Frequency-division Multiplexing
applied when the
bandwidth of a link is greater than the combined bandwidths of the signals
to be transmitted
signals generated by
each sending device modulate a different carrier frequency
, once they are not too similar they can be sent over the link at the same time
examples include
cable television and FM Radio
, where we a number of stations transmitted simultaneously over the same link
Wavelength-divison Multiplexing
Similar to FDM, frequency x wavelength = speed of light
main difference is that in
WDM
the multiplexing and demultiplexing involves l
ight signals transmitted through fibre-optic channels
Narrow beams of light are combined
at the multiplexer,
at the receiver the signals are separated
by a demultiplexer
Digital
Time-division Multiplexing
TDM is used for
digital transmission - PSTN, GSM
requires that the data rate of the medium is
at least equal to or exceeds the sum of data rates of signal to be transmitted
time slots are organised into frames
Interleaving - if there are
n
input lines, the
data rate of the links is n times faster
, and the
unit duration is n times shorter
Asynchronous TDM
- time slots can be taken on a FIFO basis
Synchronous TDM
- each device is pre-assigned a time slot, this is wasteful as device transfer very often and some very little
Modulation
Modulation is
converting an information signal (analog or digital)
into a HF signal by modulating it with a
HF signal called a carrier
a device that is both a
modulator
and
demodulator
is known as a
MODEM
Asymmetric Digital Subscriber Line (ADSL)
