• What is multiplexing?
— Frequency-Division
Multiplexing
— Time-Division
Multiplexing (Synchronous)
— Statistical
Time-Division Multiplexing, etc.
Different type of Multiplexers
• Frequency division
multiplexing (FDM)
·
Time
division multiplexing (TDM)
Frequency Division Multiplexing
(FDM)
Frequency
spectrum is divided among multiple logical channels.
• Each user can have access to its own assigned frequencies or
logical channels at all the time the individual user is active.
• Sends the signal in several distinct frequency ranges.
• Used in cable TV.
• Carry multiple video channel on a single cable
• Each signal is modulated on
the different carrier frequency and the carrier frequencies are separated by
guard‐bands.
FDM
System
FDM of Three
Voiceband Signals
Disadvantages of
FDM
• The problem with the FDM is that it cannot utilize the full
capacity of the system.
• We need to ensure that the adjacent band do not overlap each other otherwise signal mayefect
other Signal.
Time Division Multiplexing
(T DM)
• In time division multiplexing the time is divided into multiple
smaller units called slots and each user is given a slot to transmit it signal.
• Each user have the entire bandwidth of the channel for short
duration of time.
• T‐1 and ISDN telephone lines are common examples of time
division multiplexing.
Used for
digital signals or analog signals carrying digital data.
• Data rate of the transmission medium exceeds the required data
rate of all the signals.
• Clock synchronization is one of the major issue in
TDM systems as
a small mismatch in the timing may ruin the utility of overall system.
An
important feature of sampling process is a conservation of time.
• In principle, the communication link is used only at the sampling
time instances.
• Hence, it may be feasible to put other message’s samples
between adjacent samples of this message
on a time‐shared basis.
•
This
forms the time‐division multiplex (TDM) system.
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