TDM CONCEPT
👉 What is TDM?
* When Time Division Multiplexing (TDM) technique is used, multiple signals share the same channel by taking turn transmitting.
* Data is broken up into frames and assigned to time slots. (Like cars merging in a lane).
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* This technique is primarily used for digital data.* Each signal uses the entire bandwidth of the channel when transmitting.
* On the receiving end, the demultiplexing process requires synchronization of the frames. This is often accomplished through a sync pulse.
👉 What is STDM?
* A process of time division multiplexing* Time slots on the time division multiplexing (TDM) medium between Statistical time division multiplexing (STDM) multiplexers are dynamically allocated.
* STDM multiplexers are known as asynchronous multiplexers to differentiate them from the synchronous time division multiplexers.
👉 How does STDM work?
* To accomplish dynamic allocation of any input to any time slots, the TDM multiplexer places a header on the data that each input channel has ready to transmit.
👉 Comparison Between Synchronous TDM & Asynchronous TDM
|
PARAMETER NAME |
SYNCHRONOUS TDM |
ASYNCHRONOUS TDM |
|
TIME SLOT ALLOCATION |
Multiplexer allocates
same time slots to each device. |
Multiplexer does not
allocate same time slots to each device. |
|
NUMBER OF SLOTS PERFRAME |
Equal to
number of input lines. |
Less than
number of input lines. |
|
CAPACITY |
There is no guarantee
that full capacity link is used. |
There is guarantee
that full capacity link is used. |
|
TOTAL SPEED |
Total speed
of input lines cannot be greater than capacity of path |
Total speed
of input lines can be greater than capacity of path. |
|
DEVICE USAGE |
Usage of devices is less. |
Usage of devices
is more. |
|
NUMBER OF TIME SLOTS |
Number of
time slots in a frame as always based on number of input lines. |
Number of
time slots in a frame as always based on statistical analysis of number
of input lines that are likely to be transmitting at any given time. |
|
TIME SLOTS |
Time slots are fixed
and pre- defined. |
Time-slots are not pre-defined. |
👉 ADVANTAGES
&
DISADVANTAGES 👈
👉 Advantages of STDM
* More efficient bandwidth use* Frame can contain control information
* Packet can be of varying sizes
👉 Disadvantages of STDM
* Complex structure* Very expensive
👉 Applications
* Statistical multiplexing is used to allow several video, audio and data streams of different data rates to be transmitted over a bandwidth-limited channel.