In fig, same
link is used as in FDM. However, here the link is shown sectioned by time
rather than
frequency In TDM
fig, portions of
signals 1, 2,
3 and 4
occupy the link
sequentially
¾ Implementation of TDM
TDM
can be implemented in two ways:
-Synchronous
TDM
-Asynchronous
TDM
Synchronous TDM
o The
term synchronous has
a different from
that used in
other areas of
telecommunication
o Here
synchronous means that
MUX allocates exactly
the same time
slot to
each
device at all device whether or not the device has any thing to transmit
Synchronous TDM Example
o Time slot A for example is assigned to
device A alone and cannot be used by
any
other device
o Each time its allocated time slot comes
up a device has the opportunity to send
a
portion of its data
o If
a device is
unable to transmit
or does not have
data to send
time slot
remains
empty
Synchronous TDM Frames
o Time slots are grouped into Frames
o A
frame consists of one
complete cycle of
Time slots including
one or more
slots
dedicated to each sending device
o In
a system with
'n' I/p lines,
each frame has
atleast 'n' slots
with each slot
allocated
to carrying data from a specific I/p line
o If all the I/p devices sharing a link are
transmitting at the same data rate,
each
device
has 1 timeslot per frame
o However it is possible to accommodate
varying data rates
o A TX with two slots per frame will arrive
twice as quickly as one with 1 slot
per
frame
o The
time slots dedicated
to a given
device occupy the
same location in
each
frame
and constitute that device's channel
o In
figure, we have
5 I/p lines
Multiplexed onto a
single path using
synchronous
TDM
o In this example all of the I/p's have the
same data rate, so the number of time
slots
in each frame is equal to the number of I/p lines
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