- The basic idea of the
Time Division multiplexing
technique:
- Divide the
time into
slots:
- The time slots are
shared among
multiple users:
- Divide the
time into
slots:
- The 2 variants of the
TDM technique:
- Synchronous TDM
- Asynchronous TDM
- The Synchronous TDM
technique:
- Time is
divided into periods:
- Each period is
further divided into
time slots:
- A user
is assigned
one or more time slots
in every period:
The slot assigned is always in the same position inside a period
Example:
- Time is
divided into periods:
- Advantage of
Synchronous TDM:
- Simple
(= easy) to
implement
- Synchronous
TDM can provide
performance guarantee
- Asynchronuous TDM
can provide
a fixed waiting time
guarantees
(The waiting time is at most 1 period !!!)
Because the Synch. TDM method works like a converyer belt:
A waiting message will be guaranteed to be transmitted during the next period !!!
- Asynchronuous TDM
can provide
a fixed waiting time
guarantees
- Simple
(= easy) to
implement
- Disadvantage of
Synchronous TDM:
- Time slots
is wasted if
the user
has no data
to transmit.
- Because a time slot can only be used by that specific user !!!
Example: if user 2 idle, its slows are wasted
- Time slots
is wasted if
the user
has no data
to transmit.
- The Asynchronous TDM
technique:
- Time
is divided into
slots:
- The slots are
shared by
all active users:
Note:
- A slot will only be wasted, if all users do not have data to transmit,
Example:
- Time
is divided into
slots:
- Implementation note:
- I used colors to
indicate the sender of the
packets....
- In reality, the
packet
are distinguished
by a source/destination ID:
- I used colors to
indicate the sender of the
packets....
- Advantage of
Asynchronuous TDM:
- Efficient use
of the
transmission capacity.
- Asynchronuous TDM will only waste transmission capacity (= slots) when all senders are idle
- Efficient use
of the
transmission capacity.
- Disadvantage of
Asynchronuous TDM:
- It's much harder to provide
service guarantees
because:
- The time
that a user
gets a transmission slot
depends on the
current number of
active users
- Asynchronuous TDM can not provide a fixed waiting time guarantee
Note:
- Asynchronous TDM
can provide a
soft guarantee on
delay (waiting time)
using
flow control
Flow control = controlling the transmission rate of data over a communication link
- The time
that a user
gets a transmission slot
depends on the
current number of
active users
- It's much harder to provide
service guarantees
because: