1. Synchronization
By:
Loren K. Schwappach
Prepared for:
Dr. Santiago
CTU, EE463, Communications 1
October 2010

2. First some quick notes about TDM:
• As the number of independent message
sources increases the intervals allotted to
each source by the commutator is reduced, all
sources must be accommodated into a time
interval equal to the sampling interval.
• Since pulses too short can impair transmit it is
necessary in practice to reduce the number.
In a North American Public Switched
Telephone Network (PSTN) T1 System, TDM is
used to transmit 24 Channels.

3. Synchronization:
• In a PCM system with TDM it is essential that
timing operations at the receiver (exempting
time lost in transmission and regenerative
repeating) closely follow the operations of the
transmitter, this amounts to requiring a local
clock at the receive operating in
synchronization with the transmitter.
• Used in Time Division Multiplexing to lock the
commutator and decommutator and keep the
transmitter and receiver in phase.

4. Synchronization:
• One method of synchronization is to set aside a
code word at the end of a frame (transmitted
every other pulse) and send it via the
Commutator. Thus a clock is created at half the
frame rate and can be used by the receiver to
establish synchronization.
• Re-synchronization is the procedure of examining
code elements one by one by setting the clock
back by one code element until the synchronizing
pulse is detected, this process often includes the
use of buffers.

5. Low-pass
Low-pass
(reconstruction)
Message
(anti-aliasing) TDM system block diagram filters
filters Message
Inputs By Loren K. Schwappach Outputs
1 LPF LPF 1
Synchronized
Pulse Communication Pulse
2 LPF Modulator Channel Demodulator LPF 2
Commutator DeCommutator
N LPF LPF N
Clock pulses Clock pulses
Anti-aliasing filter: Removes non-essential frequencies.
Commutator: Uses electronic switching to (1) take a narrow sample of each of the N input messages at a rate
fs (slightly higher than 2W), (2) sequentially interleave these N samples inside the sampling interval.
Pulse Modulator/Demodulator: Transforms the multiplexed signal into a suitable form. Often requires
equalization to ensure satisfactory operation.
Decommutator: Performs opposite operation of commutator. Operates in synchronization.
Example of Synchronization:
Sync Sync Sync Sync Sync Sync
M1 Element M2 Element MN Element M1 Element M2 Element MN Element
1 0 1 0 1 0

6. Questions:

7. References:
Haykin, S., “Analog and Digital Communications 2nd Edition” John
Wiley & Sons, Haboken, NJ, 2007.