Frame alignment ensures that PCM frames from different sources entering a digital exchange are synchronized. A frame alignment buffer stores incoming digits and reads them out at the exchange clock rate to align frames. Synchronization networks use reference clocks to control exchange clocks and ensure frames are aligned across the entire network. Unilateral and bilateral synchronization links can control clock frequencies between two exchanges, while double-ended systems can distinguish between frequency drift and propagation delays better than single-ended systems. An integrated digital network uses a synchronizing auxiliary network to link all exchanges.

1. Synchronization
Frame alignment

2. • the PCM frames on all incoming highways must
be exactly aligned.
• incoming junctions come from different places,
their signals are subjected to different delays.
• all exchange clock-pulse generators are in perfect
synchronism,
• there will be time differences between the
starting instants of different PCM frames entering
a digital exchange.

3. PCM junction store the incoming digits in a
frame-alignment buffer

4. • The digits are read into this buffer at the rate, fa, of the
incoming line, beginning at the start of each frame.
• They are then read out at the rate, fb, of the exchange
clock, beginning at the start of the PCM frame of the
exchange.
• The fill of buffer is constant and its level depends on
the phase difference between incoming line system
and the exchange.
• It will also cope with a misalignment that changes
slowly between limits

5. Synchronization network.
• one or two atomic reference clocks control the
frequencies of clocks, of all exchanges in the
network.
• This is called despotic control.
• Synchronizing links may be unilateral or bilateral.
• first case, there is master-slave relationship;
• the clock frequency of the exchange influences
the frequency of the other.

7. • A unilateral sync system is shown in figure 4.9(a)
exchange A is the’ master ‘ and exchange B is the ‘slave
’.
• Exchange B determines the phase difference between
its own clock and that of the exchange A by fill of the
aligner buffer on the incoming link.
• single ended bilateral sync link, as shown in figure
4.9(b) the above decision process is made at each end
of the link.
• As a result, both exchange clocks achieve the same
average frequency.

8. • A disadvantage of single ended unilateral and
bilateral synchronous system is that phase
comparators are unable to distinguish
between phase changes due to frequency drift
and those due to changes in propagation time.
• disadvantage of single ended unilateral and
bilateral synchronous system is overcome by
double ended system as shown in figure 4.9(c)
and (d).

9. • A synchronizing network for an integrated
Digital Network (IDN) is shown in figure 4.10
since this auxiliary network must link all
exchanges in the IDN.