5.26. REFTA Synchronization 
The synchronization of TDM telecommunication
networks has become very important with their digitalization. Voice,
fax and modem signals are digitalized and transferred in real time. If
the clock frequencies for digital coding of the A-side and for decoding
on the B-side are not synchronous a bit slip occurs after a certain time,
causing the loss or doubling of data. The resulting deterioration in
quality varies on the basis of the service. Depending on the difference
in frequency, the bit slip occurs after a certain time, i.e. at a certain
bit slip rate and leads to total loss of service. Frequency synchronization
can be achieved in various ways. The most cost-effective is master/slave
synchronization, a procedure employed worldwide.
As transmission is asynchronous in Voice over Internet Protocol (VoIP),
there is no need for synchronization plans here. However, the techniques
specified here still apply to the remaining TDM connections with the
result that multiple island networks often have to be considered. Each
individual IPDA can have its own REFTA.
The following two tables are provided to aid understanding. The first
shows the effect of a bit slip on the quality of the required service.
The second shows the space of time with regard to the frequency difference.
Table 14. Effect of an individual bit slip
|
Service
|
Effect
|
|
Encrypted data
|
Without subsequent ARQ procedure no data can
be decoded after the bit slip
|
|
Video
|
The picture is frozen for a few seconds, the
sound produces a loud "crack"
|
|
Digital data
|
Data is lost or doubled,
the frame must
be repeated,
data throughput reduced
|
|
Fax
|
4 to 8 scan lines lost (strips)
sometimes
loss of connection
|
|
Modem
data transfer within voice band
|
Transmission errors for a duration of 10 msec
to 2 sec,
sometimes loss of connection depending on the transfer
rate
|
|
Speech compressed
|
Speech distorted, cracking sounds
|
|
Voice
|
Sometimes a "crack"
|
These tables show how quickly one small deviation in frequency can
lead to considerable loss in quality and to the breakdown of the connection.
It is possible to deduct the frequency difference and the most probable
error cause from the interval between bit slips. Frequency differences
of 10 to the power of minus nine and 10 to the power of minus eight are
caused by free-running local exchanges or tracking oscillators. Frequency
differences of 1 ppm have been noticed with smaller ISPBXs in hold over/free-run
mode. Frequency differences greater than 5 ppm are caused by free-running
transmission devices, i.e. it is likely that a synchronization chain
is not thoroughly closed. Considerable frequency differences occur if
regulation loops are caused. However, it can take several days for a
regulation loop to result in an excessive frequency difference.
In the following table, E1 connections have a 125-µs buffer and
30 B channels.
T0 connections have a 24-µs buffer and 2 B channels here.
Table 15. Interval between two bit slips
|
Frequency departure
delta
f/f
|
Interval between
two bit
slips
|
||
|
absolute
|
in Hz (at 2 MHz)
|
for E1
|
for T0
|
|
50 ppm
|
100
|
2.4 sec
|
0.48 sec
|
|
10 ppm
|
20
|
12.5 sec
|
2.4 sec
|
|
5 ppm
|
10
|
25 sec
|
4.8 sec
|
|
1 ppm
|
2
|
2 min
|
24 sec
|
|
1 x 10-7
|
0.2
|
20 min
|
4 min
|
|
1 x 10-8
|
3 1/2 h
|
40 min
|
|
|
1 x 10-9
|
1 day
|
7 h
|
|
|
1 x 10-10
|
2 weeks
|
3 days
|
|
|
1 x 10-11
|
4 months
|
1 month
|
|
|
As long as the interface is synchronous
and "jitter" and "drift" are within the standard range, no bit
slip occurs.
|
|||
As VoIP transmissions are digital but asynchronous, every connection
must incorporate a buffer to make up for minor clock deviations and larger
delay fluctuations, known as jitter. The buffer size is no longer in
the µsec range but the msec range.