5.26.5.2. Performance for Network Operation/Normal Clock Generators 
Hierarchical levels are important when choosing clock generators because
systems with multiple successive layers encounter a large amount of
transit traffic. Therefore greater clock stability must be ensured to
counteract the increased probability of malfunctions, see ISO standard
11 573 or ETSI or TIA standards. Clock generators for transit systems
(e.g. MTSCG+RTM type + clock box in class 68 or 78), moreover, have a
longer time constant than standard clock generators (e.g. MTSCG/RTM type
in class 38) which in turn have a longer time constant than clock generators
for smaller systems.
IPDA (HG3550) uses class-38 clock generators which can no longer have
an external clock box but which can be configured with a "frontref" as
of OpenScape 4000 V4 (R1). This is important in connection with Cordless
and ISS (i.e. seamless handover) because the time information for ISS
can also only be used in the case of precise clocking from the GPS.
For reasons of stability, the time constants in a network should be
progressively shorter.
The "Networking clock generator" feature (class 68 or 78) provides
a system with a more accurate frequency in free-run mode, a more stable
hold over mode for a longer period of time, a longer time constant for
the PI regulator, a more thorough check of the reference frequencies,
faster recognition of reference frequency changes and a trimming range
smaller than with the standard clock generator (class 38). For the "Networking
clock generator" feature to work, an AECB, an AICB or an SASE must be
connected to the front panel of the MTSCG board or the MCM module in
the cPCI controller. The clock generator can then be set to the corresponding
class using the AMO REFTA. Class 38 corresponds to the standard clock
generator. Classes 68 and 78 are networking clock generator classes,
the class 68 corresponding to the previous NCG-X and class 78 providing
a further limitation of the trimming range, thus making the system resemble
an SASE, even replacing an SASE in some cases.
Table 16. Quality differences for clock generators
|
Values
|
Standard clock generator
|
Networking clock generator
|
|
Free-run,
(delta f/f) for 15 years
|
10 x 10 -6
|
4,6 x 10 -6 .. 1 x 10 -10 1)
|
|
hold over mode
(initial offset)
|
4 x 10 -9
|
2 x 10 -9
|
|
hold over mode
(3 days)
|
1 x 10 -6
|
3 x 10 -9
|
|
hold over mode
(10 days)
|
1 x 10 -6
|
8 x 10 -9
|
|
Time constant
(l-part)
|
approx. 15 minutes
|
approx. 1 hour
|
|
Trimming range (absolute)
|
min. +/- 32 ppm
|
min. +/- 10 ppm
|
|
Trimming range for ATM
|
min. +/- 20 ppm
|
min. +/- 5 ppm
|
|
Trimming range for class 78
|
- -
|
min. +/- 0.1 ppm
|
1) depending on the external clock box used