Synchronization Methods; Hierarchical Synchronization - Nortel Meridian 1 Administration And Maintenance

Page 140 of 492

Synchronization Methods

Hierarchical synchronization

553-3011-311 Standard 7.0
Chapter 4 — Network clocking
There are two common methods of maintaining timing coordination between
switching systems.
a Plesiochronous operation: nodal clocks run independently (free
run) at the same nominal frequency. There will be frequency
differences between clocks resulting in frame slips. The magnitude
of frame slips will be directly proportional to the frequency
difference. Slips are inevitable but can be minimized by using very
stable clocks and elastic stores or buffers. These buffers will be
capable of absorbing a certain number of data bits to compensate for
slight variances in clock frequencies.
b Mesochronous operation: nodal clocks are continuously and
automatically locked to an external reference clock. With this
method, frame slips can be eliminated if elastic stores are large
enough to compensate for transmission variances. Mesochronous
operation should be virtually slip free.
Whenever possible, the Meridian 1 PBX will operate in the
Mesochronous mode by configuring the Clock Controller circuit
cards to lock onto an external reference source. The above statement
is true unless the Meridian 1 is used as a master in an
independent/private network. An independent network has no
digital links to a higher Node Category.
In an isolated private network, the Meridian 1 clock controller can
operate in free run mode and act as a master clock to be tracked by
other PBX systems in the private network.
The figure on the following page provides a general view of a Digital
Network Synchronization including the four stratum levels Node Categories
of clocks (Stratum 1 being the highest--most accurate to Stratum 4 being the
lowest). Meridian 1 clocking meets Node Category E--Stratum 4
requirements. Also shown are ways of providing a Secondary Clock Source
while preventing timing loops.
May 1999