System information

FCD 901 48
Issue R2A, 07.2009
XMP1 Release 5.5 System Description
Synchronous Equipment Timing Source SETS
Page 3-12 Proprietary Information Aastra
3.6.1 Synchronous Equipment Timing Source SETS
In the Synchronous Digital Hierarchy, the clock generator is referred to as
SETS (Synchronous Equipment Timing Source). In the XMP1 system, the
SETS of the SDH expansion is located on the SCU module.
It provides the following features and functions:
• Clock generation and clock recovery according to G.813.
• When doubling the SCU module, the SETS is available redundantly
(1+1 Protection, both SETS active).
• All reference timing sources are monitored (the failure or return of a
clock is signalled to the controller; in case of a failure of the active
reference timing source, it will switch over to a standby timing source).
Regarding the monitoring function and resulting measures, the SETS
distinguishes between transmission defects and clock defects.
• Transmission of alarms and status messages to the Network
Management System or Operator Terminal.
• Operating modes of the clock generator (SETS):
— Tracking mode (synchronization to an external reference signal)
— Holdover mode (if reference clock is too bad)
— Free-run mode (startup mode).
3.6.2 Synchronous Status Message SSM
To permit easy automatic protection switching, the SETS of the SDH
expansion supports the use of the Synchronization Status Message SSM.
The SSM indicates the quality of the timing source used as basis for a
certain signal.
In case of STM-N signals, the Synchronization Status Message SSM is
transmitted in the S1 byte of the Section Overhead (SOH). S1 byte
transmission can be switched off.
The SSM can assume the values indicated in the following table.
In addition, it is also possible to permanently assign a certain SSM value to
incoming STM-N signals configured as reference timing source,
irrespective of the SSM value supplied with these signals.
Table 3.C: SSM values specifying the clock quality
SOH BYTE S1,
BIT 5 TO 8
CLOCK QUALITY OF THE
SELECTED REFERENCE CLOCK
0000 Quality unknown
0010 Clock source: G.811 / PRC
0100 Clock source: G.812 transit
1000 Clock source: G.812 local
1011 Clock source: G.813 / SETS
1111 “Don´t use for sync”