User's Manual

64 ALFOplus 24GHz (North America) - Release 01.05.0x - MN.00395.E - 004
7.15 G.8264 SSM – SYNCHRONISATION STATUS MESSAGE
The SSM is a protocol that transmits the quality of the synchronization message throughout the network
(G.781).
In the synchronization network the transmission of the quality of the clock allows the network to be scal-
able and to provide redundancy. In case of failure of SIAE equipment or cable, it is helpful to provide re-
dundancy for clock propagation in order to avoid synchronization loops.
Synchronisation loops could happen after a failure when two equipment are synchronising each other on
the same physical connection. If this happens the equipment will not present any alarm on the synchroni-
zation but will generate and propagate a not reliable clock. The quality of the clocks that are propagated
are:
• PRC: Primary reference Clock – Best quality clock reachable (Cesium Clock)
• SSUT: Synchronization Supply Unit Transit (Rubidium Clock)
• SSUL: Synchronization Supply Unit Local
• SEC: SDH Equipment Clock (Crystal Clock)
• DNU: Do not Use – This signal informs the receiver to do not use this clock
Here above are listed in order from the better quality clock (PRC – Primary Reference Clock) to the worse
quality clock (DNU – Do Not Use). The better the quality is the more time can the SIAE equipment stay in
holdover (Internal Clock) without a degradation of the payload. In other words, better is the quality, longer
can remain reliable the clock in holdover cases.
The DNU quality is always propagated on the source on which the equipment is locked for synchronization.
In this way the Loops of synchronization are easily avoided.
Unless the user forces the CK quality input/output, SIAE equipment reads the quality present in the input
interfaces. This means that, unless there is some user’s modifications, SIAE equipment propagates the
quality of the clock as it is. I.e. the output quality is the same as the input quality.
E.g.: If in the input interface (POC Site) there is a quality of SSUT of the CK signal, SIAE equipment will
pass throughout the network this synchronization signal with a quality message of SSUT.
In case the SIAE equipment is in Holdover (internal clock) it changes the quality of the synchronization to
“SEC” quality. With SSM enabled, SIAE equipment will choose the sources of the synchronization based on
the better quality received.
In general, the equipment selects the synch source with the following criteria:
• it chooses the sources with the highest quality
• if more than one source have the same quality, the one with the highest priority is selected (i.e. the
ones that have the smaller priority value, from 1 up to 9)
• in any case, if a DNU quality is received on the highest priority source, this latter is discarded and
the equipment selects an alternative source.
Example 1
: if one SIAE equipment has the two following sources of synchronization, with the same priority:
a. LAN1 with quality SEC (Priority 1)
b. Radio with quality SSUT (Priority 1)
In case the sources have the same priority, the SIAE equipment will switch the source of synchroniza-
tion to the best quality, in this case (b) the Radio with SSUT quality.
Example 2
: if one SIAE equipment has the two following sources of synchronization, with the different qual-
ity and different priority:
a. LAN1 with quality SEC (Priority 1)
b. Radio with quality SSUT (Priority 2)
In case the sources have different priority and quality, the SIAE equipment will switch the source of
synchronization with higher quality, in this case (b) the Radio with Priority 2 and quality SSUT. Howev-
er, if a DNU quality would be received on Radio, the SIAE equipment will switch the synch source to
LAN1 interface.
Example 3
: if one SIAE equipment has the two following sources of synchronization, with the same quality:
a. LAN1 with quality SSUT (Priority 1)