Industrial Managed
Ethernet Switch – EH9711
User Manual
Page
104
of
223
Network elements use synchronization status messages (SSM) to inform the neighbouring elements about the quality level
(QL) of the clock. SSM is used by non-Ethernet interfaces, such as the optical interfaces and the SONET/T1/E1 SPA framers.
SSM functionality provides the following key benefits.
•
Prevents timing loops
•
Provides fast recovery when a part of the network fails
•
Ensures that a node derives timing from the most reliable clock source
To maintain a logical communication channel in synchronous network connections, Ethernet relies on a channel called the
Ethernet synchronization messaging channel (ESMC), based on IEEE 802.3 organization-specific slow protocol standards.
ESMC relays the SSM code that represents the quality level of the Ethernet equipment clock (EEC) in a physical layer.
The ESMC packets are received only for those ports configured as clock sources and transmitted on all SyncE interfaces in the
system. These packets are then processed by the clock selection algorithm and used to select the best clock. The transmitted
frame is generated based on the QL value of the selected clock source and sent to all the enabled SyncE ports.
The clock selection algorithm selects the best available synchronization source from the nominated sources. The clock selection
algorithm has a non-revertive behaviour among clock sources with the same QL value and priority. It always selects the signal
with the best QL value. The following parameters contribute to the selection process.
•
Quality level
•
Signal fail (QL-FAILED)
•
Priority
•
External commands (manual, auto-revertive, and so on)
Figure 2.85 illustrates the
SyncE Configuration
webpage. There are five parts on this webpage which are
Clock Source
Nomination and State
,
Clock Selection Mode and State
,
Station Clock Configuration
and
Clock hardware
,
SyncE Ports
,
and
PTP Ports (8265.1)
.
Figure 2.85 Webpage to Configure SyncE
For each possible clock source, the user can configure the parameters under
Clock Source Nomination and State
part.
Table 2.70 summarizes the descriptions of parameters of Clock Source Nomination and State under SyncE.
