System Management
7210 SAS M, X Basic System Configuration Guide Page 159
Synchronization
The 7210 SAS M implements distribution of timing information through the following methods:
• Adaptive Clock Recovery(ACR)
• Line timing mode
Adaptive Clock Recovery
Adaptive Clock Recovery (ACR) is a timing-over-packet technology that transports timing
information via periodic packet delivery over a pseudowire. ACR is used when there is no other
Stratum 1 traceable clock available. ACR is supported on the 7210 SAS-M T1/E1 ports.
ACR technique utilizes the packet arrival rate of a TDM pseudowire within the 7210 SAS to
regenerate a clock signal. It does not incur any additional equipment cost. The nodes in the
network that are traversed between endpoints do not need special ACR capabilities, but the TDM
pseudowire is transported over Layer 2 links, therefore, the packet flow is susceptible to PDV.
A good ACR performance can be derived by the following recommendations:
• A packet rate of 1000 pps to 4000 pps is recommended, as lower packet rates cause ACR
to be more susceptible to PDV in the network.
• Limit the number of nodes traversed between the source-end and the ACR-end of the
TDM pseudowire.
• Enable QoS in the network with the TDM pseudowire enabled for ACR classified as NC
(network control).
• Maintain a constant temperature, as temperature variations affect the natural frequency on
the internal oscillators in the 7210 SAS.
• Ensure that the network does not contain a timing loop when it is designed.
Line Timing Mode
Line timing from Synchronous Ethernet port provides the best synchronization performance
through a synchronization distribution network. Line timing mode derives the timing information
from the Ethernet ports. This mode is immune to any Packet Delay Variation (PDV) occurring on
Layer 2 or Layer 3 links. Line timing is supported on the Ethernet SFP ports with SFPs that
support Synchronous Ethernet.
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