The 7705 SAR supports Recommendation ITU-T G.8275.1 and ITU-T G.8275.2, which specify the architecture that allows the distribution of time and phasing. ITU-T G.8275.1 supports full timing support from the network and ITU G.8275.2 supports partial timing support and assisted partial timing support. It is assumed that these profiles will be used in well-planned cases where network behavior and performance can be constrained within well-defined limits, including limits on static asymmetry. When configured for the G.8275.1 or G.8275.2 profile, the 7705 SAR can operate as boundary clock, an ordinary master clock, or an ordinary slave clock.
Support for control of static asymmetries is not yet available with G.8275.2 and assisted partial timing support.
When the 7705 SAR is configured for the G.8275.1 or G.8275.2 profile, it uses an alternate BMCA for best master clock selection. This BMCA includes a PTP dataset comparison that is defined in IEEE 1588-2008, but with the following differences:
the priority1 attribute value is removed from the dataset comparison
the master-only parameter value must be considered
multiple active grand master clocks are allowed; therefore, the BMCA will select the nearest clock of equal quality
a port-level local-priority attribute value is used to select a slave port if two ports receive an Announce message. This attribute is used as a tiebreaker in the dataset comparison algorithm if all other previous attributes of the datasets being compared are equal.
the local-priority parameter value is considered for the default dataset
The ITU-T G.8275.1 and G.8275.2 profiles have the following characteristics.
The default domain setting is 24 for G.8275.1; the allowed range is 0 to 255.
The default domain setting is 44 for G.8275.2; the allowed range is 0 to 255.
Both one-step and two-step clocks are supported on slave capable PTP ports.
G.8275.2 supports IP encapsulation.
G.8275.1 supports IP encapsulation and Ethernet encapsulation. When Ethernet encapsulation is used, the following points apply.
Ethernet multicast addressing is used for transmitting PTP messages. Both the non-forwardable multicast address 01-80-C2-00-00-0E and forwardable multicast address 01-1B-19-00-00-00 are supported.
Virtual local area network (VLAN) tags within Ethernet frames carrying PTP messages are not supported. When a PTP clock receives a PTP message within a frame containing a VLAN tag, it discards this frame. A PTP clock that is compliant with the profile described in Recommendation ITU-T G.8275.1 must comply with IEEE 1588 – 2008 Annex F.
Synchronization messages are sent at a rate of 16 packets/s; Announce messages are sent at a rate of 8 packets/s.
On the 7705 SAR, the priority1 value is set to the default value (128) and cannot be changed.
On the 7705 SAR, if the clock-type parameter is set to ordinary slave, the priority2 value is set to the default value (255) and cannot be changed.
For further details, refer to Recommendation ITU-T G.8275.1/Y.1369.1 and ITU-T G.8275.2/Y.1369.2.