Information About G.8275.1
The IEEE 1588-2008 standard, in which the Precision Time Protocol (PTP) is defined, allows for separate profiles to be defined in order to adapt PTP for use in different scenarios. ITU-T G.8275.1 with full timing support from the network is a PTP profile for use in telecom networks where phase or time-of-day synchronization is required and where each network device participates in the PTP protocol and provide PHY-layer frequency support.
Synchronization Model
The model adopted for G.8275.1 is “hop-by-hop” synchronization. Each network element on the path from master to slave synchronizes its local clock to upstream devices and provides synchronization to downstream devices.
There are three types of clocks allowed in G.8275.1:
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Telecom Grandmaster (T-GM): A telecom grandmaster provides timing for other devices in the network, and is usually connected to a primary reference time source, such as a GPS antenna. It does not synchronize its local clock to other network elements.
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Telecom Time Slave Clock (T-TSC): A slave clock synchronizes its local clock to another PTP clock, and does not provide synchronization via PTP to any other devices.
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Telecom Boundary Block (T-BC): A telecom boundary clock synchronizes its local clock to a T-GM or upstream T-BC, and provides timing information to downstream T-BCs or T-TSCs. If at a given point in time there are no higher-quality clocks available to a T-BC to synchronize to, it may act as a grandmaster. G.8275.1 introduces the concept of a virtual port on the T-BC. A virtual port is an external frequency, phase and time input interface on a T-BC, which can participate in the source selection. Sumatra has gnss module on the timing card. When gnss is locked we can configure the T-BC to have a virtual port which takes time, phase and frequency from GNSS.
Non-participant devices (devices that merely forward PTP packets) and PTP transparent clocks are not permitted.
PTP is used to provide phase/time-of-day synchronization throughout the network. It is expected (although not required) that SyncE will be used in G.8275.1 deployments to provide better frequency stability for the phase/time-of-day synchronization (so called “hybrid mode”).
PTP Domains
The allowed PTP domain numbers for use within G.8275.1 networks are in the range 24-43, inclusive. The default domain is 24.
PTP Messages and Transport
The following PTP transport parameters are defined in G.8275.1:
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Multicast PTP over Ethernet must be used. Either the forwardable multicast MAC address (01-1B-19-00-00-00) or the non-forwardable multicast MAC address (01-80-C2-00-00-0E) may be used. The MAC address in use is selected on a per-port basis via configuration.
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Either one- or two-step clock mode may be used.
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Two-way PTP operation is required, in order to allow phase/time-of-day delivery. The Delay-request-response mechanism is used for propagation delay measurement; the peer-delay mechanism is not used.
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The minimum packet rate for Announce messages is 8 packets-per-second. For Sync, Follow-Up, Delay-Req and Delay-Resp messages, the minimum rate is 16 packets-per-second.
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Signaling and management messages are not used.
 Note |
G8275.1 is not supported for sub-interface, dot1q and port-channel. |
Best Master Clock Algorithm
G.8275.1 specifies an alternate best master clock algorithm (BMCA), used by each device to select which clock (if any) to synchronize to, and to decide the port states of its local ports.
The following new parameters are defined as part of G.8275.1’s alternate BMCA:
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“notSlave” flag: The notSlave flag is a per-port, configurable Boolean value which indicates whether a port can be placed in slave mode. If this is set on one of a PTP clock’s ports, the clock will never synchronize to a clock received on that port.
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Local priority: Local priority is a per-port configuration item which is used as a tie-breaker when a PTP clock is selecting between clocks received on different ports, within the scope of a single network element. The network element’s local clock also has a configurable local priority.
The G.8275.1 BMCA’s clock comparison algorithm is based on the following parameters:
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Clock Class: The profile defines a set of clock-classes for use by compliant clocks. The clock class selected depends on the type of clock, the clock’s traceability and holdover status.
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Clock Accuracy: The following clock accuracy values are used by G.8275.1:
0x21: A T-GM locked to a PRTC uses this value.
0xFE: A T-GM in holdover, or a T-BC, uses this value.
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Offset Scaled Log Variance: The following offset scaled log variance values are used by G.8275.1:
0x4E5D: A T-GM locked to a PRTC uses this value.
0xFFFF: A T-GM in holdover, or a T-BC, uses this value.
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Priority 2: Used as in the original 1588v2 BMCA. Note that priority 1 is not used.
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Local Priority: Used as described above.
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Clock Identity: The clock identity is used as a tie-breaker between different clocks (as in the original 1588v2 BMCA).
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Steps Removed: The steps removed value is used to select between different ports which are receiving the same clock (as in the original 1588v2 BMCA).
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Port Identity: The port identity is used as a tie-breaker between different ports on the same clock.
A G.8275.1-compliant clock ignores the following values in received Announce messages:
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The alternate master, unicast and the profile-specific members of the flag field.
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The control field.
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Priority1
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