Precision Time Protocol - Best Master Clock Algorithm

Best Master Clock Algorithm

The best master clock (BMC) algorithm performs a distributed selection of the best candidate clock based on the following clock properties.

  • Identifier - A universally unique numeric identifier for the clock. This is typically constructed based on a device's MAC address.
  • Quality - Both versions of IEEE 1588 attempt to quantify clock quality based on expected timing deviation, technology used to implement the clock or location in a stratum schema, although only V1 knows a data field stratum. PTP V2 defines the overall quality of a clock by using the data fields clockAccuracy and clockClass.
  • Priority - An administratively assigned precedence hint used by the BMC to help select a grandmaster for the PTP domain. IEEE 1588-2002 used a single boolean variable to indicate precedence. IEEE 1588-2008 features two 8-bit priority fields.
  • Variance - A clock's estimate of its stability based on observation of its performance against the PTP reference.

IEEE 1588-2008 uses a hierarchical selection algorithm based on the following properties in the order indicated.

  1. Priority 1
  2. Class
  3. Accuracy
  4. Variance
  5. Priority 2
  6. Unique identifier (tie breaker)

IEEE 1588-2002 uses a selection algorithm based on similar properties.

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