Precision Time Protocol (PTP)

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The Precision Time Protocol specified in IEEE standard 1588v2 is the latest in packet-based timing technology. Originally designed to provide precise timing for critical industrial automation applications, it is now providing the highest level of accurate frequency, phase, and time of day to wireless backhaul networks. PTP overcomes the Ethernet NTP latency and delay variation issues, providing an unprecedented accuracy in the nanosecond range. The effects of network latency are greatly reduced by using a technique whereby the master and slave communicate with one another to cancel out a measured delay between the two nodes. The IEEE1588 standard makes several assumptions about the network being used (e.g. multicast support) but the key assumptions that affect clock accuracy are:

  • The transmission delays are almost constant over time (or at least change slowly). The transmission delays are symmetrical between master and slave (i.e. time to travel from master to slave is the same as from slave to master).
  • Non-compliant devices require additional equipment
  • When carried over a Carrier Ethernet Network (CEN), 1588v2 requires a dedicated CoS or even a dedicated EVC – with stringent requirements on Frame Loss Ratio, Frame Delay and Inter-frame Delay Variation.