Grand Master Clock Holdover via Reverse Timing Transfer
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Solution Overview
Problem
In packet-switched communications networks, particularly in Precision Timing Protocol (PTP) systems, the Grand Master clock losing its GPS reference leads to holdover mode, causing significant time delays and potential outages in mobile telephony systems due to slave clock drift, as they may need to switch to an alternate GM, resulting in unacceptable application outages.
Innovation Solution
The method involves characterizing the properties of the packet-based communication link between a server clock and a client clock using a time reference available at both, allowing the Grand Master to receive additional timing information from alternate devices with access to GPS, thereby reducing the need for slave devices to switch over to an alternate GM and minimizing network outages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the Grand Master clock loses GPS reference and enters holdover mode, then the local oscillator maintains timing, but slave clocks drift significantly requiring switch to alternate GM causing network outage
Solution Approach 1:
The system pre-characterizes the communication link properties (delay, jitter, asymmetry) using bidirectional time-stamped packets while GPS is available. This preliminary characterization is stored and later used during holdover mode to compensate for timing drift, eliminating the need for slave clocks to switch to alternate GM and preventing network outage.
Solution Approach 2:
The system uses bidirectional time-stamped packet exchanges to continuously monitor and characterize link properties. During holdover, the Grand Master uses the pre-characterized link parameters to generate compensated timing information, creating a feedback mechanism that maintains accuracy without external GPS reference.
2Reliability
If slave clocks switch to alternate Grand Master when primary GM loses GPS, then timing reference is restored, but significant time delay occurs during switching
Solution Approach 1:
The system pre-characterizes the communication link properties (delay, jitter, asymmetry) using bidirectional time-stamped packets while GPS is available. This preliminary characterization is stored and later used during holdover mode to compensate for timing drift, eliminating the need for slave clocks to switch to alternate GM and preventing network outage.
3Duration of action of stationary object
If the Grand Master uses local oscillator in holdover mode, then timing continuity is maintained, but accuracy degrades over time
Solution Approach 1:
The system pre-characterizes the communication link properties (delay, jitter, asymmetry) using bidirectional time-stamped packets while GPS is available. This preliminary characterization is stored and later used during holdover mode to compensate for timing drift, eliminating the need for slave clocks to switch to alternate GM and preventing network outage.
Solution Approach 2:
The system uses bidirectional time-stamped packet exchanges to continuously monitor and characterize link properties. During holdover, the Grand Master uses the pre-characterized link parameters to generate compensated timing information, creating a feedback mechanism that maintains accuracy without external GPS reference.
Data Source
AI summary
A packet network that includes the distribution of timing information (time and frequency) between server and client devices can continue to operate in a holdover mode even when the server loses its primary timing reference based on GNSS. This is achieved by populating the packet network with some client devices that also have access to the same timing reference. These devices are used in a reverse timing transfer mode to provide a hack-up reference to the server and thereby provide a graceful solution to the problem of loss of reference.