White Rabbit Calibration
Relative calibration
-
White Rabbit devices
calibration
- provides coarse calibration values for the official releases of WR Switch and WR PTP Core firmware. You can use them as defaults for matching hardware and gateware version. However, to achieve the best possible synchronization results you should perform the full calibration procedure for your WR hardware.
- WR Calibration Procedure
Absolute calibration
Absolute calibration is achieved by measuring the time relationship (Δ TXcal, Δ RXcal) between the external electrical phase planes of the PPS signal and the interface to the Electrical-Optical / Optical-Electrical converter (EO/OE, usually the electrical SFP connector) as shown in Figure 1. Note that EO/OE converters have their own calibration parameters that define the relationship between their electrical and optical phase planes. Calibration of EO/OE converters is not covered on this wiki page.
Figure1: White Rabbit link delay model for absolute calibration*
Absolute calibration enables independent developers and/or vendors to
exchange their WR gear while achieving sub ns timing synchronisation.
Absolute calibrated devices can be used as "golden standards" for the
relative calibration
procedure.
Precision Time Protocol (PTP) packets that are used for the link delay
calculation are timestamped inside WR devices and hence are defined on
the “internal time” scale (the red arrow in Figure 2). Delays exist
between the internal time scale and the electrical interfaces that are
defined on the “external time” scale (the blue arrow in Figure 2).
The external time scale uses the PPS signal as a time reference point
(t=0). The time it takes for a t1 timestamped Tx packet to traverse the
external electrical reference plane is:
- Δ TXcal = A - t 1 = Δ PPS + Δ tx
When applying a loopback from Tx to Rx then:
- Δ RXcal = B - t 4p = Δ PPS + Δ rx
defines the time it takes for a t4p timestamped Rx packet to traverse the external electrical reference plane. Note that t4p includes fine delay phase information.
time-scales.png
Figure2: Time scales and their relations*
-
WR calibration - ASTERICS -
Nikhef
- Aims to provide a method of absolute calibration of all elements of a WR device (preliminary).
For absolute calibration special hardware is needed:
19 May 2016