White Rabbit devices calibration
White Rabbit link established between two devices is characterized with
hardware delays
and fiber propagation latencies presented in figure
1.
Figure1: White Rabbit link delay model*
Each WR Master and WR Slave has some constant transmission and reception delays:
- Δ TXM - transmission delay of WR Master
- Δ RXM - reception delay of WR Master
- Δ TXS - transmission delay of WR Slave
- Δ RXS - reception delay of WR Slave
They are the summed result of SFP transceiver’s, PCB traces’ and
electronic components’
delays as well as the delays inside FPGA chip. Additional reception
delay is also caused
on both sides by aligning recovered clock signal to the inter-symbol
boundaries of data
stream. It is called bitslide value and is marked in figure 1 as:
- ε M - bitslide of Master's RX path
- ε S - bitslide of Slave's RX path
Except from hardware delays, packets transmitted in fiber are affected
with propagation
latencies in both directions:
- δ MS - Master-to-Slave fiber latency
- δ SM - Slave-to-Master fiber latency
Those fiber propagation latencies are not equal since different light
wavelengths are used
to communicate simultaneously in both directions using single fiber
(Wavelength Division
Multiplexing). For estimating fiber asymmetry White Rabbit uses α
parameter, which
is constant for given wavelengths and type of fiber.
WR users should perform the WR Calibration
Procedure before deploying WR
hardware to determine correct values of delays and fiber asymmetry
factor.
Default calibration values for WR Devices
White Rabbit Switch
Port No. | Hardware version | Firmware version | Δ TX | Δ RX |
1 - 4 | 3.3 | 3.3 |
White Rabbit PTP Core
Hardware version | Firmware version | Δ TX | Δ RX | α |
SPEC v4 | 2.1 |