Chapter 5:
Timing
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v3.3-a 12/07/2022
Figure 5:9 - Reference topology with different presets.
•
External Reference Layer:
It includes the devices that will be fed by several external
references (in grey), such as an Atomic Clock or a GNSS receiver, and will receive ToD
(Time of Day) from an NTP server (external or embedded). These devices will act as Grand-
Master (GM) in the timing network and their timing information will be forwarded to all
the timing nodes.
•
The Fan-Out Layer
or Middle Layer: The devices in this layer are mainly dedicated to
spread (fan-out) the timing synchronization to more devices on the down layers. In order
to ensure continuous operation, they can be configured with redundant timing sources
(e.g., BC FO wr0-wr1) or could incorporate the Holdover option (e.g., BC wr0 slave HO).
•
The interoperability Layer:
The devices that belong to this layer are also known as last-
hop devices. Typically, one of these devices is placed per rack cabinet and is in charge of
distributing the ultra-accurate timing provided by the White Rabbit network to other 3
rd
party devices in the cabinet via PTP, via 10MHz/PPS (legacy devices), etc.
This reference topology is a simplified version of a real timing network and the proposed
structure in layers might not be respected: A last-hop device could be connected
directly to the GM or an external GNSS reference could be used as backup in the fan-
out/interoperability layer.
