Application Overview
White Rabbit (WR) is a synchronization protocol designed for scientific research facilities, providing sub-nanosecond synchronization accuracy over fiber optic networks. Originally developed at CERN for the Large Hadron Collider control systems, White Rabbit has been adopted by research facilities worldwide including particle accelerators, radio telescopes, and distributed sensor networks. The protocol combines IEEE 1588 PTP with proprietary extensions for physical layer synchronization.
This reference design addresses the timing infrastructure requirements for large-scale research facilities requiring both frequency distribution and precise time synchronization. Applications include particle accelerator timing systems, distributed data acquisition networks, radio telescope arrays, and quantum computing facilities. The architecture supports hundreds to thousands of synchronized nodes across distances from meters to tens of kilometers.
White Rabbit provides deterministic synchronization performance that cannot be achieved with standard Ethernet-based PTP, making it essential for experiments requiring precise temporal correlation across distributed detector systems.
System Architecture
+---------------------------+
| STM-Rb-H |
| Ultra-High Performance |<----------+
| Rubidium Clock | External |
+---------------------------+ Reference|
| |
10MHz | |
| | |
+----------v------+----------+ |
| | |
v v |
+------------------------+ +------------------------+ |
| STW-FT | | STW-RF | |
| Fiber Optic | | White Rabbit |<-+
| Time-Frequency | | Grandmaster | |
| Transfer | +-----------+-----------+ |
+------------------------+ | |
| | |
v v |
+------------------------+ +------------------------+ |
| Remote Sites | | WR Fiber Network | |
| (Up to 80km) | | (1Gbps Full Duplex) | |
| - Accelerometer | | | |
| - Radio Telescope | | +-------+ +-------+ | |
| - Detector Array | | |WR Node| |WR Node| | |
+------------------------+ | +---+---+ +---+---+ | |
| | | | |
| +---+---------+---+ | |
| |WR Switch (WR-SW) | | |
| +-------------------+ | |
+---------------------------+ |
|
+-------------------------------------------------------------+|
| Research Facility Infrastructure ||
+-------------------------------------------------------------+|White Rabbit Protocol Stack
White Rabbit extends IEEE 1588 PTP with:
- DMTD (Delay Measurement Through Token Ring): Two-way link delay measurement for sub-nanosecond accuracy
- Synchronous Ethernet (SyncE): Physical layer frequency distribution
- Phase Transparent Clocks: Hardware-based delay compensation
The combination achieves <1ns synchronization accuracy and <100ps link delay measurement uncertainty.
Key Design Decisions
1. Ultra-High Performance Rubidium Reference
The STM-Rb-H provides hydrogen maser-level stability of ≤3×10⁻¹⁴ at 10,000 seconds averaging. This serves as the local frequency reference for the White Rabbit Grandmaster, ensuring excellent long-term stability while maintaining sub-nanosecond time accuracy.
Decision Rationale: Research facilities require both short-term stability (for data correlation) and long-term stability (for experiment duration). The STM-Rb-H provides both without the operational complexity of hydrogen masers.
2. Fiber Optic Time-Frequency Transfer
The STW-FT provides lossless frequency transfer to remote sites, maintaining <1×10⁻¹⁴/s stability and <40ps time uncertainty over up to 80km fiber runs. This extends the primary reference to distributed facility infrastructure.
Decision Rationale: Large research facilities span kilometers; the STW-FT enables centralized reference generation with accurate distribution to all facility areas.
3. White Rabbit Grandmaster
The STW-RF White Rabbit Grandmaster implements the full WR protocol stack, providing hardware-based synchronization with <1ns accuracy. It serves as the primary time source for all WR nodes in the facility network.
Decision Rationale: Software-based WR implementations cannot achieve the required accuracy. Hardware grandmasters with FPGA-based protocol processing provide the deterministic performance research applications demand.
4. Cascaded Network Architecture
Large facilities require multiple levels of WR switches for network scaling. The architecture supports multiple grandmaster levels with hardware path delay compensation at each switch.
Decision Rationale: Single-level networks cannot scale beyond ~50 nodes due to timing budget constraints. Cascaded architectures enable thousands of synchronized nodes.
Bill of Materials (BOM)
| Item | BRIDZA Model | Function | Qty | Notes |
|---|---|---|---|---|
| ------ | ------------- | ---------- | ----- | ------- |
| Ultra-High Performance Rubidium | STM-Rb-H | Primary frequency reference | 1 | ≤3×10⁻¹⁴ @ 10ks |
| White Rabbit Grandmaster | STW-RF | WR PTP Grandmaster | 1 | Hardware implementation |
| Fiber Optic Transfer | STW-FT | Remote site distribution | 1 | ≤40ps uncertainty, 80km |
| Frequency Distributor | STZ-PF | Reference distribution | 1 | ≤3fs channel jitter |
| Phase Noise Tester | STT-PN | System characterization | 1 | ≤-135dBc/Hz floor |
| White Rabbit Switch | WR-SW | Network switching (not supplied) | As required | WR-compatible managed switch |
| Single-Mode Fiber | - | Network transport (not supplied) | As required | Duplex LC connectors |
| GPS Receiver | - | External reference (not supplied) | 1 | Optional, for UTC trace |
| PPS Distribution Amp | - | Pulse distribution (not supplied) | 1 | Optional |
Performance Targets
| Parameter | Requirement | Achieved | Notes |
|---|---|---|---|
| ----------- | ------------ | ---------- | ------- |
| Node-to-Grandmaster Sync | <1ns | <1ns | Hardware timestamp |
| Link Delay Measurement | <100ps | <40ps std dev | DMTD measurement |
| Frequency Stability (fiber) | <1×10⁻¹⁴/s | <1×10⁻¹⁴/s | STW-FT transfer |
| Frequency Stability (Rb ref) | <1×10⁻¹³/s | ≤8×10⁻¹³/s | STM-Rb-H @ 1s |
| Time Uncertainty (fiber) | <100ps | ≤40ps | STW-FT spec |
| Maximum Network Depth | 100 switches | 100+ switches | Cascaded architecture |
| Maximum Fiber Distance | 80km | 80km | Direct, longer with amplifiers |
Implementation Notes
Facility Planning
White Rabbit network planning requires careful consideration of:
- Network topology (tree, ring, or hybrid)
- Fiber path lengths between all nodes
- Timing budget allocation at each level
- Redundancy requirements for critical experiments
The STW-RF grandmaster supports multiple PTP domains for separating different experiment timing requirements.
Fiber Infrastructure
Install single-mode fiber with LC/PC or LC/APC connectors for WR network. The system uses standard 1000BASE-LX10 optics with up to 10km per span. For longer distances, add fiber amplifiers.
WR requires duplex fiber (separate transmit and receive paths) for full-duplex delay measurement.
Network Configuration
Configure WR switches for:
- Transparent Clock mode (transparent to grandmaster)
- WR protocol enable on all ports
- VLAN isolation for timing vs. data traffic
- Link delay calibration during network setup
Node Synchronization
Each WR node (detector, sensor, DAQ) requires:
- WR network interface (PCIe or embedded)
- FPGA-based WR protocol stack
- Hardware timestamp capability
- Link delay calibration
Calibration Procedures
Regular calibration ensures maintained performance:
- Link delay measurement verification monthly
- Grandmaster time accuracy check against UTC (if required)
- Fiber path length verification after network changes
Test & Verification Approach
Grandmaster Validation
- UTC Traceability Test: Compare WR time to GPS receiver time over 24 hours
- Stability Analysis: Calculate Allan deviation of grandmaster time
- Holdover Test: Disconnect external reference, measure time drift
Network Synchronization Test
- End-to-End Test: Measure synchronization error from grandmaster to furthest node
- Link Delay Verification: Use DMTD to verify all link delays
- Cascade Test: Verify synchronization through multiple switch levels
System Performance Test
- Multi-Node Correlation: Trigger distributed DAQ systems, measure time correlation
- Long-Term Stability: Monitor synchronization over extended periods
- Failure Recovery: Test behavior under grandmaster failover
Performance Benchmarking
- Timestamp Accuracy: Compare WR timestamps to reference counter
- Jitter Measurement: Measure time error variation under load
- Throughput Test: Verify network performance under data + timing traffic
Alternative Configurations
Hydrogen Maser Upgrade
For facilities requiring ultimate short-term stability, replace STM-Rb-H with active hydrogen maser. Provides 10-100x better stability at τ < 1000s.
Component Changes: Replace STM-Rb-H with hydrogen maser (third-party)
Cesium Primary Standard
For metrology-grade applications requiring primary standard traceability, replace STM-Rb-H with BD1024-P cesium atomic clock.
Component Changes: Replace STM-Rb-H with BD1024-P
Compact WR Network
For small facilities (<20 nodes), simplify to single WR grandmaster without fiber transport. The STW-RF includes direct fiber outputs for small-scale deployment.
Component Changes: Remove STW-FT
Hybrid WR/Standard PTP
For facilities with mixed requirements (some nodes need WR accuracy, others standard PTP), implement parallel networks with WR for critical systems.
Configuration: Deploy separate WR and standard PTP networks
Distributed Reference
For facilities spanning >80km, add intermediate timing hubs with STW-FT to extend range. Each hub includes local rubidium reference for holdover.
Additional Components: STW-FT + STM-Rb-N at intermediate hubs