# Precision Timing Concepts

## Time Synchronization Protocols

### NTP (Network Time Protocol)
- **Accuracy**: Typically 1-10ms over Internet, <1ms on LAN
- **Protocol**: UDP port 123
- **Versions**: NTPv3 (RFC 1305), NTPv4 (RFC 5905)
- **Stratum Levels**: 0 (reference) to 15
- **Key Features**:
  - Client/server and peer modes
  - Symmetric key authentication
  - Broadcast/multicast support

### PTP (Precision Time Protocol)
- **Standard**: IEEE 1588-2008 (PTPv2)
- **Accuracy**: Sub-microsecond with hardware support
- **Transport**: Ethernet (Layer 2) or UDP (Layer 3)
- **Clock Types**:
  - Ordinary Clock (OC)
  - Boundary Clock (BC)
  - Transparent Clock (TC)
- **Messages**:
  - Sync, Follow_Up
  - Delay_Req, Delay_Resp
  - Announce, Management

### IRIG Time Codes
- **Types**: IRIG-A, B, D, E, G, H
- **IRIG-B**: Most common (100 pps, 1kHz carrier)
- **Encoding**: Amplitude modulated or DC level shift
- **Information**: Time of year, year, status bits
- **Accuracy**: Microsecond level

### GPS Time
- **Source**: Global Positioning System satellites
- **Accuracy**: ~40ns relative to UTC
- **Outputs**:
  - NMEA sentences (serial data)
  - PPS (Pulse Per Second)
  - 10MHz reference (some receivers)
- **Considerations**:
  - GPS-UTC offset (leap seconds)
  - Antenna placement
  - Signal acquisition time

### PPS (Pulse Per Second)
- **Signal**: Rising edge aligned to second boundary
- **Accuracy**: Typically <100ns jitter
- **Uses**:
  - Clock discipline
  - Frequency reference
  - Time interval measurements

## Clock Synchronization Algorithms

### Phase-Locked Loop (PLL)
- Adjusts local clock frequency
- Minimizes phase error
- Smooth corrections

### Frequency-Locked Loop (FLL)
- Adjusts clock rate
- Good for initial synchronization
- Handles large offsets

### Kalman Filter
- Optimal state estimation
- Handles noisy measurements
- Predicts clock behavior

### Clock Discipline Algorithm
1. Measure time offset
2. Calculate frequency error
3. Apply corrections
4. Monitor stability

## Time Scales

### UTC (Coordinated Universal Time)
- International time standard
- Includes leap seconds
- Based on atomic clocks

### TAI (International Atomic Time)
- Continuous time scale
- No leap seconds
- UTC = TAI - leap_seconds

### GPS Time
- Started January 6, 1980
- No leap seconds
- GPS = TAI - 19 seconds

## Precision Timing Metrics

### Accuracy
- Closeness to true time
- Absolute time error

### Precision
- Repeatability of measurements
- Standard deviation

### Stability
- Allan deviation
- Time deviation (TDEV)
- Maximum time interval error (MTIE)

### Jitter
- Short-term variations
- Peak-to-peak or RMS

### Wander
- Long-term variations
- Low-frequency drift

## Implementation Considerations

### Hardware Timestamping
- PHY or MAC layer stamps
- Reduces software latency
- Critical for PTP

### Interrupt Latency
- PPS edge detection
- Deterministic response
- Priority management

### Clock Sources
- Crystal oscillators (XO)
- Temperature compensated (TCXO)
- Oven controlled (OCXO)
- Chip scale atomic clocks (CSAC)

### Network Considerations
- Asymmetric delays
- Packet delay variation
- Queue management
- Traffic prioritization