In SMPTE ST 2110, media is transported as separate IP essence flows (video via ST 2110-20, audio via ST 2110-30, ancillary data via ST 2110-40). Unlike SDI’s continuous bitstream, these flows consist of RTP/UDP packets that must be sent with precise, controlled timing.
Packet pacing refers to the spacing and regularity of these packets. Poor pacing leads to bursts that can overwhelm network switches, cause jitter, increase buffering requirements, or result in packet loss and video artifacts.
ST 2110-21: Traffic Shaping and Delivery Timing
ST 2110-21 defines the rules for sender behavior using two models:
- Network Compatibility Model — Ensures streams don’t overflow switch buffers (uses a scaling factor β ≈ 1.1).
- Virtual Receiver Buffer Model (VRX) — Models how a receiver processes packets at the exact video line rate.
Sender Types Defined in ST 2110-21
| Sender Type | Description | Pacing Behavior | Typical Use |
|---|---|---|---|
| Narrow (N) | Tightest control, often with short gaps during vertical blanking | Steady or gapped (isochronous) | FPGA/ASIC hardware for live production |
| Narrow Linear (NL) | Fully even spacing across the entire frame | Completely linear, no gaps | High-precision hardware |
| Wide (W) | More relaxed timing requirements | Can be bursty | Software-based or COTS PC implementations |
Common Packet Pacing Issues
- Bursty transmission from Wide senders due to OS scheduling, CPU interrupts, or software processing.
- Non-compliance with ST 2110-21 models, leading to buffer overflows or underflows.
- Network-induced jitter that turns even narrow streams into bursts after multiple hops.
- Mismatch between gapped and linear pacing expectations.
- Extremely high packet rates in 4K/8K streams making microsecond deviations critical.
- Interoperability problems when mixing narrow and wide devices in the same system.
Real-World Impacts
- Video glitches, frame drops, or black screens
- Increased end-to-end latency (especially with Wide receivers)
- Network congestion even when bandwidth is sufficient
- Difficult troubleshooting without deep packet inspection
Mitigation Strategies
- Prefer Narrow (N/NL) senders for critical live paths
- Ensure all receivers support Wide mode if any Wide senders are present
- Use dedicated media networks with QoS, deep-buffer switches, and proper PTP timing
- Employ hardware packet pacing (FPGA or NIC offload)
- Test thoroughly with ST 2110-21 compliance tools (CMAX, VRX measurements)
