ST 2110-40 – Ancillary Data in SMPTE ST 2110 Systems


BL Study Plan  2110 Topo 

What you will learn on this page


This lesson explains how ST 2110-40 – Ancillary Data in SMPTE ST 2110 Systems fits into the SMPTE ST 2110 stack.
• What a 2110 System Designer Must Know
• Why Ancillary Data Flows Are Separate
• ST 2110-40 Ancillary Data – Common Types & Implementation
• How ST 2110-40 Handles Packet Transmission
• ST 2110-40 RTP Clock Frequency


ST 2110-40 defines the transport of ancillary data (often called “ANC data”) as a separate essence flow over IP. This includes captions, timecode, SCTE 104/35 triggers, AFD, and other metadata.

What a 2110 System Designer Must Know

Why Ancillary Data Flows Are Separate

This is one of the core principles of ST 2110 — essence separation:

Design Rule of Thumb:
Treat ANC data (2110-40) as a first-class citizen. It must be synchronized to the same PTP clock as video and audio. Plan your multicast addressing, bandwidth, and redundancy for it just like you do for the main video and audio essences.

ST 2110-40 Ancillary Data – Common Types & Implementation

Bottom Line   ST 2110-40 carries ancillary data as its own independent essence flow over IP. Below is a breakdown of the most common types, what typically generates them, and how they are converted into 2110 IP streams.

1. Closed Captions / Subtitles (CEA-608 / CEA-708)

Example: Extracting CTA-708 Closed Captions from ST 2110-40

Bottom Line   To extract CTA-708 Closed Captions from an ST 2110-40 ancillary data stream, the receiver must filter for a specific Data Identifier (DID) value inside the packet payload.

The Required DID / SDID Values

DID = 0x61
SDID = 0x01

These two values together identify CEA-708 digital closed captions in the ancillary data stream.

Parser Code Snippet (Python Example)

Why This Combination?

Bottom Line:
A 2110-40 parser looking for closed captions should scan for DID = 0x61 and SDID = 0x01. This is the standard identifier used in both traditional SDI VANC and in ST 2110-40 IP streams.

Missing or Broken Closed Captions in ST 2110-40

Bottom Line   If a downstream device (multiviewer, compliance logger, monitor, etc.) reports that closed captions are completely missing or broken, but a packet capture confirms the ST 2110-40 stream is physically arriving at the port, the problem is almost always in the SDP (Session Description Protocol).

The Most Common Cause

The receiver is not parsing the ancillary data because the SDP is either:

Correct SDP for CTA-708 Captions

		m=application 50000 RTP/AVP 96
		a=rtpmap:96 ancillary/90000
		a=fmtp:96 DID=0x61,SDID=0x01

Why This Happens

Bottom Line:
If packets are arriving but captions are missing → Check the SDP first. Make sure it correctly declares DID=0x61, SDID=0x01 for CTA-708 captions. This is a very common configuration error in ST 2110 deployments.

2. Timecode (LTC / VITC via SMPTE ST 12-1 / ST 12-2)

How Ancillary Timecode (ATC) is Carried in ST 2110-40

Bottom Line   Ancillary Timecode (ATC) is transported in ST 2110-40 as a dedicated ancillary data packet, following the same structure used in traditional SDI VANC.

Technical Identifiers

DID = 0x60
SDID = 0x60
Standard: SMPTE ST 12-2 (Ancillary Timecode)

ATC packets are carried inside 2110-40 RTP streams and use the same 90 kHz RTP clock and PTP timing reference as video (2110-20) and audio (2110-30).

Problems This Resolves Compared to Legacy LTC / VITC

Issue Legacy LTC / VITC ST 2110-40 ATC
Distribution Separate cables or embedded in SDI Same IP network as video & audio
Routing Flexibility Limited (point-to-point) Any-to-any via multicast
Synchronization Prone to drift and jitter Locked to facility PTP clock
Redundancy Difficult ST 2022-7 hitless protection
Scalability Poor in large facilities Excellent
Bottom Line:
ST 2110-40 ATC replaces fragile, hard-to-route legacy timecode distribution with a clean, PTP-synchronized, network-native solution. Because it uses the same timing reference as video and audio, timecode stays frame-accurate across the entire facility without manual delay compensation.

3. SCTE 104 / SCTE 35 Triggers (Ad Insertion & Program Signaling)

4. Active Format Description (AFD)

5. Camera Metadata, Tally, and Production Control Data

How ST 2110-40 Handles Packet Transmission

Bottom Line   Unlike ST 2110-20 video, which sends massive continuous streams of pixel data, ST 2110-40 (Ancillary Data) operates very differently.

Key Difference in Transmission

How 2110-40 Packet Transmission Works

ST 2110-40 does not send a packet for every video line. Instead:

Practical Examples

Bottom Line:
ST 2110-40 is an event-driven essence. It only uses network bandwidth when there is data to send. This makes it far more bandwidth-efficient than video (2110-20) or audio (2110-30), but it still requires proper PTP timing and reliable multicast delivery so the occasional packets arrive correctly synchronized with the main program.

Why ST 2110-40 Packets Are Susceptible to Jitter — and How QoS Fixes It

Because ST 2110-40 ancillary data packets are very small and sent infrequently (bursty), they are highly vulnerable to network issues on a busy switch.

The Main Problem: Jitter & Head-of-Line Blocking

On a congested switch, a large 2110-20 video packet (which can be several thousand bytes) may already be in the transmit queue when a tiny 2110-40 ANC packet arrives.

The small ANC packet is forced to wait behind the large video packet(s). This introduces unpredictable queuing delay and jitter.

This jitter can cause problems with timing-sensitive data such as:

How Quality of Service (QoS) Prevents This

Bottom Line:
ST 2110-40’s bursty, low-bandwidth nature makes it vulnerable to jitter caused by large video packets. Proper QoS marking and priority queuing ensure these critical but tiny packets are not delayed, maintaining reliable delivery of captions, timecode, and triggers across the network.

ST 2110-40 RTP Clock Frequency

According to SMPTE ST 2110-10 (System Timing and Definitions), the RTP clock frequency used by ST 2110-40 Ancillary Data is:

90,000 Hz (90 kHz)

This is the same RTP clock rate used by ST 2110-20 Video.

Why 90 kHz? Why does it match the video stream?

  • Video Compatibility — Most professional video frame rates (especially 59.94, 29.97, etc.) divide cleanly into 90,000. This enables precise frame-accurate timestamping.
  • Common Timing Plane — Using the same 90 kHz clock for both video and ancillary data allows all RTP timestamps to be derived from the shared PTP clock. This makes synchronization much simpler.
  • Seamless Lip-Sync & Alignment — Captions, timecode, SCTE triggers, and other ancillary data stay perfectly aligned with the video frames without complex timestamp conversion.
Bottom Line:
ST 2110-40 uses a 90 kHz RTP clock by design so that ancillary data remains tightly synchronized with the video essence using the same PTP timing reference. This unified timing approach is one of the biggest advantages of the ST 2110 architecture.
Design Recommendation:
In modern 2110 facilities, most ancillary data is converted from legacy physical interfaces (SDI, RS-422, GPIO) into dedicated 2110-40 IP flows using gateways or software-based encoders.

Plan separate multicast addresses and ST 2022-7 redundancy for critical data such as captions and SCTE triggers.

Design Best Practice:
Treat 2110-40 ancillary data as a first-class essence. Give it proper PTP synchronization, dedicated multicast addressing, and ST 2022-7 redundancy where needed. Many facilities run separate ANC flows for captions, timecode, SCTE triggers, and production metadata to maximize routing flexibility.

 

UPDATED
5/25/26
V260525-1.0