ST 2110-40 DID / SDID Mapping Explained

SMPTE ST 2110-40 defines the real-time transport of ancillary (ANC) data over IP using RTP packets, based on SMPTE ST 291-1 ANC packets from traditional SDI systems.

What is DID/SDID Mapping?

In SDI and ST 2110-40, every ancillary data packet is identified by two key fields:

• DID (Data Identification) — 10-bit word that identifies the general type or category of the ancillary data.
• SDID (Secondary Data Identification) — 10-bit word that provides a more specific sub-type (used with Type 2 packets).

These DID/SDID pairs act like a “type code” or label that tells receivers exactly what kind of data is inside the packet (e.g., closed captions, timecode, AFD, etc.).

Packet Types in ST 291-1 / 2110-40

How DID/SDID Works in 2110-40

• Each ANC packet from SDI is wrapped into an RTP payload per RFC 8331 and ST 2110-40.
• The original DID and SDID values are preserved inside the RTP packet.
• Receivers use the DID/SDID combination to route, process, or extract specific ancillary services (e.g., only closed captions or only timecode).
• This enables true “break-away” routing — you can send captions on one 2110-40 flow and AFD on another, independently of the video stream.
• SDP files describe which DID/SDID combinations are present in a particular 2110-40 flow (using the did_sdid parameter).

Common DID/SDID Examples

Full list of registered DID/SDID values is maintained in the official SMPTE Registry for Data Identification Word Assignments.