Digital Storage Media

Command and Control

Data Transmission & DSM-CC

DSM-CC is a toolkit for developing control channels associated with MPEG-1 and MPEG-2 streams. It uses a client/server model connected via an underlying network (carried via the MPEG-2 transport multiplex or independently if needed).

DSM-CC may be used for controling the video recouping, providing features normally found on Video Cassette Recorders (VCR) (fast-forward, rewind, pause, etc). It may also be used for a wide variety of other purposes including packet data transport. It is defined by series of weighty standards, principally MPEG-2 ISO/IEC 13818-6 (part 6 of the MPEG-2 standard).

DSM-CC may work in conjunction with next generation packet networks, working alongside such protocols as RSVP, RTSP, RTP and SCP.

Data Transmission

Data is already being sent over DVB networks using a variety of proprietary encoding schemes. In an effort to standardise these services, the DVB specification allows DSM-CC to carry traditional network traffic using one of five profiles:

Data Piping - where discrete pieces of data are delivered using containers to the destination.
Data Streaming - where the data takes the form of a stream which may be Asynchronous (i.e. without timing) Synchronous (i.e. tied to a fixed rate transmission clock) or Synchronised (i.e. tied via time stamps to the decoder clock and hence to other streams)
Multiprotocol Encapsulation - the technique intended for packet data and using DSM-CC sections.
Data Carousels - a scheme for assembling data sets into a buffer which is played-out cyclically on a DVB stream. The data sets may be of any format or type.
Object Carousels - resembling data carousels, but primarily intended for data broadcast services. The data sets are defined by the DVB Network Independent Protocol specification.

The multi-protocol extension encapsulates the data to be sent using a Medium Access Control (MAC) header (and optionally a Sub Network Access Protocol (SNAP) header). The bytes of the MAC header are re-arranged (to ease decoding by MPEG receivers with limited capability) and other section header information added. The encapsulated packet forms the first part of a DSM-CC section to which a trailer is added to control processing of the received section and also includes a CRC-32 checksum to protect the data. The entire DSMCC section is padded to an integral number of transport packets, and packetised into the MPEG transport stream.

Backwards compatibility with legacy transmission schemes is provided by using registered Service Information (SI) codes which are only recognised by equipment able to process the proprietary encodings. SI codes act as the labels to the data being sent, enabling the receiver to determine whether the data may be processed (i.e. whether the receiver has the required capability) and indicating the type of processing to be performed.

Digital Storage Media Command and Control (DMS-CC) Download

Compared to other download protocols, DSM-CC Download is designed for lightweight and fast operation in order to meet the needs of devices that contain limited memory. The DSM-CC download operates over heterogeneous connections and is applied to a number of network models one of which is the broadcast model with no upstream channel. The mechanisms used in download are:

Sliding window
No ACK for use in broadcasting
Maps to MPEG-2 Transport Stream for hardware multiplexing

Links to other sites: FAQ about DSM-CC