DSM-CC Object Carousel Protocol
m (Correct all ObjectCarousel to "Object Carousel" ... don't know why the original authour used a conjoined word, as I don't see such in any other references)
Latest revision as of 16:29, 17 January 2008
The DSM-CC standard/toolkit, which is Part 6 of the MPEG-2 Standard, defines or extends five distinct protocols to cover the many different aspects of broadband service delivery. One of those five includes the "Download" Protocol, which itself contains several variations that detail the transfer of content from a server to a client, either within the MPEG-2 TS or independently in a separate channel. One of these "Download" protocol variations is an autonomous "data carousel", and an extension to the data carousel itself is the "object carousel".
Stated another way, the DSM-CC Object Carousel Protocol is a broadcast protocol on top of MPEG-2. It is used for transmission of filesystem hierarchies over a MPEG-2 channel. Like other protocols (NFS, SAMBA etc. via IP), the Object Carousel protocol allows mounting of external filesystems that are broadcast within a MPEG-2 program. Of particular interest is its use for the application of interactive multimedia formats, such as MHP or MHEG-5, for Digital TV services.
A. MPEG-2 Transport Stream
Basically, to mount an Object Carousel, of course you have to know where to find it.
First, given a stream location on dvb://ONID.TSID.SID, you have to tune to the service and check some (P)SI tables for presence of an Object Carousel there (which tables is the subject of the next layers).
As you can see, an Object Carousel can be distributed over multiple PIDs (i.e. all elemantary streams belong to the same CarouselID), and there is nothing more special to say about the MPEG-2 layer of the Object Carousel protocol. TS-Packets are merged into Sections as usual, and DSMCC-Sections also follow the usual PrivateSection format.
B. Data Carousel
The PMT points to elementary streams with StreamType 0xb, on which DSM-CC streams are located, but that does not mean a DataCarousel is really there. You can find various descriptors in the 2nd ES-loop of the PMT which indicate the presence of a DataCarousel, e.g. the DataBroadcastDescriptor and CarouselIDDescriptor. The latter one associates a DataCarousel with a CarouselID, so that an Object Carousel can include multiple DataCarousels, each one located on a separate elementary stream.
Well then, you already know all elementary streams including a DataCarousel, but what pieces of data are broadcast there and how to filter them?
A DataCarousel includes at least one Download Control Message which is repeated more frequently than the broadcast data. This message is called DII (DownloadInfoIndication) and describes a set of data belonging to the broadcast DataCarousel.
C. Object Carousel