DVB Data Services: MHP and SSU - Digital Video and Audio Broadcasting Technology - page 500

Object carousels (Fig. 27.3.) allow complete file and directory structures
to be transmitted from a server to the terminal via the MPEG-2 transport
stream. A restriction imposed by the data carousels is that they only allow
a relatively flat directory structure and flat logical structure. Object and
data carousels are described both in the standard [ISO/IEC 13818-6] (a
part of MPEG-2) and in the DVB data broadcasting document
[EN301192].
Firstly, data/object carousels have a logical structure which owes noth-
ing to the content actually to be transmitted (directory tree plus files). The
entry point into the carousel is via the DSI (Download Server Initializing)
message, or via a DII (Download Information Identification) message in
the case of the data carousel. It is retransmitted cyclically with a ta-
ble_ID=0x3B in a DSM-CC section. Cyclically because this is broadcast-
ing and it must be possible to reach a large number of terminals time and
again and the terminals are unable to request messages from the server.
The DSI packet then uses IDs to refer to one or more DII messages (Fig.
27.4.) which are also retransmitted cyclically in DSM-CC sections with a
table_ID = 0x3B. The DII messages, in turn, refer to modules in which the
actual data are then repeatedly broadcast cyclically via many data
download blocks (DBB) with a table_ID=0x3C in DSM-CC sections.

DSI: Download Server Initializing
gi: Group Info Bytes
DII: Download Info Identification
mi: Module Info Bytes
DDB: Data Download Blocks

PSI/SI

data_broadcast_desc

Super Group

transaction_ID
DSI

gi gi

transaction_ID transaction_ID

DII DII

mi mi mi mi

DDB DDB DDB DDB

Block

DDB DDB DDB DDB

Module

DDB DDB DDB

DDB DDB

Group

Fig. 27.4. Logical structure of an object carousel