ATSC Data Broadcast Standard

The Advanced Television Systems Committee (ATSC) [2] is an international, non-profit organization developing voluntary standards for DTV. Many of the ATSC standards are adopted by the US Federal Communications Commission (FCC) [15] as standards for the DTV industry in US. The ATSC standard on datacast is called ``ATSC Data Broadcast Standard (A/90)'' [3], which deals with data transmission on top of the MPEG-2 Transport Streams. This standard is first released in 2000. It mainly covers the following three aspects:
(1) Transport Protocols The transport protocols support the transmission of three categories of data with different timing models. They are called asynchronous data (no presentation time stamps associated with the data), synchronous data (presentation time stamps are associated with the data and refer to a common audio-visual program timeline), and synchronized data (presentation time stamps are associated with the data and are tied to no common timeline). According to the timing model of data and the content type of data (file, IP datagram, multimedia stream, or user-defined data), the transmissions use different methods of encapsulations. However, in all methods, data are finally encapsulated using the MPEG-2 [26] transport streams packets, which have fixed length 188 bytes. These methods are listed as follows (for illustration, see Figure 1):

Figure: ATSC data broadcast encapsulations.

1. Data files are transfered by the DSM-CC Download Protocol. Support is available for one-time or carouseled delivery, asynchronous streaming download, and synchronized nonstreaming download.
2. Protocol datagrams such as IP datagrams are encapsulated using ``DSM-CC addressable sections'', which contain a six-byte device ID (having the same format as MAC address) in the section header. This packet format is extended from a kind of DSM-CC encapsulation called the ``DSM-CC section''.
3. Synchronized and synchronous streaming data are transfered by MPEG-2 PES, which encompasses the streaming data model as well as the timing model.
4. The privately user-defined data are transfered by the Data Piping protocol, which is designed to carry any arbitrary form of data items.

(2) Application Signaling Application signaling is performed by means of Service Description Framework (SDF), a scheme extended from that of DSM-CC. Simply put, ATSC SDF is a general application signaling framework that allows receivers to discover the detailed composition of a data broadcast service about the following information: the number of data elementary streams, the encapsulations, the protocols, and the minimum receiver resources required. This information is needed by a data receiver to assess whether it is capable of receiving and processing part of a data service. It is also needed to link any receiver application component to a set of transmitted data elements that it may consume.
(3) Buffer Models The purpose of a buffer model is to regulate the delivery of the elementary stream packets and the data packets from the head-ends to the receivers. It is important for both head-end equipment manufacturers and consumer electronics manufacturers to conform to the buffer model to produce smooth packet transmissions. The MPEG-2 standard has defined a Transport Stream Target Decoder Buffer model (T-STD) for the delivery of video and audio elementary streams. However, this model is not enough for the data broadcast protocols. In response, the ATSC data broadcast standard includes the definition of an extended T-STD for the delivery of asynchronous, synchronous, and synchronized data elementary streams respectively. Concretely, different buffer models are defined according to the four data service profiles G1, G2, G3, and A1. G1, G2, and G3 correspond to a data rate up to 384 Kbps, 3.84 Mbps, and 19.3 Mbps respectively. A1 corresponds to an opportunistic data service with rate up to 19.3 Mbps. An opportunistic data service is a service in which data is transmitted on a best-effort basis based on the instantaneous bandwidth available in the transport stream.