The Network Information Table is specified in the MPEG-2 standard (ISO/IEC 13818-1), section 2.4.1, as an optional Program Specific Information (PSI) table, but the syntax of the NIT is left for others to specify. This is the first indication that the table may not be all that necessary. ATSC, for example, sees no need for a Network Information Tble.
A "network" in this context is one or more ways of delivering transport streams. A cable system may be a network, or it could even be configured as more than one network. A broadcast station could be a network, or even several coordinated broadcast stations could be considered a network. "Satellite transmission" could be considered a network, or the comprehensive manner in which a Direct-to-Home or Direct Broadcast Satellite system implements their transmission system on multiple frequencies could be a network.
A network information table, then, is a PSI table that conveys information about the way that multiplexes carrying transport streams are organized. ETSI EN 300 468 (the DVB-SI specification) provides for a mandatory NIT providing information about the network containing the NIT, and one or more optional NITs conveying information about other networks.
The format of the NIT is simple enough; a single loop with a transport_stream_id field, followed by a original_network_id field, a descriptor length field, followed by descriptors that can total 8192 bytes in length. All sorts of descriptors are possible, including the network_name_descriptor, service_list_descriptor, satellite_delivery_system_descriptor, cable_delivery_system_descriptor, terrestrial_delivery_system_descriptor, linkage_descriptor, frequency_list_descriptor, even the multilingual_network_name_descriptor.
Unfortunately, the NIT -- at least when describing the network carrying the NIT -- is basiclly worthless, superfloous and perhaps even cynical. This is due to the simple fact that before a receiver can acquire the NIT, the receiver has to tune into and demultiplex the transport stream. So, the NIT -- when describing the network carrying the NIT -- is only available after the receiver has actually made full use of the network and therefore the only information that might be useful is the original_network_id, presuming the receiver makes use of it, and the value was set correctly at the origination point.
When a receiver needs information in the NIT is before tuning in the network; not after. In addition, we are told that in practice the information contained in NITs is not definitive. For example, when a terrestrial signal is picked up and put into a cable delivery network, the NIT is seldom changed, so one cannot even rely on information in the NIT to know whether a signal was picked up off the air, or via satellite or cable.
The table_id for a NIT describing the network carrying the NIT is 64/0x40. The table_id for a NIT describing a different network is 65/0x41.
This page was last modified on January 10, 2010.