日志

深入NIT

已有 2032 次阅读| 2011-5-4 14:37 |个人分类:DVB

1 NIT 基本概念：what is NIT used for?
Network Information Table （NIT）:
It is intended to provide information about the physical network. 【hsy75 但是，到底是那些physical信息呢，接下来讨论！】
1 .1
NIT的作用主要是对多路传输流的识别，NIT提供多路传输流，物理网络及网络传输的相关的一些信息，如用于调谐的频率信息以及编码方式。
调制方式等参数方面的信息。根据此信息设置IRD（Integrated Receiver Decoder）可以进行多路传输流之间的切换。
【hsy75案】说白了就是方便快速做查找物理的网络和切换：
1.2
NIT 被EPG的应用所要求：
1.3 NIT 概括地说：

The first entry in the PAT (program 0) is reserved for network data and contains the PID of the network information table (NIT).
【hsy75案】PAT的表0 包含了NIT的索引：

The NIT includes information of other transport streams that may be available, for example,by 1 tuning to a different RF channel or 2 satellite.
Each transport stream may include a descriptor that specifies the radio frequency, satellite orbital position, etc.
【hsy75案】NIT 主要包含网络的调谐信息，包含频率，卫星的位置等等。
In MPEG-2, only the NIT is mandatory for this purpose.
In DVB, additional metadata, known as DVB-SI, is used, and NIT is considered to be part of DVB-SI. Therefore, the term PSI/SI is a commonly used term.
【hsy75案】NIT 在DVB 中基本上可以被DVB-SI 取代。

2 NIT 的应用和结构：En300468
The NIT (see table 3) conveys information relating to the physical organization of the multiplexes/TSs carried via a given network, and the characteristics of the network itself.
The combination of original_network_id and transport_stream_id allow each TS to be uniquely identified throughout the ETS application area.
【hsy75案】每个TS都可以被唯一的标识
Networks are assigned individual network_id values, which serve as unique identification codes for networks. The allocation of these codes may be found in ETR 162 [6].
In the case that the NIT is transmitted on the network on which the TS was originated（appear for the first time in a particular place or situation）,
the network_id and the original_network_id shall take the same value.
【hsy75案】每个Networks有自己独立的标识：network_id，当NIT一开始传输时，两个值的标识应该相同的值。
Guidelines for the processing of SI at transitions between delivery media boundaries, e.g. from satellite to cable or SMATV systems, can be found in ETR 211 [7].
IRDs may be able to store the NIT information in non-volatile memory in order to minimize the access time when switching between channels ("channel hopping").
It is also possible to transmit a NIT for other networks in addition to the actual network.
Differentiation between the NIT for the actual network and the NIT for other networks is achieved using different table_id values (see table 2).

标准定义的表的值如下：
0x40 network_information_section - actual_network
0x41 network_information_section - other_network

【hsy75案】一个TS流传输其他网络的信息，可以通过NIT others来进行，不同的是NIT的table_id不同。
下图是一个实际流的例子：

The NIT shall be segmented into network_information_sections using the syntax of table 1.
NIT, ST 0x0010
Any sections forming part of an NIT shall be transmitted in TS packets with a PID value of 0x0010.
Any sections of the NIT which describe the actual network
(that is, the network of which the TS containing the NIT is a part) shall have the table_id value 0x40
with the network_id field taking the value assigned to the actual network in ETR 162 [6].
Any sections of an NIT which refer to a network other than the actual network shall take a table_id value of 0x41 and the network_id shall take the value
allocated to the other network in ETR 162 [6].

ref:
1 video demystify
2 En300468