Network Technologies DVB-ASI to IP Data Flow Block Diagrams, Forward Error Correction FEC

ASI-IP-GTW User's Manual

1.1.1.34 UDP Mode

To allow interoperability with legacy equipment, the ASI-IP-GTW can stream video over IP without using the RTP protocol encapsulation. This is also called UDP mode or UDP only. UDP mode is manually configured on the transmitting ASI-IP-GTW unit, and automatically detected on the receiving ASI-IP-GTW.

Note: FEC relies on information in the RTP protocol, and will not be available in UDP mode. FEC is explained in 5.4.1.

5.4 Data Flow Block Diagrams

The following figure shows the data flow between two ASI-IP-GTW over an IP network.

Figure 35. Data flow between two units

5.4.1 Forward Error Correction (FEC)

Unfortunately, real networks are not perfect and packet losses occur. In order to cope with packet losses, the ASI-IP-GTW has implemented Pro- MPEG FEC according to Pro-MPEG Code of Practice #3 rev. 2. Pro- MPEG FEC is carried out on RTP packets. The mechanism is based on the insertion of additional data containing the result of an XOR (exclusive OR)-operation of packets over a time window.

The generation of FEC packets is based on the use of a matrix. The matrix size is defined by the number of columns (L) and the number of rows (D). The FEC packets are calculated as an XOR operation over the packets in a column and the packets in a row. Figure 36 shows an example of the FEC scheme. Three packets are lost and corrected.

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