IP video broadcasting
Oct 1, 2008 12:00 PM, By Wes Simpson
Flawless IP video over imperfect networks
In spite of the obvious appeal of IP networks for many video transport applications, some broadcasters are not completely comfortable with the technology. Many of these concerns derive from experiences with the public Internet, which admittedly is not a perfect channel for mission-critical, time-sensitive material. However, when private networks are employed, IP technology can be used to deliver signal quality and reliability that significantly exceeds that available with traditional time-division multiplex (TDM) technologies employed in standard SONET/SDH networks.
In any real network, errors are going to happen. Data bits can be corrupted, IP packets lost and fibers cut, in addition to many other faults. In this environment, it is important to design a video transport system that tolerates these faults and recovers from them.
Bit errors
Bit errors can be introduced from a number of sources. Optical components will occasionally misfire and change values. Electronic devices will also experience errors. These errors are typically transient and often last for only a few bits. These types of errors are typically corrected using forward error correction (FEC) with specially designed redundancy codes.
Lost packets can occur from longer sequences of bit errors, congestion and other errors. Some network equipment deliberately discards packets when faced with network congestion. This situation can often be avoided for video streams by increasing the priority of the packets. Lost packets are also caused by random bit errors that corrupt the header or other portions of a packet. Packets with invalid headers (as indicated by a bad checksum value) are normally discarded by networking equipment. To correct these types of errors, FEC codes can be used, or packet retransmission schemes can be employed.
Network interruption happens when more than one packet in a row is lost. Because these outages last from a few milliseconds to many days, the only totally effective way to correct for these failures is to build a redundant network.
Interestingly, IP technology is uniquely suited for handling all of these different error types. Reed-Solomon and row-column FEC handle minor bit errors and occasional lost packets. In transmission systems with even higher error rates, packet doubling and automatic packet re-sending replace lost data. And finally, hitless protection switching, which uses packets to compensate for network delay differences and to precisely select an appropriate switchover point, corrects even long-term network interruptions.
Reed-Solomon error detection
Reed-Solomon coding is a popular method for detecting and correcting errors, particularly burst errors that occur in data transmission. In Reed-Solomon systems, the data stream is broken into discrete blocks, typically a number of bytes (n) that is less than 256. Within the data block, some of the bytes are designated as information bytes (k), and the remaining bytes (n-k) are calculated values that form the entire data block into a polynomial. These (n-k) values are calculated so as to add redundancy to the information bytes (k) to form a total data block (n). It can be shown that as long as no more than [(n-k)/2] of the bytes are corrupted in the transmission (either in the original information bytes or in the calculated bytes) then the original data block can be recovered perfectly.
As an illustration, consider a 240-byte overall data block (n=240) with a 224-byte information payload (k=224). Using standard Reed-Solomon calculations, 16 bytes of error correction data would be added to each payload (n-k=240-226=16) to form a data block prior to transmission. At the receiving end of the circuit, calculations would be performed that could recover the original block as long as no more than 8 bytes were corrupted along the way [(n-k)/2=16/2=8]. This gives a robust block transportation scheme, where all errors that occurred in a transmitted block could be repaired, so long as no error exceeded 8 bytes in length and no more than 8 bytes were corrupted in any 240-byte block. This corresponds to an error rate of 3.33 percent, which would be exceedingly high for any modern communication link, such as SONET or IP.
Figure 1. Reed-Solomon FEC adds additional data packets to allow for error correction.
Click to enlarge
This same Reed-Solomon scheme could be creatively applied to correct for complete lost packets instead of simple bit or byte errors. In this scheme, each 240-byte block would be spread across 40 consecutive IP packets, wherein 6 bytes from each block would be included in each packet, as shown in Figure 1. Other blocks would be added to this group of packets so that each of the 40 IP packets is a reasonable size for transmission. Using a 1500-byte total packet size (including IP headers, etc.), it works out that 240 of these data blocks could be spread across the 40 IP packets. Because 6 bytes from each block are in each packet, the total payload size of each IP packet is 1440 bytes (6 × 240), leaving plenty of room for the required IP headers. These IP packets could then be transmitted across a network even in the presence of errors that would cause packets to become lost.
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