Application-Level forward error correction

PS: Each layer has error correction, the physical layer has error correction level, belongs to the channel FEC code

Data link layer with CRC checksum, which belongs to the channel FEC code

Application layer, also has the application layer of FEC, that is, the source of FEC code

Application-Level forward error correction
Based on these shortcomings, the use of these error control technology does not guarantee the video transmission quality of IPTV services, but if the use of end-to-end application layer reliability solution on the IP network, it can provide the IPTV service very well [5]. In fact, the application of Layer forward error correction (AL-FEC) technology provides a simple and powerful method to solve the network packet loss problem, thus guaranteeing the end-to-end reliability. Compared to the link layer and the physical layer FEC (mainly for error correction), the AL-FEC transparently works on the application layer above the IP layer, and it is applied to special end-to-end application flows such as IPTV streaming. A typical application scenario is that the AL-FEC sender handles all or a selected set of IPTV streams at the IPTV server, and then the corresponding AL-FEC receiver is processed at the end device. The value of AL-FEC is that IPTV streaming can be as high as the original video stream, even in the case of packet loss. The AL-FEC in IPTV can be broadly divided into 3 kinds of [6]:raptor coding, Pro-mpeg CoP3 coding, and FEC and retransmission combination.

The basic idea of AL-FEC is the use of erasure codes. In the AL-FEC transmitting end, the IPTV stream is segmented, divided into the same size packets, through the Erasure Code encoder to the packet encoding to generate repair packets, the repair packet and the original packet sent to the receiving end. At the AL-FEC receiver end, the Erasure code decoder recovers all the original IPTV streams by repairing the packet and the original packet. Ideally, the receiver will be able to fully recover all the data if it receives any k packets from n encoded packets, as shown in Figure 1.

2.1 Raptor Encoding
From the whole AL-FEC mechanism, the operation is relatively simple, do not need to make big changes to the original network protocol stack. But the type of erasure code is many, choose which erasure code to become a key factor of AL-FEC success or failure. Fountain code as a new type of innovative erasure code can be used for any probability of the deletion of the channel. The first effective and practical fountain code is the LT code. And on this basis, the expansion of the LT code after the formation of the Raptor code. The Raptor code has low reception overhead and system code characteristics. It is an attractive choice to apply raptor code to IPTV business. [7-8] The IPTV application of digital Video Broadcasting (DVB) and the 3GPP Multimedia Broadcast Multicast Service (MBMS) have chosen the Raptor code as its AL-FEC specification.

The encoder of the Digital fountain Code [9], like a fountain, is capable of generating infinitely multiple encoded packets. Assuming that the size of the source file is KL bit, the encoder output each time the size of the L-bit, and receive the receiving process can be figuratively likened to a water bucket to the fountain under the same time, when the number of elements received more than K, you can recover the source file.

Fountain code is a bit-rate-independent, from the source file encoding generated by the number of code elements are unlimited, can generate an infinite number of code elements, such a code-independent encoding method determines that it has a very important practical application value, it does not need to like the traditional erasure code as the deletion of the channel has a pre-judgment, flexibility greatly enhanced.

Michael LUBY[10] Found a coding method of digital fountain code with the complexity of linear compiler code, called LT Code. The cost of the LT Code encoding and decoding is only K Logek (where k is the file size), while the Raptor code is a fountain code with a lower complexity than the code of the LT coding, which can be used to further reduce the probability of the LT code distribution under the condition of Lossless error performance (to form the code generation matrix). Complexity, the coding complexity is independent of k, and the decoding complexity is linearly related to K in the condition of big data packet. In other words, because of the pre-coding process, it is only necessary to restore a portion of the indirect packets through the LT code decoding, and then use the traditional erasure code to recover all the packets. Figure 2 is the Raptor code encoding process [11].

By using AL-FEC (Raptor), the end-to-end reliability of the application layer is provided [12].

AL-FEC (Raptor) Advantage: by Dvb-h, 3GPP, IETF adoption, enough to demonstrate the advantages of this method, can resist high packet loss rate, with flexible error correction ability (for different files and different packet loss rate set different protection parameters), high efficiency, Only a few repair data to ensure the reconstruction of the source files, efficient coding efficiency, simple software implementation, easy to deploy, shorten the entire file transfer time, unconstrained source block size to adapt to various files, improve coverage, suitable for broadcast and multicast environment, with the physical layer FEC, Become one of the most efficient solutions; good backwards compatibility.

Disadvantages of AL-FEC (Raptor):

Must be used in conjunction with the physical layer and the FEC of the link layer; The network bandwidth cost is generally 5%~10%; the file is too small to apply.

2.2 pro-mpeg CoP3 encoding
Pro-mpeg cop3[13] Encoding is actually a simple parity code that can be divided into CoP3 1D (encode rows or columns) and CoP3 2D (encode rows and columns). Figure 3 is the encoding process: each row or column of packets is different or produces a coded packet.
When using only CoP3 1D, the resulting encoded packet is the bottom row or rightmost column in Figure 3, which can only protect a single packet loss in one row or column, if two packets are lost in the original packet sequence (such as packet 0 and packet 1 or packet 0 and packet L). The lost two packets will not be recoverable. However, this situation will not occur when using CoP3 2D.

If you use CoP3, you typically need 20% redundancy. Its greatest advantage is that it is simple to operate, but its utilization rate is very low because of its great consumption of network bandwidth for the success of transmission.

2.3 FEC combined with retransmission
When using this solution, most of the lost data is recovered by FEC, and then the remaining small portions of the recovered data are retrieved through the re-transmitted. Because of the different network conditions, the choice of FEC technology is the key to the quality of this scheme.
This technology has been used in IPTV services through the Asymmetric Digital Subscriber Line (ADSL) [14], where the vast majority of them opt to use pro-mpeg CoP3 encoding as FEC technology (the codec process is simple). It has the advantage of increasing the probability of recovering lost data and reducing the burden of the original retransmission server. Its disadvantage is that there is no fast channel switching scheme, and when using Pro-mpeg CoP3 encoding, there is not enough protection against random packet loss situations.
3 concluding remarks
With the further development of IPTV business, how to ensure video quality becomes the key to the success of IPTV business. Whether it is the forward error correcting code at the coding end, the post-processing error concealment at the decoding end, or the interactive error control technology at the encoder and decoding end, although it can improve the robustness of video transmission to some extent, their shortcomings make them impossible to be the perfect solution to the problem of IPTV business. In recent years, the standardization organization has evaluated the AL-FEC and adopted this technique due to its excellent performance. And with the research of Raptor Code, the AL-FEC technology has been turned into practical from theory.

Transferred from: http://www.zte.com.cn/cndata/magazine/zte_communications/2009/6/articles/200912/t20091219_178855.htm