Lecture 4 — September 17 4.1 Digital Fountain Characterization 4.1.1 Limitations of Building Digital Fountain with Rs Codes

A Digital Fountain is an approach used to provide reliability in multicast networks without the use of receiver initiated packet retransmissions. Prior work to Digital Fountain has shown that use of receiver initiated retransmissions to recover lost data is unscalable in multicast networks and the impact is significant in lossy networks. Instead, like many other techniques that transmit redundant codewords, Digital Fountain uses the concept of encoding data using Tornado and LT codes such that reconstruction of data at the receiver is done with minimal overhead. The intuition behind Digital Fountain is that there is a tradeoff between decoding time and decoding efficiency. By accepting an almost negligible penalty of decoding inefficiency, significant improvements are achieved for encoding and decoding times . In the following section, we discuss the limitations of RS codes to make the advantages of Tornado and LT codes clear.