Skuola Screenshot Piece Christos Bouras is Professor in the University of Patras, Department of Computer Engineering and Informatics. Also he is a scientific advisor of Research Unit 6 in Computer Technology Institute and Press - Diophantus, Patras, Greece. His research interests include Analysis of Performance of Networking and Computer Systems, Computer Networks and Protocols, Mobile and Wireless Communications, Telematics and New Services, QoS and Pricing for Networks and Services, e-learning, Networked Virtual Environments and WWW Issues. He has extended professional experience in Design and Analysis of Networks, Protocols, Telematics and New Services. He has published more than 450 papers in various well-known refereed books, conferences and journals. He is a co-author of 9 books in Greek and editor of 2 in English. He has been member of editorial board for international journals and PC member and referee in various international journals and conferences. He has participated in R&D projects.

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A crucial point on the provision of reliability over mobile multicast delivery is the use of a Forward Error Correction (FEC) scheme on the application layer. FEC, unlike the common methods for error control is not based on lost or corrupted packets retransmission, since the error correction is "forward" in the sense that redundant data are transmitted in advance with the source information, in order to obtain the receivers the ability to overcome packet losses. The application of FEC on ptm reliability protocols provides particular advantages. The most important property of FEC codes is the ability to use the same FEC packets to repair simultaneously different independent packet losses at multiple receivers, without the need of the costly or impossible procedure of packets retransmission. In order to meet the error free transmission requirement of demanding applications, 3GPP recommends the use of the systematic, fountain Raptor code as an Application Layer FEC (AL-FEC) protection mechanism exclusively for MBMS.

The 3GPP multicast services standard, named MBMS, is a unidirectional ptm service in which data are transmitted from a single source to a group of multiple mobile endpoints in a specific service area. 3GPP defines two delivery methods namely, download and streaming.

Download uses the FLUTE protocol which is carried over UDP/IP and is independent of the IP version and the underlying link layers used. In order to apply AL-FEC protection on the MBMS download delivery, the transmitted file is partitioned in one or several source blocks each consisting of k source symbols. For each source block, redundant repair symbols are generated through FEC encoding with a unique ID assigned on each resulting encoding symbol. Subsequently, one or more encoding symbols are placed in each FLUTE packet payload with the resulting packets encapsulated in UDP and distributed over the IP multicast flow.

On streaming delivery, RTP is the application layer protocol which provides means for sending real-time or streaming data over UDP transport layer. The MBMS AL-FEC streaming framework operates on RTP/UDP flows. A copy of the source packets is forwarded to the Raptor encoder and arranged in a source block with each packet occupying a new row of T bytes. The source block is filled up to k rows, where the value of k can be different for each source block. After forming a FEC source block from the packets to be protected together, the Raptor encoder generates the desired repair symbols which are then sent using the FEC repair packet format.

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