Motivated by a problem of transmitting data over broadcast channels (Birk and Kol, INFOCOM 1998)

We present the Informed-Source Coding-On-Demand (ISCOD) approach for efficiently supplying non-identical data from a central server to multiple caching clients through a broadcast channel. The key idea underlying ISCOD is the joint exploitation of the data already cached by each client, the server's full awareness of client-cache contents and client requests, and the fact that each client only needs to be able to derive the items requested by it rather than all the items ever transmitted or even the union of the items requested by the different clients. We present a set of two-phase ISCOD algorithms. The server uses these algorithms to assemble ad-hoc error-correction sets based its knowledge of every client's cache content and of the items requested by it; next, it uses error-correction codes to construct the data that is actually transmitted. Each client uses its cached data and the received supplemental data to derive the items that it has requested. This technique achieves a reduction of up to tens of percents in the amount of data that must be transmitted in order for every client to be able to derive the data requested by it. Finally, we define k-partial cliques in a directed graph, and cast the two-phase approach in terms of partial-clique covers. As a byproduct of this work, bounds and a close approximation for the expected cardinality of the maximum matching in a random graph have been derived and are outlined.

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Motivated by a problem of transmitting data over broadcast channels (Birk and Kol, INFOCOM 1998)

This paper shows how to improve the classic multichannel slotted ALOHA protocols by judiciously using redundant transmissions. The focus is on user-oriented requirements: a deadline along with a permissible probability of failing to meet it. Subject to satisfying those, maximization of capacity is the optimization goal. When there is no success/failure feedback prior to the deadline, the use of information dispersal with some redundancy provided by error-correcting codes for the data in conjunction with a replicated, separately transmitted synchronization preamble, is proposed. It is shown to sharply reduce the overhead resulting from the use of shorter packets and to significantly increase capacity. When the deadline permits several transmission-feedback rounds, we propose a novel replicationbased retransmission policy: all attempts except the final one entail the transmission of a single or very few copies, and a larger number of copies are transmitted in the final attempt. This sharply increases channel capacity, even with a single transmitter per station. The proposed approaches are particularly suitable for high-bandwidth satellites with on-board processing.