On-demand broadcasting under deadline

In broadcast scheduling multiple users requesting the same information can be satisfied with one single broadcast. In this paper we study preemptive on-demand broadcast scheduling with deadlines on a single broadcast channel. We will show that the upper bound results in traditional real-time scheduling does not hold under broadcast scheduling model. We present two easy to implement online algorithms BCast and its variant BCast2. Under the assumption the requests are approximately of equal length (say k), we show that BCast is O(k) competitive. We establish that this bound is tight by showing that every online algorithm is Omega(k) competitive even if all requests are of same length k. We then consider the case where the laxity of each request is proportional to its length. We show that BCast is constant competitive if all requests are approximately of equal length. We then establish that BCast2 is constant competitive for requests with arbitrary length. We also believe that a combinatorial lemma that we use to derive the bounds can be useful in other scheduling system where the deadlines are often changing (or advanced).