A Polynomial Time Approximation Scheme for Minimum Cost Delay-Constrained Multicast Tree under a Steiner Topology

This paper is concerned with a restricted version of minimum cost delay-constrained multicast in a network where each link has a delay and a cost. Given a source vertex $s$ and $p$ destination vertices $t_1, t_2, \ldots, t_p$ together with $p$ corresponding nonnegative delay constraints $d_1, d_2, \ldots, d_p$, many QoS multicast problems seek a minimum cost multicast tree in which the delay along the unique $s$--$t_i$ path is no more than $d_i$ for $1 \le i \le p$. This problem is NP-hard even when the topology of the multicast tree is fixed. In this paper we show that every multicast tree has an underlying Steiner topology and that every minimum cost delay-constrained multicast tree corresponds to a minimum cost delay-constrained realization of a corresponding Steiner topology. We present a fully polynomial time approximation scheme for computing a minimum cost delay-constrained multicast tree under a Steiner topology. We also present computational results of a preliminary implementation to illustrate the effectiveness of our algorithm and discuss its applications.     

欧氏空间货郎担问题的一个多项式时间近似方案的改进与实现

货郎担问题的实例是给定n个结点和任意一对结点{i,j}之间的距离di,j,要求找出一条封闭的回路,该回路经过每个结点一次且仅一次,并且费用最小,这里的费用是指回路上相邻结点间的距离和.货郎担问题是NP难的组合优化问题,是计算机算法研究的热点之一.在过去几十年中,这一经典问题成为许多重要算法思想的测试平台,并促使一些研究领域的出现,如多面体理论和复杂性理论.欧氏空间上的货郎担问题,结点限制在欧氏空间,距离定义为欧氏距离.即使是这样,欧氏空间上的货郎担问题仍然是NP难的.1996年,Arora提出欧氏空间上货郎担问题的第1个多项式时间近似方案.对其中货郎担问题的算法进行了改进：提出一种新的构造方法,使应用于该算法的“补丁引理”结论由常数6改进到常数3,从而使算法的时间复杂度大幅减少;同时,编程实现了该算法,并对实验结果进行了分析.     

Improved Approximation Algorithms for the Quality of Service Multicast Tree Problem

The Quality of Service Multicast Tree Problem is a generalization of the Steiner tree problem which appears in the context of multimedia multicast and network design. In this generalization, each node possesses a rate and the cost of an edge with length l in a Steiner tree T connecting the source to non-zero rate nodes is l · r_e, where r_e is the maximum node rate in the component of T-{e} that does not contain the source. The best previously known approximation ratios for this problem (based on the best known approximation factor of 1.549 for the Steiner tree problem in networks) are 2.066 for the case of two non-zero rates and 4.212 for the case of an unbounded number of rates. In this paper we give improved approximation algorithms with ratios of 1.960 and 3.802, respectively. When the minimum spanning tree heuristic is used for finding approximate Steiner trees, then the previously best known approximation ratios of 2.667 for two non-zero rates and 5.542 for an unbounded number of rates are reduced to 2.414 and 4.311, respectively.     

An O(n log n) Average Time Algorithm for Computing the Shortest Network under a Given Topology

In 1992 F. K. Hwang and J. F. Weng published an O(n ² ) time algorithm for computing the shortest network under a given full Steiner topology interconnecting n fixed points in the Euclidean plane. The Hwang—Weng algorithm can be used to improve substantially existing algorithms for the Steiner minimum tree problem because it reduces the number of different Steiner topologies to be considered dramatically. In this paper we present an improved Hwang—Weng algorithm. While the worst-case time complexity of our algorithm is still O(n ² ) , its average time complexity over all the full Steiner topologies interconnecting n fixed points is O (n log n ). Received August 24, 1996; revised February 10, 1997.