تجزیه و تحلیل عملکرد از الگوریتم های چندپخشی برای شبکه های ارتباطات ماهواره ای تلفن همراه؛ بخش نخست

With the rise of mobile computing and an increasing need for ubiquitous high-speed data connections, Internet-in-the-sky solutions are becoming increasingly viable. To reduce the network overhead of one-to-many transmissions, the multicast protocol has been devised. The implementation of multicast in these low earth orbit (LEO) constellations is a critical component to achieving an omnipresent network environment. This paper examines the system performance associated with two terrestrial-based multicast mobility solutions, distance vector multicast routing protocol (DVMRP) with mobile IP and on demand multicast routing protocol (ODMRP). These protocols are implemented and simulated in a satellite LEO constellation. Results from the simulation trials show the ODMRP protocol provided greater than 99% reliability in packet deliverability, at the cost of more than 8 bits of overhead for every 1 bit of data for multicast groups with multiple sources. In contrast, DVMRP proved robust and scalable, with data-to-overhead ratios increasing logarithmically with membership levels. DVMRP also had less than 70 ms of average end-to-end delay, providing stable transmissions at high loading and membership levels.

The Internet was initially conceived to provide a means of transferring data from one machine to another. Today, it is frequently being used as a mechanism for sending the same data to many users. In an effort to provide efficient communications, multicasting was developed to ease the pressure that duplicate message transfers place on corporate, educational, and military network bandwidth constraints. As Internet usage becomes more ubiquitous and in demand, typical connection methods will not be able to meet the demand for either bandwidth or mobility. With the advent of low earth orbit (LEO) satellite networks such as Globalstar, Teledesic, and Iridium, the potential for having a constant broadband connection to the Internet becomes more realistic. Transmitting multicast information through a dynamic global network is a capability these future networks will need to have. However, wireless multicasting is more complex than its wired equivalent. Whereas in the wired case, the user's location and network topology is relatively fixed, in the wireless case, the location of the multicast members is constantly changing. Adding the dynamic environment of a LEO constellation removes the assumption of a static topology. This paper examines two very antipodal protocols for LEO multicast communication, On Demand Multicast Routing Protocol (ODMRP) and Distance Vector Multicast Protocol (DVMRP), in light of these special challenges and evaluates their appropriateness for use.

This paper has examined the efficiency, reliability, and timeliness performance of ODMRP and DVMRP under different loading levels and membership conditions. During this discussion, the two protocols have been compared in an effort to determine an ideal protocol for satellite-based multicasting. Both DVMRP and ODMRP have performance strengths. For use of bandwidth, the DVMRP protocol outperforms ODMRP by over 200% in every case. The ability to selectively choose the next hop of a multicast route requires less overhead than the mini-broadcast ODMRP performs at every hop. Even with the additional overhead of determining the shortest route from every source to every destination, DVMRP maintains a data-to-overhead ratio that increases as more subscribers join the multicast group. The extra overhead of ODMRP, however, pays off well when it comes to it's nearly 100% reliability. With many sources, the replication of the mesh-based architecture provides enough redundancy that nearly every packet created is successfully delivered. Both protocols provide adequate performance in the end-to-end metric. ODMRP has a slight advantage at low membership and low loading levels, while the lower response of DVMRP to loading and membership level gives it an edge at higher loading and membership levels.