Robustness Control for Network-Wide Broadcast in Multi-Hop Wireless Networks

Mobile Ad hoc networks (MANETs), mesh networks, and multi-hop sensor networks are instances of multi-hop wireless networks where nodes cooperate to forward traffic among each other. Such networks are important whenever infrastructure is unavailable or expensive, and quick deployment is desired. They play an important role in many existing and emerging applications in the military, industry, research and civilian domains. Network-Wide Broadcast (NWB) algorithms provide a mechanism to deliver information to nodes in a multi-hop network without depending on routing state. This makes them ideal for initial self-configuration or for operation under mobility where the routing state becomes stale. Therefore, NWB is a heavily used primitive at the core of most multihop routing [1, 2] and group communication protocols [3, 4]. A common approach to performing NWB is flooding: a process where every node that receives a packet for the first time, rebroadcasts it. Flooding has been shown to be wasteful, especially in dense networks – a problem called the broadcast storm [5]. Ni et al, who identified the broadcast storm problem, also proposed several solutions to it based on nodes locally determining whether their rebroadcast is likely to be needed. An alternative, topology-sensitive, approach to the problem attempts to construct a virtual backbone that is tasked with disseminating the broadcast. Such a virtual backbone must form Connected Dominating Sets (CDS) comprising of a connected subset of the nodes that together cover all the nodes in the network; this set can then be tasked with rebroadcasting the NWB packet while all other nodes just receive it significantly reducing the number of retransmissions relative to flooding. We survey these approaches in more detail in Section 3. While the focus of NWB algorithms has mostly been on reducing the overhead, a different problem occurs that affects the robustness of most NWB protocols. This problem affects NWB protocols that rely on MAC level broadcast operations. This includes virtually all existing NWB algorithms, both flooding based protocols [5] and virtual backbone approaches (e.g., [1, 6, 7]). More specifically, because MAC broadcasts are unreliable, it is possible for rebroadcasts to be lost due to interference or transmission errors. The loss rate can be considerable if high interference exists or if link quality is bad [8, 9]. These losses may lead to the NWB reaching only a subset of the nodes. A particularly damaging example of this problem occurs when the initial transmission of the NWB packet is lost at all receivers (e.g., due to a collision with another packet). In this case the NWB will not reach any nodes. In Section 5 we present a classification of existing NWB algorithms, focusing on their robustness characteristics. In addition, we analyze the robustness properties of selected algorithms representing different points of the space under different network densities, loss rates, and mobility characteristics; the algorithms are described in Section 4. We show that under high loss rates and in networks that are sparse coverage for existing NWB algorithms, including flooding