23-06-2014, 03:48 PM
SOAR: SIMPLE OPPORTUNISTIC ADAPTIVE ROUTING PROTOCOL FOR WIRELESS MESH NETWORKS
SOAR SIMPLE OPPORTUNISTIC ADAPTIVE.docx (Size: 1.04 MB / Downloads: 14)
INTRODUCTION
MULTI-HOP wireless mesh networks are becoming a new attractive communication paradigm. A wireless mesh network (WMN) is a communications network made up of radio nodes organized in a mesh topology. Wireless mesh networks often consist of mesh clients, mesh routers and gateways. The mesh clients are often laptops, cell phones and other wireless devices while the mesh routers forward traffic to and from the gateways which may, but need not, connect to the Internet. A mesh network is reliable and offers redundancy. When one node can no longer operate, the rest of the nodes can still communicate with each other, directly or through one or more intermediate nodes. Many cities across the world have deployed or are planning to deploy them to provide the Internet access to residents and local businesses.
Routing protocol design is critical to the performance and reliability of wireless mesh networks. A natural approach to routing traffic in wireless mesh networks is to adopt techniques similar to those in wireline networks, which select a best path for each source destination pair (according to some metric) and send traffic along the predetermined path. Most of the existing routing protocols, such as DSR, AODV, DSDV, and LQSR are examples of traditional routing protocols. But traditional routing faces difficulties in coping with unreliable and unpredictable wireless medium
Motivation
Motivated by these observations, researchers developed opportunistic routing protocols for wireless mesh networks. Opportunistic routing exploits the broadcast nature of the wireless medium and does not commit to a particular route before data transmission. Instead, the sender broadcasts its data; among the nodes that hear the transmission, the one closest to the destination is selected to forward the data. In this way
BACKGROUND
1Traditional routing protocols
Routing has been an active area in wireless networking research. Most of the original work in this area targeted high-mobility scenarios such as battlefield networks. Therefore, the focus was on establishing and maintaining routes under frequent and unpredictable changes in network connectivity. A number of on-demand routing protocols have been proposed for this purpose, as exemplified by DSR and AODV, where packets are routed along paths with the shortest hop count.
Recently, wireless mesh networks have emerged as a new dominant application of multihop wireless networks. Nodes in such networks have little or no mobility and often are not constrained by short battery life or limited computational power. Therefore, improving network performance becomes the primary focus. Researchers have found that the hop-count metric, as used in DSR and AODV, does not provide good performance since not all hops are equal. To address this issue, various link quality metrics have been proposed. These metrics quantify the quality of links using link loss rate, packet transmission time, or signal-to-noise ratios.
PRIOR OPPORTUNISTIC ROUTING PROTOCOLS
In this section we describe the main features of some representative opportunistic routing protocols. The Extremely Opportunistic Routing (ExOR) is the most popular opportunistic routing protocol, while both MORE and MIXIT generalize the opportunistic routing paradigm by adopting, respectively, packet-level and symbol-level network coding. Simple Opportunistic Adaptive Routing (SOAR) proposes a simple packet-level responsibility transfer process based on time division multiple access (TDMA). The same mechanism is adopted by Opportunistic DHT-based Routing (ODR), but it is the first protocol to propose a scalable mechanism to distribute loss rate estimates across the network. Finally, the Multi-Channel Extremely Opportunistic Routing (MCExOR) protocol extends opportunistic routing to multichannel environments.
DESIGN CHALLENGES
The goal of opportunistic routing is to maximize the progress each transmission makes without causing duplicate (re)transmissions or incurring significant coordination overhead. In order to achieve this goal, several important design issues should be addressed
OVERVIEW OF SOAR
As ExOR and MORE, SOAR is a proactive link state routing protocol. Every node periodically measures and disseminates link quality in terms of ETX. Based on this information, a sender selects the default path and a list of (next hop) forwarding nodes that are eligible for forwarding the data. It then broadcasts a data packet including this information. Upon hearing the transmission, the nodes not on the forwarding list simply discard the packet. Nodes on the forwarding list store the packet and set forwarding timers based on their proximity to the destination. A node closer to the destination uses a smaller timer and forwards the packet earlier. Upon hearing this transmission, other nodes will remove the corresponding packet from their queues to avoid duplicate transmissions. Like all the existing opportunistic routing protocols, SOAR broadcasts data packets at a fixed PHY data rate.
CONCLUSION
SOAR, a novel opportunistic routing protocol. SOAR effectively realizes opportunistic forwarding by judiciously selecting forwarding nodes and employing priority-based timers. It further incorporates adaptive rate control to dynamically adjust sending rates according to network conditions and recovers lost packets using efficient local feedback and recovery. The combination of these techniques enables SOAR to achieve high efficiency and effectively support multiple bulk transfer flows. By judiciously selecting forwarding nodes and avoiding diverging paths are important especially for supporting multiple simultaneous flows. The rate control is important to the performance of a routing protocol. It improves both aggregate good through put and fairness among multiple competing flows. The joint design of routing and rate limiting as in SOAR is useful, and may be useful to the design of other opportunistic routing protocols. Finally, the default path selection algorithm is important to the performance of opportunistic routing. We use the ETX metric to select the default path, but SOAR can easily accommodate other default path selection metrics.