Opportunistic Routing in Wireless Multi-hop Networks: A Tutorial

Two major factors that limit the throughput in multi-hop wireless networks are the unreliability of wireless transmissions and co-channel interference. One promising technique that combats lossy wireless transmissions is opportunistic routing (OR). OR involves multiple forwarding candidates to relay packets by taking advantage of the broadcast nature and spacial diversity of the wireless medium. Furthermore, recent advances in multi-radio multi-channel transmission technology allows more concurrent transmissions in the network, and shows the potential of substantially improving the system capacity. However, the performance of OR in multi-radio multi-channel systems is still unknown, and the methodology of studying the performance of traditional routing (TR) can not be directly applied to OR. In this paper, we present our research on computing an end-to-end throughput bound of OR in multi-radio multi-channel systems. We formulate the capacity of OR as a linear programming (LP) problem which jointly solves the radio-channel assignment and transmission scheduling. Leveraging our analytical model, we gain the following insights into OR: 1) OR can achieve better performance than TR under different radio/channel configurations, however, in particular scenarios, TR is more preferable than OR; 2) OR can achieve comparable or even better performance than TR by using less radio resource; 3) for OR, the throughput gained from increasing the number of potential forwarding candidates becomes marginal.

Routing in multihop wireless networks is challenging, mainly due to unreliable wireless links/channels. Geographic opportunistic routing (GOR) was proposed to cope with the unreliable transmissions by exploiting the broadcast nature of the wireless medium and the spatial diversity of the network topology. Previous studies on GOR have focused on networks with a single channel rate. The capability of supporting multiple channel rates, which is common in current wireless systems, has not carefully been studied for GOR. In this paper, we carry out a study on the impacts of multiple rates, as well as candidate selection, prioritization, and coordination, on the performance of GOR. We propose a new local metric, i.e., the opportunistic effective one-hop throughput (OEOT), to characterize the tradeoff between one-hop packet advancement and packet forwarding time. We further propose a local rate adaptation and candidate-selection algorithm to approach the optimum of this metric. The simulation results show that the multirate GOR (MGOR) incorporating the rate adaptation and candidate-selection algorithm achieves higher throughput and lower delay than the corresponding single-rate and multirate traditional geographic routing and single-rate opportunistic routing protocols.

Opportunistic routing (OR) involves multiple forwarding candidates to relay packets by taking advantage of the broadcast nature and multi-user diversity of the wireless medium. Compared with Traditional Routing (TR), OR can evidently improve the end to end throughput of Wireless Mesh Networks (WMNs). At present, there are many achievements concerning OR in the single radio wireless network. However, the study of OR in multi-radio wireless network stays the beginning stage. In this paper, we focus on OR in multi-radio multi-channel WMNs. Directly using the existing channel assignment leads to decreasing the number of candidate forwarder and the performance of OR. According to OR with one-to-more transmission mode, the Candidate Forwarder Set based Channel Assignment (CFSCA) for OR is proposed. Firstly, according to the candidate forwarder selected by routing, we bound the sender node and its candidate forwarders as a Candidate Forwarder Set(CFS). Then we calculate the interference among all the CFSs. Then the channel assignment for OR is described as a minimum CFS interference optimization problem, which must obey the constraints of the channel number of CFS and the number of radio interfaces. The evaluation results show that CFSCA improves 25.2%, 10%, 19% of the aggregative throughput than random, uniform and tradition channel assignment, respectively.Keywordswireless mesh networksmulti-radio and multi-channelopportunistic routingchannel assignment

Wireless channels have a broadcast nature based on which opportunistic routing protocols work. In opportunistic routing protocols, packets are forwarded by the intermediate nodes that hear their transmissions, which are called candidate forwarders. In most of these algorithms, the forwarder list is pre-determined. However, the energy-efficient selection of the forwarder list is a research topic that is not considered well. Energy Efficient OPportunistic Routing algorithm (EEOPR) is presented in this paper in which the forwarders are determined on the packets’ fly. EEOPR is a flexible method that performs the routing process locally, and the candidate forwarders are selected during the routing and for each packet. The process of the candidate nodes’ selection and their packet forwarding are managed by the Genetic algorithm according to the nodes’ remaining energy and their regions. Simulation results show that network performance is improved in EEOPR compared to ROMER and CORP-M in terms of throughput, the number of duplicate packets, and the network nodes’ residual energy.

Opportunistic Routing (OR) has been proposed to improve the efficiency of unicast protocols in wireless networks. In contrast to traditional routing, instead of preselecting a single specific node to be the next-hop forwarder, an ordered set of nodes (referred to as candidates) is selected as the next-hop potential forwarders. In this paper, we propose a new multicast routing protocol based on OR for wireless mesh networks, named Multicast OR Protocol (MORP).We compare our proposal with the well known ODMRP Multicast protocol. Our results show that Multicast-OR outperforms ODMRP, reducing the number of transmissions and increasing the packet delivery ratio.

A general framework for efficient geographic routing in wireless networks

We consider the problem of quality of service (QoS) routing in multi-hop wireless networks where data are transmitted from a source node to a destination node via multiple hops. The routing component of a QoS-routing algorithm essentially involves the link and path metric calculation which depends on many factors such as the physical and link layer designs of the underlying wireless network, transmission errors due to channel fading and interference, etc. The task of link metric calculation basically requires us to solve a tandem queueing problem which is the focus of this paper. We present a unified tandem queue framework which is applicable for many different physical layer designs. We present both exact and approximated decomposition approaches. Using the queueing framework, we can derive different performance measures, namely, end-to-end loss rate, end-to-end average delay, and end-to-end delay distribution. The proposed decomposition approach is validated and some interesting insights into the system performance are highlighted. We then present how to use the decomposition queueing approach to calculate the link metric and incorporate this into the route discovery process of the QoS routing algorithm. The extension of the queueing and QoS routing framework to wireless networks with class-based queueing for QoS differentiation is also presented.

Multi-hop Wireless Mesh Networks (WMNs) is a promising new technique for communication with routing protocol designs being critical to the effective and efficient of these WMNs. A common approach for routing traffic in these networks is to select a minimal distance from the source to destination as in wireless networks. Opportunistic Routing (OR) makes use of the broadcasting ability of wireless network and is especially very helpful for WMN because all nodes are static. Our proposed scheme of Multicast Opportunistic Routing (MOR) in WMNs is based on the broadcast transmissions and Learning Automata (LA) to expand the potential candidate nodes that can aid in the process of retransmission of the data. The receivers are required to be in sync with one another in order to avoid duplicated broadcasting of data which is generally achieved by formulating the forwarding candidates according to some LA based metric. The most helpful aspect of this protocol is that it intelligently learns from the past experience and improves its performance. The results obtained via this approach of MOR show that the proposed scheme outperforms some existing schemes and is an improved and more effective version of opportunistic routing in a mesh network.

Recent research has shown that interference can make a significant impact on the performance of multihop wireless networks. Researchers have studied interference-aware topology control recently [M. Burkhart et al., 2004]. In this paper, we study routing problems in a multihop wireless network using directional antennas with dynamic traffic. We present new definitions of link and path interference that are suitable for designing better routing algorithms. We then formulate and optimally solve two power constrained minimum interference single path routing problems. Routing along paths found by our interference-aware algorithms tends to have less channel collisions and higher network throughput. Our simulation results show that, compared with the minimum power path routing algorithm, our algorithms can reduce average path interference by 40% or more at the cost of a minor power increase. We also extend our work towards survivable routing by formulating and solving the power constrained minimum interference node-disjoint path routing problem.

Opportunistic Routing (OR) has recently been proposed to improve the efficiency of unicast in multi-hop wireless networks. OR exploits the broadcast nature of wireless transmission medium and opportunistically selects a relay path to deliver a packet to its receiver. To adopt OR in wireless multicast, the main challenge is to efficiently share opportunistic relay paths between multiple receivers. In this paper, we propose an opportunistic overlay multicast design for wireless networks, named Minimum Steiner Tree with Opportunistic Routing (MSTOR). In MSTOR, the source and receivers are connected by an overlay Steiner tree. The source multicasts packets along the overlay links of the Steiner tree to reach all receivers. The transmission of packets on each overlay link is controlled by unicast OR. We first propose an overlay construction algorithm based on the optimal ``OR distance

We have developed a cost-effective wireless multi-hop network system for accessing core networks. Cost-effectiveness can be provided to users by offering them a wireless multi-hop network in which a wireless link enables multiple hops between access points (APs) and reduces the number of APs connected to a wired network. The key issues of wireless multi-hop networks are routing and security. We proposed a hybrid routing for wireless multi-hop networks that combines on-demand and table-driven routing, a packet check scheme between APs, and an authentication scheme. To ensure both correct routing and security, the system makes use of a wireless multi-hop (WM) layer between the IP and medium access control (MAC) layers. This paper evaluates the system based on its FTP throughput, round-trip time, and route creation time. The FTP throughput was about 3.5 Mbps even when there were four hops in an 802.11b wireless LAN. We believe that our wireless multi-hop network system is useful for providing cost-effective broadband access service.

Routing in wireless mesh networks presents a great challenge due to unreliable wireless links and collisions. Recently, a new routing method, opportunistic routing (OR), is proposed to deal with them by exploiting broadcast nature of wireless networks. OR relies on the global knowledge of networks to select relays and determine priorities. In this paper, we propose a new routing scheme, OxDSR, which combines OR and dynamic source routing (DSR) and performs opportunistic transmissions with the help of the local information and the traditional route obtained by DSR. By taking the duplicates and additional wireless resource consumed by multiple ACK frames into consideration, we propose a new metric, equivalent data rate, to select and prioritize relays. Extensive simulation study shows that the proposed OxDSR is more resilient and achieves higher throughput than DSR based on the expected transmission count routing metric.

Opportunistic routing – A review and the challenges ahead

Recently, opportunistic routing has gained much popularity for the ability to exploit the broadcast nature of the wireless medium with several practical protocols been proposed over wireless mesh networks (WMNs). These protocols usually add many mechanisms to introduce a dramatic increase in the overall network throughput, such as reliability promise and rate control. However, the evaluation of these protocols did not show how much performance benefit can be expected from each mechanism. In this paper, we provide a systematic performance evaluation on the effectiveness of reliability promise and rate control mechanisms that commonly used in opportunistic routing protocols. Our study shows that: (i) batch acknowledgment used in opportunistic routing significant improves the transmission performance over traditional routing, the true gain from opportunistic routing is not as much as claimed in previous works. (ii) rate control is important to the opportunistic routing. It ensures opportunistic routing to utilize its advantage sufficiently, but also prevents the performance degradation when the network is congested. These results give us new understanding in the effectiveness of the opportunistic routing. We then analyze and uncover reasons for these results and propose some specific schemes.

Testbed results of an opportunistic routing for multi-robot wireless networks