HCOR: a high-throughput coding-aware opportunistic routing for inter-flow network coding in wireless mesh networks

Long Hai 0 Hongyu Wang 0 Jie Wang 0 Zhenzhou Tang 0 0 Faculty of Electronic Information and Electrical Engineering , DUT, Linggong Road, Dalian 116024, China Coding-aware routing is an effective approach to create more coding opportunities in inter-flow network coding. To the best of our knowledge, most of the coding-aware routing schemes focus on maximizing the coding opportunities. However, for opportunistic transmission, the throughput is not always increased with the increase of coding opportunities. In this paper, we explore why this case will happen and how to measure the benefits of network coding in the opportunistic routing. According to the above conclusions, we propose a novel high-throughput coding-aware opportunistic routing (HCOR) to achieve the maximal throughput gain in wireless mesh networks. HCOR is based on anypath routing and takes advantage of the network coding gain to find out the route with minimal anypath cost reasonably. Meanwhile, it is also a 'multihop' network coding and changes the route with dynamical data loads adaptively. Simulation results demonstrate that HCOR has better performance than coding opportunity-aware routing and also obtains a significant throughput gain in wireless mesh networks. - Different from IANC, IRNC is an efficient transmission method. Intermediate nodes encode the native packets from different flows and broadcast the encoded packets to different destinations. By exploiting the broadcast nature of wireless channel, destinations can overhear some of side information of the encoded packets before decoding them. So the coding nodes can broadcast the encoded packets to different destinations for different flows at the same time. By making use of this free-ride transmission [3], IRNC saves many time slots to send different flows at intermediate nodes simultaneously. In this paper, we only focus on and discuss IRNC. So the term network coding which appears in the following only means IRNC. Much work points out that the benefits of network coding are different for the same flow by using different routes. So the question of how to find out the best route with network coding is still open until now. Most of the coding-aware routing schemes are based on deterministic routing protocols [3,4]. Due to the characteristic of network coding, these schemes cannot work well when the data flows change frequently. The reason is that route must be preprogrammed in deterministic routing protocols. However, network coding depends on the flows and routes simultaneously. So doing coding-aware routing in deterministic routing protocols must bind flows and routes together. This causes low stability of routes when the state of flows (such as on or off, or the rate of flow) is changed frequently. For example, assume that the best NC-aware route is Ra for flow a to be encoded with flow b. When flow b is end, Ra may not be the best route for a. The follow-on problems are that high delay will be caused if a does the rerouting at this time or the throughput will be degraded if a still uses Ra as its route. Network coding-aware opportunistic routing (NCOR) has been proposed recently [5-8]. Unlike classical deterministic routing, opportunistic routing (OR) always finds potentially feasible paths as the route. In OR, every packet is forwarded by a forwarding list which is composed of the neighbors closer to the destination. Hence, unstable routing problem can be solved in opportunistic routing. Meanwhile, due to the forwarding list adopted, which means many nexthops can be considered in a coding structure, more coding opportunities may be found in an opportunistic transmission. Conventional NCOR schemes [5,8] only consider the maximum coding opportunities as the selection mechanism for the coding node. For example, the authors in [5] propose that they use the number of the neighbor nodes that can decode a coding pattern as the coding gains of that coding pattern. In IRNC, the maximum number of the neighbor nodes that can be the decoding nodes suggests the maximum coding opportunities. The authors in [8] claim that if a node with the most coding opportunities can be chosen as the forwarder in each packet transmission, the throughput of network will be greatly improved as a result. From our study, however, this mechanism is not very suitable for incorporating NC into OR. When multiple packets are encoded together, the receivers of this encoded packet should be determined. After that, the set of new receivers may be possibly shrunken from the set of original receivers. As we know, the shrinkage of a forwarding list will reduce the transmission gain in an opportunistic transmission. So in this case, is the network coding still beneficial for this transmission? How should we decide which is the best choice, coding or not? In addition, if we consider the gain of opportunistic transmission in doing network coding, how to determine the forwarder list is complicated. In this paper, aiming to address the above problems, we study the network coding in anypath routing [9] which provides a calculation method to compute the gain of opportunistic transmission. Our main contributions are summarized as follows: According to the definition and coding condition of IRNC, we figure out two following solutions to help design a coding-aware opportunistic routing. The first is which nodes should be in the forwarding list for sending an encoded packet. The second is how to measure the price of network coding in an opportunistic transmission. We propose a novel coding-aware opportunistic routing scheme called HCOR, which is based on anypath routing and finds out the minimal anypath cost path as the route with network coding. Compared with conventional NCOR schemes, HCOR is more feasible on choosing the coding opportunities. Meanwhile, HCOR is designed on a multihop network coding structure, in which much more coding opportunities can be collected than with a local coding structure [3]. We implement the HCOR scheme in ns-2 and carry out extensive evaluation to show the performance of HCOR. The remainder of this paper is organized as follows. Section 2 reviews some related work. Section 3 is the overview of HCOR. In Section 4, we show some basic knowledge and key technologies used in this paper. In Section 5, we discuss the characteristic of incorporating NC into opportunistic routing and give a computing method to calculate the cost of network coding. We present the details of HCOR implementation in Section 6. The simulations performed to evaluate the performance of HCOR are discussed in Section 7. Finally, we conclude this paper in Section 8. 2 Related work Ahlswede in [1] presented that the network capacity can be increased significantly by employing network coding. Since then, many various studies have proved the benefits of network coding [10,11] and given many coding methods to achieve the gain of network coding for different networks [12-17]. For inter-flo (...truncated)