Link-state routing without broadcast storming for multichannel mesh networks

Abstract

A link-state routing protocol tailored for multichannel mesh networks is proposed. One drawback of using multichannel communications is the high overhead involved in broadcast operations: a transmitter should transmit a broadcast packet to all channels that may be occupied by receivers. This makes certain broadcast-intensive mechanisms, such as link-state routing, difficult to implement. The link-state routing protocol proposed in this paper is tailored for multichannel mesh networks by minimizing the broadcast overheads. This is achieved by a special set of nodes, called cluster-heads. We have implemented our protocol on a multichannel mesh network test bed and compared its performance with an AODV-like reactive routing protocol, also tailored for multichannel mesh networks. The measurements show that the proposed link-state routing protocol provides transient communications with comparable or better performance. Ways to improve the performance of the proposed routing with infrastructure access is also discussed.

Introduction

Wireless networks have been successful in both commercial and residential deployments. While last-mile, single-hop wireless networks, such as wireless local area networks and cellular networks, have achieved significant popularity, mobile ad-hoc (MANET) and multihop networks have gained little practical attention despite significant academic contributions in the area. This is due mainly to two reasons: (1) purely ad-hoc networks without connectivity to the Internet do not have much practical application and (2) data in an ad-hoc network may have to be routed through vulnerable intermediate nodes, making it less attractive for commercial deployments. Wireless mesh networks overcome these difficulties by allowing for a gateway node that has connectivity to the Internet. Furthermore, relay nodes (different from client nodes) deployed by the operators provide reliable multihop connections between client nodes and gateway nodes.

In their seminal paper [1], Gupta and Kumar derived the capacity bounds of a wireless ad-hoc network and showed that placing additional relay nodes in the network helps to achieve significant capacity gains. Additionally, in [2], Kyasanur and Vaidya have analyzed the capacity bounds of a wireless network when multiple channels are used. Accordingly, though there are no asymptotic capacity benefits with multiple channels (rather than a single channel), in practical networks they have several advantages. This is because multichannel networks encourage more transmissions in the network, as the amount of contention per channel is significantly reduced. Much of the existing literature on multichannel networks proposes use of multiple interfaces (or wireless radios) to provide simpler multichannel operation [2], [3], [4].

The main challenges in a multichannel wireless network lie in developing protocols for exploiting the flexibility offered by using all the channels available for operation. Due to the difficulties involved in coordinating the nodes in a multichannel network, each of which may operate on a different channel, traditional protocols designed for single-channel networks may not directly apply in a multichannel setting. In this paper, we discuss a link-state routing mechanism, called MCLSR (Multi-Channel Link-State Routing), tailored for a multichannel mesh network.

Link-state routing has been a popular routing methodology in wired networks. In this approach, each node maintains the link-state information of the entire network and makes route decisions based on this information. To keep link-states fresh, each node periodically announces its link-state to all the other nodes. Although link-state routing has relatively high control overheads due to the associated link-state information exchange, it provides quicker route discoveries without additional route discovery packets. Moreover, because the routes are constructed using a global knowledge of the network, they are highly optimized. While link-state routing has been less popular in MANETs because of the associated topology dynamics caused by node mobility,¹ it has recently been reconsidered for mesh networks due to their relatively stable topology compared to MANETs. Moreover, the optimized routing achievable using a link-state routing protocol is more attractive for a multichannel environment, as routing decisions can be based not just on the channel quality of each link but also on the sequence of the channels to be used in a route. This can significantly affect end-to-end throughput [3].

Most of the existing designs of wireless link-state routing protocols exploit the broadcast nature of the wireless medium for propagating the link-state information [5]. However, since the broadcast overheads in a multichannel network are much higher than in a single-channel environment, the existing implementations of the link-state routing protocols are best not directly applied to multichannel networks. Notice that in a multichannel network, a transmitter should transmit a broadcast packet on all channels that might be occupied by a receiver, thereby increasing the overhead involved. To minimize broadcast overheads, we propose a set of cluster-head nodes, elected dynamically for exchanging the link-state information over the network. The cluster-head nodes collect the link-state information from their dependent nodes and share it with other cluster-heads. The receiving cluster-heads rebroadcast the shared link-state information to their dependents to propagate the link-states over the entire network. We discuss the procedure involved in more detail in Section 4.

We have implemented our protocol on a real multichannel mesh network test bed with two IEEE 802.11a interfaces per node. We discuss the implementation issues and the experimental results in Section 5. Our results show that the link-state routing mechanism provides transient communication with comparable or better performance. The mechanisms for improving the performance of the proposed routing approach using infrastructure access are discussed in Section 6. Section 7 concludes the paper.