Wireless mesh networks (WMNs) have in recent years provided an alternative technology for last-mile broadband Internet access service. They have attracted much research interest because of their numerous potential applications. Some of these applications, such as video-on-demand, video-conferencing, preferably involve multicasting. The limited capacity and the effect of interference in WMNs, the large number of users and the development of real-time multimedia applications, have all led to the need to improve network throughput. This work studies the problem of multicast routing and channel assignment using wireless broadcast advantage in multi-hop and multi-channel wireless networks.
In recent years, wireless equipment has been extensively applied in local area networks (LANs) and is becoming increasingly popular because of its lower cost.
Despite considerable advances in physical layer technologies, wireless LANs today still do not offer the same sustained bandwidth as wired LANs. As a result of the MAC contention, 802.11 schemes, protocol overheads and other issues, the actual bandwidth that is available to applications is almost halved even when the peak data rate of 802.11 a/g is 54Mbps. This problem is worsened for multi-hop ad hoc networks by interference from neighbor nodes and links. Fortunately, the IEEE 802.11a/b/g and 802.16 standards provide multiple non-overlapping (orthogonal) channels. They allow various simultaneous transmissions only if different orthogonal channels are used. Even though 802.11a/b/g standards provide this possibility, most ad hoc network implementations involve only a single frequency channel, wasting the rest of the spectrum.
Wireless mesh networks are being developed as an alternative technology for last-mile broadband Internet access service. WMNs comprise two types of node -
mesh routers and mesh clients, as displayed in Fig. 1. The mesh router in the WMN functions not only as a host but also as a wireless router, forwarding packets for the node pairs that cannot communicate directly with each other. A WMN is conceptually similar to a mobile ad hoc network, but with several differences. The main characteristics of WMNs and the differences between WMNs and ad hoc networks are summarized as follows.
(1) Mesh routers are typically stationary but mesh clients can be stationary or mobile nodes. Accordingly, topological changes are infrequent.
(2) Mesh routers are generally equipped with multiple NICs, which are assigned to different orthogonal channels. Therefore, mesh routers can send multiple data simultaneously, enormously increasing the bandwidth of the whole network.
(3) Traffic in WMNs is normally aggregated from a large number of flows and does not change very often. Accordingly, routes and channels can be assigned according to the traffic profile.
(4) Channels can be assigned to NICs by a static or dynamic method. In the static mode, each communication link is bound to a channel and this assignment does not vary over time. In the dynamic mode, the assignment of each link can vary with the network situation.
Figure 1: An example of wireless mesh network that consists of several mesh routers and mesh clients.
Multicasting is a technique that routes data from a set of source nodes to a set of destination nodes, in a way that minimizes the overall utilization of network resources.
In the 80s, Deering et al. [1] investigated the efficient distribution of traffic from a set of senders to a set of receivers in a datagram network problem. Thereafter, many routing protocols were proposed in wired networks [2], such as MBONE [3], MOSPF [4], PIM [5], core-based, trees [6], and shared tree technologies [7, 8], among others.
However, the use of protocols in ad hoc networks has not been discussed as they do not take into account mobility. For a wireless mesh network, these protocols may be solutions but they do not take advantage of the wireless property, broadcast. Wireless broadcast advantage the use of broadcast transmission rather than unicast transmission to send one datum to multiple receivers at one time. In Fig. 2, a source is to send one datum to its three neighbors, and only one channel is available. The left of the figure reveals that three transmissions are required when unicast is used, but the right of the figure reveals that only one transmission is required when broadcast is used.
Figure 2: An example illustrates that a source sends data to its neighbors with and without wireless broadcast advantage.
The channel assignment problem of WMN is an NP-hard optimization problem [9]; therefore, routing and channel assignment is also NP-hard. For static and dynamic traffic, the channel assignment can be static or dynamic. More specifically, in static channel assignment, each link employs the channel that is assigned at the beginning of the operation of the network. However, dynamic channel assignment may change the link’s channel at any time. In this (work OR study OR investigation), for simplicity, only static channel assignment is considered. First, two ILPs are proposed to solve the routing and channel assignment problem for single-channel and multi-channel wireless mesh networks, respectively. Next, the problem is divided into two sub-problems – the routing sub-problem and the channel assignment sub-problem and a heuristic algorithm is developed to solve these two sub-problems.
The rest of this article is organized as follows. Section 2 reviews related work on multi-channel WMNs and the presented network model. Section 3 presents the statement of the problem and the two proposed ILPs. Section 4 proposes a heuristic algorithm and describes the strategies of the routing phase and the channel assignment phase. Section 5 explains the simulation results. Section 6 draws conclusions.