1Department of Electrical and Computer Engineering, International Islamic University Malaysia, 50728 Kuala Lumpur, Malaysia
2Malaysia-Japan International Institute of Technology (MJIIT), Department of Electronic Systems Engineering, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Jalan Semarak, Malaysia
Copyright © 2016 Omar M. Zakaria et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Multiradio wireless mesh network is a promising architecture that improves the network capacity by exploiting multiple radio channels concurrently. Channel assignment and routing are underlying challenges in multiradio architectures since both determine the traffic distribution over links and channels. The interdependency between channel assignments and routing promotes toward the joint solutions for efficient configurations. This paper presents an in-depth review of the joint approaches of channel assignment and routing in multiradio wireless mesh networks. First, the key design issues, modeling, and approaches are identified and discussed. Second, existing algorithms for joint channel assignment and routing are presented and classified based on the channel assignment types. Furthermore, the set of reconfiguration algorithms to adapt the network traffic dynamics is also discussed. Finally, the paper presents some multiradio practical implementations and test-beds and points out the future research directions.
Wireless Mesh Network (WMN) is a multihop wireless network characterized by low deployment cost that consists of mesh routers and Internet gateways. It is a special ad hoc network with static topologies where unlike the general ad hoc networks mesh routers have no energy limitations. WMN deployments are found in surveillance [1, 2], building automation, remote healthcare delivery [3, 4], and smart grids [5–7]. Mesh routers are the fundamental part of wireless mesh network, which gives routing support for network traffic of mesh clients. Mesh routers can also be equipped with multiple radios to increase the network capacity and to reduce the interference level over the network. The availability and flexibility of IEEE 802.11 components make it a good candidate for wireless mesh deployment. 802.11-based networks provide a cheap and flexible wireless access capability and are easy to deploy in campuses, airports, and hospitals. Backhaul links in 802.11-based WMN can be operated on one of the several nonoverlapping channels (i.e., 12 channels for 802.11a and 3 channels for 802.11b). Furthermore, the cost-effectiveness of network interface cards made it possible to use multiple radios and channels to increase the throughput. Multiple radio configurations allow the utilization of the available multiple orthogonal channels (partially overlapped channel also can be considered as in [8, 9]). The network architecture of multiradio wireless mesh networks (MR-WMNs) is illustrated in Figure 1. Multiradio mesh routers are connected through wireless backhaul links over multiple orthogonal channels. Mesh clients are connected to mesh routers through different set of links referred to as network access links. Mesh clients are user entities with no routing functionality. In the rest of this paper, the term link will refer to backhaul link. The gateway is a mesh router that connects the mesh components with external networks.
Figure 1: The multiradio wireless mesh networks architecture.
Recently, MR-WMN has attracted numerous numbers of research efforts to utilize the advantages that this network is offering. Several proposed approaches on channel assignment (CA) algorithms, multichannel MAC protocols, multichannel routing metrics, links scheduling (LS), multichannel multicast protocols, power and topology control, and network planning exist in the literature. However, the designs that are considering combinations of these issues have shown more efficient performance such as joint routing and link scheduling , joint CA and power control , joint QoS multicast routing and CA , joint gateway selection, transmission slot assignment, routing and power control , joint CA, power control and routing , joint CA, power control and rate assignment , joint routing and topology control with directional antennas , and partially overlapped CA [8, 9].
CA algorithms aim to assign channels to the radio interfaces and links with the objective of minimizing the overall contention and interference over the wireless links. CA can either be solved as a separated problem as in [17, 18] or jointly solved with routing as in [19, 20]. Furthermore, it can be developed as centralized solutions [19, 20] and distributed solutions [21, 22]. The availability of the entire network view makes centralized solution more effective than distributed solutions, which only relayed on local information. Typically, CA algorithms must have knowledge on network load in order to assign channels to links. However, some other algorithms do not require any traffic information, as in , where the interference is minimized by conserving a -connected topology.
Joint design approaches in WMNs are surveyed in many works in the literature such as [24–27]. However, none of the works in the literature have comprehensively investigated the Centralized Joint Channel Assignment and Routing (C-JCAR) approaches. This motivates us to undertake an in-depth review of different C-JCAR proposals in the literature. Figure 2 shows the research direction of this paper. The rest of the paper is organized as follows. Section 2 discusses key design issues, models, and approaches for C-JCAR algorithms. Section 3 presents reviewed works under two subsections of C-JCAR approaches and reconfiguration CA/routing algorithms. Section 4 investigates some practical implantations and test-beds for MR-WMNs. Finally, Section 5 describes the future research directions and concludes the paper.
Figure 2: Organization of the paper.
2. Design Issues, Modeling, and Approaches for C-JCAR
This section identifies the key design issues from link and network layers for C-JCAR approaches to address and clarifies interference model and network model to be used in problem formulation and algorithms. It also presents the mathematical formulation of the problem including optimization objectives, constraints, and fairness. Besides, this section also gives the general classifications to C-JCAR algorithms.
2.1. Key Design Issues
2.1.1. Channel Assignment Schemes
Based on the underling hardware, CA can be implemented in one of three schemes. If the implemented technology does not support any link-level synchronization then only Static CA (SCA) can be implemented and channels are assigned to radio interfaces for a long term. However, if synchronous coordination is supported at link layer then SCA can be implemented with link scheduling (LS) and the wireless link can be active on specific time slots only; let us refer to this case by SCA-LS. Dynamic CA (DCA) is the general case where radio interfaces are capable of switching their channel in a small time compared with the time slots. Thus, wireless links can operate on different channels at different time slots. To illustrate the difference between the three CA schemes, Figure 3 shows the possible channel allocation on a wireless link. As it is shown in Figure 3, for SCA links are allowed to be active on one channel only at all times. In SCA-LS links are allowed to be active on one channel only and on specific time slots. However, if DCA is adopted links can operate on different channels on different time slots.
Figure 3: CA types on a wireless link.
2.1.2. Traffic and Routing Issues
As in load-aware algorithms, the aggregated traffic loads over mesh routers are required as an input. This traffic information can either be measured/estimated online as in [28, 29] or assumed based on a historical profile . WMNs can be used to deliver two types of traffic, the Internet traffic and peer-to-peer traffic. Internet traffic is the traffic received or directed to the Internet [20, 30–33]. Either outbound traffic (to the gateway) [20, 30] or inbound traffic (from the gateway)  can be considered in finding the network configuration. In , specific software such as IPFIX system  is used at the gateway to collect the traffic information. The upper and lower bound of the traffic demand are assumed to be available for each mesh router . A traffic profiler at each mesh router collects the traffic information and sends it to a centralized entity [29, 35] where peer-to-peer traffic is assumed. References [36, 37] assume the traffic load is elastic and only information on source-destination communication pairs are considered.
Routing algorithms in wireless ad hoc networks are categorized as proactive (table-driven), reactive (on-demands), and hybrid algorithms. In proactive algorithms such as [38–40], routing table is built individually through exchanging routing information with other nodes in the network. In reactive algorithms such as [41–43], no routing tables are maintained and instead each node triggers the route discovery process whenever it has traffic to deliver to a destination. Hybrid algorithms [44–47] implement the concepts of proactive and reactive routing protocols. Ad hoc routing protocols are extended to be used in WMNs. The routing decisions in these protocols are decentralized process and each node is responsible to make its routing decision. On the other hand, in centralized routing paradigm routing decision is performed at a centralized entity and nodes build their routing/forwarding table based on the updates received from a centralized entity. In WMNs, if traffic is identified per mesh client then mesh routers must maintain a forwarding entry for each mesh client and layer-2 addresses are used to identify each flow. This is similar to 802.11s standard with Hybrid Wireless Mesh Protocol (HWMP) where path selection is based on layer two addressing. However, path selection is usually associated with scalability issues. On the other hand, if the traffic is identified per mesh router and flow is defined as aggregated traffic that originated from one mesh router to another mesh router, then layer-3 routing is used. In most cases, centralized routing approaches follow the latter definition of flow. A TCP-level flow path selection is assumed in . However, this introduces more overheads to the routing layer. Centralized routing proposals built routing tables based on source-destination manner, where both source and destination addresses are required in the routing decision. With centralized routing in MR-WMN, modification is required on the routing table structure and the Address Resolution Protocol (ARP). However, a complicated routing scheme is required for dynamic CA especially if no static binding is applied between radio interfaces and neighbors. For better exploitation of the topology structure of MR-WMN, multipath routing is assumed in most related works to achieve load balancing. However, the out-of-order problem and the configuration complexity are the main drawbacks of multipath scheme.
2.1.3. Topology and Connectivity Issues
Several mechanisms have been used in the literature to control the topology formations in WMNs. This includes power control [48–50], the use of directional antennas [16, 51], and routing [23, 52–54]. Furthermore, in multiradio architecture CA is another factor that determines the physically connected topology. An overview of the topology control mechanisms and issues in MR-WMNs is presented in . In centralized algorithms, the full network topology is assumed to be available at the centralized entity. Topology can be obtained using existing routing protocol such as OLSR  or the network management protocol as in . In order to define different possible types of topology formation, let us define logical links as the set of potential links that directly connect mesh routers if proper channels are assigned to their radios, and let us define the physical links as the actual wireless links with a designated channels assigned to it. Since inappropriate mapping can lead to disjoint topology, mapping the logical link onto physical link must be carefully determined. Furthermore, assigning channels to all logical links affects the channel diversity on the network. This is due to node-radio constraint (elaborated more in next subsections), more specifically with a small number of radio interfaces. On the other hand, physical links need to be mapped onto the active links, where active links are the set of links carrying traffic and are determined by routing algorithm. Figure 4 shows two types of physical and logical links mapping. In Figure 4(a), channels are assigned to each logical link as in [19, 29, 37]. In Figure 4(b) channels are assigned to a number of logical links only as in [20, 36, 56, 57]. However, some works allow multiple physical links to exist between adjacent mesh routers [30, 56] which need to be considered in the routing procedure. Meanwhile, connectivity must also be addressed in CA algorithms to prevent isolating parts of mesh routers from receiving the control messages and the configuration updates from the centralized controller. Connectivity is achieved in [19, 29, 37] by assigning a channel to each virtual link or by dedicating a radio on a common channel at each mesh router .
Figure 4: Topology formation, logical-to-physical links mapping.
2.2. Interference and Network Modeling
2.2.1. Interference Modeling
Interference plays an important role in wireless networks and has a significant impact on the network performance. MR-WMNs are proposed to reduce the contention on the communication channel and to distribute the transmission over several channels. CA algorithms tend to exploit the channel diversity to reduce the interference levels in the network. Several interference models are proposed in the literature to model the interface in the wireless networks. Three different interference models are presented in this section, namely, the interference-range model, the protocol model of interference, and the physical model of interference. In order to explore these interference models let us assume that all radios in the network have fixed transmission power with omnidirectional antennas and the signal propagation model is based only on frequency and distance. Each radio will have a fixed transmission-range () and two radios can form a wireless link if they are within the transmission-range of each other; see Figure 5(a). The receiver radio at each wireless link can correctly receive and decode the transmission in the absence of any interfering radio.
Figure 5: (a) Transmission-range and (b) interference-range model.
The interference-range model is the simplest model where each radio has its interference range () where : (default ). A directional transmission on a wireless link is successfully completed if the receiving radio is not within the interference range of another active transmitting radio, as shown in Figure 5(b). Thus, in order to successfully receive the transmitted signal from radio () to radio () on wireless link (, ), (1) must be true:
On the other hand, interference in protocol model does not assume a fixed interference range. Instead, the interference is determined based on the Euclidean distance between interference radios, transmitting radio, and receiver radio. Thus, a directional transmission on a wireless link can be successfully completed if the distance between the interfering radio and the receiving radio is larger than the link’s length. Thus, in order to successfully receive a signal transmitted from radio to radio on wireless link in protocol model (see Figure 6(a)), (2) must be true:
Figure 6: Interference based on (a) protocol model and (b) physical model.
In comparison with the previous two models, the physical model is the most realistic model, where concurrent transmissions from multiple interfering radios are accounted in this model. In order to successfully receive the transmitted signal the SINR (signal-to-interference-and-noise ratio) at the receiving radio must be greater than a predefined threshold. Thus, radio can successfully receive the transmitted signal from radio on wireless link ) (see Figure 6(b)), if (3) is true:where is the thermal noise power in the frequency channel, is an SINR-threshold which is determined based on the considered modulation and coding schemes, and is the received power from at . SINR-threshold is chosen in a way that the resulting BER is higher than acceptable BER by the modulating technique. SINR-threshold is based on the experimental observation and can be mapped onto a bit error probability . The physical model of interference is extended in  to include the shadowing effect of the RF signals. The work of  studied the minimum required number of channels to achieve interference-free channel assignments under realistic interference model called SINR model with shadow.
In the conventional 802.11 MAC protocol with Carrier Sense Multiple Access-Collision Avoidance (CSMA/CA), wireless links are bidirectional since the sender still needs to receive acknowledgement messages from the receiver (if RTS-CTS is enabled, it also receives CTS message). Therefore, all transmissions causes that interfere with the sender or the receiver must be avoided for successful transmission. In this model, sending and receiving nodes on a link are potential source of interference to another link transmission. Figure 7 shows the set interfering radios to wireless link () based on the 802.11 bidirectional model.
Figure 7: Interference based on interference-range model for 802.11.
2.2.2. Network Modeling
Network in MR-WMN architecture can be modeled using either a router-to-router model [20, 56, 57] or a radio-to-radio model [31, 36, 61]. In router-to-router model, the network is modeled as a directed graph , where represents the set of mesh routers in the networks and is the set of directed links. A directional link exists between two mesh routers if direct communication is possible between them. In radio-to-radio model, vertices represent the radio interfaces. A link exists between two vertices, if radios can communicate directly and are placed in two adjacent mesh routers. Radio-to-radio model is usually used if the radio interfaces are heterogeneous and support different technologies or if different radios operate on different subsets of channels. Figure 8 illustrates the difference between radio-to-radio and router-to-router model for single channel. For multichannel scenarios, each link between two radios must be replicated into the number of available orthogonal channels supported by the mesh router or radio pairs .
Figure 8: Router-to-router versus radio-to-radio models.
2.3. Problem Formulation and Approaches for C-JCAR
2.3.1. Optimization Objective Functions and Fairness
The C-JCAR algorithms have been designed in the literature for different optimization objectives and metrics; below is the set of optimization objective functions and metrics:(i)Maximize aggregated network throughput [29, 31, 33, 36, 62].(ii)Maximize achievable scaling factor () [20, 30, 31, 56, 61].(iii)Maximize the aggregated utility of flows .(iv)Minimize the maximum interference on all channels [20, 63].(v)Maximize the minimum unutilized capacity on links .(vi)Maximize the cross-section good-put .(vii)Minimize the path length and link contention [32, 64].
Fairness is also another objective to be considered. Fairness ensures a fair resource allocation among network users or traffic. Several fairness constraints are introduced for C-JCAR. To elaborate the fairness constraints let us assume FL is the set of flows in the network and let , be the demanded load and the actual achieved load of flow ; the four types of fairness constraints in the literature are as follows.
(i) Min- Fairness. This constraint is to ensure that each flow achieves at least of its demands. The Min- fairness constraint is presented in
(ii) Max-to-Min Fairness. This constraint is to constrain the deference between the highest and the lowest achieved loads. Then, for a given μ the Max-to-Min fairness constraint is given in where is the lowest and is the highest load.
(iii) Proportional Fairness. This fairness can be used as an optimization objective function and it is given as in
(iv) Bounded Fairness. This fairness configuration bounds each flow with specific upper and lower bound.
Min- fairness is considered in [20, 29–31, 56, 61] where flows may have different demands. Other fairness constraints such as Max-to-Min fairness, proportional fairness, and bounded fairness are used in [37, 62] and , respectively.
2.3.2. Mathematical Formulation Constraints
The general set of constraints to be considered in the mathematical formulation of the C-JCAR problem can be divided into six sets as follows.
(1) Flow Routing Conservation Constraints. These constraints ensure that for each traffic flow from source to destination the net amount of traffic out of each mesh router is equal to the flow rate () if the mesh router is the source of the traffic and () if it is the destination and otherwise 0. More constraints can be added to the problem to determine the routing scheme (for multipath or single-path).
(2) Radio Constraints. These constraints ensure that the number of channels assigned to each incident link on a mesh router (at any given time slot for dynamic CA) does not exceed the number of radio interfaces at that node. This integer-value constraint can be relaxed to linear constraints to ensure that the total load on the links on each node is not higher than the number of radios on that node multiplied by the channel capacity.
(3) Interference Constraints (or Capacity Constraints/Schedule-Ability Constraints). These constraints ensure that the amount of data flows on interfered links does not exceed a specific value. This is to constraint the maximum contention level over the collision domains if contention-based MAC is considered or to ensure a feasible interference-free link scheduling if contention-free MAC is used.
(4) Link Capacity Constraints. These constraints ensure that the traffic load on a wireless link is not exceeding the link capacity. This constraint is implicitly considered under the interference constraints if only sufficient condition for schedule-ability is considered.
(5) Fairness Constraints. These constraints ensure fairness in allocating resources to different traffic flow demands.
Apart from the above-mentioned constrained, other constraints on the topology can also be introduced. The remaining part of this section will be discussed with regard to the radio and interference constraints. The Notations depicts a set of notations for mathematical formulation employed in the derivation.
If CA and interference-free scheduling is computed for all links in time slotted system, the allocated capacity of a wireless link can be obtained by
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