Mesh routers build the core network architecture of a WMN. A mesh router can be equipped with multiple interfaces, and can operate in multiple channels. Every mesh router forms a static mesh connectivity with the neighboring routers. With such a connectivity, multiple paths exist between any pairs of mesh routers separated by multiple hops. Mobile ad-hoc networks (MANET) [32] provide similar type of multi-hop architecture, however have some key differences from mesh networks, that demand separate protocol designing challenges. First, the underline topology in a MANET changes with the time due to the mobility, and therefore the mesh path selection protocols are designed based on the reactive or on-demand approaches. On the other hand, WMN provides fixed mesh architecture where the proactive approaches are more preferable. The second difference is that in case of MANET, all nodes are of equal priority in terms of traffic forwarding characteristics. In a mesh network, mesh routers are heterogeneous in nature as they need
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Figure 1.1: Forwarding Load and Capacity Calculation of a Mesh Network
to forward the relayed traffic along with the traffic from associated clients. Therefore, mesh routers need to be prioritized based on the traffic load they forward, such that a proper traffic distribution can be maintained. In a typical community mesh network, most of the traffics are towards or from mesh gateways. Therefore, mesh routers near the gateway are more loaded compared to the edge-routers, which are far from the gateway. With such differences, the well designed MANET protocols do not scale well in a mesh network, and a new set of protocols are required to be designed to support MAC layer mesh activities.
Capacity degradation due to heterogeneous load distribution among mesh routers is a fundamental problem of a mesh network built over the IEEE 802.11 technology [33, 34].
To illustrate the behavior of mesh routers, let us consider Fig. 1.1. The dark node is the mesh gateway, and all other nodes are mesh routers. For simplification in illustration, let us consider that there are G numbers of clients associated with every mesh router, and every client forwards unit amount of traffic towards the gateway. The arrow denotes the forwarding path from the mesh router towards the mesh gateway, and the values written beside the arrows denote the total traffic load to be forwarded by that mesh router, based on the above simplified assumption of uniform client distribution. Let us analyze the behavior of a mesh router R1. In a mesh network, two transmissions are called to be interfered, if they can not be performed simultaneously because the packets from one
transmission may collide with the other transmission, resulting in frequent packet losses.
Interference generally happens if at least one of the two receivers fall within the common range of both the transmitters. In Fig. 1.1,R1 acts as one of the transmitters. The shaded region along R1 denotes the set of routers with which the transmission of R1 interferes.
Let us call this region as the ‘interference region’ for R1. The total traffic load to be forwarded byR1 is 9G, whereas, the total traffic load for all the routers in the interference region of R1 is 34G. Therefore, for a maximum capacity channel access, R1 should get (9/34)×100% = 26.47% of the total bandwidth. However, as IEEE 802.11 provides equal time channel share to all contending nodes, R1 actually gets (1/9)×100% = 11.11%
of the channel share, as a total of 9 routers (excluding the gateway) are there in the interference region of the router R1. This bandwidth under-utilization results in severe performance degradation in a mesh network. As a consequence, the clients are suffered from the unfairness, when network load is high. The performance of mesh clients degrades as the hop distance between the associated router and the gateway increases, because of the insufficient bandwidth availability at the forwarding routers.
Every mesh router needs to contend with its neighboring routers to gain access to the shared media. The region of contention is defined by theinterference range, the range upto which the transmission can interfere. In a wireless media, the transmitted signal gets faded as it propagates more from the transmitter. The communication range of a router is denoted by the distance upto which a transmission from that router can be decoded successfully. In general, the interference range is more than the communication range, that makes the contention issue oppressive for the mesh network. This increased contention reduces the effective capacity of a mesh network [34]. The mesh capacity can be improved by using high gain directional antennas to restrict the interference among the neighbors of a specific directions [35–38]. In directional mesh networks, mesh routers are equipped with multiple-beam multi-interface directional antennas to support more than one transmissions or more than one receptions simultaneously. Directional communications can improve the network performance by allowing multiple simultaneous communications in space (called thespatial reuse). However, the directional communication introduces the problem of interdependency among the link scheduling and the forwarding activities in a mesh network. The maximum transmission rates of individual directional links depend on the end-to-end path characteristics. On the contrary, the forwarding decision should result in a path that provides maximum end-to-end bandwidth with the minimum delay guarantee.
Therefore, a joint link scheduling and mesh path selection protocol need to be designed to effectively schedule multiple directional links with the capacity constraint.
Quality of Service (QoS) is another important issue for community networks, both from the perspective of the service provider and end-users. Different types of network traffic requires different amount of network resources for sustaining. For example, the voice traffic requires strict delay and bandwidth guarantee, whereas the video traffic requires minimum jitter (variation in end-to-end delay) support. Therefore, for assuring QoS, reservation of the network resources are required to be allocated dynamically based on the specific service requirements. Two types of service provisioning architectures have been incorporated in the Internet design - the integrated service architecture [39], and the differentiated service architecture [40]. The integrated service architecture provides strict QoS requirements, however demands a centralized controller for the service provisioning. On the contrary, the differentiated service architecture provides soft service guarantee, and can work in a distributed fashion. IEEE 802.11 is more preferable to the differentiated service architecture compared to the integrated service architecture, because of its inherent inability of centralized control. The IEEE 802.11e amendment over the basic standard supports service differentiation using Enhanced Distributed Channel Access(EDCA) [41,42]. Unlike EDCA, the mesh channel access protocol in IEEE 802.11s, called the Mesh Coordinated Channel Access (MCCA), does not provide any service differentiation architecture1. Designing an efficient service differentiation technique over MCCA is challenging, because the priority based flow reservation technique follows a non- convex function, which is hard to solve in a distributed environment.
Topology control is another crucial aspect for the performance optimization in a multi-hop mesh architecture. IEEE 802.11s uses Mesh Peer Management (MPM) protocol to support the topology control in a mesh environment. Two neighbor mesh routers can not communicate directly until they establish a mesh peering using the MPM protocol.
However, the standard MPM protocol supports fixed data rates only. On the other hand, most the physical layer technologies, like IEEE 802.11a/b/g/n support multiple physical data rates based on modulation techniques. Different data rates can sustain at different channel conditions, measured in terms of the ‘Signal to Noise Ratio’ (SNR). Low data rates can sustain for low SNR values, whereas high data rates require high SNR values for the correct decoding of the received signal. The current industry standard wireless access technologies support rate adaptation based on physical layer channel conditions [43]. On
1It can be noted that IEEE 802.11s supports EDCA as a mandatory MAC layer protocol and MCCA as an optional MAC layer protocol. However, MCCA provides better performance in a WMN compared to EDCA. EDCA is kept as compulsory protocol to support the backward compatibility, as discussed in the next chapter.
the contrary, IEEE 802.11s MPM uses a fixed data rate for peer establishment, therefore can not avail the advantages of the physical layer rate adaptation. This limits the performance of the IEEE 802.11s mesh technology to a fixed data rate support.
Topology control, channel access and mesh path selection are interdependent in multi-hop mesh networks. As discussed, topology control requires the physical data rate information to establish peering among the neighbors. Selection of an optimal physical layer data rate depends on the network interference and the channel quality of the forwarding path. Conversely, channel access requires the topology information and the interference characteristics of the forwarding path. At the same time, mesh path selection should find out an end-to-end path that provides maximum bandwidth based on the channel access information and minimum end-to-end delay depending upon the topological connectivity. This interdependency makes the performance optimization aspects of a mesh network more challenging. Further, the mesh path establishment in a mesh network faces several challenges. The proactive path selection mechanism finds out the optimum path before the actual forwarding requirements, and it may use the stale information for future decisions. As discussed earlier, traffic characteristics in a mesh network change arbitrarily with the variation of the number of mesh clients and their individual traffic demands. In addition, wireless channel dynamics, like the signal blocking, fading, and the shadowing may result in a stale path information in case of the proactive selection. Conversely, the reactive path selection introduces extra network overhead by flooding control packets every time a path is required to be established.
IEEE 802.11s uses Hybrid Wireless Mesh Protocol (HWMP) for the mesh path selection and maintenance, that is based on a combination of both the proactive and the reactive approaches [44–46]. Therefore the shortcomings of both the proactive path selection and the reactive path selection are inherent in HWMP.
A number of proposals have been published in the literature for joint topology control, channel access and mesh path selection in a multi-hop wireless mesh network, like [47–66] and the references therein. These works model the problem of topology control, channel assignment and mesh path selection as a joint multivariate optimization problem, with either capacity maximization, or throughput maximization, or end-to-end delay minimization as the objective function. The fundamental demerits of these proposals can be summarized as follows,
The solution of a multivariate optimization is known to be NP-hard [67]. Therefore, different heuristics are used to solve the problem from different perspectives. As a consequence, no uniform framework exists that gives a complete solution on the
basis of the mesh networking fundamentals.
Most of the existing solutions assume either general contention based channel access, or time division multiple access (TDMA) channel reservation strategies. IEEE 802.11s mesh networking framework, being a new standard in the wireless domain, is not characterized by any of the above mentioned works. The earlier works are difficult to implement over the IEEE 802.11s framework, as it provides a complete new set of protocols for the topology management, channel access and mesh path selection. Compatibility becomes a serious issue when dealing with multivariate optimization based solutions for the performance optimization in an IEEE 802.11s mesh network.
Therefore IEEE 802.11s, the new standard for mesh networking over the well-established Wireless Local Area Network (WLAN) technologies, demands focused researches to opti- mize the performance of a WMN and to provide extra functionality over the basic proto- cols. The standard has the capability of providing last mile ubiquitous broadband access through a cost-effective wireless solution, however need to be enhanced with modern de- mands of QoS assurance and new features like multi-rate support, for effective usage of the available channel resources.