Chapter 7. Conclusions & Future Work
In this thesis a new protocol for Wireless Mesh Networks (WMN) has been designed, analysed, and evaluated through extensive simulations. This protocol adopts some concepts form the IEEE 802.11s draft standard. In particular, this new protocol relies on the idea to provide routing functionalities at MAC layer and the terminology and logical functionalities for the WMN components: MPP (Mesh Portal), MP (Mesh Point), and MAP (Mesh Access Point) which are used to deploy a WMN.
Three research aspects have been covered in this work: (i) defining optimal criteria for channel selection in a wireless mesh network with multi-interface mesh nodes, (ii) designing a simple and scalable routing/forwarding protocol for MAC layer, and (iii) design a Load Balancing (LB) protocol that operates MAC layer in conjunction with the forwarding protocol.
Based on an extensive number of simulations it appears that it is possible to identify an optimal solution for the channel selection issue based not only of the SNIR (Signal Noise Interferences Ratio) in the channel but also of the information concerning the network topology such as the number of MPP (mesh portal) available and how far a MPP is to the new node. Each solution is obviously tied to the specificities of the network topology and typical traffic patterns. This information should thus be included into the metric to select the best channel for each interface. In order to obtain a lightweight routing protocol that does not involve excessive flooding and traffic overhead, we have decided to allow nodes to route frame only to the MPPs and in the opposite direction (from a MPP to the destination outside the mesh backbone). To increase the network utilization and obtain benefit from multiple interfaces, a LB protocol has been designed as an option in the routing protocol.
From our WMN simulations using the LB protocol we have detected the possibility of a loop existing in the data forwarding process. This event is due to the lack of information concerning the network topology and the limited global network topology view available at each wireless mesh node. This loop doesn’t exist if the only the metric used for the LB algorithm is the load at each MP. However, this type of metric is not suitable to fully exploit the entire network’s capacity. In fact an MAP, that
adopts this metric, tends to select the same path to reach each MPP. Therefore, every time a new user requires a connection to the external resource; its MAP selects the best MPP (that which has the least users connected).Therefore, it will adopt the same path to forward the new traffic through the WMN without evaluating the number of users or the congestion along that path. In conclusion only a small portion of the mesh network will be used to forward traffic. Thus a more distributed approach is required.
As illustrated in Chapter 5, each MAP has to choose the best Mesh Portal and its first hop toward the portal. The other nodes have to select only the next hop to the prefixed Mesh Portal. To reduce the presence of loops in the backbone, a lightweight protocol, based on two management messages, have been defined. A graphical tool, designed to simplify the analysis of the network traffic, shows that not all of the loops can be solved by the protocol, so some of the user's traffic is lost in the wireless backbone.
Thus this traffic is not able to arrive to any MPP.
The traffic analyzer tools developed to analyze the network traffic has been realized as an external element for post processing because it was not possible to access QualNet graphical interface source code to insert eventual modifications.
The traffic analyzer tool is able to display a simple animation for the traffic flowing through the network. It is possible to evaluate the delay to establish a connection between a node and its neighbours, and to display each traffic path in the wireless backbone. An interesting behaviour has been revealed when processing simulation results through the traffic analyzing tool, which helped dimension some of the load balancing major parameters.
An explanation for the second case can be found in the particular layout of the scenario, where the MAPs are all at the opposite side from the mesh portal. In this situation some regions in the scenario, close to the MAP, could test an absence of beacons because almost all of the nodes are engaged in creating peer links so only few nodes are able to send beacons to extend the network. Thus, in this scenario, only a subset of MAPs is able to receive beacons.
Based on the simulation results it is possible to observe a strong correlation between the channel selection and the load balancing in terms of number of local links available for the single hop. This interaction between the load balancing mechanism and the
channel selection procedure also impacts the number of mesh portals that mesh nodes can detect through the beacon information.
This research leaves some points open to future works, starting from the results analysis as in Chapter 6. First, it is important to devise a better solution for the load balancing and tackle the traffic routing loop problem. Ideally a typical solution to this problem should leverage a little bit more the information available in the routing table and eventually evaluate the possibility to add extra routing-related information in the beacon frame.
Another possible future research direction could reside in developing more efficient load balancing –specific metrics to be used for both the load balancing and channel selection. In fact, using SINR measurement just on the last hop is not very revealing of the achieveable throughput throughout the whole path upstream to the MP. Another possible improvement for the LB algorithm would be investigating mechanisms to estimate the mesh backbone dimension and use this information to skip long paths.
This information could be propagatred downstream in the beacon, and used in the weight function for the load balancing as a dynamic parameter for its normalization.
In our current design and implementation, all end-users (Wi-Fi stations) are assumed as stationary users. It would be interesting to introduce mobility management for end-users and their respective traffic. The mobility issue requires redesigning the routing protocol to introduce a handover mechanism to allow users to freely move from one MAP to another without interrupting their serving sessions. The two MAPs involved in the handover should exchange a new management frame to maintain the same final mesh portal and calculate the new path to it. The process to find the new path toward the assigned mesh portal should select a path which is as close as possible to the old one in order to minimize possible negative effects on the load balancing mechanism.
There are a couple of projects that aims to create a Linux Kernel module for the new 802.11s draft standard. These pre-release for 802.11s are partial implementation and work only with one interface with the only availability of MPP and MP logical elements. It would be interesting to deploy a test-bed with one of these pre-releases (open80211s is the most interesting) to carry out some performance test and evaluate the possibility to extend the code to work with more than one interface implementing the routing protocol presented in this research with load balancing functionalities.
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