1.3 Organization of the Thesis
2.1.1 Channel Access and Scheduling Optimizations for WMN
Two different types of channel access and scheduling protocols are widely studied in the literature for WMN design - the TDMA based channel access and the contention based channel access. Both of these mechanisms have their own merits and demerits. The TDMA based channel access and scheduling protocols provide collision free channel access, and are efficient in terms of fairness. However, in a distributed mesh environment, hard TDMA is difficult to implement, because it requires perfect coordination and synchronization among mesh routers. On the contrary, the contention based channel access are distributed in nature, however does not guarantee collision free channel access. Further, it may result severe unfairness in a multi-hop environment.
TDMA based Channel Access and Link Scheduling
A number of works exists in the literature that discuss about link scheduling and channel access in TDMA based WMN. Most of the works in this domain models the scheduling and channel access problem as a graph theoretic problem of finding maximum number of conflict free nodes that can be scheduled simultaneously [76]. For this purpose, the network is modeled as a communication graph, where every node represents the mesh routers, and the links correspond to the ongoing communication. An interference graph is extracted from the communication graph, where the links of the communication graphs are represented as individual nodes, and there exists an edge between two nodes if the corresponding links interfere. The interference graph is used to find out an interference free scheduling in the network, and accordingly the TDMA time slots are allocated.
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(a) Communication Graph (b) Interference Graph
Figure 2.1: Communication and Interference Graphs
As an example, let us consider Figure 2.1. The dotted edge denotes the physical link between two nodes and the arrow denotes an ongoing communication. An arrow in the communication graph is represented as a node in the interference graph. For example, the communication between the node 1 and the node 3 is represented as the node{1,3}in the interference graph. There exist and edge between the node{1,3} and the node {2,3} in the interference graph, because these two communications interfere with each other (both the node 1 and the node 2 can not transmit simultaneously to the node 3). With the help of this interference graph, the problem of finding an interference free scheduling is similar to find the maximum independent set from the interference graph. As an example, the node{1,3}and the node{5,6}from the interference graph forms a maximum independent sent, and hence they can transmit simultaneously. Therefore, these two communications can be scheduled in the same TDMA time slot.
The performance of the scheduling algorithms based on pure graph theoretic conjectures depends on how efficiently the interference graph is constructed from the communication graph. In [77], the authors propose a graph based solution for minimum delay scheduling in TDMA mesh networks. They have interpreted scheduling delay as the cost of transmission order of the links, and formulate an optimization that finds a transmission order with the min-max delay across a set of multiple paths. They have shown that the problem is NP-complete for arbitrary network graphs, and proposed a solution to find the transmission order with the min-max delay for a tree based network topology. Brar et al.[78] have proposed a graph based heuristics to find a near-optimal solution for spatial TDMA based mesh network, while considering the physical interference model. In [79], the authors have proposed a distributed link scheduling algorithm over the TDMA based WMN. Their algorithm runs in two phrases. In the first phrase, an iterative procedure is used to find locally feasible schedules by exchanging link scheduling information between
nodes. The iterative procedure uses a modified Bellman-Ford algorithm over the network conflict graph. The second phrase uses a wave based termination procedure to detect the termination of the local schedules. The algorithm results in a conflict free schedule within finite convergence time. TDMA based link scheduling can provide conflict free schedules, however have some disadvantages, as follows:
The scalability is a serious issue for these scheduling algorithms. Every mesh router needs to have complete network information to design the communication graph and interference graph. Global communication graph is dynamic in nature, and changes when a new flow is introduced or an existing flow terminates. Most of the scheduling algorithms do not sustain for the dynamic communication and interference graphs.
Similarly, the distributed scheduling algorithms, as proposed in [79] and others, may infer long convergence time when the network size is large.
The second issue with these scheduling algorithms is the reliability. Though the algorithms theoretically guarantee conflict-free scheduling, however, in practice it may show problems for dynamic communication and interference graphs. A small variation from the conflict-free property may results in severe unfairness among the end-users because of the uncontrolled packet losses.
Contention based Channel Access and Link Scheduling
Most of the works in the literature propose to design protocol enhancements over the contention based channel access and link scheduling, as it resembles the well-established IEEE 802.11 Distributed Coordination Function (DCF) that uses a contention based channel access using binary exponential back-off (BEB) mechanism. Two different types of optimization procedures are designed for contention based channel access. First, the graph based optimization which is similar to the TDMA based access, as discussed earlier. The second type is to design a non-linear optimization based on the interference constraints, where the objective is to maximize the network performance. The objective function for such optimizations can have several variations, like maximizing the network throughput and fairness, or minimizing the end-to-end delay. In [80], the authors have used a dynamic programming mechanism for optimized link scheduling in a WMN. They have modeled the link scheduling problem as an integer programming problem. As integer programming is known to be NP-hard, they have proposed an approximate dynamic programming method to reduce the dimensionality in the integer programming, and provide a near-optimal solution. Yuet al.[81] have proposed a channel assignment and link scheduling mechanism
for multi-channel multi-radio WMN. They have modeled the scheduling problem as an optimization to maximize the network capacity with minimum bandwidth demand. Kumar et al.[82] have defined a ‘Link Cost Metric’ to order the scheduling of links for maximizing the overall capacity and throughput in a WMN. They have considered the joint problem of channel assignment and link scheduling over contention based protocols, that minimizes the network interference. Several other protocols have been designed in the literature, like [83–86] that uses non-linear optimization based methods for solving the channel assignment and scheduling in contention based WMN.
The fundamental problem of designing a non-linear optimization to model the channel access of a WMN is that, the distributed or randomized versions of the solutions require sufficient time to converge, therefore, not scalable for large networks. Further, most of these schemes assume equal communication and interference ranges for a mesh router, which is far from the practice. Recently, ‘back-pressure scheduling’ [87] based channel access protocols have attracted much attention for the researchers. In this class of scheduling algorithms, whenever a mesh router detects congestion in the network, it back- propagate this information by falsely injecting some packets in the network, therefore, creating a temporal congestion effect that propagates towards the edge-routers. In [87], the authors have shown that back-pressure scheduling can significantly improve the performance of IEEE 802.11 based WMN. The advantage of this scheduling mechanism is that it is completely distributed in nature and does not make any assumption over the interference range of a mesh router. However, for the proper implementation of such access mechanisms, improved loss detection techniques are required, that can segregate normal packet errors from congestion related losses, as shown in [88].