Transmission coordination mechanism (TCM) aids opportunistic routing (OR) to reduce the total number of packet transmissions and improve end-to-end throughput. Existing paradigms based on batch map partitions packets of communication session into segments, and transmit packet segments in batch mode sequentially. However, the rate of successful transmission coordination oscillates due to the oscillation of the number of packets batch transmitted. In this paper, we propose batch sliding window-based TCM to improve the performance of OR. By transmitting packets in continuous batch mode, batch sliding window-based TCM can hold the rate of successful transmission coordination steady. Simulation results show the average end-to-end throughput gain of the proposed TCM is 15.4% over existing batch map-based TCM.

TCP's performance significantly degrades in multi-hop wireless networks because TCP's retransmission timeouts (RTOs) are frequently triggered regardless of congestion due to sudden delay and wireless transmission errors. Such RTOs non-related to congestions lead to TCP's unnecessary behaviors such as retransmitting all the outstanding packets which might be located in the bottleneck queue or reducing sharply its sending rate and increasing exponentially its back-off value even when the network is not congested. Since traditional TCP has no ability to identify if a RTO is triggered by congestion or not, it is unavoidable for TCP to underutilize available bandwidth by blindly reducing its sending rate for all the RTOs. In this paper, we propose an algorithm to detect the RTOs non-related to congestion in order to let TCP respond to the RTOs differently according to the cause. When a RTO is triggered, our algorithm estimates the queue usage in the network path during the go-back-N retransmissions, and decides if the RTO is triggered by congestion or not when the retransmissions end. If any RTO non-related to congestion is detected, our algorithm prevents TCP from increasing unnecessarily its back-off value as well as reducing needlessly its sending rate. Throughout the extensive simulation scenarios, we observed how frequently RTOs are triggered regardless of congestion, and evaluated our algorithm in terms of accuracy and goodput. The experiment results show that our algorithm has the highest accuracy among the previous works and the performance enhancement reaches up to 70% when our algorithm is applied to TCP.

Multihop network is an approach utilizing distributed wireless stations for relaying. In this system, area size, coverage and total transmit power efficiency can be improved. It is shown by computer simulations that the cooperative relaying scheme provides transmit diversity effect, and can offer much better performance compared with that of non-cooperation case. To confirm this superior performance in actual environments, field trials using real time communication equipments are now being planned. This paper reports the design and the performance of wireless equipments for field trials.

When TCP operates in multi-hop wireless networks, it suffers from severe performance degradation. This is because TCP reacts to wireless packet losses by unnecessarily decreasing its sending rate. Although previous loss differentiation algorithms (LDAs) can identify some of the packet losses due to wireless transmission errors as wireless losses, their accuracy is not high as much as we expect, and these schemes cannot avoid sacrificing the accuracy of congestion loss discrimination by misclassifying congestion losses as wireless losses. In this paper, we suggest a new end-to-end loss differentiation scheme which has high accuracy in both wireless loss discrimination and congestion loss discrimination. Our scheme estimates the rate of queue usage using information available to TCP. If the estimated queue usage is larger than 50% when a packet is lost, our scheme diagnoses the packet loss as congestion losses. Otherwise, it diagnoses the packet loss as wireless losses. Because the estimated queue usage is highly correlated to congestion, our scheme has an advantage to more exactly identify packet losses related to congestion and those unrelated to congestion. Through extensive simulations, we compare and evaluate our scheme with previous LDAs in terms of correlation, accuracy, and stability. And the results show that our scheme has the highest accuracy as well as its accuracy is more reliable than the other LDAs.

In multi-hop wireless networks, since source and destination nodes usually have some candidate paths between them, communication quality depends on the selection of a path from these candidates. For network design, characterizing the best path is important. To do this, in [1], [2] we used expected transmission count (ETX) as a metric of communication quality and showed that the best path for ETX is modeled by a path that consists of links whose lengths are close to each other in static one-dimensional multi-hop networks with a condition that the ETX function of a link is a convex monotonically increasing function. By using the results of this characterization, a minimum route ETX can be approximately computed in a one-dimensional random network. However, other metrics fail to satisfy the above condition, like medium time metric (MTM). In this paper, we use MTM as a metric of communication quality and show that we cannot directly apply the results of to the characterization of the best path for MTM and the computation of minimum route MTM. In this paper, we characterize the path that minimizes route MTM in a different manner from [1] [2] and propose a new approximate method suitable for the computation of minimum route MTM.

In multi-hop wireless networks, communication quality depends on the selection of a path between source and destination nodes from several candidate paths. Exploring how path selection affects communication quality is important to characterize the best path. To do this, in [1], we used expected transmission count (ETX) as a metric of communication quality and theoretically characterized minimum route ETX, which is the ETX of the best path, in a static one-dimensional random multi-hop network. In this paper, we characterize minimum route ETX in static two-dimensional multi-hop networks. We give the exact formula of minimum route ETX in a two-dimensional network, assuming that nodes are located with lattice structure and that the ETX function satisfies three conditions for simplifying analysis. This formula can be used as an upper bound of minimum route ETX without two of the three conditions. We show that this upper bound is close to minimum route ETX by comparing it with simulation results. Before deriving the formula, we also give the formula for a one-dimensional network where nodes are located at constant intervals. We also show that minimum route ETX in the lattice network is close to that in a two-dimensional random network if the node density is large, based on a comparison between the numerical and simulation results.

In multi-hop wireless networks, communication quality depends on the route from a source to a destination. In this paper, we consider a one-dimensional multi-hop wireless network where nodes are distributed randomly and theoretically analyze the relation between communication quality and routing policy using a measure called the Expected Transmission Count (ETX), which is the predicted number of data transmissions required to send a packet over that link, including retransmissions. First, we theoretically analyze the mean length of links, the mean number of hops, and the mean route ETX, which is the sum of the ETXs of all links in a route, of Longest Path Routing (LPR), and Shortest Path Routing (SPR). Second, we propose Adjustable Routing (AR), an approximation to Optimum Routing (OR), which minimizes route ETX. We theoretically compute the above characteristic values of AR. We also theoretically compute a lower bound of the mean route ETX of OR. We compare LPR, SPR, and OR using the results of analyses and show differences between these algorithms in the route ETX.

This paper presents a new QoS model for end-to-end service provisioning in multi-hop wireless networks. In legacy IEEE 802.11e based multi-hop wireless networks, the fixed assignment of service classes according to flow's priority at every node causes priority inversion problem when performing end-to-end service differentiation. Thus, this paper proposes a new QoS provisioning model called Dynamic Hop Service Differentiation (DHSD) to alleviate the problem and support effective service differentiation between end-to-end nodes. Many previous works for QoS model through the 802.11e based service differentiation focus on packet scheduling on several service queues with different service rate and service priority. Our model, however, concentrates on a dynamic class selection scheme, called Per Hop Class Assignment (PHCA), in the node's MAC layer, which selects a proper service class for each packet, in accordance with queue states and service requirement, in every node along the end-to-end route of the packet. The proposed QoS solution is evaluated using the OPNET simulator. The simulation results show that the proposed model outperforms both best-effort and 802.11e based strict priority service models in mobile ad hoc environments.

In recent years, studies on multi-hop wireless networks have been made by many brilliant researchers. Such a network consists of a set of mobile nodes having wireless communication devices, and is constructed by the nodes autonomously. The most serious problem on the network is the difficulty of network topology management. All nodes are freely movable and their topology is dynamically changing continuously, so it is difficult to determine the paths to any nodes in the network. Although a classical flooding algorithm is a robust algorithm in that situation, the number of retransmitting nodes increases beyond what is necessary. In this paper, a new and more efficient information dissemination algorithm called WDD is proposed, which can substitute for various broadcasting algorithms by just selecting an appropriate waiting-time function. The algorithm is implemented and its applicability is evaluated on a network simulator.

To enhance throughput and reuse bandwidth, clustering techniques can effectively manage nodes in multi-hop wireless networks. However, in such networks, hidden terminal interference degrades the system performance and increases the average packet delay time. Therefore, this work presents novel two-level cluster-based code assignment techniques to resolve the hidden terminal problems. At the low level, five basic and an optimized intra-cluster code assignment schemes are developed to calculate the number of codes used in each cluster. At the high level, two inter-cluster code assignment schemes (coarse-grained and fine-grained controls) are proposed to obtain the minimal number of code sets. The merits of our schemes include low execution time, low probability of code re-assignment, and low overhead. Furthermore, the proposed schemes allow us to regionally update orthogonal codes when the topology varies. Experimental results demonstrate that the proposed schemes outperform conventional techniques. Among the five basic intra-cluster code assignment schemes, the ordering criteria of increasing number of one-hop neighbors is the best in terms of the number of orthogonal codes to avoid hidden terminal interference. The optimized intra-cluster code assignment scheme generally obtains fewer orthogonal codes than other schemes. For inter-cluster code assignment schemes, the coarse-grained control has a lower code allocation time. However, the fine-grained control requires fewer orthogonal codes.