Seokjae Ha

Bio: Seokjae Ha is an academic researcher from Seoul National University. The author has contributed to research in topics: Routing protocol & Wireless network. The author has an hindex of 2, co-authored 3 publications receiving 65 citations.

K-hop cluster-based dynamic source routing in wireless ad-hoc packet radio network

Abstract: The existing wireless networks, such as cellular networks, personal communication services and mobile Internet protocol use the fixed network as their backbones. However in the situations like disaster rescues, wireless conferences in the hall, or battlefields, there exists no fixed communication infrastructure. Therefore, routing to send data packets to their destinations becomes very difficult. Distance vector and link state protocols used in the existing fixed networks are not suitable for supporting host movements. Variations of distance vector protocol, dynamic source muting schemes, and cluster-based schemes have been suggested to solve the muting problem in this entirely wireless network. However, since the link channel resource is very scarce, the muting scheme must consider the overall channel utilization as well as host mobility. We present a generalized approach called "k-hop cluster-based dynamic source routing". This approach is simple and can take actions on the host movements quickly.

Cluster-based dynamic source routing protocol in wireless ad-hoc packet radio network with variable-sized clusters and variable transmission ranges

Abstract: Many research efforts have been made for supporting mobile users in wireless ad-hoc network where there is no infrastructure such as wired base stations or central entities used in cellular wireless network and wireless LAN, respectively. Although many routing protocols have been proposed, the proposed routing protocols assumed bidirectional wireless links, which means that the transmission ranges of all nodes are the same. The assumption is not realistic, and in this paper we deal with the environment where the transmission ranges of all nodes may be different due to differences in wireless radio power. The routing protocol suggested previously by our group, called "K-hop cluster-based dynamic source routing protocol" is modified to handle the situation. In addition, we propose a scheme where each node has its adaptive K value according to the movement rate. By simulation, the proposed solutions proved to be efficient and realizable.

Fast update of forwarding tables in Internet router using AS numbers

Abstract: The updates of router forwarding tables can be made faster using the autonomous system number corresponding to a prefix as an intermediate number between the prefix and the next-hop address. At the cost of fast update, one table lookup introduces small additional delay, which can be eliminated by pipelining. This scheme is applicable to several routing table lookup algorithms for fast update.

Connectivity based k-hop clustering in wireless networks

Abstract: In this paper we describe several new clustering algorithms for nodes in a mobile ad hoc network. We propose to combine two known approaches into a single clustering algorithm which considers connectivity as a primary criterion and lower ID as secondary criterion for selecting cluster heads. The goal is to minimize the number of clusters, which results in dominating sets of smaller sizes (this is important for applications in broadcasting and Bluetooth formation). We also describe algorithms for modifying cluster structure in the presence of topological changes. Next, we generalize the cluster definition so that a cluster contains all nodes that are at a distance of at most k hops from the cluster head. The efficiency of four clustering algorithms (k-lowestID and k-CONID, k=1 and k=2) is tested by measuring the average number of created clusters, the number of border nodes, and the cluster size in random unit graphs. The most interesting experimental result is stability of the ratio of the sum of CHs and border nodes in the set. It was constantly 60-70% for 1-lowestID and 46-56% for 1-ConID, for any value of n (number of nodes) and d (average node degree). Similar conclusions and similar number were obtained for k=2. We also proposed a unified framework for most existing and new clustering algorithms where a properly defined weight at each node is the only difference in the algorithm. Finally, we propose a framework for generating random unit graphs with obstacles.

Connectivity Based k-Hop Clustering in Wireless Networks

Abstract: In this paper we describe several new clustering algorithms for nodes in a mobile ad hoc network. The main contribution is to generalize the cluster definition and formation algorithm so that a cluster contains all nodes that are at distance at most k hops from the clusterhead. We also describe algorithms for modifying cluster structure in the presence of topological changes. We also proposed an unified framework for most existing and new clustering algorithm where a properly defined weight at each node is the only difference in otherwise the same algorithm. This paper studied node connectivity and node ID as two particular weights, for k=1 and k=2. Finally, we propose a framework for generating random unit graphs with obstacles.

A access-based clustering protocol for multihop wireless ad hoc networks

Abstract: The ad hoc network is a temporary wireless system without a fixed (wired or wireless) infrastructure. Many clustering algorithms have been proposed to partition mobile users into clusters to support routing and network management. Most previous studies, however, focus on algorithm design, lacking an overall evaluation of clustering overheads. We design a multiple access scheme for the broadcast of control messages, and propose a new access-based clustering protocol (ABCP) whose cluster formation is heavily influenced by the outcome of the multiple access. By taking into account many delicate aspects in the clustering process, the ABCP provides a generic, flexible, rapidly deployed and stable cluster architecture for the upper layer protocols. Simulation is used to compare ABCP with the other clustering strategy in terms of cluster stability and overheads. Since ABCP makes clustering decision directly based on the result of channel access, it requires fewer control overheads and has shorter convergence time than the other clustering criteria. We also demonstrate that the resulting cluster structure by ABCP behaves stable in face of topology changes.

TCP-BuS: Improving TCP performance in wireless Ad Hoc networks

Abstract: Reliable data transmission over wireless multi-hop networks, called ad hoc networks, has proven to be non-trivial TCP (Transmission Control Protocol), a widely used end-to-end reliable transport protocol designed for wired networks, is not entirely suitable for wireless ad hoc networks due to the inappropriateness of TCP congestion control schemes Specifically, the TCP sender concludes that there is network congestion upon detecting packet losses or at time-outs However, in wireless ad hoc networks, links are broken as a result of node mobility and hence some time is needed to perform route reconfiguration During this time, packets could be lost or held back Hence, the TCP sender could mistake this event as congestion, which is untrue A route disconnection should be handled differently from network congestion In this paper, we propose a new mechanism that improves TCP performance in a wireless ad hoc network where each node can buffer ongoing packets during a route disconnection and re-establishment In addition to distinguishing network congestion from route disconnection due to node mobility, we also incorporate new measures to deal with reliable transmission of important control messages and exploitation of TCP fast recovery procedures Our simulation compares the proposed TCP-BuS approach with general TCP and TCP-Feedback Results reveal that TCP-BuS outperforms other approaches in terms of communication throughput under the presence of mobility

TCP-BuS: improving TCP performance in wireless ad hoc networks

Abstract: Reliable data transmission over a wireless multi-hop network, called the ad hoc network, has proven to be non-trivial. The TCP (transmission control protocol), a widely used end-to-end reliable transport protocol in a wired network, is not entirely suitable when applied to a wireless ad hoc network due to TCP's congestion control schemes. In particular, the TCP at the source considers the network as congested when detecting packet losses or timeouts. However, in a wireless ad hoc network when a route disconnection occurs because of node movement, the network mistakes this as a congestion. Therefore, the conventional TCP congestion control mechanism cannot be applied, because a route disconnection must be handled differently from a network congestion. We propose a new mechanism that improves the TCP performance in a wireless ad hoc network where each node can buffer packets during a route disconnection and reestablishment. Additionally, we incorporate new measures to deal with the reliable transmission of important control messages. Our simulation results further confirmed these advantages.