Shou-Chih Lo

Architecture for mobility and QoS support in all-IP wireless networks

Mobility management and quality-of-service (QoS) provisioning are the important tasks on the future development of wireless networks. The high host mobility makes these tasks more challenging. In this paper, we propose an architecture which supports both mobility and QoS management in Internet protocol (IP)-based wireless networks. In mobility management, the fast handoff, which the packets are forwarded in advance to the neighboring locations where a mobile node (MN) may move to, is provided to reduce the service disruption. Also, the fast location lookup, which the routing information about a MN is replicated to some routers, is provided to avoid the triangular routing problem incurred by the protocol of mobile IP. In QoS provisioning, we enable the end-to-end QoS guarantee by using the resource reservation protocol (RSVP) signaling. In particular, the RSVP aggregation technique is used to avoid the scalability problem. Also, the technique of passive resource reservation is used to reduce the influence of host mobility on the resource reservation delay. We emphasize the integration of mobility and QoS management in the architecture design. A performance analysis is given to justify the benefits of our proposed architecture.

Optimal index and data allocation in multiple broadcast channels

The issue of data broadcast has received much attention in mobile computing. A periodic broadcast of frequently requested data can reduce the workload of the up-link channel and facilitate data access for the mobile user. Since the mobile units usually have limited battery capacity, the minimization of the access latency for the broadcast data is an important problem. The indexing and scheduling techniques on the broadcast data should be considered. We propose a solution to find the optimal index and data allocation, which minimizes the access latency for any number of broadcast channels. We represent all the possible allocations as a tree in which the optimal one is searched, and propose a pruning strategy based on some properties to greatly reduce the search space. Experiments are performed to show the effectiveness of the pruning strategy. Moreover, we propose two heuristics to solve the same problem when the size of the broadcast data is large.

An efficient multipolling mechanism for IEEE 802.11 wireless LANs

To expand support for applications with QoS requirements in wireless local area networks (WLANs), the 802.11 E Task Group was formed to enhance the current IEEE 802.11 Medium Access Control (MAC) protocol. The multipolling mechanism was discussed in the task group, but some problems remain unsolved. In this paper, we show a design of the multipolling mechanism with the advantages of high channel utilization and low implementation overhead. In our proposed mechanism, wireless stations use a priority-based contention scheme to coordinate in themselves the transmission order on the channel. Moreover, we propose a polling schedule mechanism for our proposed multipoll to serve real-time traffic with constant and variable bit rates. The bounded delay requirement of the real-time traffic can be satisfied in our scheduling model. We establish an admission test to estimate the system capacity and to determine whether a new connection can be accepted. We study the performance of our proposed mechanism analytically, as well as through simulated experiments. The results show that the proposed mechanism is more efficient than the one discussed in the IEEE 802.11 E task group.

Broadcast data allocation for efficient access of multiple data items in mobile environments

The issue of data broadcast has received much attention in mobile computing. A periodic broadcast of frequently requested data can reduce the workload of the up-link channel and facilitate data access for the mobile user. Many approaches have been proposed to schedule data items for broadcasting. However, the issues of accessing multiple data items on the broadcast channel are less discussed. Two problems are discussed in this paper, that is, deciding the content of the broadcast channel based on the queries from the clients, and scheduling the data items to be broadcast. We will show that these two problems are NP-complete. Different heuristics to these problems are presented and compared through performance evaluations.

A strategy for efficient access of multiple data items in mobile environments

Many approaches have been proposed to schedule data items for broadcasting in a mobile environment. However, the issues of accessing multiple data items on the broadcast channel are less discussed. In this paper, an efficient strategy for accessing multiple data items in mobile environments is proposed. Two issues are investigated in this paper, that is, deciding the content of the broadcast channel based on the queries from the clients, and scheduling the data items to be broadcast. Experiments are performed to compare our proposed methods for deciding the content and to show that our data scheduling methods can reduce the average access time without considerable increase on processing time.

An adaptive access method for broadcast data under an error-prone mobile environment

A ubiquitous information environment can be achieved by the mobile computing technologies. In this environment, users carrying their portable computers can retrieve local or remote information anywhere and at anytime. Data broadcast, with its advantages, has become a powerful means to disseminate data in wireless communications. Indexing methods for the broadcast data have been proposed to speedup access time and reduce power consumption. However, the influence of access failures has not been discussed. For the error-prone mobile environment, the occurrence of access failures is often due to disconnections, handoffs, and communication noises. Based on the distributed indexing scheme, we propose an adaptive access method which tolerates the access failures. The basic idea is to use index replication to recover from the access failures. One mechanism, named search range, is provided to dynamically record the range where the desired data item may exist. According to the search range, an unfinished search can be efficiently resumed by finding an available index replicate. A performance analysis is given to show the benefits of the method. Also, the concept of version bits is applied to deal with the updates of the broadcast data.

Data allocation on wireless broadcast channels for efficient query processing

Data broadcast is an excellent method for efficient data dissemination in the mobile computing environment. The application domain of data broadcast will be widely expanded in the near future, where the client is expected to perform complex queries or transactions on the broadcast data. To reduce the access latency for processing the complex query, it is beneficial to place the data accessed in a query close to each other on the broadcast channel. In this paper, we propose an efficient algorithm to determine the allocation of the data on the broadcast channel such that frequently co-accessed data are not only allocated close to each other, but also in a particular order which optimizes the performance of query processing. Our mechanism is based on the well-known problem named optimal linear ordering. Experiments are performed to justify the benefit of our approach.

A Multi-Head Clustering Algorithm in Vehicular Ad Hoc Networks

Clustering is an important research topic in wireless networks, because cluster structures can facilitate resource reuse and increase system capacity. In this paper, we present a new clustering algorithm that considers both node position and node mobility in vehicular ad hoc environments. The proposed algorithm intends to create stable clusters by reducing reclustering overhead, prolonging cluster lifetime, and shortening the average distance between cluster heads and their cluster members. Most important, this algorithm supports single and multiple cluster heads. Simulation results show the superiority of our clustering algorithm over the other three well-known algorithms.

An adaptive retransmission scheme with QoS support for the IEEE 802.11 MAC enhancement

The medium access control (MAC) protocol of the IEEE 802.11 standard is based on carrier sense multiple access with collision avoidance (CSMA/CA). The basic retransmission mechanism, binary exponential backoff, may cause large packet delay and jitter values that are not suitable for real-time traffic. In this paper, we first investigate some MAC enhancement mechanisms discussed in the IEEE 802.11 task group E, which was formed for enhancing the current 802.11 MAC protocol to support for applications with quality of service (QoS) requirements. Then, we propose a jamming-based retransmission mechanism that is compatible with the 802.11 standard and could reduce the packet delay of real-time traffic. Besides, this mechanism performs stably when the traffic load is heavy. The optimal setting of our proposed mechanism is discussed analytically. We perform simulated experiments by comparing our proposed retransmission mechanism with the other two mechanisms discussed in the 802.11 task group E. We show that the jamming-based retransmission mechanism can reduce the packet delay and the packet dropping rate.

Routing and Buffering Strategies in Delay-Tolerant Networks: Survey and Evaluation

Delay Tolerant Networks (DTNs) have attracted considerable attention in recent years. This kind of network works in communication environments subject to delays and disruptions. Traditional end-to-end routing fails in DTNs due to intermittent connections. A variety of routing strategies for DTNs have been proposed in the past. In this paper, we present a survey of these strategies and provide a classification. Moreover, we evaluate their use in social contact networks. Buffer management strategies are also needed to support routing operations. The technical issue is to design sorting policies that determine the transmission and drop order of messages in the buffer. We identify several sorting indexes and evaluate their performance.