Amulti-channel model for initial and handoff accesses in a mobile computing environment

The communication infrastructure of a mobile computing environment can be based on the structure of a cellular/microcellular telephone system or a PCS network. In such a system, the occurrence of handoffs cannot be avoided and when handoffs occur, wireless links held by mobile computers crossing cell boundaries may be forced to terminate. The probability that a handoff access request will result in forced termination has a significant effect on the performance of a mobile computing environment, as does the probability that an initial access request will be blocked. Although some research has been done on initial and handoff accesses in cellular/microcellular telephone systems and PCS networks, the analytical models used in this research are not appropriate for mobile computing, since unlike a telephone, a mobile computer may use several channels at once. In this paper, we develop an analytical model to study initial and handoff accesses in a mobile computing environment. The model is based on a multi-dimensional continuous time Markov chain. The accuracy of the model is verified by comparison with simulation results. We then use the model to find a practical approach to balancing the initial access blocking probability and avarage forced termination probability of a connection in a mobile computing network.This research was supported by the National Science Council, ROC, under grant NSC 85-2213-E-009-063.     

Fast Duplicate Address Detection for Seamless Inter-Domain Handoff in All-IPv6 Mobile Networks

In All-IP networks, each computer or communication equipment needs an IP address. To supply enough IP addresses, the new Internet protocol IPv6 will be used in next generation mobile communication. Although IPv6 improves the existing Internet protocol (IPv4), Duplicate Address Detection (DAD) mechanism may consume resources and suffer from long delay. DAD is used to make sure whether the IP address is unique or not. When a mobile node performs an inter-domain handoff, it will first generate a new IP and perform a DAD procedure. The DAD procedure not only wastes time but also increases the signaling load on Internet. In this paper, we propose a new DAD mechanism to speed up the DAD procedure. We create a DAD table in access or mobility routers in All-IP networks and record all IP addresses of the area. When a new IP address needs to perform DAD, it can just search in the DAD table to confirm the uniqueness of the address. Furthermore, we propose a new method, Fast Duplicate Address Detection (Fast DAD), to reduce data search time. Simulation results show that our method outperforms the existing DAD methods.     

The design and implementation of a distributed image understanding system

Computer vision is concerned with extracting information about a scene by analyzing images of that scene. Performing any computer vision task requires an enormous amount of computation. Exploiting parallelism appears to be a promising way to improve the performance of computer vision systems. Past work in this area has focused on applying parallel processing techniques to image-operator level parallelism. In this article, we discuss the parallelism of computer vision in the control level and present a distributed image understanding system (DIUS).In DIUS, control-level parallelism is exploited by a dynamic scheduler. Furthermore, two levels of rules are used in the control mechanism. Meta-rules are concerned mainly with which strategy should be driven and the execution sequence of the system; control rules determine which task needs to be done next. A prototype system has been implemented within a parallel programming environment, Strand, which provides various virtual architectures mapping to either a shared-memory machine, Sequent, or to the Sun network.     

A Fair Scheme for Multi-channel Selection in Vehicular Wireless Networks

In recent years, the IEEE 802.11p/1609 wireless access in vehicular environments standards adopt the dedicated short-range communications multi-channel architecture for vehicular wireless networks. To utilize the multi-channel architecture, each vehicle equipped with two sets of transceivers can operate concurrently on three different channels. For example, in cluster-based multi-channel schemes, a cluster head vehicle coordinates and assigns an appropriate channel to its cluster members. However, these schemes are unsuitable for a single channel device performing on only one RF channel at a time which would waste channel resource and increase time to allocate a channel. Another approach, called LEACH-based scheme, selects channels randomly and ensures that each channel is selected once within a round in each vehicle. However, this leads to a situation that different vehicles might select the same channel in short-term duration. In this paper, we propose a multi-channel selection scheme, called minimum duration counter (MDC) scheme, which could apply to a single channel device, while utilizing the multi-channel architecture of an 802.11p/1609 network. In addition, we compare the MDC scheme with pure random (PR) and LEACH-based schemes in terms of fairness index (FI) and utilization to emphasize the fairness and to balance the traffic of multi-channel usage. Furthermore, we analyze the counter overflow probability distribution and propose solutions to the MDC scheme. Numerical results show that our scheme outperforms the PR and LEACH-based schemes in terms of multi-channel usage, traffic balancing, and fairness.     

Enhanced binary search with cut-through operation for anti-collision in RFID systems

RFID systems allow contactless identification of objects using radio frequency. When there is more than one transponder within the interrogation area of a reader, all the transponders may send data (or tags) at the same time which may lead to mutual interference. This event causes data loss and is referred to as a collision. In this letter, we propose an enhanced binary search with cut-through operation to minimize the anti-collision cost. The analytic results show that the proposed scheme achieves low delay in the collision resolution with low power consumption.     

Two Auction-Based Protocols for Fair and Fast Resource Assignment in Wireless PCS

The Resource Auction Multiple Access (RAMA) protocol was recently proposed for fast resource assignment in wireless PCS networks. The protocol assigns communication resources to subscribers using a collision resolution method based on subscriber ID's. In each auction cycle, the subscriber with the highest ID will obtain the resource. Thus the RAMA protocol encounters the unfairness problem since subscribers with lower ID's might be delayed forever. In this paper, we propose two modifications to the RAMA protocol to solve the unfairness problem. The first modification forces the RAMA protocol to process requests in bursts, i.e., new requests are not processed until the current burst of requests are all satisfied. The second modification asks the base station to select subscribers randomly instead of the one with the highest ID. We derive mathematical models to compare the performances of proposed and original protocols. Performance results indicate that the modification of random selection is fairer than other protocols in terms of the waiting time distribution.     

HLR Replication Protocols for Global Mobility Management in PCS Networks

The new trend for PCS networks is to provide mobile users with large-scalemobile capability across many service areas. In this scenario, global databasemanagement for PCS networks has become an increasingly important researchissue.In this paper, we examined two replicated database strategies,single-replica (SR) and multiple-replica (MR), for large-scalemobility of per-user data management in personal communicationnetworks. The SR strategy uses a single replica approach of HLR.The MR strategy replicates the per-user data of HLR in manyregions. The two strategies are based a partial replicationscheme, and a primary copy method is used to maintain replicas'consistency. Our numerical results show that the MR strategyoutperforms the SR strategy in most situations; however, it may beworse when the probability of a mobile user visiting a foreignregion is high and the query rates from other foreign regions arelow. Additionally, the number of replicas should be compact in theMR strategy in order to achieve a reasonable query response time.Therefore, we propose an adaptive multiple replication protocol tochoose a suitable replication strategy and to decide the optimizednumber of replicas.     

Resource reservation with pointer forwarding schemes for the mobileRSVP

We propose a pointer forwarding scheme for the mobile resource reservation protocol (MRSVP) to reduce the resource reservation cost on the wireless Internet. We show that the pointer forwarding scheme could significantly degrade the reservation cost when a mobile host performs locality movement     

A fair protocol for fast resource assignment in wireless PCS networks

Efficient sharing of communication resources is essential to PCS networks since the wireless bandwidth is limited. The Resource Auction Multiple Access (RAMA) protocol was recently proposed for fast resource assignment and handover in wireless PCS networks. The RAMA protocol assigns available communication resources (e.g., TDMA time slots or frequency channels) to subscribers one at a time using a collision resolution protocol based on subscriber ID's. However, the RAMA protocol encounters an unfairness problem; furthermore, performance results also indicate that it is inefficient at transmitting fixed-length subscriber ID's. Moreover, the emerging services such as teleconferencing have been presenting new challenges to dynamic-priority resource assignment. In this paper, we propose a modification to the RAMA protocol to improve its performance and resolve the unfairness problem. The proposed protocol also adopts dynamic priority assignment to improve the QOS for subscribers in overload environments.     

Q-RAMA: QoS-guaranteed deterministic packet access protocol in PCSnetworks

The RAMA (resource auction multiple access) protocol and its variations use a collision resolution protocol based on subscriber IDs to allocate available communication resources. However, these protocols suffer from unfairness or high packet dropping probability. The authors propose and analyse a novel deterministic packet access protocol, Q-RAMA, to improve the quality of service in PCS networks. Simulation results show that Q-RAMA has the lowest packet dropping probability and better overall performance