UWB WiMedia MAC has been widely used in wireless multimedia networks, but it suffers from inefficient channel utilization. In this paper, we propose a reservation based MAC protocol for multimedia data to increase the channel utilization efficiency. The proposed scheme has two components. First, the protocol lets flows with large amounts of data be reserved early among the nodes. Second, when anode allocates slots, the protocol uses wasted slots. We evaluate the performance of the proposed scheme through simulations that compares channel utilization and drop probability with an existing scheme. The simulation results show that the measured performance metrics are improved.

Recent empirical studies have shown that clients gather together in popular locations, which results in load-imbalance, in a network. One of the key wireless LANs (WLANs) challenges is alleviating the load-imbalance while maximizing the bandwidth utilization. However, the IEEE 802.11-based standard does not provide any formal solution to the load-imbalance problem. Several load-balancing schemes have been proposed. Unfortunately, most of them are of limited-benefit under only certain network conditions. In this paper, we propose a joint approach adopting both dynamic-association and adaptive channel-width to enhance the system performance but significantly reducing computational complexity involved. We show that the proposed approach outperforms any individual approaches without increased computational burden. Furthermore, performance measure obtained using the proposed approach is very close to the best result obtained by using an exhaustive search.

Carrier Sense Multiple Access/ID Countdown(CSMA/IC) is a collision-free ad hoc MAC protocol.CSMA/IC avoids simultaneous medium access by competing with a local unique ID assigned to each node. However, assigning the local unique ID to each node is a very complicated problem. In this paper, we propose a simple distributed ID assignment scheme for CSMA/IC. Investigating CSMA/IC, we found that the largest ID screens all IDs of contenders during the contention process. Due to this ldquoID screen effectrdquo, every node can estimate the largest ID among its contenders. Using this knowledge,we propose a self unique ID assignment scheme which does not require message exchanges. To complement our ID assignment scheme, we also suggest a "well arranged rotation ID system". The proposed ID assignment scheme suffers from an ID increasing problem and an unfair medium access problem. However, the new ID system mitigates these problems in a very simple way. Simulation results show the proposed scheme operates quite efficiently and provides medium access opportunities fairly.

In opportunistic networking, networks are sparse and nodes are moving around, so it is hard to predict when a node gets and how long it keeps in contact with another. For prompt neighbor discovery, a node is assumed to broadcast continuously probing messages to discover another in its vicinity. This kind of persistent probing consumes too much energy for battery- operated devices to afford. One way to save energy for neighbor discovery is simply to turn off radio during non-contact time. In this paper, as the first step toward the general solution, we simplify the problem consisting of one sender and one receiver moving around and present a novel neighbor discovery scheme with radio "on" and "off' which is optimal in a sense that it does not miss a contact with the minimum energy consumed.

Adaptive bit allocation for orthogonal frequency division multiplexing (OFDM)-based systems are critical for avoiding performance degradation due to additional bit allocations when channel conditions are bad. In this paper, we present two novel bit-loading algorithms for OFDM-based systems with target bit rate and fixed energy constraints. The proposed algorithms converge to the same bit allocation as the optimal discrete bit-filling and bit-removal algorithms. The proposed algorithms can approach a near-optimal bit allocation with less computational steps by using a water-filling solution at the initial bit allocation step, and then a multiple-bit loading procedure to satisfy the target bit rate. The proposed algorithms were evaluated by comparing with bit-filling and bit-removal algorithms through numerical analysis. Numerical results demonstrate the computational efficiency, fast convergence and optimality of the proposed algorithms.

The point coordination function (PCF) of the IEEE 802.11 WLAN facilitates deterministic frame transfer by a polling mechanism. This enables to support quality of service. Despite this benefit the PCF could be inefficient at low-to-medium traffic loads because it always polls all stations regardless of their actual demand. To overcome this drawback, we introduce a novel polling scheme where all stations simultaneously report their status by sending orthogonal signals, and a BS identifies and polls only those stations having data to send. The feasibility of orthogonal signalling is evaluated in a realistic channel model where multipath fading and additive white Gaussian noise are applied. Moreover, channel utilization of the proposed scheme is investigated analytically, and compared with other schemes through simulation. The evaluation results show that the proposed scheme considerably outperforms other schemes at low-to-medium traffic loads.

In opportunistic networking, it is hard to predict when a node gets and how long it keeps in contact with another. The persistent probing for prompt neighbor discovery consumes too much energy for battery-operated devices to afford. The most important issue is to carefully manage the power states at all stages of network operations. For the problem consisting of one sender and one receiver, we have presented a novel neighbor discovery scheme which is optimal in a sense that it does not miss a contact with the minimum energy consumed. In this paper, we extend the previous result to the optimal neighbor discovery scheme for the general case.

In-network data aggregation is one of the key technologies in wireless sensor networks. Because data aggregation inherently incurs a tradeoff between energy efficiency and delivery delay, several solutions have been proposed to control the waiting time before transmission of an aggregated data (i.e. data aggregation timeout). In this work we consider the case when one packet can carry multiple sensing data elements and this aggregation is limited to a finite number due to the limitation of packet size. Based on the idea that the timeout may be determined by the current number of waiting data elements, we propose a scheme for varying timeout, analyze its performance using a stochastic model, and show how to customize the varying timeout in the proposed scheme for energy efficiency or low delay. Experiment results show that our analytical model accords well with the real world implementation and our scheme facilitates efficient tradeoff between energy and delay, that is, high energy efficiency can be achieved with little overhead in delay, and vice versa.

Orthogonal frequency division multiplexing (OFDM) has been thoroughly investigated as an enabling technology for future broadband multimedia communication. In this paper, we suggest adaptive bit allocation algorithms that operate in a frequency selective fading channel environment by exploiting channel state information obtained through a feedback channel. The proposed algorithms try to maximize the overall throughput of the system with significantly reduced complexity while guaranteeing that mean bit error rate (BER) of all sub-channels remains below the pre-defined BER threshold. To do that, the first proposed algorithm divides all sub-channels into several groups according to their signal-to-noise ratio (SNR). The grouping criteria are adaptive to the current channel state and BER constraint. The second algorithm tries to find the appropriate constellation size for each sub-channel. The proposed algorithms were compared with existing algorithms through simulation. The simulation results show that the proposed algorithms are close to optimum solution with significantly lower complexity.

In sensor routing algorithms, energy efficiency is considered as the main concern since every node is batterypowered. However, some sensor applications require high priority feature like delay-guaranteed data deliveries if it is critical to notice important events within a predefined deadline such as in surveillance systems. In this paper, we set up a data delivery scheme making use of variable transmission range and propose a way to determine the optimal transmission range to provide statistical delay bound and energy efficiency. Note that these two metrics are in the tradeoff relation. And then, we apply our scheme to GEAR [8] to achieve energy awareness which is critical to extend the lifetime of sensor networks. Furthermore, we identify the delay reaction phenomenon, a big obstacle in providing guaranteed delay, and also propose two experimental counter-plans to remove this phenomenon.

Unlike host mobility support, network mobility is concerned with situations where an entire network changes its point of attachment to the Internet. Furthermore, we should consider the situation of several mobile networks recursively attached together. The goal of network mobility support is to provide continuous and optimal Internet access to all nodes located in the mobile network. In this paper we present a solution to support nested network mobility by extending MIPv6. Note that the IETF MIPv6 protocol is for mobile nodes and not for mobile networks according to D. B. Johnson et al. (2002). The main idea is to register a prefix binding in HA and CN with a chain of intermediate mobile router's (MRs) care-of-addresses (CoAs). To the main idea, we introduce new option; the nested care-of-address option (NCO) is used to carry a sequence of MR's CoAs in the header. The router alert option (RAO) is used to indicate that NCO is set. When RAO is set, it is not necessary to encapsulate an outgoing packet. The HA and CN can use the addresses in its binding cache to construct a Type 2 Routing Header to send packets into the mobile network. This extension allows our scheme to give an optimal routing path, avoid the tunnel-in-tunnel problem, save the bandwidth resource and reduce the computation overhead in the home network.

The DQDB (distributed queue dual bus) MAC (media access control) protocol (IEEE 802.6 MAN) is modified to use a single slot as many times as possible on the fly, based on the assumption that all the nodes have their own addresses in an ascending (or descending) order in the direction of data flow. Every node compares the destination address of a slot with its own address to determine the reusability at the cost of one-bit nodal delay which is minimal for active tapping. If a node recognizes that a slot is reusable and it has a proper payload, the slot is reused. This increases the opportunity to access the bus for each node and enhances the network throughput up to 1.61 times that of the original protocol because a slot can be reused several times by multiple nodes. The throughput of the modified DQDB MAC is also compared with that of a slot reuse method employing erasure nodes by simulations using the SLAM language.>

A fork-join queue is considered as a typical model of parallel processing systems with arrival and departure synchronizations. An approach for obtaining the transient and the steady-state solutions of the fork-join queue in terms of the virtual waiting times, which can be used to obtain the response time and the delay between the fork and the join instants, is presented. With the restriction of the service time being the Erlangian distribution, the formulations result in a form of a series, the worth of which is in actually computing numerical results. The joint-state-occupancy probabilities can be easily deduced from the results. The approximate solutions in the steady state are presented for the cases where the interarrival time distributions are exponential and hyperexponential, and the service-time distributions are exponential and two-state Erlangian. The parallelism achievable with synchronization constraints is examined from the approximations.>