Wanjiun Liao

The Split and Merge protocol for interactive video-on-demand

A true video-on-demand (VoD) system lets users view any video program, at any time, and perform any VCR-like user interactions. To reduce the per-user video delivery cost, multiple users may be batched and share the same video stream. Existing sharing schemes do not allow true VoD. A new protocol, called Split and Merge (SAM), does allow true VoD. SAM also provides an innovative way to merge these individuals back into the batching streams when they resume normal play mode.

Performance model of interactive video-on-demand systems

An interactive video-on-demand (VoD) system allows users to access video services, such as movies, electronic encyclopedia, interactive games, and educational videos from video servers on a broadband network. This paper develops a performance evaluation tool for the system design. In particular, a user activity model is developed to describe the usage of system resources, i.e., network bandwidth and video server usage, by a user as it interacts with the service. In addition, we allow batching of user requests, and the effect of such batching is captured in a batching model. Our proposed queueing model integrates both the user activity and the batching model. This model can be used to determine the requirements of network bandwidth and video server and, hence, the trade-off in communication and storage costs for different system resource configurations.

Throughput and delay performance of IEEE 802.11e enhanced distributed channel access (EDCA) under saturation condition

In this paper, we analyze the saturation performance of IEEE 802.11e enhanced distributed channel access (EDCA), which provides contention-based differentiated channel access for frames of different priorities in wireless LANs. With EDCA, quality of service (QoS) support is provided with up to four access categories (ACs) in each station. Each AC behaves as an independent backoff entity. The priority among ACs is then determined by AC-specific parameters, called the EDCA parameter set. The behavior of the backoff entity of each AC is modeled as a two-state Markov chain, which is extended from Bianchis model to capture the features of EDCA. The differences of our model from existing work for 802.11e EDCA include: (i) virtual collisions among different ACs in an EDCA station are modeled, thus more accurately capturing the behavior of EDCA; (ii) the influence of using different arbitrary inter-frame spaces (AIFS) for different ACs on saturation performance are considered; (iii) delay and delay jitter are derived, in addition to saturation throughput. The analytical model is validated via ns-2 simulations. The results show that our analytical model can accurately describe the behavior of IEEE 802.11e EDCA

Design of light-tree based logical topologies for multicast streams in wavelength routed optical networks

In this paper, we formulate an optimization problem for the design of light-tree based logical topology in wavelength division multiplexing (WDM) networks. The problem is comprised of two parts: (1) multicast routing and wavelength assignment of light-trees, and (2) the design of light-tree based logical topology for multicast streams. In the first part, we use mixed integer linear programming (MILP) to solve the optimal routing and wavelength assignment problem of light-trees with an end-to-end delay bound, and obtain the optimal placement of power splitters and wavelength converters. The numerical results show that networks with just a few power splitters and wavelength converters can efficiently carry multicast data. In the second part, we extend the above formulation to design the logical topology based on light-trees for multicast streams. In our approach, a light-tree can carry data of multiple multicast streams, and data of a multicast stream may traverse multiple light-trees to reach a receiver. The numerical results show that our approach use network resources more efficiently, as compared to the approach with a separate light-tree for a multicast stream and to the approach of transporting multicast streams over lightpath based logical networks.

A Region-Based Clustering Mechanism for Channel Access in Vehicular Ad Hoc Networks

Several contention-based Medium Access Control (MAC) protocols have been proposed for the vehicles to gain the radio channels to distribute active safety messages timely, e.g., Safety Critical Application (SCA) information, for inter-vehicle communications in Vehicular Ad Hoc Networks (VANETs). In these MAC protocols, a contention period is introduced before the vehicle for channel access so that we may not have the timely and reliably message dissemination in VANETs. To reduce the contention period, this paper proposes a Region-based Clustering Mechanism (RCM) to be applied in these MAC protocols. We propose analytical models to investigate the performance of the RCM, which are validated by the simulation experiments.

Utility-based resource allocation in wireless networks

In this paper, we study utility-based maximization for resource allocation in the downlink direction of centralized wireless networks. We consider two types of traffic, i.e., best effort and hard QoS, and develop some essential theorems for optimal wireless resource allocation. We then propose three allocation schemes. The performance of the proposed schemes is evaluated via simulations. The results show that optimal wireless resource allocation is dependent on traffic types, total available resource, and channel quality, rather than solely dependent on the channel quality or traffic types as assumed in most existing work.

Interference-aware QoS routing for multi-rate multi-radio multi-channel IEEE 802.11 wireless mesh networks

QoS routing in multi-channel wireless mesh networks (WMNs) with contention-based MAC protocols is a very challenging problem. In this paper, we propose an on-demand bandwidth-constrained routing protocol for multi-radio multi-rate multi-channel WMNs with the IEEE 802.11 DCF MAC protocol. The routing protocol is based on a distributed threshold-triggered bandwidth estimation scheme, implemented at each node for estimating the free-to-use bandwidth on each associated channel. According to the free-to-use bandwidth at each node, the call admission control, which is integrated into the routing protocol, predicts the residual bandwidth of a path with the consideration of inter-flow and intra-flow interference. To select the most efficient path among all feasible ones, we propose a routing metric which strikes a balance between the cost and the bandwidth of the path. The simulation results show that our routing protocol can successfully discover paths that meet the end-to-end bandwidth requirements of flows, protect existing flows from QoS violations, exploit the capacity gain due to multiple channels, and incurs low message overhead.

Relay Station Placement Strategy in IEEE 802.16j WiMAX Networks

In this paper, we study the relay station (RS) placement strategy in IEEE 802.16j WiMAX networks. Specifically, the impact of RS placement on IEEE 802.16j network performance is analyzed. A throughput maximization RS placement problem is mathematically formulated as a binary integer programming problem. We prove the NP-hardness of the formulated problem. To find the sub-optimal solution to the problem with huge input size, we propose an efficient near-optimal placement solution for IEEE 802.16j WiMAX networks. Simulations on the IEEE 802.16j network performance with our RS placement strategy are conducted. The throughput performance shows that with the deployment strategy we proposed, the IEEE 802.16j network capacity can be tremendously enhanced, especially when hotspots are present in the network.

Location-Dependent Throughput and Delay in Wireless Mesh Networks

In this paper, we model the location-dependent throughput and delay in wireless mesh networks. We analyze the packet-arrival and the packet-departure rates for the forwarding queues at relaying nodes and then derive the per-user throughputs and packet delays experienced by nodes at different hop-count distances from the gateway. Based on this analytical model, we propose two network design strategies to provide fair resource sharing and minimize the end-to-end delay in wireless mesh networks. We then conduct simulations to validate the correctness of our analytical model and evaluate the performance of the proposed network design strategies. The results show that our analytical framework can accurately model the location-dependent throughput and delay in wireless mesh networks and that our proposed strategies effectively and efficiently provide fair throughputs for nodes and reduce the end-to-end packet delay. This paper not only provides a framework for studying the performance of wireless mesh networks but also gives insights into the network design strategy for wireless mesh networks.

Intersection-based routing for urban vehicular communications with traffic-light considerations

In this article, we explore the impact of traffic lights on routing protocol design in vehicular ad hoc networks. Specifically, we focus on intersection-based routing protocols designed for vehicular communications in urban areas. We find that traffic lights greatly impact routing in urban areas, but this issue has not been well treated in existing routing protocols. In this article, we propose a new protocol called Shortest-Path-Based Traffic-Light-Aware Routing (STAR) for VANETs with traffic light considerations. For STAR, the signals of traffic lights on the intersection, together with the traffic patterns, determine how packets are forwarded. We show via analysis and simulation that our proposed STAR protocol is superior to Green Light First forwarding (i.e., packets are forwarded on the path with green lights). We also conduct ns-2 simulations to evaluate the performance of STAR. The results show that STAR achieves shorter average delay, higher delivery ratio and higher TCP throughput than existing routing protocols for urban VANET communications.