Vol.2 No.4 December 15, 2006 
Mobile Multimedia Networking and Context-Aware Computing

Editorial (ppi-ii)
        H.-H. Hsu, M.C. Angelides, and T.K. Shih                       

Research articles:
Distributed Coordination Protocols to Realize Scalable MultimediaStreaming in Peer-to-Peer Overlay Networks (pp283-296)
         T. Enokido, Y. Tanaka, S. Itaya, and M. Takizawa 
Multimedia contents are distributed to peers in various ways in peer-to-peer (P2P) overlay networks. A peer which holds a content, even a part of a content can provide other peers with the content. Multimedia streaming is more significant in multimedia applications than downloading ways in Internet applications. We discuss how to support peers with multimedia streaming service by using multiple contents peers. In our distributed multi-source streaming model, a collection of multiple contents peers in parallel transmit packets of a multimedia content to a requesting leaf peer to realize the reliability and scalability without any centralized controller. Even if some peer stops by fault and is degraded in performance and packets are lost and delayed in networks, a requesting leaf peer receives every data of a content at the required rate. We discuss a pair of flooding-based protocols, distributed and treebased coordination protocols DCoP and TCoP, to synchronize multiple contents peers to reliably and efficiently deliver packets to a requesting peer. A peer can be redundantly selected by multiple peers in DCoP but it taken by at most one peer in TCoP. We evaluate the coordination protocols DCoP and TCoP in terms of how long it takes and how many messages are transmitted to synchronize multiple contents peers.

Client-Centric Usage Environment Adaptation Using MPEG-21 (pp297-310)
         A. A. Sofokleous and M. C. Angelides 
Enabling universal multimedia access either focuses research on content adaptation or on the usage environment adaptation. When a client device makes a single video streaming request, either the streaming server adapts the video or the client device adapt its own properties. Where a client device makes multiple concurrent video streaming requests across several streaming servers, if the streaming servers adapt the videos, then concurrent local video adaptation may easily result in a network bottleneck and if the client device adapts its own properties without any consideration of the bandwidth requirements of each video request, of the local usage environment, of the streaming servers’ resources and available adaptation solutions then this may easily result in a device bottleneck. In this paper we propose a client-centric usage environment adaptation framework which allows a client device to generate multiple concurrent video streaming requests across several video streaming servers and using MPEG-21 tools describe the client’s and servers’ usage environment, constraints and resource adaptation policies, determine optimal adaptation solutions and bandwidth requirements for each video request and distribute adaptation across the servers.

Resource Adaptation for Mobile AV Devices in the UPnP QoS Architecture (pp311-326)
         M. Ditze 
Distributed multimedia applications increasingly populate Audio-Visual (AV) mobile and desktop devices. Crucial to the success of these applications is the delivery of Quality of Service (QoS) which often refers to resource reservation on devices and network links. Resource reservation is usually ensured by an appropriate admission control (AC) that determines if another request for resources can be granted without interfering already accepted traffic flows and processing resources. If an AC cannot be processed or cannot grant the requested resources, proactive measures like resource adaptation may control the resource requirements of particular flows. Resource adaptation adjusts the resource demands of the application according to the available resources. This paper presents a flexible and hybrid resource adaptation framework for multimedia applications that is incorporated into the UPnP QoS architecture. Furthermore, a simple methodology for controlling resource adaptation in mobile networks and devices is presented. An implementation of resource adaptation proves that the resource utilization is optimized and the QoS can be effectively maintained even in overloaded conditions.

Performance Evaluation of Real-Time Transport with Link-layer Retransmissions in Wired/Wireless Networks (pp327-343)
         P. Papadimitriou and V. Tsaoussidis 
Real-time transport over wired/wireless networks is challenging, since wireless links exhibit distinct characteristics, such as limited bandwidth and high error rates, due to fading or interference. We focus on the efficiency of mechanisms that bind operationally wired and wireless links. In this context, local error control is attractive, due to the remarkable feasibility of wireless link protocols in terms of wide range deployment. We investigate whether local retransmissions enable TCP to efficiently utilize wireless resources under the constraint of bounded end-to-end delay. Based on an analytical approach, as well as extensive simulations, we show that local recovery prevents wasteful end-to-end retransmissions and allows the transport protocol to utilize a higher fraction of the available bandwidth. However, we uncover undesirable effects of local error control which degrade the performance of real-time delivery in several occasions. Furthermore, we investigate whether local error control compares favorably with selected transport-layer mechanisms.

Qos-Energy Aware Broadcast for Heterogeneous Wireless Ad Hoc Networks (pp344-358)
          A. Durresi, V. Paruchuri, M. Durresi, and L. Barolli
We present QoS Geometric Broadcast Protocol (QoS-GBP), a novel broadcasting protocol for heterogeneous wireless ad hoc networks. The growing number of multimedia applications over wireless ad hoc networks require low delay from network protocols and in particular from broadcasting. While broadcasting is a very energy-expensive protocol, it is also widely used as a building block for a variety of other network layer protocols. Therefore, reducing the energy consumption by optimizing broadcasting is a major improvement in heterogenous wireless ad hoc networks networking. QoS-GBP is a distributed algorithm where nodes make local decisions on whether to transmit based on a geometric approach. QoS-GBP enables a tradeoff among the need for neighborhood information (communication overhead) and the delay. QoS-GBP is scalable to the change in network size, node type, node density and topology. Through simulation evaluations, we show that QoS-GBP is very scalable and guarantees minimum delay.

GUARD: a GUide, Alarm, Recovery and Detection System on a Wireless Sensor Network for the Blind (pp359-370)
         H.-C Keh, K.-P. Shih, C.-Y. Chang, H.-C. Chen, and C.-M. Chou 
The paper proposes a GUARD (GUide, Alarm, Recovery, and Detection) system for the blind. The main goal of the GUARD system is to construct an obstacle-aware wireless sensor network and provide guide, alarm, recovery, as well as detection functions for the blind to guarantee his safety and convenience at Tamkang University. Some hardware components as well as communication and management protocols are implemented in the GUARD system.