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A Two-Phase Handoff Management Scheme for Synchronizing Multimedia Units Over Wireless Networks Azzedine Boukerche, Sungbum Hong and Tom Jacob Dept. of Computer Science, University of North of Texas Email : boukerche, hong, jacob@cs.unt.edu
Abstract This paper presents a two-phase handoff management scheme for synchronization algorithms of wireless multimedia systems. This scheme allows mobile hosts to receive multimedia units without delivery interruption while they are moving from one cell to another cell. The messages passing among mobile hosts, base station and servers include a time stamp to compute delay time for each multimedia unit for each server. The scheme always search for a Quasi-receiver among base stations with which mobile hosts are communicating to synchronize multimedia units. We discuss the algorithms and also present a set of simulation experiments to evaluate the performance of our scheme using message complexity and buffer usage at each frame arrival time. Our results indicate that our scheme exhibits no underflow within the bounded delivery time.
1 Introduction Advances in wireless technology allow mobile clients to carry multimedia traffic and have served as an impetus to the emergence of new applications like wireless digital news service and video on demand. Multimedia units can be time dependent or non-time dependent objects. The time dependent objects are video and voice streams. The basic abstraction for a time constrained media element is a timed stream of media components. The term “media component” means a video frame or audio sample. These components normally must be kept in temporal order when they are being played. The process of maintaining this ordering of the media components is referred to as multimedia synchronization [2]. There are two timing aspects for time constrained elements: intra-media continuity is subject to a real time constraint in handling media elements; and inter-media synchronization is subject to temporal correlation during a playback of media elements [13]. The synchronization problem under a combination of wireless/wired networks is more complicated than the problem for wired networks because wireless net This work was supported by the Texas Advanced Research Program Grant TARP/ARP 003594-0092-2001.
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Figure 1. Network with soft Handoff works must use a base station to deliver packets, and have much fewer resources [6]. Furthermore, a combination of wired/wireless network might provide a high-quality access to mobile users. Recall that a cellular wireless communication network has its coverage area partitioned into cells, where each cell is served by an antenna or base station (BS). Cell size and shape depend on signal strength and the presence of obstacles to signal propagation. As shown in Figure 1, a set of BSs are controlled by a base station controller (BSC). Several BSCs are connected to a mobile switching center (MSC), which manages all calls in a large geographical area, doing call set-up and handoff. The links between the BSs and MHs are wireless, whereas those between the BSs and their MSCs are wired. This architecture allows a BS to communicate with many MHs concurrently. All of the MHs can move from one BS to another. Even though the synchronization problem for distributed stored multimedia streams based on fixed networks has been well studied [1, 9], little has been done for cellular wireless networks.
1.1 Related Work Most of the research work to synchronize play-out and delivery of distributed stored multimedia streams has been done for wired network. In this paper, we review research work related to the multimedia synchronization problem. Interested readers may wish to consult [1, 9, 16] for multimedia synchronization work done in a wired network. As we all know, most wireless multimedia services still
Proceedings of the Eighth IEEE International Symposium on Computers and Communication (ISCC’03) 1530-1346/03 $17.00 © 2003 IEEE
need more network resources than are currently available on a wireless network. Depending on resource sufficiency for wireless networks, access speed and mobility can vary so that synchronization schemes must be developed differently to fit varying network conditions. In this paper, we shall review the synchronization schemes for several wireless network types. A Wireless LAN provides wide bandwidth within geographical areas and supports low speed mobility. In [14], four lip synchronization schemes are studied with single transport streams. In [15], a set of lip synchronization schemes are studied with single-stream approach, where the schemes are implemented on a wireless LAN to measure their performance. In both [14, 15], a a single server is used to store multimedia units for all models. PHS (Personal Handy Phone System) is a kind of micro cellular digital cordless telephone system that covers a slightly wider geographical area with medium mobility than Wireless LAN, but covers much less area then PCS (Personal Communication System). Kato et al.[10, 11] proposed applications of a slide control scheme. The first paper [10] showed the slide control scheme to be effective when audio and video streams are transmitted over two wireless channels for mobile hosts. In the second paper [11] the authors studied the interleaved transmission of audio and video for wireless multimedia synchronization as well as a QoS (Quality of Service) control scheme.
nicate using two BSs that belong to different CDMA (Code Division Multiple Access) carriers. In this paper, we propose a two-phase handoff scheme to support synchronization of multimedia units (MMU) for wireless clients and distributed multimedia systems that use BSs as Quasi-receivers to control MMU synchronization. The proposed solutions with MoSync [4] cope with network jitter, end-system jitter, clock drift, and changing network conditions during soft handoff. Hence, our scheme are suitable both for synchronizing inter- and intra-streams during playback video and for re-synchronizing streams in a wireless network. The scheme is proposed for managing MMUs to deliver them to mobile hosts on time. We also present a set of simulation experiments to study the performance of our algorithm. The remainder of the paper is organized as follows. Section 2 introduces the network and system model and handoff model we use in this paper. Section 3 presents a description of the synchronization algorithm for handoff management. Section 4 describe our handoff management scheme, which we refer to as two-phase scheme. Section 5 presents pseudo codes for our handoff management scheme. Section 6 reports simulation experiments evaluating the performance of our algorithms for two different arrival patterns. Section 7 details our conclusions and future research topics.
2 System Model
1.2 Contributions of This Paper
In this section, we describe our system model and introduce the data model used here. The routing problem solved in this paper is initiated by advances in mobile computing and multimedia studies. The system consists of K scalable server nodes, N base station nodes and L wireless clients (mobile hosts). Unlike in fixed networks, most of the time there is only one route between a base station and a mobile host, and a base station can directly communicate with many mobile hosts. Generally, the system contains a variety of servers, depending on the needs of the mobile hosts. In our model, all mobile hosts get services from the same set of servers. Their communication is through base stations, where many servers connect to each base station, and each base station connects to many mobile hosts. Also at certain times, two or three base stations can communicate with a mobile host simultaneously, see Fig. 1. Because of this, we need to assign special roles to the base stations to represent mobile hosts within their cells. During the handoff period, there should be a management function that can handle ordering of MMU arrivals at the base stations. A video consists of many video frames that can be divided into K equal-size parts, called subframes. These Subframes are equally located at K different server nodes using a technique called subframe stripping [12]. During a playback, wireless clients continually receive subframes from a server. Servers
A combination wired/wireless network for multimedia servers differs significantly from a wired network: in delivering packets for mobile hosts, all packets must pass through at least one BS to reach a mobile host; a MH has small memory and small screen size; there is low network bandwidth between a BS and a MH; and a MH can get two or three communication channels from different BSs. Because of such differences, conventional synchronization strategies for delivering MMUs (Multimedia Units) to MHs within a time certain can not be applied in the mixed environment. A single base station must pass many MMUs and much control information to MHs, which can cause congestion at the base station. Because of the small memory and low bandwidth at a MH, this traffic may also cause MH memory underflow. A MH can move from one cell to another cell which means the communication channel can change. A MH can hold more than one communication channels in a certain area. The Mosync algorithm[4] copes with only hard handoff. As shown in Figure 1, there four types of handoffs: Intersector handoff where MHs communicate with two sectors of the same cell; Intercell handoff where MHs communicate with two or three sectors of different cells; Three way soft handoff where MHs communicate with two sectors of a cell and a sector of another cell; and soft-softer handoff where MHs commu2
Proceedings of the Eighth IEEE International Symposium on Computers and Communication (ISCC’03) 1530-1346/03 $17.00 © 2003 IEEE
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