Real-time resource reservation for synchronized

Real-Time Resource Reservation for Synchronized Multimedia Object over Wireless LAN Husni Fahmi†, Mudassir Latif† , Basit Shafiq †, Ray Paul‡ , Arif Ghafoor † † School of Electrical and Computer Engineering Purdue University, West Lafayette, IN 47907 ‡ Department of Defense [email protected]

Abstract The increasing use of wireless networks has necessitated the development of a wireless networking infrastructure that supports quality of service (QoS) for multimedia communications. Proxy servers are gaining prominence in facilitating QoS assurance and intelligent resource allocation for multimedia services. In this paper, we propose a proxy-based framework that provides QoS support to mobile users. We describe the overall QoS architecture and implement a number of key QoS functions, including bandwidth allocation and synchronization. In this architecture, admission control is exercised to ensure the long-term availability of resources to mobile clients, where the resources include bandwidth for supporting required presentation rates and buffer for synchronizing media streams. Bandwidth allocation is implemented in Linux Traffic Control using Stochastic Fairness Queueing and Token Bucket Filter disciplines to provide bandwidth guarantees for the multimedia connections. An adaptive synchronization technique is also implemented in the proxy to synchronize multiple streams originating from different sources before transmitting them to the clients for presentation. Finally, a number of quality adaptation strategies that accommodate the diverse requirements and capabilities of clients are described.

1. Introduction There is increasing demand for the development of mobile network technologies addressing the needs of emerging multimedia applications in business, education, telemedicine, and entertainment. Such network services are expected to provide quality of service (QoS) to heterogeneous clients. QoS assurance in real-time mobile communications poses unique technical challenges, due to the rapid variations in bandwidth, as well as the movement of users between wireless cells [2]. Delivery of high throughput multimedia data is problematic, not only due to bandwidth limitation but also due to the heterogeneity of portable devices necessitating the

delivery of different levels of multimedia document quality. These devices may move from one cell to another in the middle of an ongoing session. This mobility of clients can lead to lapses in communication. Intelligent resource allocation and quality adaptation protocols are relied upon for ensuring high fidelity and synchronized delivery of data over wireless channels [2]. In this regard, the role of proxy servers has gained prominence in facilitating multimedia services that support QoS guarantees [8, 6, 18]. Recent work has extended the role of proxies from caching textual documents to caching streaming media for real-time interactive function support [6, 18]. In [6], the proxy stores the salient objects of a document, denoted as hotspots, to enable clients to interact seamlessly with multimedia documents by reducing the network latency experienced by users. Proxies have also been used in wireless network environments for quality adaptation required due to resource limitation. For example in [8], on-the-fly adaptation using transformational proxies, is shown to be a cost-effective and flexible technique for addressing variations in data quality. Another example is the Conductor framework, which implements a generalpurpose adaptation technique to handle both lossy and lossless adaptations by deploying agents at specially enabled nodes in the network [16]. We propose a proxy -based wireless communication framework that provides QoS support and allows mobile users to access multimedia ni formation from distributed multimedia servers. In this framework, a proxy provides comprehensive QoS support over a wireless LAN, supporting admission control, bandwidth allocation, synchronization, and quality adaptation. We describe the overall architecture and implement a number of critical QoS functions, including bandwidth allocation and synchronization. Bandwidth allocation is implemented in Linux Traffic Control [1] using Stochastic Fairness Queueing (SFQ) [12] coupled with Class-Based Queueing (CBQ) [7] which provides bandwidth guarantees for multimedia

Proceedings of the Fifth IEEE International Symposium on Object-Oriented Real-Time Distributed Computing (ISORC’02) 0-7695-1558-4/02 $17.00 © 2002 IEEE

2. Multimedia Document Model The presentation of multimedia documents requires specification of the spatial layout and temporal relations among component objects. Synchronized Multimedia Integration Language (SMIL) [15] has emerged to become the standard for comp osing and presenting synchronized multimedia documents. SMIL aims at modeling the structure and behavior of multimedia documents composed of a set of independent multimedia objects, and presenting them on the Web in a synchronized manner. Users can specify the spatial layout and control the accurate timing of either consecutive or concurrent presentations of multiple media objects such as audio and texts, or video and music. The language also facilitates interactive controls such as stop, fast-forward, and rewind, which interrupt the presentation and move forward or backward in the sequence of presentation. A SMIL document can be logically depicted using a Petri-Net model [10], as shown in Figure 1(a). In this figure, a place, shown as a circle, represents the play-out process of a multimedia object, which may be textual data, image, or an audio/video segment of certain duration. Attributes associated with an object include its type, size, throughput requirements, and the duration of

its presentation. A transition, shown as a vertical bar, represents a synchronization point. It marks the play-out start time of new concurrent objects. The bandwidth requirement profile of this document, shown in Figure 1(b), is known prior to its presentation at the clients. Components of this document can be stored at distributed remote sites. Its presentation requires real-time synchronized delivery of multiple media streams to the end-user. Text

Text

Text Video

Image Video Finish

Start Audio

(a) Temporal structure of multimedia document Bandwidth

connections. Our experimental results show that the proxy can effectively guarantee bandwidth for multimedia connections. An adaptive synchronization technique based on PARK [9] is implemented to synchronize multiple streams in a proxy before transmitting them to clients for presentation. We examine several techniques in admission control, aiming to allow long-term availability of resources to mobile clients moving between cells during their connection lifetime. Considering an indoor networking environment, we suggest the admission control scheme proposed in [11], which exploits the regularity of users’ mobility patterns in a typical office environment to predict the direction of hand-offs. Similarly, we review several quality adaptation techniques and offer guidelines for their implementation. The rest of the paper is organized as follows. In Section 2, a multimedia document model is presented that specifies spatio-temporal relationships among component objects. In Section 3, we present a proxy -based framework for supporting QoS among wireless clients. We review several admission control techniques and suggest a suitable scheme for an indoor wireless network environment. The implementation of bandwidth allocation and synchronization modules is described and experiments are conducted to examine the effectiveness of these modules. We envision several quality adaptation techniques that can be implemented in our framework. Section 4 concludes the paper.

(b) Bandwidth profile of multimedia document Time

Figure 1. Temporal structure and bandwidth profile of a multimedia document A proxy server can play a critical role in delivering multimedia documents to wireless clients by performing QoS functions. In the following section, we discuss in detail our proxy architecture for supporting QoS functions in wireless network environments.

3. Proxy-based Mobile Communication Framework for QoS Support We propose a proxy -based framework, shown in Figure 2, for performing QoS functions in a wireless network. The proxy provides centralized QoS control and manages multiple base stations in its network service area. The QoS architecture at the proxy consists of several key components, including admission control, bandwidth allocation, synchronization, and quality adaptation, depicted in Figure 3. We will discuss in detail each component of the QoS architecture in the following subsections.

3.1. Admission Control Users of wireless networks can roam in arbitrary directions and migrate to any neighboring cell. This migration will cause an ongoing session to be terminated if the newer base station does not have sufficient resources to serve the migratory connection. In order to provide continuous service to mobile users and avoid


hand-off failures, an admission control policy needs to be enforced to filter the connection requests based on the long-term availability of resources.

Server

Base Station

Wide WideArea Area Network Network

Proxy Server

Base Station Server

Server

Figure 2. Proxy-based multimedia services for wireless network The admission control module residing in a proxy accepts a new connection request if sufficient resources are available in the requesting mobile’s current cell and next cells during the connection lifetime or cell residence time in a particular cell. Admission control for the current cell follows a simple method that sums the reserved rates and buffer space. Mobility estimation of users can be determined by using either probabilistic or deterministic mobility estimation techniques. Probabilistic techniques assume no prior information regarding user mobility, and rely on statistical information to predict hand-offs during the connection lifetime. A predictive scheme proposed in [4] estimates the directions and hand-off times of an ongoing connection based on its previous cells and the aggregate history of hand-offs in each cell. Accordingly, bandwidth is reserved in each cell to serve future hand-off connections. Another predictive technique developed in [13] attempts to minimize hand-off drops by estimating the possible hand-offs in a certain time window. The scheme approximates the hand-off dropping probability of a radio cell by the overload probability, which is defined to be the probability of more than N connections handing off to a cell that can support an infinite number of connections. N is the total number of connections a single cell can support in a real system. A higher value of cell overload probability implies a greater number of hand-off failures because of the unavailability of resources. In contrast to the probabilistic techniques, the deterministic counterparts assume a priori knowledge of the users’ mobility throughout their connection lifetime. Either the mobile user provides his/her mobility specification at the time of connection request [14] or the mobility information can be accurately predicted based on the regularity of the users’ mobility patterns and the known relationship between the users and certain cells

[11]. In [11], a per-connection resource reservation scheme is proposed for an indoor mobile computing environment. This scheme maintains per-user and per-cell hand-off profiles, and uses this information to predict the next cell where resources need to be reserved in order to provide continuous service to mobile users. The prediction of the next cell depends on the type of the current and neighboring cells. For some types of cells, such as office, corridor, and meeting room cells, the prediction is deterministic because of the regularity in the users’ mobility patterns. For other types, such as cafeteria or lounge cells, where the users’ movements are random, the next cell is probabilistically estimated as in [4, 13]. Unlike previous schemes, which only address admission control and resource reservation at the wireless entry points, this scheme uses admission control and resource reservation in both wireless and wired networks. In a wired network, multicast routes are established for a mobile user to all neighboring cells, where the packets destined for the mobile user are also transmitted, so that the handoff is smooth and transparent to the user. These multicast routes are established after performing end-toend admission control tests and reserving required resources along the route. To other proxies

Document cache

Document Profiles

Handoff Manager

Data flows from servers

Quality Adaptation

Synchronization

Proxy Server

Bandwidth Allocation

To base station

Admission Control

Figure 3. QoS architecture at proxy server. The probabilistic mobility estimation techniques presented in [4, 13] are more suitable in outdoor environments, where mobile users can roam in any arbitrary direction and a user’s next cell cannot be accurately determined. The per-connection reservation scheme described in [14] can work in any environment, but it requires a priori information of the users’ mo bility. The scheme proposed in [11] is designed for indoor mobile computing environments. It exploits the regularity of users’ mobility patterns specific to a typical office environment. Since our work is mainly focussed on providing quality of service in an indoor wireless LAN, we believe that [11] will be more suitable in our wireless QoS framework.


3.2. Bandwidth Allocation In a typical presentation of multimedia documents over wireless networks, data flows are dominated by data transfer from servers to clients [5]. In our wireless framework, clients occasionally send data to the proxy in order to open or close connections and request documents. However, the amount of this data transfer is marginal compared to that of the data transfers from the proxy to the clients. Hence, the shared bandwidth is mainly contended among data flows from the proxy to the clients. Providing bandwidth guarantees for data flows from the proxy to the clients can guarantee the bandwidth commitments to multimedia sessions. In current packetswitched networks, switches offer only a best-effort service in which packets are transmitted on available bandwidth. The performance of a communication session can degrade significantly when the network is overloaded. Hence, an appropriate packet service discipline needs to be selected at the proxies to provide such guarantees. A service discipline controls the order in which packets are serviced and determines how packets from different connections interact with each other [17]. Once a service discipline is selected, a performance bound can be derived for each connection in the presence of heterogeneous and bursty traffic. We aim at providing bandwidth guarantees for multimedia connections. We assume that the multimedia document profile is known a priori. Users send different levels of bandwidth reservation requests to the proxy for supporting their presentation rates. The proxy then performs dynamic bandwidth allocation according to these requirements. Our system uses Linux Traffic Control to support bandwidth provisioning [1]. This protocol consists mainly of filters that classify packets, and queues into which the classified packets are placed for transmission. A hierarchical link-sharing mechanism, called Class-Based Queueing (CBQ), is used for monitoring and controlling the bandwidth allocations between various classes of traffic [7]. Each class in this link-sharing structure corresponds to either an aggregation of traffic or an individual connection. CBQ ensures that each class receives its allocated link-sharing bandwidth over the relevant time interval. It prevents lower priority traffic from starvation while satisfying the bandwidth requirement of higher priority traffic. CBQ serves as a super-queue, which contains other queues for scheduling the transmission of packets. It does not attempt to implement service disciplines, which schedule packets from leaf classes to satisfy their bandwidth requirements. Several service disciplines are implemented in Linux Traffic Control, including Token Bucket Filter (TBF), Weighted Round Robin (WRR), and Stochastic Fairness Queueing (SFQ). TBF is a simple queue that limits the transmission rate of a session. It is characterized by a

token rate and a bucket depth. The bucket is continuously filled by tokens at a specified token rate. The token rate determines the speed at which the network can transmit packets. A data packet can be sent only if there is a token in the bucket. This type of queue is useful for traffic shaping. WRR consists of a number of dynamically allocated FIFO queues, one queue for each connection. Each queue is visited in a round-robin manner and a certain number of packets, proportional to its assigned weight, are transmitted. Even though this approach can apportion the requested bandwidths among connections, it may not be scalable, as each session is mapped onto its own queue. When the number of sessions becomes very large, maintaining commitments to all flows can become computationally expensive, as it requires numerous memory references that are not suitable for high-speed networks [12]. A service discipline should be simple enough to implement at very high speeds. SFQ reduces the computational requirements of the fair queueing implementation because it does not require one-to-one mapping of a flow to a queue [12]. It is a probabilistic variant of fair queueing. A simple hash function is used to map the connections to a set of fixed queues that are serviced in a round-robin manner. Two connections can collide into a queue, resulting in each connection receiving less than its assigned share of bandwidth. The hash function can be periodically perturbed to prevent connections that have collided during one period from colliding again in the next period. In this case, CBQ can prevent a colliding connection from taking over the bandwidth share of another connection by monitoring and controlling its bandwidth usage. Increasing the number of queues can improve the performance of SFQ because it reduces the number of colliding connections. To provide bandwidth guarantees, the proxy not only reserves bandwidth for the registered multimedia flows, but also ensures that best-effort traffic not registered with the proxy server does not cause congestion on the wireless link, resulting in packet losses. The total available bandwidth on the wireless subnet is divided between registered and non-registered flows. Bandwidth is allocated for each registered flow, while all nonregistered flows are combined into a single “best-effort” class, which is allotted any residual bandwidth. In contrast with the bandwidth allocation to a single multimedia object, the bandwidth allotted to the best-effort class changes as new flows are registered or torn down, and a new value of the residual bandwidth is calculated. A token bucket filter is used to shape the traffic of this besteffort class to prevent non-registered flows from causing congestion on the wireless link.


Bandwidth received (Mbps)

6 5 4 3 2 1

1.1 0 2.1 0 3.1 0 4.1 0 5.1 0 6.1 0 7.1 0 8.1 0 9.1 0 10 .10

0

network traffic into the wireless network (Mbps)

Figure 4. Bandwidth capacity of the wireless LAN For the experimental setup, we use a 700 MHz Pentium machine as the server, a quad-processor 700 MHz Pentium machine as the proxy, and a Dell Latitude C800 laptop as the client in the wireless network. All machines have the RedHat Linux 7.1 operating system. The server and the proxy are connected through a 10Mbps Ethernet LAN while the proxy and the client are connected through an 11 Mbps wireless LAN. The effective capacity of the wireless link was measured by generating a steadily increasing amount of UDP traffic and measuring the bandwidth received by the wireless client. It is observed in Figure 4 that the effective capacity of the wireless link is 5.6 Mbps. We allocate 5 Mbps for the incoming bandwidth from proxy to clients, and 0.6 Mbps for traffic generated by the clients. We examine the effectiveness of the bandwidth reservation scheme for ensuring that QoS flows receive their required bandwidth and preventing best-effort traffic from causing QoS flows to experience packet losses. This experiment is conducted by reserving a QoS flow and setting up a number of best-effort flows. The QoS flow comprises a presentation of multimedia document consisting of a video and audio file with an average bandwidth requirement of 1.20 Mbps. Subsequently, the same presentation was started at multiple times without registering the bandwidth requirements to the proxy. In Figure 5, the QoS flow consistently receives its requested bandwidth. The link-scheduling mechanism ensures that the bandwidth requirements of the registered pres entation are always met. The best-effort flows do not experience losses when the total bandwidth usage is below the effective capacity. As more best-effort flows arrive in the network, each flow experiences reduction in its bandwidth share as evident in Figure 5. In Figure 6, we show in close up the difference between the QoS flow and a best-effort flow as the network traffic increases.

Figure 5. QoS vs. best -effort flows in wireless LAN

Figure 6. QoS vs. best -effort flow in wireless LAN (two flows only)

3.3. Synchronization Multimedia documents are characterized by strict temporal constraints, which specify that the data needs to be presented according to its temporal presentation schedule. Presenting documents across a network faces leads to technical challenges in preserving the temporal relations among different media objects. Current besteffort networks have unpredictable delay and bandwidth characteristics that can disturb the temporal relations of a multimedia document. Multimedia synchronization techniques are used to satisfy the temporal constraints, aiming to achieve smooth coordination and cooperation among various media [9]. Such synchronization techniques are categorized into two types: intra-stream a n d in ter-stream. Intra -stream synchronization aims at preserving the temporal relations between media units


within a media stream, while inter-stream synchronization attempts to preserve the temporal relations among related media streams. Researchers have proposed adaptive synchronization techniques [3, 9], which send feedback messages from clients to servers to adjust the transmission rates. This approach utilizes buffers to temporarily store data streams at clients prior to presentation, in order to maintain the temporal relations between media streams. In our framework, the proxy temporarily holds related portions of the media streams that have arrived. The synchronization module then interleaves the data streams so that they can be transmitted to the clients in a synchronized manner. We have implemented three variations, schemes A, B, and C, of the adaptive synchronization technique proposed in [9]. In this approach, the proxy sends a feedback message to the server describing the buffer conditions. The server adjusts its transmission rate based on the status of the proxy buffer. The server has three different transmission rates, namely, high, normal and slow/paused transmission. Based on the level of buffer occupancy, namely, High Threshold (HT), Run Threshold (RT), and Low Threshold (LT), the proxy sends one of three types of feedback messages to the server, Feedback Slowdown (FB_SLWDN), Feedback Normal (FR_NORM), and Feedback Speedup (FB_SPDUP), respectively. The proxy sends these feedback messages according to the following principles: • If the buffer occupancy is above HT, and the last feedback message was not FB_SLWDN, the proxy sends feedback message FB_SLWDN. • If the buffer occupancy crosses RT, and the last feedback message was FB_SPDUP or FB_SLWDN, the proxy sends feedback message FB_NORMAL. • If the buffer occupancy falls below LT, and the last feedback message was not FB_SPDUP, the proxy sends feedback message FB_SPDUP. • On the server side, the following actions are taken upon reception of the feedback messages: • Upon reception of FB_SPDUP, the server increases the transmission rate by 100% in scheme A, and 50 % in schemes B and C. • Upon reception of FB_NORMAL, the server resumes the transmission of the corresponding media stream at its natural rate. • Upon reception of FB_SLWDN, the server pauses the transmission of the media stream in schemes A and B, or reduces the transmission rate by 50 % in scheme C. Table 1 illustrates the responses of the server to different feedback messages in the three variations of the proposed synchronization mechanism. f is the normal frame rate of the media stream.

Table 1. Feedback messages and frame rates in adaptive synchronization Feedback Message

Resultant State

Frame Rate(A)

Frame Rate(B)

Frame Rate(C)

FB_NORMAL

Normal Underflow Overflow

F 2f 0

f 1.5f 0

f 1.5f 0.5f

FB_SPDUP FB_SLWDN

We conducted experiments to study the synchronization module using the same experimental setup as the one used in the bandwidth allocation experiments. In this experiment, we simulate a scenario where the proxy and the server are connected with a multi-hop WAN link with sufficient bandwidth to support both the video and audio streams. To simulate the multihop nature of the link, the server injects a random delay into its packet transmissions. The network delay is assumed to be exponentially distributed with an average of 60 ms, a standard deviation of 40 ms, and a minimum cutoff of 20 ms. A random delay is calculated before scheduling each frame, as well as for simulating the latency of response to feedback messages. The experiments were conducted using a 10-minute MPEG1 video clip from the movie U-571. The 192kbps audio and 1.15 Mbps video bit streams were demultiplexed from the original MPEG system bit stream, and served as separate streams. The audio is constant bit rate, while the video is variable bit rate with an average frame size of 5749 bytes, transmitted at 30 fps. The standard deviation of the frame size is 3621 bytes, indicating a bursty media stream. For a given buffer size of video and audio, we vary the low and high buffer threshold values. A threshold window is defined as the difference between the high and the low threshold values. For example, a 80% threshold window is derived from threshold values of 90% high and a 10% low. In Figure 7, we illustrate the effect of varying these threshold values on the number of feedback messages for the three synchronization schemes. It can be observed that a threshold window of 40% is too small, and causes an excessive number of feedback messages; hence, this threshold value is eliminated from further analysis. Considering threshold windows of 60% and 80%, scheme B yields the minimum number of feedback messages as shown in Figure 8. Scheme A is too aggressive in transmission speedup and thus causes excessive overflow. For a small threshold window, the number of buffer overflows increases because there is an increase of fluctuation in the transmission rates, due to more frequent feedback messages. On average, schemes B and C are almost equally effective, requiring a small number of feedback messages and causing a only small number of packet drops as a result of overflow.


Video Buffer = 128 KB Audio Buffer = 32 KB 1130 1005

No of feedback messages

1200 1000

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Scheme A Scheme B Scheme C

600 400 200

178 40 8

24

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Threshold window

Figure 7. Effect of the threshold window size on the number of feedback messages Video Buffer = 128 KB, Audio Buffer = 32 KB

No of Overflows

30

26

25

19

20

Scheme A

15

Scheme B 7

10

Scheme C

3

5

0

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0 80%

60%

is increased from 256 KB to 512 KB. Thus, a buffer size of 256 KB (equivalent to approximately 1.5 seconds of video) is sufficient for this scenario having a bursty video and high jitter. We also measure the number of buffer underflows while varying the video buffer size, as shown in Table 2. Buffer underflow occurs when a packet is due to be scheduled for transmission but it is not available in the proxy buffer. It is observed that only the smallest buffer sizes, 128 KB and 192 KB, cause underflow. Table 2. Buffer underflows in adaptive synchronization Video Threshold Scheme Scheme Scheme Buffer window (A) (B) (C) 128 80% 9 0 0 128 60% 0 0 1 128 40% 0 2 0 192 80% 2 0 0 192 60% 0 0 0 192 40% 0 0 0 256 80% 0 0 0 256 60% 0 0 0 256 40% 0 0 0 512 80% 0 0 0 512 60% 0 0 0 512 40% 0 0 0

Threshold window

3.4. Quality Adaptation Figure 8. Buffer overflow due to variation in transmission rate

No of feedback messages

120 100

Scheme B(80%) Scheme C(80%)

80

Scheme B(60%) Scheme C(60%)

60 40 20 0 128

192

256

512

Video Buffer(KB)

Figure 9. Effect of video buffer size on the number of feedback messages We measure the effect of varying the size of the video buffer on the number of feedback messages, while maintaining a fixed audio buffer, because audio packets are transmitted at a constant bit rate. The results are shown in Figure 9. The number of feedback messages decreases as the buffer size increases. Except for 128 KB buffer size, the number of feedback messages is similar for schemes B and C. The decrease in the number of feedback messages is not significant when the buffer size

This section describes a number of strategies that address problems associated with network heterogeneity . This heterogeneity is due to the wide range of subscribers, who differ in network access medium, communication speed, processing power, storage capacity, physical memory, display characteristics, and rendering applications. For example, some mobile users can receive high quality images and video because they have access to high bandwidth and high quality presentation devices; whereas, others have limited bandwidth capacity, which allows the presentation of only lower resolution images and video. To support a wide range of clients, a proxy can perform quality adaptation of data to support low-end clients because maintaining multiple versions of data suffers from high storage overhead. Performing the quality adaptation at the proxies favors incremental deployment within the existing network infrastructure [2]. In addition, the servers are relieved from the computationally intensive task of extracting low fidelity but semantically intact data contents from a high quality content. We envision a quality adaptation module that performs on-the-fly data adaptation to support clients with slow network connections. The quality adaptation functions include lossy data compression for reducing


bandwidth requirements and data transformation for extracting lower fidelity components from high quality data content. The transformed data can be cached in the proxy for future requests of the same version from another user. Another function is the partial delivery of objects by a proxy, which is carried out by dropping some packets when a link is congested [16]. Close monitoring of link conditions is essential for deciding when to drop frames. A measure of tolerable loss of packets can be expressed in terms of the reliability requirement. In MPEG videos, for example, predictive or backward frames can be dropped to reduce transfer rates. On the other hand, when sufficient bandwidth is available, the data can be transmitted without dropping frames, resulting in higher fidelity.

4. Conclusions In this paper, we have shown that a proxy -based framework can support QoS-based multimedia services in wireless networks. We have evaluated the performance of QoS functions including bandwidth allocation and synchronization. Bandwidth reservation enables synchronized streams at the proxy to be transmitted to the clients at their required presentation rates. The bandwidth allocation module effectively maintains bandwidth commitments to QoS flows. Despite delay variations in the network links between the server and the proxy, the adaptive synchronization technique effectively maintains the temporal relations between media streams without causing an excessive number of feedback messages and buffer overflows or underflows. Our experiments show that an aggressive rate adjustment and a small threshold window can cause drastic transmission rate fluctuations leading to unnecessary feedback messages and buffer overflow.

5. References [1] W. Almesberger, “Linux traffic control --- implementation overview,” Technical Report SSC/1998/037, EPFL, Nov. 1998. [2] B. Badrinath, A. Fox, L. Kleinrock, G. Popek, P. Reiher, and M. Satyanarayanan, “A conceptual framework for network and client adaptation,” ACM MONET Journal, vol. 5 (4), pp. 221– 231, Dec. 2000.

[5] D. A. Eckhardt and P. Steenkiste, “Effort-Limited Fair (ELF) scheduling for wireless networks,” in Proceedings of IEEE INFOCOM 2000, vol. 3, pp. 1097–1106, 2000. [6] H. Fahmi, M. Latif, S. Sedigh-Ali, A. Ghafoor, P. Liu and L. Hsu, “Proxy servers for scalable interactive video support,” IEEE Computer, pp. 54–60, Sept. 2001. [7] S. Floyd and V. Jacobson, “Link-sharing and resource management models for packet networks,” IEEE/ACM Transactions on Networking, vol. 3 (4), Aug. 1995. [8] A. Fox, S. D. Gribble, Y. Chawathe and E. A. Brewer, “Adapting to network and client variation using active proxies: lessons and perspectives,” IEEE Personal Communications (invited submission), Sept. 1998. [9] C. M. Huang, H. Y. Kung and J. L. Yang, “PARK: a PausedAnd-Run K-stream multimedia synchronization control scheme,” in Proceedings of ICDCS 2000, pp. 272–279, Apr. 2000. [10] T. D. C. Little and A. Ghafoor, “Synchronization and storage models for multimedia objects,” IEEE Journal on Selected Areas in Communications, vol. 8 (3), pp. 413–427, Apr. 1990. [11] S. Lu and V. Bharghavan, “Adaptive resource management algorithms for indoor mobile computing environments,” in Proceedings of ACM SIGCOMM '96, pp. 231–242, Aug. 1996. [12] P. E. McKenney, “Stochastic fairness queuing,” in Proceedings of the IEEE INFOCOM ’90, vol. 2, pp. 733–740, Jun. 1990. [13] M. Naghshineh, M. Schwartz, “Distributed call admission control in mobile/wireless networks,” IEEE JSAC, vol. 14, pp. 711-717, May 1996. [14] A. K. Talukdar, B. R. Badrinath and A. Acharya, “On accommodating mobile hosts in an integrated services packet network,” in Proceedings of IEEE INFOCOM '97, pp. 1048– 1055, Apr. 1997. [15] “Synchronized Multimedia Integration Language (SMIL 2.0) Specification,” W3C Recommendation, Aug. 2001. [16] M. Yarvis, P. Reiher and G. Popek, “Conductor: a framework for distributed adaptation,” Proceedings of the 7th Workshop on Hot Topics in Operating Systems (HotOS-VII), Mar. 1999.

[3] S. Chatterjee and M. Brown, “Adaptive QoS resource management in dynamic environments,” IEEE International Conference on Multimedia Computing and Systems, vol. 2, pp. 997–998, 1999.

[17] H. Zhang, “Service disciplines for guaranteed performance service in packet-switching networks,” in Proceedings of the IEEE, vol. 83 (10), pp. 1374–96, Oct. 1995.

[4] S. Choi and K. G. Shin, "Predictive and adaptive bandwidth reservation for hand-offs in QoS-sensitive cellular networks," in Proceedings of ACM SIGCOMM '98, Vancouver, British Columbia, pp. 155–166, Sept. 1998.

[18] Z. Zhang, Y. Wang, D. H. C. Du and D. Su, “Video staging: a proxy -server-based approach to end-to-end video delivery over wide-area networks,” IEEE/ACM Transactions on Networking, vol. 8 (4), Aug. 2000.
