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«End-to-End QoS Provision over Heterogeneous IP and non IP Broadband Wired and Wireless Network Environments A dissertation submitted in satisfaction ...»

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Authors in [?] discuss details of the mapping of the traffic classes offered by UMTS and the RCL architecture, which is a prototypical implementation of the next-generation Internet. The existence of the traffic classes aims at the differentiation of user traffic in order to provide the individual QoS guarantees required by different types of applications. In order to achieve the desired end-to-end performance, the traffic classes must be appropriately associated at the point where the two networks converge. The authors propose not only the association of the traffic classes of one network to those of the other, but also a possible methodology to appropriately transform the QoS service attributes. The simulations performed in this context proved that the proposed mapping achieves the end-to-end service requirements, which are, in a few words, delay sensitivity and minimal packet loss for the conversational and streaming classes, and differentiation based on priorities for the interactive and background classes of UMTS. These results have been accomplished even though the traffic classes of the RCL architecture are not in full compliance with those of UMTS, especially for delay-insensitive traffic classes. However, this situation is pragmatic, since in a real scenario the traffic classes of the external IP network will not be defined according to the traffic classes of UMTS. Furthermore, the article very briefly discusses the control and user plane interworking issues, and the authors intend to focus on this part for future work. It would be challenging, however, to study the interworking issues not in particular for the examined networks, but in the broad context of interdomain signaling in the IP world.

2.8 Conclusions

In this chapter there is a review available solutions which address the problem of end-to-end quality of service for multimedia streaming applications over heterogeneous networks, including wireless and wired network domains. In particular, it describes advanced QoS-enabled multimedia streaming techniques from the application perspective. These include layered video coding, packet prioritization and packetization, semantic description and QoS profiling of each stream, and addressed the generic areas of content adaptation and context awareness in an end-to-end system context. Following this, it describes a number of available network technologies that have support to class based quality of service from the network perspective, including the IntServ and DiffServ models, Multiprotocol Label Switching and UMTS. Finally, it provides a review of selected techniques that combine application and network layer QoS mechanisms in order to provide the desirable end-to-end quality of service. This research area is highly active because the demands of emerging applications in the field of real-time multimedia streaming are increased and because of the wide use of mobile networks.

Many topics/issues are still open in this area, like quality of service and mobility, terminal equipment heterogeneity and intellectual property management and protection. Despite this, it concludes that for QoS-enabled multimedia streaming, the user and network heterogeneity requires highly scalable video coding, prioritized packetization, wherever possible, and flexible QoS enabled delivery techniques to smoothly and transparently interoperate.

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3.1 Introduction This chapter discusses a heterogeneous cluster of networks comprising of two IP and one DVB domains. The developed testbed encompasses DiffServ technology in the IP domain and bandwidth management over MPEG-2 Transport Stream in the DVB domain. The IP domains are realized with PCs running Linux Operating System acting as border and core routers. These Linux-based routers are employing DiffServ capabilities implemented through open source software. The DVB domain is realized through commercial equipment patched with the ability to discriminate the DiffServ traffic aggregates and apply bandwidth management.

The major goal of these experiments is to demonstrate that the common operation of IP DiffServ, DVB BM mechanisms and scalable MPEG-4 FGS prioritized video streaming offer quality gains for continuous media applications.

This chapter is organized as follows: Section 3.2 discusses in detail a linuxbased heterogeneous IP/DVB testbed for MPEG-4 FGS video streaming traffic delivery experimentation. The testbed configuration details for the media delivery experimental studies and the results of these studies are discussed in Sections

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3.3 and ?? respectively. Finally, Section ?? draws conclusions and discusses directions for further work and improvements.

3.2 Proposed Arrhitecture

3.2 depicts the implemented heterogeneous IP/DVB testbed for MPEG-4 FGS media delivery experimentation studies. The testbed includes two IP autonomous systems interconnected via a DVB MPEG-2 autonomous system, acting as a trunk network.

The implementation of both IP and DVB autonomous systems is mainly based on open source software. The choice of open source software enables the configuration of the testbed according to the needs and the specific requirements of the experimental studies that are carried out. The major goal of these experiments is to demonstrate that the common operation of IP DiffServ, DVB BM mechanisms and scalable MPEG-4 FGS prioritized video streaming offer quality gains for continuous media applications. The next three subsections discuss implementation details of DiffServ, BM mechanisms and MPEG-4 FGS coding and packetization approaches, respectively.

3.2.1 IP Domain - DiffServ Implementation The DiffServ [11] framework aims to provide service differentiation within backbone IP networks. DiffServ technology enables the deployment of IP traffic discrimination in a scalable manner, by providing QoS guarantees only for aggregated traffic classes rather than for specific flows. Essentially, when entering a network, packets are placed into a broad service group by a classification mechanism that reads the DiffServ Code Point (DSCP) in the IP packet header and the source and destination address. The advantage of such a mechanism is that several different traffic streams can be aggregated to a small number of Behavior Aggregates (BA), thereby simplifying processing and associated storing and forwarding processes at the routers. Furthermore, there is no need for signaling and the traffic differentiation is obtained on a packet-by-packet basis.

Differentiated Services model as proposed by IETF support two different services: (1) the Expedited Forwarding (EF) that supports low loss and delay/jitter, and (2) the Assured Forwarding (AF) that provides better QoS than the best effort, but without guarantee. For streaming video applications, in which the encoding and decoding process is more resilient to packet loss and delay variations, besides Premium Service, the Assured Service can be employed. Note that MPEG-4 FGS originally assumes guaranteed delivery to Base Layer (BL) and leaves the Enhancement Layer (EL) to the mercy of best effort service of Internet.

The international literature presents a number of DiffServ implementations

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[23] [24]. However, most of them are poorly documented and/or quite outdated.

In this context, I implemented our own version running over Linux OS with kernel version 2.6.11 [25]. Our implementation follows the generic architecture of Figure 3.2.1. Specifically, according to a set of filters, the incoming packets are separated into a number of classes, where each one of them maintains its own queuing/scheduling discipline for serving its packets, as well as its own policing scheme for controlling the amount of its packets.

The implemented DiffServ mechanism is incorporated into the two IP autonomous systems of the heterogeneous IP/DVB testbed. Each autonomous system consists of three PCs (at least PIII CPU with 512MBytes of RAM) running Linux OS (kernel version 2.6.11) with iproute2 package and tc utility support.

Each IP domain includes two edge routers and one core router. The supported BAs are EF, AF1x and BE. The Hierarchical Token Bucket (HTB) packet scheduler with three leaf classes is used for the realization of the supported BAs. In particular, a pFIFO queuing discipline is adopted for the EF BA. Three GRED virtual queues with different drop percentages are implemented for the AF1x BA.

The BE BA is served through a RED queuing discipline. This setting is depicted

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in Figure 3.2.1. The maximum bandwidth allocated at the parent HTB class is 13Mbps shared among the BAs. Each leaf class can borrow excess bandwidth from another leaf class.

The configuration of the GRED virtual queues requires the adjustment of the following parameters: AvgQmin which is the maximum average queue size after which all packets get dropped, BS, which is the percentage of the bandwidth share, L, which is the desired latency, BW, which is the total link bandwidth, AvgQmax which is the minimum average queue length after which packets get dropped, AvgP kt which is the average packet size, B, which is the burst value in number of packets and Qlimit which is the actual queue length never to be exceeded.

The AvgP kt size is 1024 bytes. The percentage of the Bandwidth Share (BS) is 33%. The desired Latency L is either equal to 100ms for all AF1x BAs, or 100ms for AF11, 200ms for AF12 and 500ms for AF13 BA. The remaining parameters are calculated based on the following formulas 3.1-3.4 and the corresponding

results are given in 3.1:

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3.2.2 DVB Domain - BM Implementation The bandwidth reallocation among the IP virtual channels of a DVB MPEG-2 TS [26] uplink is based on a set of predefined priority policies. In this work three priority policies are implemented, namely: (1) Static guaranteed - This policy guarantees a static bandwidth to each virtual channel. A guaranteed bit rate value has to be specified so that the actual bit rate is guaranteed up to this boundary value. The unused bandwidth (guaranteed bit rate - instant bit rate) is reserved and cannot be allocated to other virtual channels. (2) Dynamic guaranteed - This policy guarantees a dynamic bandwidth to each virtual channel. A guaranteed bit rate value has to be specified so that the actual bit rate is guaranteed up to this boundary value. On the contrary to the static guaranteed policy, the unused bandwidth (guaranteed bit rate - instant bit rate) is not lost, but can

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be allocated to other virtual channels. (3) Best effort - This conventional policy allocates bit rate to various virtual channels based on the available bandwidth.

Two full uplink/downlink configurations comprise the DVB domain. The uplink involves an encapsulator, a multiplexer and a DVB modulator. The downlink is realized via a DVB/IP gateway, which is a standard PC running Linux operating system equipped with a standard Ethernet controller and a DVB PCI card capable of demodulating the DVB signal and de-encapsulating the IP packets.

The DVB domain employs the implemented priority policies in order to preserve BAs defined in the IP domains. The binding among BAs and the corresponding

priority policies is given in Table II:

Note that in order to deal with the IP to MPEG-2 encapsulation overheads, the total link bandwidth is 14 Mbps, which is 1 Mbps bigger than the IP domains one. While AF1x BAs and BE BA can borrow bandwidth beyond the guaranteed, the EF BA is statically allocated a maximum value and therefore cannot borrow unused bandwidth.

3.2.3 MPEG-4 FGS video coding and transmission

MPEG-4 FGS [27]scalable video coding constitutes a new video coding technology that increases the flexibility of video streaming. Similar to the conventional scalable encoding, the video is encoded into a Base Layer (BL) and one or more (ELs). For MPEG-4 FGS, the EL can be efficient truncated in order to adapt transmission rate according to underlying network conditions. This feature can be used by the video servers to adapt the streamed video to the available bandwidth in real-time (without requiring any computationally demanding re-encoding). In addition, the fine granularity property can be exploited by the intermediate network nodes (including base stations, in case of wireless networks) in order to adapt the video stream to the currently available downstream bandwidth. In contrast to conventional scalable methods, the complete reception of the EL for successful decoding is not required [28]. The received part can be decoded, increasing the overall video quality according to the rate-distortion curve of the EL as described in [29] [30].

The most widely used scheme, in order to packetize MPEG-4 video streams, is fixed-length packetization, where video packets of similar length are formed. The packet size of video stream is also related to efficiency and error resiliency because a smaller packet size for example requires a higher overhead but has a better performance in error prone networks. By evaluating the expected loss impact of each packet to the end-to-end video quality, by assign priority to each packet according to its importance in video sequence. With assigned priorities, the packets are sent to underlying network and receive different forwarding treatments.

3.3 Testbed Configuration

Eight YUV QCIF 4:2:0 color video sequences consisting of 300 to 2000 frames and coded at 25 frames per second are used as video sources. Each group of pictures is structured as IBBPBBPBB. and contains 25 frames, with maximum UDP packet size of 1000 bytes (payload only). The Microsoft MPEG-4 FGS encoder/decoder is used for encoding YUV sequences. A number of background flows is transmitted in the network, in order to lead the DiffServ/DVB system in congestion. The background traffic is always running and is assigned to the BE BA. The latter BA has always the following characteristics: Poisson distribution with 1472 bytes packet size and constant rate of 8 Mbps. Correspondingly, EF BA is generated at 3Mbps rate and each AF1x BA at 2Mbps rate. The assigned bandwidth to AF1x BA is equally segmented to support three-drop percentages, which are 2% for AF11, 4% for AF12 and 6% for AF13.

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