«End-to-End QoS Provision over Heterogeneous IP and non IP Broadband Wired and Wireless Network Environments A dissertation submitted in satisfaction ...»
Nowadays, continuous media applications over heterogeneous IP networks, such as video streaming and video-conferencing, are become very popular. Several approaches have been proposed in order to address the end-to-end QoS both from network perspective, like DiﬀServ, DVB BM and UMTS QoS traﬃc classes, and from application perspective, like scalable video coding. In this chapter, I show that the common operation of IP DiﬀServ and DVB BM mechanisms and UMTS QoS traﬃc classes can oﬀer quality gains for media delivery across heterogeneous IP/DVB/UMTS settings, and addresses the end-to-end QoS of MPEG-4 FGS streaming traﬃc delivery over a heterogeneous networks. Towards this purpose, the paper presents experimental results of an empirical study of a heterogeneous IP/DVB/UMTS network supporting continuous media applications. The development of new service categories increases the need for a diﬀerentiated networklevel treatment of the information packets, according to their diﬀerent relevance to within each type of service.
6.1 Introduction IP technology seems to be able to resolve the inter-working amongst the diverse ﬁxed core and wireless access technologies. At the network level, the end-to-end QoS provision could be established through the appropriate mapping amongst the QoS traﬃc classes/services supported by the contributing underlying networking technologies  . A QoS cross layer architecture based on error resilience features of H.264/MPEG-4 AVC can be applied for further improvements on endto-end QoS. Building on this background, this work involves a DiﬀServ-aware IP core network and a 802.11e access network and examines end-to-end QoS issues regarding scalable video streaming and prioritized packetization based on data partitioning (DP) for delivering multimedia traﬃc across ﬁxed and wireless network domains.
The Diﬀerentiated Services (DiﬀServ)  approach proposed by IETF supports (based on the DiﬀServ Code Point (DSCP)  ﬁeld of the IP header) two diﬀerent services, the Expedited Forwarding (EF) that oﬀers low packet loss and low delay/jitter and the Assured Forwarding (AF), which provides better QoS guarantees than the best-eﬀort service. Diﬀerences amongst AF services imply that a higher QoS AF class will give a better performance (faster delivery, lower loss probability) than a lower AF class.
The 802.11e  standard addresses the issue of QoS support in wireless LANs.
The MAC protocol of 802.11e supports multiple access categories (ACs). A higher priority access category has a smaller minimum contention window thus has a higher probability to access the channel. Diﬀerent access categories can have a diﬀerent maximum contention window and inter-frame spacing interval (IFS).
The 802.11e deﬁnes four access categories; AC3 corresponds to the highest access priority, and AC0 to the lowest.
The basic coding scheme for achieving a wide range of spatio-temporal and quality scalability is scalable video. For Signal-to-Noise Ratio (SNR) scalability the most appropriate technique for video delivery over heterogeneous networks, is the scalable extension of H.264/MPEG-4 AVC . In order to support ﬁne-granular SNR scalability, progressive reﬁnement (PR) slices have been introduced in the scalable extension of H.264 . A base representation of the input frames of each layer is obtained by transform coding similar to H.264 .
The corresponding Network Abstraction Layer (NAL) units (containing motion information and texture data) of the base layer are compatible with the single layer H.264/MPEG-4 AVC. Furthermore, by employing data partitioning, the H.264 encoder partitions the compressed data in separate units of diﬀerent importance. The packets, with assigned priority, are sent to a QoS-aware network to receive diﬀerent forwarding treatments. Mapping these prioritized packets to diﬀerent QoS levels causes them to experience diﬀerent packet loss rates with this diﬀerential forwarding mechanism. The quality of the base representation can be improved by an additional coding of the so-called PR slices. The corresponding NAL units can be arbitrarily truncated in order to support ﬁne granular quality scalability or ﬂexible bit-rate adaptation.
To address end-to-end QoS problem scalable video streaming traﬃc delivery over a heterogeneous IP/802.11e network, this chapter proposes and validates through a number of NS2-based simulation scenarios an architecture that explores the joint use of packet prioritization and scalable video coding together with the appropriate mapping of 802.11e access categories to the DiﬀServ traﬃc classes.
This work extends previous authors’ papers   dealing with joint scalable video coding and packet prioritization over IP/UMTS and IP/DVB heterogeneous networks.
The rest of the chapter is organized as follows. In Section 6.2, the proposed scalable video coding techniques and prioritization framework for providing QoS guarantees for scalable video streaming traﬃc delivery over a heterogeneous DiﬀServ/WLAN network is presented. In Section 6.3, it is demonstrates how videostreaming applications can beneﬁt from the use of the proposed architecture.
Finally, Section 6.4 draws the conclusions and discusses directions for further work and improvements.
6.2 Proposed Architecture
The proposed architecture integrates the concepts of scalable video streaming, prioritized packetization based on the H.264 data partitioning features and mapping DiﬀServ classes to MAC diﬀerentiation of 802.11e. The proposed architecture is depicted in Figure 7.2. It consists of three key components: (1) Scal
Figure 6.1: Overall Architecture
able video encoding (Scalable extension of H.264/MPEG-4 AVC), (2) prioritized packetization according based on data partitioning, and (3) DiﬀServ/802.11e class mapping mechanism in order to assure the optimal diﬀerentiation and to achieve QoS continuity of scalable video streaming traﬃc delivery over DiﬀServ and 802.11e network domains. Each one of these components is discussed in detail in the following subsections.
6.2.1 Scalable Video Coding
Scalable Video Coding should meet a number of requirements in order to be suitable for multimedia streaming applications. For eﬃcient utilization of available bandwidth, the compression performance must be high. Also, the computational complexity of the codec must be kept low to allow cost eﬃcient and real time implementations. When compared against other scalable video coding schemes, the ﬁne granular scalability coding method is outstanding due to its ability to adapt to changing network conditions more accurately.
184.108.40.206 Scalable Extension of H.264/MPEG-4 AVC In order to provide FGS scalability, a picture must be represented by an H.264/AVC compatible base representation layer and one or more FGS enhancement representations, which demonstrate the residual between the original predictions residuals and intra blocks and their reconstructed base representation layer. This basic representation layer corresponds to a minimally acceptable decoded quality, which can be improved in a ﬁne granular way by truncating the enhancement representation NAL units at any arbitrary point. Each enhancement representation contains a reﬁnement signal that corresponds to a bisection of the quantization step size, and is directly coded in the transform coeﬃcient domain.
For the encoding of the enhancement representation layers a new slice called Progressive Reﬁnement (PR) has been introduced. In order to provide quality enhancement layer NAL units that can be truncated at any arbitrary point, the coding order of transform coeﬃcient levels has been modiﬁed for the progressive reﬁnement slices. The transform coeﬃcient blocks are scanned in several paths, and in each path only a few coding symbols for a transform coeﬃcient block are coded .
6.2.2 Prioritized Packetization
I deﬁne two groups of priority policies, one for BL and one for EL. These policies are used from the Edge Router of the DiﬀServ-aware underlying network to map the packets to the appropriate traﬃc classes. The packetization process can aﬀect the eﬃciency as well as the error resiliency of video streaming. In the proposed framework, by assuming best eﬀort delivery of the EL.
For the BL, at the Video Coding Layer (VCL), an additional type of slice, besides the three partitions (A, B, and C) obtained when DP is enabled, that represents Instantaneous Decoding Refresh (IDR) pictures. The IDR access units contain information that cannot be included into the three partitions, like the intra-picture (coded picture that can be decoded without needing information from previous pictures) where no data partitioning can be applied.
The order in which the slice units are sent is constant. The ﬁrst transmitted slice units transmitted contain the Packet Set Concept (PSC) information, such as picture size, display window, optional coding modes employed, macroblock allocation map, etc. This higher-layer meta information should be sent reliably, asynchronously, and before transmitting video slices.
The next transmitted slice units contain the IDR picture. Since IDR frames may contain only I slices without data partitioning, they are usually sent at the start of video sequences (just after the PSC). The slice units following the IDR frames contain one of the three partitions (A, B, or C).
The NAL is responsible for the encapsulation of the coded slices into transport entities of the network. Each NAL unit (NALU) could be considered as a packet that contains an integer number of bytes, including a header and a payload. The header speciﬁes the NALU type, and the payload contains the related data. The most important ﬁeld of the NAL header is the Nal Ref Idc (NRI) ﬁeld .
The NRI contains two bits that indicate the priority of the NALU payload, where 11 is the highest transport priority, followed by 10, then by 01, and ﬁnally, 00 is the lowest. Accordingly, the incoming VCL layer slices are diﬀerentiated and encapsulated into NALUs by enabling the NRI ﬁeld in the NAL header. Table
6.1 depicts the relation between the type of the BL content and the corresponding DiﬀServ classes. The ﬁrst digit of the AF class indicates forwarding priority and
the second indicates the packet drop precedence.
The PSC packets obtain the highest priority. Furthermore, as information carried in both partition A and IDR are essential for decoding an entire video frame, it is important to give these slices more priority than partition B and C.
Based on these rules, the NAL layer marks the diﬀerent NALUs.
6.2.3 DiﬀServ/802.11e QoS Classes Coupling
In order to integrate the 802.11 network domain with the core network domain, and to achieve QoS consistency across the DiﬀServ IP and 802.11e network, by mapping 802.11e access categories to predeﬁned DiﬀServ classes. A direct mapping apprach as proposed by  is adopted. Table 6.2 shows the mapping of the predeﬁned DiﬀServ classes according to the DiﬀServ speciﬁcation, where the ﬁrst digit of the AF class indicates forwarding priority and the second indicates the packet drop precedence, and the 802.11e access categories for the proposed mapping approach.
The packets, with assigned priority, are sent to the DiﬀServ network to receive diﬀerent forwarding treatments. Mapping these prioritized packets to diﬀerent QoS DS levels causes them to experience diﬀerent packet loss rates with this diﬀerential forwarding mechanism. In addition to the prioritized dropping performed by DiﬀServ routers, traﬃc policing can be carried out at intermediate video gate
ways (between diﬀerent network domains), using packet ﬁltering. When the IP packets are encapsulated in MAC frames, each frame should be allocated to a priority queue, or an access category.
6.3 Framework Evaluation This section evaluates the performance of the proposed framework through a set of simulations. A NS-2 based simulation environment with the appropriate extensions  for simulating 802.11e WLANs is adopted. Figure 6.3 depicts
Four YUV QCIF 4:2:0 color video sequences consisting of 300 to 2000 frames and coded at 30 frames per second are used as video sources. Each group of pictures (GOP) is structured as IBBPBBPBB. and contains 36 frames, and the maximum UDP packet size is at 1024 bytes (payload only). The scalable extension of H.264/MPEG-4 AVC encoder/decoder provided by  is used for encoding YUV sequences. The video frames are then encapsulated into RTP packets using a simple packetization scheme  (by one-frame-one-packet policy). The size of each RTP packet is maximally bounded to 1024 bytes. The generated video packets are delivered through the DiﬀServ at the form of UDP/IP protocol stack. The 802.11b is employed for the physical layer, which provides four diﬀerent physical rates. In our simulation, the physical rates are ﬁxed to 11 Mbps for data and 2Mbps for control packets. Table 7.1 depicts the MAC Parameters for the simulations.
Additionally, the streaming node station generates background traﬃc (500 kbps) using constant bit rate (CBR) traﬃc over User Datagram Protocol (UDP).
This allows us to increase the virtual collisions at the server’s MAC layer. Furthermore, by including ﬁve wireless stations where each station generates 300 kbps of data using CBR traﬃc in order to overload the wireless network.
A unique sequence number, the departure and arrival timestamps, and the