basic 802.11 – based channel access procedure is nearly sufficient to deliver nonreal-time traffic. The delivery should be augmented by appropriate mechanisms to better consider different QoS requirements and ultimately adjust the medium access parameters to the video data content characteristics.

First part of this work reviewed H.264 video coding standard and WLAN IEEE802.11. Next, need for cross-layer optimization in wireless video transport is explained. Finally, we evaluated the QoS cross-layer architecture relies on a data partitioning (DP) technique at the application layer and an appropriate QoS mapping at the 802.11e-based MAC layer. 2 MPEG/ITU AVC/H.264 standard Transport of multimedia content over wireless networks is very challenging because the wireless channels are usually severely impaired due to multi-path fading, shadowing, inter-symbol interference, and noise disturbances. The channel error rate varies with the time varying channel environments. This imposes some necessary tradeoff between robust video quality of service (QoS) and adaptive wireless network resource utilization. Therefore, wireless video faces many challenges in both coding techniques and transmission mechanisms. 1) advanced scalable video source coding; 2) low power coding and transmission techniques; 3) error control mechanisms including error resilience,

error correction, and error concealment; 4) networked video, including rate control and streaming; 5) encryption for wireless video. The recently developed H.264 video standard achieves efficient encoding over a bandwidth ranging from a few kilobits per second to several megabits per second. Hence, transporting H.264 video is an important component of many wireless multimedia services, such as video conferencing, real-time network gaming, and TV broadcasting.

H.264 consists of two conceptually different layers [2]. First, the video coding layer (VCL) contains the specification of the core video compression engines that achieve basic functions such as motion compensation, transform coding of coefficients, and entropy coding. This layer is transport-unaware, and its highest data structure is the video slice - a collection of coded macroblocks (MBs) in scan order. Second, the network abstraction layer (NAL) is responsible for the encapsulation of the coded splices into transport entities of the network. In this

H.264 overview, we particularly focus on the NAL layer features and transport possibilities (Fig.1).

The NAL defines an interface between the video codec itself and the transport world. It operates on NAL units (NALUs) that improve transport abilities over almost all existing networks. An NALU consists of a one-byte header and a bit string that represents, in fact, the bits constituting the MBs of a slice. The header byte itself consists of an error flag, a disposable NALU flag, and the NALU type. Finally, the NAL provides a means to transport high-level syntax (i.e., syntax assigned to more than one slice, e.g., to a picture or group of pictures) to an entire sequence.

One very fundamental design concept of the H.264 codec resides in its ability to generate self-contained packets, making mechanisms such as header duplication and MPEG-4’s header extension code (HEC) unnecessary. The way this is achieved is to decouple information relevant to more than one slice from the media stream. This higher-layer meta information should be sent reliably, asynchronously, and before transmitting video slices. Here, provisions for sending this information in-band are also available for applications that do not have an out-of-band transport channel appropriate for the purpose. The combination of higher-level parameters is called the parameter set concept (PSC). The PSC contains information such as picture size, display window, optional coding modes employed, MB allocation map, and so on. In order to be able to change picture parameters without necessarily retransmitting PSC updates, the video codec can continuously maintain a list of parameter set combinations to switch on. In this case each slice header would contain a codeword that indicates the PSC to be used.

The H.264 standard includes a number of error resilience techniques. Among these techniques, DP is an effective applications-level framing technique that divides the compressed data into separate units of different importance. Generally, all symbols of MBs are coded together in a single bit string that form a slice. However, DP creates more than one bit string (partition) per slice, and allocates all symbols of a slice into an individual partition with a close semantic relationship. In H.264 three different partition types are used: Partition A, containing header information such as MB types, quantization parameters, and motion vectors. This information is the most important because without it, symbols of the other partitions cannot be used.

Proceedings of the 10th WSEAS International Conference on COMMUNICATIONS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp235-240)

Partition B (intra partition), carrying intra coded block pattern (CBP) and intra coefficients. The type B partition requires the availability of the type A partition in order to be useful at the decoding level. In contrast to the inter information partition, intra information can stop further drift and hence is more important than the inter partition. Partition C (inter partition), contains only inter CBPs and inter coefficients. Inter partitions are the last important because their information does not resynchronize the encoder and decoder. In order to be used it requires the availability of the type A partition, but not of the type B partition.

Usually, if the inter or intra partitions (B or C) are missing, the available header information can still be used to improve the efficiency of error concealment. More specifically, due to the availability of the MB types and motion vectors, a comparatively high reproduction quality can be achieved as only texture information is missing.

3 WLAN IEEE 802.11 standard The IEEE 802.11 standard along with the b-annex define a 2.4-GHz spread-spectrum wireless LAN capable of operation at bit rates of 1, 2, 5.5, and 11 Mbits/s (using a spread-spectrum BPSK modulation) up to 11 Mbits/s (using different modulation methods). The IEEE 802.11 standard uses the same logical link layer as other 802-series networks (including the 802.3 wired Ethernet standard), and uses compatible 48-bit hardware Ethernet addresses to simplify routing between wired and wireless networks. As in the wired Ethernet, corrupted packets are dropped at the link layer; therefore, the wireless link appears as a packet loss network to applications running on it.

However, significant differences between the properties of wired and wireless networks demand a very different media access control (MAC) layer for 802.11 wireless networks. Hence, we describe the relevant features of IEEE 802.11 [3, 4].

Using radio transceivers for the network physical layer is complicated by the inability of radio transceivers to detect collisions as they transmit, and the potential for devices outside the network to interfere with network transmissions. Communication is also hampered by the hidden node problem; two widely spaced nodes on the network may be unable to communicate with each other directly, and yet still interfere with transmissions to an intermediate point. To address these limitations, a complex media access control (MAC) that includes

retransmissions of corrupt packets and collision avoidance is used.

The distributed coordination function (DCF) is the basic mechanism for IEEE 802.11. It employs carrier sense multiple access with collision avoidance (CSMA/CA) as the access method. Before initiating a transmission, each station is required to sense the medium. If the medium is busy, the station defers its transmission and initiates a backoff timer. The backoff timer is randomly selected between 0 and contention window (CW). Once the station detects that the medium has been free for a duration of DCF interframe spaces (DIFS), it begins to decrement the backoff counter as long as the channel is idle. As the backoff timer expires and the medium is still free, the station begins to transmit. In case of a collision, indicated by the lack of an acknowledgement, the size of the CW is doubled following until it reaches the CWmax value. Furthermore, after each successful transmission, the CW is initialized with CWmax,

( ) 12min −= ixCWCW (1)

where i is the number of transmission attempts.

Under DCF, all stations compete for channel access with the same priority. There is no differentiation mechanism to provide better service for real-time and multimedia applications.

The need for a better access mechanism to support service differentiation has led Task Group e of IEEE802.11 to propose an extension of the actual IEEE802.11 standard. The 802.11e draft introduces the hybrid coordination function (HCF) that concurrently uses a contention-based mechanism and a pooling-based mechanism, EDCA, and HCF controlled channel access (HCCA), respectively. Like DCF, EDCA is very likely to be the dominant channel access mechanism in WLAN because it features a distributed and easily deployed mechanism. In the following QoS support in EDCA is realized with the introduction of access categories (ACs). Each AC has its own transmission queue and its own set of channel access parameters. Service differentiation between ACs is achieved by setting different CWmin, CWmax, arbitrary interframe space (AIFS), and transmission opportunity duration limit (optional). If one AC has a smaller arbitrary interframe space (AIFS) or CWmin or CWmax, the AC’s traffic has a better chance of accessing the wireless medium earlier. Generally, AC3 and AC2 are reserved for real-time applications (e.g., voice or video transmission), and the others (AC1, AC0) for best effort and background traffic.

Proceedings of the 10th WSEAS International Conference on COMMUNICATIONS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp235-240)

4 Video communication over WLAN As the use of wireless local area networks spreads beyond simple data transfer to bandwidth-intense, delay-sensitive, and loss-tolerant multimedia applications, addressing quality of service issues will become extremely important. Currently, a multitude of protection and adaptation strategies exists in the different layers of the open systems interconnection (OSI) stack. Hence, an in-depth understanding and comparative evaluation of these strategies are necessary to effectively assess and enable the possible trade-offs in multimedia quality, power consumption, implementation complexity, and spectrum utilization that are provided by the various OSI layers [1].

The research focus has been to adapt existing algorithms and protocols for multimedia compression and transmission to the rapidly varying and often scarce resources of wireless networks. However, these solutions often do not provide adequate support for multimedia applications because the resource management, adaptation, and protection strategies available in the lower layers of the stack - the physical (PHY), medium access control (MAC), and network/transport layers - are optimized without explicitly considering the specific characteristics of multimedia applications. Conversely, multimedia compression and streaming algorithms do not consider the mechanisms provided by the lower layers for error protection, scheduling, resource management, and so on. This layered optimization leads to a simple independent implementation, but results in suboptimal multimedia performance [8].

Alternatively, under adverse conditions, wireless stations need to optimally adapt their video compression and transmission strategies jointly across the protocol stack in order to guarantee a predetermined quality at the receiver. The cross-layer approach does not necessarily require a redesign of existing protocols, and can be performed by selecting and jointly optimizing the application layer and the strategies available at the lower layers, such as admission control, resource management, scheduling, error protection, and power control.

The cross-layer optimization problem can be solved using iterative optimization or decision tree approaches, where a group of strategies are optimized while keeping all other strategies fixed, and this process is repeated until convergence. For the optimization of each group of strategies, one can use derivative and nonderivative methods (e.g., linear and nonlinear programming). Since this is a complex

multivariate optimization with inherent dependencies (across layers and among strategies), an important aspect of this optimization is determining the best procedure for obtaining the optimal strategy • determining the initialization, • grouping of strategies at different stages, • a suitable order in which the strategies should be

optimized, and • even which parameters, strategies, and layers should be

considered based on their impact on multimedia quality, delay, or power.

The selected procedure determines the rate of convergence and the values at convergence. The rate of convergence is extremely important, since the dynamic nature of wireless channels requires rapidly converging solutions (this is illustrated in the example later). Depending on the multimedia application, wireless infrastructure, and flexibility of the adopted WLAN standards, different approaches can lead to optimal performance. A classification of the possible solutions is given in the next subsection.

Numerous solutions have been proposed for efficient multimedia streaming over wireless networks. Potential solutions for robust wireless multimedia transmission over error-prone networks include application-layer packetization, (rate-distortion optimized) scheduling, joint source-channel coding, error resilience, and error concealment mechanisms. Transport issues for wireless (multimedia) transmission have been examined in [6, 7]. At the PHY and MAC layers, significant gains have been reported by adopting cross-layer optimization, such as link adaptation, channel aware scheduling, and optimal power control [8]. Explicit consideration of multimedia characteristics and requirements can further enhance the important advances achieved in cross-layer design at the lower layers [9].

Cross-layer architectures for robust H.264 video transmission over WLAN can be classified into the following categories [8]: Top-down approach. The higher layer optimizes their parameters and the strategies at the next lower layer. This cross-layer solution has been deployed in most existing systems, wherein the application dictates the MAC parameters and strategies, while the MAC selects the optimal PHY layer modulation scheme. Bottom-up approach. In this architecture the lower layer isolates the higher layers from losses and b. This cross-layer solution is not optimal for

Proceedings of the 10th WSEAS International Conference on COMMUNICATIONS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp235-240)