Applications of “Cross-Layer” in Video Communications over Wireless Networks

Introduction

Traditional network design has largely taken place independently of the application. This form of design preserves protocol layering as a way of providing a well-defined interface between the layers and of isolating errors. Because of this, in network design it becomes easier to replace one protocol with another within the same layer. However, multimedia applications with real-time constraints and limited tolerance of data loss can significantly benefit from cross-layer design and optimization (Schaar, 2007). A cross-layer approach allows the exchange of information across protocol-layer boundaries and in some cases the approach allows system-wise optimization of performance. For example, sensor networks benefit from system-wise optimization (Jurdak, 2007), as they usually run dedicated applications such as video surveillance. The same applies to ad hoc networks in general, where video streaming can benefit (Setton, et al., 2005) from cross-layer design. Widening the remit, IEEE 802.11 wireless LANs (Schaar, et al., 2005) have also proved a fertile ground for multimedia applications of cross-layer design.

There are penalties in applying a cross-layer scheme (Kawadia & Kumar, 2003), namely it may result in a monolithic application that is hard to modify or evolve. However, for wireless communication (Srivastav & Motani, 2005) an adaptive scheme that leverages information across the layers can cope with the volatile state of the channel due to fading and shadowing and the constrained available bandwidth of the channel. It is not necessary to abandon layering altogether in a ‘layerless’ design but simply to communicate between the layers. Video applications break protocol boundaries with limited objectives in mind, though improvements in performance remain the goal. Performance may be defined variously in terms of reduction of delay, reduction of errors, throughput efficiency, and, in wireless networks, reduction of energy consumption. This list by no means exhausts the possible trade-offs that can be engineered through cross-layer exchange of information.