fundamental problem in networking is the allocation of limited resources among the users of the network. In a traditional layered network architecture, the resource to be allocated at the medium access control (MAC) and network layers is the use of communication links, viewed as "bit pipes" that deliver data at a fixed rate with occasional random errors. This bit pipe is a simple abstraction of the underlying physical and data link layers. This abstraction has, in some ways, caused the research

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... community to split into two distinct groups, which we shall refer to as the networking and communication communities. Research in the networking community has focused on allocating these bit pipes among different streams of randomly arriving traffic using approaches such as packet scheduling and collision resolution. The goal here is to efficiently utilize the bit pipes while providing acceptable quality of service (QoS) in terms of delay and throughput to each user. In contrast, the communication community has focused on building better bit pipes, i.e., improving the transmission rate or spectral efficiency for a given channel through improved detection, modulation, and coding. The random arrivals and departures of traffic are typically ignored and delay is not addressed. Though this separation has many advantages, both practically and conceptually, there is growing awareness that this simple bit-pipe view is inadequate, particularly in the context of modern wireless data networks. Indeed, as highlighted throughout this issue, significant performance gains can be achieved by various cross-layer approaches, i.e., approaches that jointly consider physical layer and higher networking layer issues in an integrated framework. In this article, we consider several basic cross-layer resource allocation problems for wireless fading channels. Here, the resources to be allocated include the transmission A © DIGITAL VISION