Call admission control (CAC) is a key element in the provision of guaranteed quality of service (QoS) in wireless networks. The design of CAC algorithms for mobile cellular networks is especially challenging given the limited and highly variable resources, and the mobility of users encountered in such networks. This article provides a survey of admission control schemes for cellular networks and the research in this area. Our goal is to provide a broad classification and thorough discussion of existing CAC schemes. We classify these schemes based on factors such as deterministic/stochastic guarantees, distributed/local control and adaptivity to traffic conditions. In addition to this, we present some modeling and analysis basics to help in better understanding the performance and efficiency of admission control schemes in cellular networks. We describe several admission control schemes and compare them in terms of performance and complexity. Handoff prioritization is the common characteristic of these schemes. We survey different approaches proposed for achieving handoff prioritization with a focus on reservation schemes. Moreover, optimal and near-optimal reservation schemes are presented and discussed. Also, we overview other important schemes such as those designed for multi-service networks and hierarchical systems as well as complete knowledge schemes and those using pricing for CAC. Finally, the paper concludes on the state of current research and points out some of the key issues that need to be addressed in the context of CAC for future cellular networks.

This paper investigates the benefit of network coding for TCP traffic in a wireless mesh network. We implement network coding in a real 802.11a wireless mesh network and measure TCP throughput in such a network. Unlike previous implementations of network coding in mesh networks, we use off-the-shelf hardware and software and do not modify TCP or the underlying MAC protocol. Therefore, our implementation can be easily exported to any operational wireless mesh network with minimal modifications. Furthermore, the TCP throughput improvement reported in this paper is due solely to network coding and is orthogonal to other improvements that can be achieved by optimizing other system components such as the MAC protocol. We conduct extensive measurements to understand the relation between TCP throughput and network coding in different mesh topologies. We show that network coding not only reduces the number of transmissions by sending multiple packets via a single transmission but also results in a smaller loss probability due to reduced contention on the wireless medium. Unfortunately, due to asynchronous packet transmissions, there is often little opportunity to code resulting in small throughput gains. Coding opportunity can be increased by inducing small delays at intermediate nodes. However, this extra delay at intermediate nodes results in longer round-trip-times that adversely affect TCP throughput. Through experimentation, we find a delay in the range of 1 ms to 2 ms to maximize TCP throughput. For the topologies considered in this paper, network coding improves TCP throughput by 10% to 85%.

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Abstract-The capacity gain of network coding has been extensively studied in wired and wireless networks. Recently, it has been shown that network coding improves network reliability by reducing the number of packet retransmissions in lossy networks. However, the extent of the reliability benefit of network coding is not known. This paper quantifies the reliability gain of network coding for reliable multicasting in wireless networks, where network coding is most promising. We define the expected number of transmissions per packet as the performance metric for reliability and derive analytical expressions characterizing the performance of network coding. We also analyze the performance of reliability mechanisms based on rateless codes and automatic repeat request (ARQ), and compare them with network coding. We first study network coding performance in an access point model, where an access point broadcasts packets to a group of K receivers over lossy wireless channels. We show that the expected number of transmissions using ARQ, compared to network coding, scales as Θ(log K) as the number of receivers becomes large. We then use the access point model as a building block to study reliable multicast in a tree topology. In addition to scaling results, we derive expressions for the expected number of transmissions for finite multicast groups as well. Our results show that network coding significantly reduces the number of retransmissions in lossy networks compared to an ARQ scheme. However, rateless coding achieves asymptotic performance results similar to that of network coding.

In covert communication, Alice tries to communicate with Bob without being detected by a warden Willie. When the distance between Alice and Bob becomes large compared to the distance between Alice and Willie(s), the performance of covert communication will be degraded. In this case, multi-hop message transmission via intermediate relays can help to improve performance. Hence, in this work multi-hop covert communication over a moderate size network and in the presence of multiple collaborating Willies is considered. The relays can transmit covertly using either a single key for all relays, or different independent keys at the relays. For each case, we develop efficient algorithms to find optimal paths with maximum throughput and minimum end-to-end delay between Alice and Bob. As expected, employing multiple hops significantly improves the ability to communicate covertly versus the case of a single-hop transmission. Furthermore, at the expense of more shared key bits, analytical results and numerical simulations demonstrate that multi-hop covert communication with different independent keys at the relays has better performance than multi-hop covert communication with a single key.

The capacity gain of network coding has been extensively studied in wired and wireless networks. Recently, it has been shown that network coding improves network reliability by reducing the number of packet retransmissions in lossy networks. However, the extent of the reliability benefit of network coding is not known. This paper quantifies the reliability gain of network coding for reliable multicasting in a wireless network where network coding is the most promising. We define the expected number of transmissions per packet as the performance metric for reliability and derive analytical expressions characterizing the performance of network coding. For a tree-based multicast, we derive expressions for the expected number of transmissions at the source of the multicast and inside the multicast tree. We also analyze the performance of error control mechanisms based on rateless codes and automatic repeat request (ARQ). We then use the analytical expressions to study the impact of multicast group size on the performance of different error control schemes. Our numerical results show that network coding significantly reduces the number of retransmissions in lossy networks compared to end-to-end ARQ scheme, however, rateless coding and link-by-link ARQ are able to achieve performance results comparable to that of network coding. Interestingly, link-by-link ARQ can outperform rateless coding depending on the network size and loss probability. We conjecture that network coding achieves a logarithmic reliability gain with respect to multicast group size compared to a simple ARQ scheme.

Abstract-There is a rich recent literature on informationtheoretically secure communication at the physical layer of wireless networks, where secret communication between a single transmitter and receiver has been studied extensively. In this paper, we consider how single-hop physical layer security techniques can be extended to multi-hop wireless networks. We show that guaranteed security can be achieved in multihop networks by augmenting physical layer security techniques, such as cooperative jamming, with the higher layer network mechanisms, such as routing. Specifically, we consider the secure minimum energy routing problem, in which the objective is to compute a minimum energy path between two network nodes subject to constraints on the end-to-end communication secrecy and goodput over the path. This problem is formulated as a constrained optimization of transmission power and link selection, which is proved to be NP-hard. Nevertheless, we show that efficient algorithms exist to compute both exact and approximate solutions for the problem. In particular, we develop an exact solution of pseudo-polynomial complexity, as well as an ϵ-optimal approximation of polynomial complexity. Simulation results are also provided to show the utility of our algorithms and quantify their energy savings compared to a combination of (standard) security-agnostic minimum energy routing and physical layer security. In the simulated scenarios, we observe that, by jointly optimizing link selection at the network layer and cooperative jamming at the physical layer, our algorithms reduce the network energy consumption by half.