Anti-Jamming Schedules for Wireless Broadcast Systems

Modern society is heavily dependent on wireless networks for providing voice and data communications. Wireless data broadcast has recently emerged as an attractive way to disseminate data to a large number of clients. In data broadcast systems, the server proactively transmits the information on a downlink channel; the clients access the data by listening to the channel. Wireless data broadcast systems can serve a large number of heterogeneous clients, minimizing power consumption as well as protecting the privacy of the clients’ locations. The availability and relatively low cost of antennas resulted in a number of potential threats to the integrity of the wireless infrastructure. The existing solutions and schedules for wireless data broadcast are vulnerable to jamming, i.e., the use of active signals to prevent data distribution. The goal of jammers is to disrupt the normal operation of the broadcast system, which results in high waiting time and excessive power consumption for the clients. In this paper we investigate efficient schedules for wireless data broadcast that perform well in the presence of a jammer. We show that the waiting time of client can be efficiently reduced by adding redundancy to the schedule. The main challenge in the design of redundant broadcast schedules is to ensure that the transmitted information is always up-to-date. Accordingly, we present schedules that guarantee low waiting time and low staleness of data in the presence of a jammer. We prove that our schedules are optimal if the jamming signal has certain energy limitations. I. I NTRODUCTION Modern society has become heavily dependent on wireless networks to deliver information to diverse users. People expect to be able to access the latest data, such as stock quotes and traffic conditions, at any time, whether they are at home, at their office, or traveling. The emerging wireless infrastructure provides opportunities for new applications such as on-line banking and electronic commerce. Wireless data distribution systems also have a broad range of applications in military networks, such as transmitting up-to-date battle information to tactical commanders in the field. New applications place high demands on the quality, reliability, and security of transmissions. In order to provide a ubiquitous and powerful communication infrastructure that can satisfy security and reliability demands, sophisticated network technology, protocols and algorithms are required. Due to their open and ubiquitous nature, wireless information systems are extremely vulnerable to attack and misuse. Wireless systems can be attacked in various ways, depending on the objectives and capabilities of an adversary. Due to high availability and relatively low cost of powerful antennas, jamming, i.e., the use of active signals to prevent data distribution, has emerged as an attractive way of attack. As the current data communication standards such as IEEE802.11 [1] and Bluetooth [2] are not designed to resist malicious interference, a small number of jammers with limited energy resources can disrupt operation of an entire network [21]. Jamming is a common method of attack in military networks, where transmissions are often performed in the presence of an adversary whose goal is to disrupt the communication to a maximum degree. For example, the Global Positioning System (GPS) relies on extremely weak signals from orbiting satellites and, as a result, is very vulnerable to jamming. This constitutes a significant threat for GPS-based weapon and navigational systems. Jamming can be viewed as a form of Denial-of-Service(DoS) attack, whose goal is to prevent users from receiving timely and adequate information. A. Wireless Data Broadcast Systems One common characteristic of wireless infrastructure is an asymmetry between the downlink and uplink channels. In cellular, 802.11, or others similar networks, the downlink channel is of much higher bandwidth than the uplink Fig. 1. A typical data broadcast system. channel. Moreover, while the downlink channel is operated by a powerful antenna, the uplink channel is driven by a mobile device with limited power resources. This intrinsic asymmetry of wireless systems impacts the way information is delivered to clients. In particular, the standardclient-serverparadigm, in which the data transfer is initiated by clients, is not adequate for wireless systems [3]. Wireless data broadcast [3], [6], [17] has recently emerged as an attractive way to disseminate data to a large number of clients. In data broadcast systems, the server proactively transmits the information on the downlink channel and the clients access data by listening to the channel. This approach enables the system to serve a large number of heterogeneous clients, minimizing client power consumption as well as protecting the privacy of the clients’ locations. Fig. 1 depicts a typical data broadcast system. The system includes the following components: the server (scheduler), the broadcast channel, the information source, and the wireless users. The server periodically accesses the information source, retrieves the most recent data, encapsulates it into a packet and sends the packet (or encoding thereof) over the broadcast channel. There are two key performance characteristics of a wireless data distribution system. The first characteristic is waiting time, i.e., the amount of time spent by the client waiting for data. Waiting time is an important parameter, as timely information delivery is essential for many practical applications. In addition, it is closely related to the amount of power spent by the client to obtain the information. The second characteristic is aleness , i.e., the amount of time that passes from the moment the information is generated, until it is delivered to the client. The staleness of the schedule usually depends on the amount of redundancy used by the system, as information become less and less relevant with time.