Accepted Manuscript
Taking Advantage of Jamming in Wireless Networks: A Survey Haithem Al-Mefleh, Osameh Al-Kofahi PII: DOI: Reference:
S1389-1286(16)30027-5 10.1016/j.comnet.2016.02.011 COMPNW 5821
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Computer Networks
Received date: Revised date: Accepted date:
24 May 2015 9 February 2016 11 February 2016
Please cite this article as: Haithem Al-Mefleh, Osameh Al-Kofahi, Taking Advantage of Jamming in Wireless Networks: A Survey, Computer Networks (2016), doi: 10.1016/j.comnet.2016.02.011
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Taking Advantage of Jamming in Wireless Networks: A Survey Haithem Al-Mefleh, and Osameh Al-Kofahi
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Department of Computer Engineering, Yarmouk University, Jordan
Abstract
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There has been an increasing interest in jamming-based solutions to improve the performance of wireless networks. In such schemes, users would deliberately jam the channel, with a special signal, to improve the efficiency of different operations related to wireless networks especially when high performance is required like in contention resolution, QoS support, and statistics estimation. Thus, in this paper we seek to describe jamming and identify positive employments of the jam signals in such networks. We identify different areas where jamming signals are exploited to improve the performance and to solve problems that may arise in wireless networks. We also attempt to categorize different protocols in some of these areas. We provide a comprehensive survey of different proposals that exploit jamming to efficiently perform operations and solve problems in a wireless network. We highlight challenges, requirements, advantages, and disadvantages of jamming. Keywords: Wireless networks, jamming, jamming-based algorithms, MAC protocols.
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1. Introduction
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Today, wireless networks and their applications are getting more interest and attention as they are available almost everywhere and anytime at a reasonable cost. Consequently, research and industry are pushing for higher performance and quality of service (QoS) to be provided by wireless networks. Algorithms and protocols are being developed and studied to address and improve solutions for different problems, like resources’ allocation, of wireless networks. Moreover, new technologies, standards, and architectures are introduced to allow wireless networks to achieve higher data rates and advanced features to enhance the performance seen by different users. However, there are still some sources of overhead that may highly reduce the gain realized by higher rates, new features, and different enhancements suggested by research. The overhead includes all bandwidth, energy, and time wasted due to different operations in wireless networks, such as contention resolution and control packets needed to exchange different information among
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users of the network. Furthermore, the overhead becomes intolerable when there are mobile devices, high traffic load, misbehaving users, and large number of users. Finally, the overhead increases because of problems, like the hidden terminal and exposed terminal problems, that emerge in wireless networks. To enhance the performance, there have been an abundant amount of proposals and research that aim at reducing different overheads seen in wireless networks of different types like WiFi, WiMAX, wireless vehicular networks, and wireless sensor networks (WSNs). In addition, different scenarios are considered like single channel, multiple channels, directional/omnidirectional antennas, infrastructure/ad-hoc topologies, and different modulation schemes. One interesting proposal was utilizing jam signals to enhance the performance of wireless networks, where we have observed that there has been an increasing interest in jammingbased solutions for reducing the overheads in wireless networks. In jamming-based schemes, users would deliberately jam the channel, with a special signal, to improve the efficiency of different operations related to wireless networks especially when high performance February 18, 2016
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convey different requests and responses in parallel with data transmissions. Various applications of this idea are presented in this paper.
• In contention resolution using jamming signals allows more efficient resolution schemes that result in shorter contention periods and higher successful medium access rates, [21, 8, 12]. Moreover, some jamming-based solutions introduce fixed length contention periods, [9, 10], which results in a more predictable network behavior. Furthermore, for the next generation wireless networks, jamming is utilized to implement contention in the frequency domain to save time wasted in counting down in the time domain, [29, 33]. The proposed methods support higher number of users and enhance the performance of wireless networks.
Other uses of jamming in wireless networks include time synchronization [114], solving hidden and exposed terminal problems [53], frame transmissions [109], and collection of statistics [84] in different wireless networks. In this paper we seek to describe jamming and identify positive employments of jam signals. To the best of our knowledge, this paper is the first to present a comprehensive survey of jamming-based protocols and solutions proposed for different problems of wireless networks. Different outstanding surveys, summarized in Section 2, may mention one or more of the jamming-based solutions. However, none of these surveys is concerned about jammingbased approaches, but may shortly describe only a few of jamming-based protocols in the principal context of the corresponding survey. We summarize the contributions of this paper as follows:
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is required. We highlight some of the possible uses of jamming signals in the literature and leave the rest to be explained in the paper:
• QoS can be enhanced. For example, it is shown that assigning different jamming lengths to different Access Categories (ACs) instead of varying the contention window guarantees service for high priority traffic and totally prevents low priority traffic from blocking it, [44, 45]. In addition, using jamming allows preemptive access schemes [49, 51].
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• We highlight challenges, requirements, advantages, and disadvantages of jamming.
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• Because of its immunity to collisions, jamming signals can be used to relay important control messages in a reliable manner [67, 92].
• We identify different areas where jamming signals are exploited to improve the performance of wireless networks and to solve problems that may arise in such networks. We also attempt to categorize different protocols in some of these areas. • We provide a comprehensive survey of different proposals that exploit jamming to efficiently perform operations and solve problems in a wireless network.
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• Jamming signals can be used to enhance security, [117, 121]. Basically, with appropriate transceivers, jam signals can be used to intentionally collide with or shoot down suspicious or unauthorized transmissions.
This paper is organized as follows. An introduction is given in Section 1, and related work is described in Section 2. Then, we provide background and discussion of jamming signals in Section 3. Thereafter, in Section 4, we classify jamming-based solutions based on what they are used for in wireless networks, and we survey different jamming-based protocols deployed for wireless networks. Then we compare different protocols in Section 5 and we point out some features of jamming signals in Section 6. Finally, the paper is concluded in Section 7.
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• Jamming can be exploited to allow users of heterogeneous networks to coexist, see [143]. The performance of users may be severely affected in the presence of users of another type. Accordingly, solutions are proposed to provide better sharing of the wireless medium, and some of these solutions are based on jamming. • Parallel transmissions are possible with jam signals, [99, 100, 101, 102]. Recent research showed that it is possible for different users to send jam signals in parallel with data signals, and yet both transmissions can be received correctly. This would highly enhance the performance of the network as users can
2. Related Work To the best of our knowledge, this paper is the first to present a comprehensive survey about the 2
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use of jamming-based solutions proposed for different problems of wireless networks. Different outstanding surveys, like [156, 157, 158, 159, 160, 1, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179], mention one or more of the jamming-based solutions. However, none of these surveys is concerned about jammingbased approaches, but may shortly describe only a few of jamming-based protocols in the principal context of the corresponding survey. We briefly describe each of these surveys in the following discussion. In [156], a detailed survey of MAC protocols for ad-hoc networks is provided. A comprehensive list of challenges and solutions of ad-hoc MAC protocols is given in [157]. The authors in [158] give a general idea on ad-hoc MAC protocols. Another fine survey of ad-hoc networks’ MAC protocols is found in [159]. Also, a review of issues and research problems related to MAC, routing, and transport layers is provided in [160]. In [1], a thorough survey of MAC protocols that provide QoS mechanisms for ad-hoc networks is presented. A survey of QoS in mobile ad hoc networks is given in [161]. A survey of vehicular ad hoc network MAC protocols is provided in [162]. Hidden terminal problem in ad hoc networks is surveyed in [170, 171]. The authors of [163] thoroughly surveyed multicasting in wireless networks. A survey of machine-to-machine (M2M) MAC protocols’ work is given in [164]. In [165], the authors attempt to provide a comprehensive survey of M2M wireless networks used to support health care and medical and services. In [172], the authors provide an overview of research on directional antennas in wireless networks including MAC, routing, and transport layers. The survey in [173] presents fine summary of MAC protocols for directional antenna wireless networks, and discusses different challenges and categories of such protocols. Also, the reader can find good surveys of research challenges and MAC protocols of wireless cognitive networks in [166, 167, 168, 169]. The authors of [174] provide a survey of adapting the threshold of carries sense to handle hidden and exposed terminal problems. In [175], the authors describe MAC requirements for wireless sensor networks (WSN) and surveyed some protocols that do not work for a WSN and protocols that are designed especially for a WSN. In [176], the authors present a survey of asynchronous MAC protocols that consider the delay measurement WSNs. A comprehensive survey of multi-channel MAC protocols for wireless sen-
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sor networks is provide in [177]. The authors of [178] survey and classify control separation in multichannel MAC protocols. In [179], different design problems of wireless mesh networks are identified and related work is surveyed. We would like to point out that destructive jamming, or jamming-based attacks, see [147, 148, 149, 150] for more details, and proposals that help against them are out of the scope of this paper. This paper focuses on jamming-based schemes that are helpful, i.e. schemes which boosts the performance of wireless operations by transmitting jam signals. 3. Background of Jam Signals
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In this section we give a background of jam signals and discuss advantages, disadvantages, and some of the challenges of using jam signals. Different names are used to describe jam signals including black bursts, bursts, and busy tones [1]. Mainly, a jam signal is a pulse of energy transmitted over the channel for a specific period of time. A jam signal does not include any decodable information; simply the transmitter turns the carrier on with no modulation. Therefore, receivers are required only to detect the presence of jam signals. Short periods of jamming are possible. Moreover, jam signals can be transmitted in parallel and are resistant to collisions; users only need to detect their existence or absence. Assume users are transmitting in parallel, then the resulting channel status is seen as an OR function of all parallel transmissions as illustrated in Figure 1. Hence using jam signals could reduce much of the time and bandwidth wasted.
Figure 1: Parallel jams
Jam signals have the following advantages: • Jam signals are resistant to collisions, and thus no need to use collision avoidance for trans3
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mitting such signals. In other words, different users can transmit jam signals simultaneously.
able to detect the jam correctly. In addition, different usage scenarios may require various energy levels for the jam signal. Therefore, it is necessary to be able to adapt the energy of the signal to take into account trade-offs between reducing errors, decreasing energy wastage, and increasing spatial reuse.
• Jam signals can travel for longer distances; receivers are required only to detect the existence of such signals as they carry no information and do not need to be decoded.
4. Description of Different Jamming-Based protocols
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• It is possible to transmit jam signals of short durations. Hence, short jam signals can save much of the time wasted in normal operations. For example, a one time slot jam signal can be transmitted as an ACK (acknowledgment) frame; one slot is much shorter than the time of a normal ACK frame.
We identify sixteen different areas for jammingbased solutions according to the usage of the jamming signal as shown in Figure 2. Furthermore, in some areas, we classify solutions to different categories, which are shown in the shaded boxes in Figure 2. In this section, for each area where jamming solutions are used, we start with a brief background and then we survey different jamming-based solutions in that area. There are plenty of jamming-based schemes, and some of them may provide solutions to more than one problem. Hence we provide what we believe is sufficient to cover all different ideas and uses of jamming to enhance wireless network performance.
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This motivated many researchers to benefit from these signals in different areas related to wireless networks especially when high performance is required. For example, jamming is utilized in contention resolution, time synchronization, and collection of statistics in different wireless networks like WSNs, WiMax, WiFi, and vehicular networks. However, jam signals have drawbacks including:
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• They can be a waste of bandwidth and energy as they carry no information.
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• Jam signals may reduce spatial reuse, the possibility of having concurrent transmissions between different pairs of nodes, as they are often transmitted with higher power levels.
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• The use of jamming adds to the complexity of wireless devices and protocols due to different challenges that need be addressed at the PHY (physical) and MAC (Medium Access Control) layers. When using jam signals, each user should have the ability to generate and detect jam signals. In addition, synchronization may be required especially when users are allowed to transmit jams concurrently. Finally, different sources of errors related to jamming should be considered including: – False alarms or detecting noise as jam signals. – Not being able to detect jam signals.
The length of a jam signal should be considered carefully for single and parallel transmissions, and should be kept minimal to meet two criteria: not wasting extra bandwidth and energy, and being 4
4.1. Contention Resolution in MAC Protocols When a number of users attempt to access the same medium in a distributed fashion, an algorithm is required to determine who has the right to access the channel. In general, accessing the wireless medium is divided into time frames with each frame consisting of two phases as explained in Figure 3. CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is the most popular wireless medium access protocol, and is the main part of the Distributed Coordination Function (DCF) in the IEEE 802.11 standard [2, 3, 4]. Figure 4a shows the basic operation of DCF which is a contention based mechanism that tries to avoid packet collisions by using random backoff delays. When a node wants to transmit data it listens to the medium for a duration called Distributed Inter Frame Space (DIF S). If the medium is free during this period, a random backoff delay is chosen from the range [0, CW ], where CW is called the contention window. The backoff timer is decreased every slot time following DIF S, and if the medium becomes busy the value of the backoff counter is retained until the medium becomes free again. The user starts to transmit when its backoff
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Figure 2: Different areas where jamming is utilized
Figure 3: Contention and data phases
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counter reaches zero. Nevertheless, collisions may still occur. In such a case, the affected nodes will double their CW and choose a new backoff delay from it. The CW starts at CWmin and doubles with every collision until it reaches CWmax . The efficiency of DCF decreases as the number of collisions increases; i.e. as the number of contending nodes increases.
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Figure 4: DCF operations
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Different proposals attempted to increase the efficiency of the contention resolution of wireless networks. A group of these proposals, which are related to the paper’s topic, utilize jamming to resolve contention. We categorize jamming-based MAC protocols into three types, slot-based, ID-based, and frequency-based protocols. Both slot-based and ID-based protocols resolve contention in time domain, and they follow a binary count-down procedure [5]. Mainly, they propose an elimination scheme where each user transmits a number of jam signals during the contention phase. As a result, the nodes receiving a jam sig-
nal discontinue their current contention phase and defer their attempt to transmit to the next frame. On the other hand, the user who sends the last jam signal is the winner of the medium, and thus starts transmitting data in the current data phase. The main differences among the elimination proposals are: 1) The number of jam signals that can be transmitted, and so the length of the contention period, 2) the decision to send a jam signal, or how each user decides whether or not to send a jam signal at a time, and 3) how to decide who the winners are. In ID-based protocols, each user first selects a binary number. Then the user sends a jam during the corresponding time slot for each bit whose value is 0 10 , and listens to the channel otherwise. For example, for ”1011”, a user would send a jam signal for slots 1, 3, and 4 and listens to channel for slot 2. On the other hand, in slot-based protocols, a user selects to send or not to send a jam for each slot during the contention period with a certain probability. Finally, frequency-based protocols move contention into the frequency domain. Instead of counting in time domain, each user sends a jam signal on a randomly selected OFDM (Orthogonal Frequency Division Multiplexing) subcarrier. Based on the jammed subcarriers, users determine who the winners of the contention procedure are. Backoff counting in time domain produces large overheads especially at higher data rates and smaller packets. On the other hand contention in frequency domain reduces much of the backoff time. In the following subsections we describe different protocols that take advantage of jamming for con-
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tention resolution.
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4.1.1. Slot-Based Protocols Hiperlan standard [6, 7] channel access is illustrated in Figure 5. After an idle period called IFS (Interframe Spacing), each user must listen to the channel for N idle slots. The smaller the value of N, the higher is the priority. The users with the lowest value of N transmit a jam signal after waiting N idle slots. Hence, all receiving users, who are of lower priority, would stop contention. The jam signal length in slots is randomly determined from 0 to some maximum number. After transmitting the jam signal, the user must listen to the channel to check if there are any longer jams. As a result, to further reduce collisions, users with the longest jam signal proceed to another contention period where each user waits for a randomly selected number of idle slots; this is called the yield period. Finally, data transmission phase starts when a user decrements its yield value to 0. In our example, Figure 5, users 1 and 2 will start to jam the channel after 0 slots as they have the highest priority. Users with higher priorities will stop contention. User 1 transmits data because of its jam length which is longer than that of user 2.
Figure 6: PREMA example, h=3
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number of the contention slot. The probabilities are chosen in a way to highly reduce the number of collisions and increase the possibility of having a winner of the channel after the contention period. The work of [11] analyzes and improves the method introduced in CONTI, i.e. using a constant period for contention.
Figure 7: Conti Protocol
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K-Round [12] is similar to CONTI, see Figure 8. However, the contention period is divided into k rounds with each round consisting of 1, 2, ..., or m slots. At the beginning of the contention period, each user randomly generates a vector indicating when to send a jam during each round; i.e., selecting which slot in that round to send a jam. Thereafter, each round ends when any user jams the channel as shown in Figure 9. Users who jam the channel go to the next round, and all others retire from contention. The authors used 7 rounds and 3 slots per round. Three slots are assumed to solve a problem, called ”contention resolution failure” in CONTI when no user jams the channel for at least 3 consecutive slots. This is because eliminated users would sense the channel to be idle for DIFS, which is shorter than 3 consecutive slots, and thus would resume contention. K-CR [13] provides a similar approach to K-Round. However, K-CR aims at increasing the probability of having one winner of the channel at the end of the contention period. CTP [14] allows users to contend using jam signals on a control channel while transmitting data on
Figure 5: Channel access in Hiperlan standard
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With PREMA [8], for each time slot, a user transmits a jam with probability p, or listens to the channel with probability (1 − p). When listening, the user is eliminated if a jam is sensed. As long as not eliminated, the user continues the same operation for each slot while counting the number of idle slots. When the number of perceived idle slots becomes h, the user starts transmitting data. Figure 6 provides an example of PREMA. In CONTI ([9, 10]), users contend for the channel using a constant number of contention slots, 6 slots, as explained in Figure 7. For each contention slot, a user either jams or listens to the channel. When listening, the user is eliminated if a jam is sensed. On the other hand, the user jams the channel with a probability that is different based on the 6
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Figure 8: K-Round Protocol
the user who waited longer. Since no two stations would have the same number, Ntr , the data transmission is free of collision. However, the scheme assumes no hidden nodes, only data traffic, and a known number of contending users. Moreover, the number of contention slots would be inefficient when number of contending users is low. In [18], a similar scheme is used but with the ID being assigned by the access point (AP). Also, it supports a mix of VoIP and data traffic. Each VoIP user starts by sending a jam after an AIF Sv period that is shorter than the interf rame space used by data users, DIF S. Moreover, after every two contention slots, another jam is sent by each VoIP user to prevent data users from attempting to access the channel since the length of DIF S is slightly larger than two slots. In WiDOM [19], the ID represents the message priority, and it is uniquely selected from a large set (210 numbers) to make collisions almost zero. The work in [20] modifies WiDOM by having the neighbors resend the jam signals to make the protocol works when there are hidden terminals. CSMA/IC [21] uses a narrow band channel for contention, and a data channel for data transmissions. Moreover, contention consists of two parts. First, each node sends a jam signal whose length is proportional to the priority of the packet to transmit. Second, users follow binary count-down using their MAC IDs to resolve contention. Accordingly, CSMA/IC provides 0% collisions and provides QoS support. PICK [22] attempts to solve the starvation issue of CSMA/IC. The binary count-down number consists of three parts in order: priority number, a random number, and MAC ID. This would allow users with lower MAC IDs to send data before users with higher MAC IDs. To solve unfairness in CSMA/IC, DFIC [23] and DIFC [24] add a fairness slot after the prioritization period. A user who believes that the situation is unfair would send a jam signal during the fairness slot. Since the fairness slot is located after the prioritization period, low priority users always loose contention. HT MAC [25] provides a collision resolution mechanism providing all collided packets with higher probability. HT MAC uses four slots to resolve collisions using jam signals. The collision resolution provides a jamming-based MAC protocol for collided users, and provides all collided users with a higher priority compared to other users as the contention period is assured to be shorter than DIFS. Each collided user selects a number and jams for bit
Figure 9: K-Round Example
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a data channel using two transceivers. The choice to send a jam signal follows a uniform distribution. CTP considers WLANs users who can overhear each other, and can transmit and listen at the same time allowing pipelined contention and data transmissions.
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4.1.2. ID-Based Protocols PBC [5] divides the contention period into a number of rounds with N slots per round. Each user randomly selects a priority number, P N , from a priority space P S. If P N is larger than 2N − 1, the user sets its contention window CW to 0; in other words it listens to the channel and defers its access to the channel to the next round. On the other hand, the user sets its CW to (2N − 1) − P N to guarantee that smaller priority numbers access the channel first. Here, the CW represents the ID used for contention. The user sends a jam during the corresponding time slot for each bit whose value is 0 10 , and listens to the channel otherwise. During contention, a user overhearing a jam is eliminated. This way, the users who jam last start data transmissions. CRA-BPC [15] optimizes the priority space of BPC based on the measured collision rate. FRRBC [16] implements a round robin scheduling scheme to provide guaranteed QoS and good fairness by mapping the allowance of each user to a binary-count down ID. In [17], each user overhears the number of transmitted packets, Ntr , since its last successful transmission. Using the binary equivalent number of Ntr and starting from the most significant bit (MSB), a jam is sent for a bit of value ’1’. All nodes receiving a jam lose contention, and thus priority is given to
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the channel. On the other hand, each other user subtracts the smallest subcarrier number from its own selected value to later contend for the channel. Back2F requires changes to the PHY layer to lessen the slot time, leakage among subcarriers, and errors in detecting the jam signals. While Back2F considers multiple collision domains, the work of [30] and [31] considers only one collision domain where all users can overhear each other. REPICK [32] is similar to Back2F but also it piggybacks the ACK frame with contention information and shortens DIFS period. In addition, the sender embeds its backoff value, number of subcarrier, in data frame. Later, the receiver uses the embedded backoff value to contend on behalf of the source of the received data. FICA [33, 34, 35] emulates OFDMA (Orthogonal Frequency-Division Multiple Access) by dividing the whole channel into a number of subchannels and allowing different users to access them concurrently, as illustrated in Figure 10. First, each user randomly selects one or more subcarriers. Second, the user sends a jam signal on each of the selected subcarriers to contend for them. The jam signals are coded to distinguish between users and their needs. The resulting concurrent transmissions of these jam signals is called M-RTS. The AP finds from the M-RTS the highest subcarriers jammed for each subchannel, and replies with an M-CTS that contains jam signals on those subcarriers. Thereafter, each user receiving the M-CTS decides whether it is a winner or not. Finally, a winner transmits data and waits for an ACK from the AP. FICA introduces a new OFDM architecture, requires synchronization, and demands a central controller to broadcast contention results. D-Fi [36] is similar to FICA but performs channel estimation and contention simultaneously to utilize frequency diversity. Medley [37] extends FICA to support a higher number of users. For fine-grained access mechanisms like that of FICA, the authors of [38] introduce dynamic tuning of the subchannel bandwidth and the number of subchannels.
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0 and listens for bit 1, and so the user with the lowest number wins the channel. All non-collided users must first wait for the channel to be idle for DIFS, and thus as long as there are jam signals they defer accessing the channel allowing all collided users to transmit. In [26], broadcast in 802.11-based vehicular networks is studied. A handshake called Request To Broadcast/Clear To Broadcast (RTB/CTB) is used to guarantee a successful broadcast. The source sends an RTB which contains the position of the source and the direction of the broadcast. Each node in the corresponding direction replies with a jam signal whose length is proportional to its distance from the source. Therefore, the farthest node wins and will be the node responsible to reply with a CTB. In [27], the authors propose a similar protocol that utilizes jamming for broadcasting emergency messages in vehicular ad-hoc networks.
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4.1.3. Frequency-Based Protocols Frequency-based protocols move contention to the frequency domain in OFDM wireless networks. The basic idea of these protocols is to save time wasted in counting down the backoff counter in time domain. MCBC [28] employs a binary count-down protocol with m rounds for OFDM wireless networks. Each round consists of two time slots, the first one is a contention slot and the second one is a feedback slot. For each contention slot, each user sends a jam on one subcarrier with probability pt where the number of the subcarrier is selected randomly from [1−nmax ]. Thereafter, users who do not send a jam signal, called referees, lose the contention, and resend a jam signal during the feedback slot on the highest subcarrier jammed in the contention slot. Hence, users who send jam signals on the highest subcarrier, during the contention slot, move to the next round. The process is repeated for each round, and the winners of last round transmit data frames. MCBC modifies the PHY layer to allow for shorter slot time and to reduce the leakage from one subcarrier to another. Also, it requires synchronization and feedback to deduce who the winners are. In Back2F [29], each user randomly selects a number, N , and sends a jam signal on the subcarrier whose number is N for one time slot. Back2F assumes that each node can transmit and receive at the same time, or each node is equipped with two antennas. Hence, the node who detects that it has jammed the lowest subcarrier would win
4.2. Support of QoS To support QoS, Enhanced Distributed Channel Access (EDCA) is introduced in IEEE 802.11e standard [39]. In EDCA, network traffic is organized into Access Categories (ACs). Instead of using a fixed DIFS, EDCA uses different Arbitration Inter Frame Spacing (AIFS). The AIFS gets shorter as the priority of an AC increases. Similarly, CWmin 8
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jam signal prevents data users from accessing the channel. Finally, real-time users must listen to the channel for a period Tobservation after jamming to assure that no other nodes have a longer jam signal. The user with the longest jam starts transmitting data. Note that Tlong , Tmed , Tslot , Tobservation , and Tshort are respectively equivalent to DIFS, PIFS, slot time, slot time, and SIFS interframe spacing in IEEE 802.11 DCF, and that Tbb , one jam slot, should at least equal a slot time; i.e., longer than the round trip time. BTPS [41, 42] attempts to provide QoS while solving the hidden terminal problem and the performance degradation of low priority users when no high priority traffic exists. BTPS utilizes two narrow-band channels and a data channel, and assumes only two priority levels. When a user has high priority data, it starts by sending BT1, a jam signal, for one slot every M slots on the first narrow band channel after sensing an idle medium for DIFS, the IFS (interframe spacing) of high priority users, and backoff periods. Thereafter, every user overhearing BT1 sends BT2, another jam signal, on the second narrow-band channel. The time of M slots is smaller than the IFS of the low priority users to prevent them from accessing the channel before high priority traffic. Also, while the transmission of BT1 and BT2 mitigates the hidden terminal problem, their absence allow low traffic priority users to access the channel. The work in [43] is similar to that of BTPS. However, instead of waiting for the channel to be idle for a backoff value, each user sends a jam signal whose length is equal to the backoff value. Hence, the fact that voice users transmit BT1 after a shorter interframe space, compared to that of data users, assures priority for voice traffic. Moreover, the node with the largest backoff transmits first. In PUMA [44], illustrated in Figure 11, a jam signal of one slot is used to inform others of high priority traffic. Users of real-time traffic simultaneously transmit a jam signal for one time slot after a period of PIFS. All users with normal traffic start their contention after DIFS period which is longer than PIFS, and thus would hear the jam signal and defer accessing the channel until an RTS or a CTS is received. Therefore, real-time traffic is guaranteed to have a higher priority. After the jam, realtime users follow DCF backoff procedure to resolve contention among them. A data packet’s priority may be changed to be the same as that of a realtime packet when certain timeout is reached. This
Figure 10: Fine-grained channel access
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and CWmax are also variable and their values decrease as the priority of an AC increases. Using this mechanism, network traffic with higher priorities gets a better chance of transmission. Both DCF and EDCA suffer from short-term unfairness because of the backoff mechanism. The source of unfairness is the way CW size is handled. Specifically, a node that transmitted successfully will maintain a small CW , while a node that experienced a collision will have a larger CW . Therefore, successful nodes are awarded with a better chance of transmission for the next packet, and colliding nodes are penalized by doubling the CW . Another problem with EDCA is that although it implements a priority mechanism, it cannot provide guaranteed service for high priority traffic. Many researchers utilized jamming to support and improve QoS in wireless networks. We discuss different service differentiation protocols that allow high traffic to always be transmitted first, and preemptive schemes that allow a higher priority traffic to interrupt an ongoing transmission of a lower priority.
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4.2.1. Service Differentiation Schemes In BB contention [40], jam signals are used for twofold: they guarantee the priority of real-time traffic, and they resolve contention among realtime traffic users. The length of the jam signal chosen increases as the waiting time of the packet increases, and thus providing higher priority for packets with closer deadlines. While real-time users start jamming after the channel is sensed idle for Tmed =Tslot +Tshort , data users decrement their backoff counters after waiting Tlong >Tmed . The 9
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and CWmax can be the same for both classes of traffic. To compensate for the longer waiting periods to get channel access in the proposed scheme, the CWmin and CWmax can have values that are much smaller than their values in the original EDCA. Finally, nodes that collide will have a better chance to access the network next time because their transmitted jam signals may have longer periods due to doubling the CW after a collision, which also solves the inherit short-term unfairness in EDCA. DPCA [48] is also similar to EDCA as a higherpriority traffic node uses a smaller AIF S. However DPCA considers hidden nodes in an infrastructure network, and allows a higher-priority traffic node to send a jam signal for a short period to block all lower priority nodes. The AP repeats the jam signal for the next slot taking care of hidden nodes. Thereafter, the higher-traffic nodes contend using random backoff values like in EDCA. With DPCA, lower priority traffic cannot access the channel as long as there are higher priority traffic.
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would prevent starvation of data traffic.
Figure 11: PUMA
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Like PUMA, the protocol provided in [45] uses jamming to classify priorities, but m priority levels are considered. For priority classification, each user sends a jam with a length that is proportional to its priority. The winners, the nodes with the highest priority level, are assigned IDs (Identity Numbers) by exchanging some control packets. Finally, transmissions are carried out in the order of the IDs assigned. A user with priority m sends m jam signals. Hence, all lower priority users defer their access to the channel. TOMAC [46] maps the waiting time of a message to an ID, such that a larger ID represents a longer waiting time. Similar to the schemes in Section 4.1.2, jamming based on the ID determines the order of the transmitted messages. A modification on the enhanced distributed channel access (EDCA) in IEEE 802.11e is presented in [47]. The objective of the modification is to provide guaranteed service to voice (or high priority) traffic over data (or low priority) traffic while improving short-term fairness. A single-hop fully connected network is assumed, in which a mix of real-time voice traffic and data traffic exists. As in EDCA, the AIFS for voice is shorter than that for data (AIF S[ACvoice ] < AIF S[ACdata ]) to give a higher priority to voice over data. The proposed scheme changes the behavior of contending nodes, where after waiting for an AIFS (voice or data) the node sends a jam signal of a duration that is equal to its backoff timer. After that, if the channel is idle the node sends the packet, otherwise it quits the current contention. This way, the node with the longest jam wins the channel. Note that the scheme guarantees voice service over data because the AIF S[ACvoice ] is shorter than the AIF S[ACdata ]. Therefore, there is no need to distinguish access categories by varying the minimum and maximum contention windows (CW ) as in the original EDCA, and CWmin
4.2.2. Preemptive Differentiated Service Schemes In [49], a MAC protocol is proposed to assure priority of emergency messages in vehicular adhoc networks while addressing the hidden terminal problem. In this protocol, the backoff counter and the duration of a jam signal depend on the priority level of the message. While the transmitter sends data on the data channel, it also sends pulses, jam signals with pauses in-between them, on the control channel. When another source has a message with higher priority, it sends pulses during the pauses of the ongoing transmission. When the current transmitter receives the new pulses, it compares the duration of the new and its own jam signals, and releases both channels if the new duration is longer than that of its own jam signals. In addition, the pulses help in mitigating the hidden terminal problem. RBTMA-NIT [50] is an enhancement of [49]. VCWC [51] is another similar protocol that uses jamming to give emergency messages the highest priority in a vehicular ad-hoc network. The authors in [52] provide service/device differentiation schemes for jamming-based MAC protocols. 4.3. Solving/Mitigating Problems Emerging in Wireless Networks Different problems emerge especially in a wireless network. First, hidden terminal problem occurs when a user, who is unaware of any ongoing transmission, starts a transmission interfering
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mission, the AP transmits a jam signal on a narrow band channel to prevent all other nodes from attempting to access the data channel. In other words, when a station have data to transmit, it listens to the jamming channel. If a jam is detected, the station defers its attempt to transmit. PAMAS [54] extends BTAM to consider power savings while solving the hidden terminal problem. In PAMAS, RTS, CTS, and jam signals are sent over the control channel. Accordingly, users know when and how long they can turn their powers off. DBSMA [55] is another extension of BTMA for directional antennas. DBTMA [56] introduces two types of busy tones; the transmit busy tone (BTt), and the receive busy tone (BTr). Before sending data, a source senses the medium for BTr or BTt, if the channel is clear the BTt is activated and an RTS is sent. However, if it senses a BTr or BTt, then there is a neighboring node expecting (or receiving) data or an ongoing transmission respectively. Therefore, the source refrains from sending the RTS and goes to backoff. After a destination receives an RTS, it senses the channel for BTt to make sure that there are no ongoing transmissions in the neighborhood, then it activates the BTr to acknowledge the RTS and to tell neighboring nodes that data is expected to be received. BTr stays active until the data is received. DOSS [57] extends [56] for cognitive wireless networks. In [58], a protocol is proposed to enhance the utilization of DBTAM by using power control of the data and BTt to allow concurrent pairs of communications. DBTMA/DA [59] applies DBTMA for nodes with directional antennas. Sending the busy tones directionally further expands the capacity since it allows more concurrent transmissions. Extending DBTMA, DSMA [60] uses three channels: jam channel to send jam signals to protect against hidden nodes, control channel to send RTS/CTS, and data channel for data transmissions. In [61], the AP, in an IEEE 802.11 WLAN, transmits a jamming signal on a jamming channel while receiving data on all other channels. Thus hidden nodes are prevented from disrupting the ongoing transmission. The protocol xRDT [62] is proposed for multichannel wireless networks where the receiver continues to send jam on a control channel as long as it is receiving data on data channel. In RCTC [63], a receiver transmits a jam signal while receiving data in full-duplex wireless networks to solve hidden terminal problem.
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with an ongoing transmission. In the standard IEEE 802.11, Request-To-Send and Clear-To-Send (RTS/CTS) messages exchange, illustrated in Figure 4b, are adopted to solve the hidden terminal problem. When a node gains access to the medium, it sends an RTS message to the receiver. The RTS message reserves the medium by announcing to the one-hop neighbors the intention of the transmitter. When the destination hears the RTS message, it replies with a CTS message to announce to its onehop neighbors that it is expecting a packet. Both RTS and CTS have duration fields to tell the neighboring nodes how long they need to wait. The duration in RTS/CTS messages sets the Network Allocation Vector (NAV), which tells a node the duration of channel occupancy. This process is usually referred to as virtual carrier sense. Note that using RTS/CTS does not eliminate collisions completely. Second, exposed terminal problem arises when a user is blocked from transmitting because of an ongoing transmission although both transmissions may proceed in parallel. Jamming-based solutions to hidden and exposed terminal problems are summarized in Section 4.3.1. Third, Deafness, a problem that arises in directional antenna wireless networks, occurs when a user, say user1, attempts to transmit to another user who is communicating in a different direction. Hence, user1 unnecessarily keeps backing off and increasing its own backoff counter. Jamming-based solutions for the deafness problem are discussed in Section 4.3.2. Fourth, erroneous reservation occurs when a user unnecessarily defer accessing the channel because of its NAV timer. Usually NAV is included in a transmitted packet to reserve the channel long enough, and listening users are required to use NAV to defer their access to the channel. The problem occurs for faulty transmissions as we will explain in Section 4.3.3. Finally, jamming is used to allow a full-duplex user to finish an ongoing transmissions as illustrated in Section 4.3.4. Most of the surveyed work in this area use the term busy tone instead of jam signal. Therefore, both terms are used interchangeably throughout this subsection. 4.3.1. Hidden and Exposed Terminal Problems BTMA [53] may be the first protocol to address the hidden terminal problem using jam channels. BTMA extends CSMA to solve the hidden terminal problem in an infrastructure network where an AP exists. As long as the AP is receiving a data trans-
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ceiver. To distinguish among different users, each user determines the duration and frequency of a jam signal using a static hash function that depends on the node ID. When any node overhears a jam signal, it concludes that the transmitter of the jam signal was unaware of the transmission attempt, and thus resets its backoff value saving bandwidth that could be lost needlessly. DSDMAC [69] attempts to solve both the hidden terminal and deafness problems in directional antenna networks. RTS, CTS, Data, and ACK are sent directionally on data channel, and two types of jam signals, BT1 and BT2, are transmitted on control channel. The transmitter sends RTS and BT1 at the same time, and then starts to send BT2. When the receiver receives RTS, it sends CTS and BT2 at the same time. While BT1 solves hidden terminal problem, BT2 solves the deafness problem. BT-DMAC [70] is similar to DSDMAC. However, the transmitter and sender send jam signals on the control channel only during the transmission of DATA and ACK frames. Moreover, each node transmits the jam signal as pulses of 1’s and 0’s to reveal the ID of its transmitter. Nodes use a hash function like in ToneDMAC [68] to determine the pattern of 1’s and 0’s. BT-DMACP [71] is another protocol that uses jam channels to solve the deafness problem, but it also uses power control to enhance efficiency and minimize energy consumption.
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DUCHA [64] proposes to use one jamming and two information, control and data, channels to solve hidden terminal and exposed terminal problems. A source, say A, first waits for the control channel to be idle for DIFS and senses no jamming. Then, A sends an RTS on the control channel and the destination, say B, replies with a CTS on the same channel. Accordingly, A starts to send data on the data channel, and B keeps sending a jam signal on the jam channel as long as it is receiving data to prevent the hidden terminal problem. DUCHA solves the exposed terminal problem by allowing another sender, say C, to transmit an RTS if it does not hear the jam from the receiver of the current transmission, B. If the intended receiver, say D, replies with a CTS (i.e., D is not a neighbor of B), C understands that it will not be interfering with the ongoing transmission and can proceed with the new transmission. SBA-MAC [65] solves the hidden terminal problem using a single channel. The transmitter sends data in fragments and inserts dummy bits between every two fragments. On the other hand, the receiver transmits a jam signal during the periods of the dummy bits while receiving data. Hence, all nodes overhearing the jam defer their access to the channel even if no RTS or CTS are received. SBA-DMAC [66] exploits SBA-MAC for directional antenna wireless networks. BusySiMOn [67] aims to minimize the collisions of signaling data in a single-channel wireless network and so it mitigates the hidden terminal problem. Before transmitting RTS and CT frames, two short jam signals are exchanged. The first jam signal serves as requesting to send RTS, and the second one works as the CTS of the first jam signal. This allows quicker channel reservation, and so reduces collisions due to hidden terminal problem. More schemes that use jam signals instead of RTS/CTS are discussed in Section 4.6.1.
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4.3.3. Erroneous Reservation Problem In addition to the hidden terminal problem, JMAC [72] addresses the erroneous reservation problem that may occur because of the RTS/CTS exchange. Erroneous reservation of the channel happens in different scenarios that are explained in [72]; we list the simplest one here. For example, if the transmitter sends the RTS message, but does not receive the CTS from the receiver due to a transmission error, then the medium around both the sender and the receiver is reserved by the NAV but is not used. The authors propose using multiple transceivers, in addition to jam signals to solve these issues. The proposed scheme relies on two ideas; the first one is to separate sender traffic (RTS and Data) from receiver traffic (CTS and ACK), and send each on a different channel, and the second one is to rely on jam signals instead of NAV for channel reservation. Therefore, two channels are used; channel S for the sender traffic, and channel R for the receiver traffic. Figure 12 shows
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4.3.2. Deafness Problem ToneDMAC [68] tackles the problem of deafness where users send RTS, CTS, Data, and ACK frames directionally. Initially, the sender and receiver finish a directional transmission (directional RTS, CTS, Data, and ACK packets) on a data channel. Then each of the receiver and transmitter converts to omnidirectional transmissions, and transmits a jam signal on a control channel. The jam teaches neighbors that deafness is the reason for being unable to reach the transmitter or the re12
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the primary sender transmits a jam to protect an ongoing secondary transmission when the primary transmitter finishes transmission earlier. Another similar approach for full-duplex is available in [77].
the details of the proposed scheme. If the R channel is idle for DIFS, the sender sends RTS and then jams the S channel while waiting for CTS on the R channel. After receiving the RTS, the receiver sends CTS on the R channel and jams it while waiting for the data on the S channel. When the CTS is received, the sender sends the data then continues to jam the S channel while waiting for the ACK on the R channel. If the data is successfully received, the receiver sends ACK on the R channel. Finally, upon receiving the ACK, the sender stops jamming the S channel. Using jam signals instead of the NAV to reserve the channel solves the erroneous reservation problem. Consider the case stated above, if the sender sends RTS, but does not receive CTS due to a transmission error then the channel will not be reserved because after CTS timeout the sender will stop jamming the channel and a neighboring node may receive an RTS from a different sender. Similarly, after data timeout, the receiver will stop jamming, and a neighboring node can send an RTS to another receiver.
4.4. Collision Detection
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Figure 12: JMAC
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RI-BTMA [78] follows BTMA, explained in Section 4.3.1, to eliminate hidden nodes and provide collision detection for wireless networks, see Figure 13. The sender first transmits only the preamble of the data packet. After receiving the preamble, the receiver starts to send a jam signal on a separate channel. Thereafter, the sender sends the rest of the data. When the sender does not overhear the jam, it stops sending data assuming a collision. RI-BTMA is receiver initiated and requires two transceiver for its operation. For MIMOOFDM networks, MARI-BTMA [79] extends RIBTMA and suggests a new transceiver architecture to allow multiple simultaneous transmissions and receptions on the same channel; it uses one jamming subchannel for each data subchannel.
Figure 13: RI-BTMA
In FAMA-PJ [80], the source first sends RTS, and then sends a jam signal if the CTS was not received. This informs others that a collision occurred. WCD [81, 82] extends RI-BTMA to detect collisions faster. WCD splits the channel into a data and a carrier detect (CD) channels to add collision detection mechanism to wireless networks while eliminating hidden and exposed terminal problems. In WCD, when a transmitter sends data it also listens to the CD channel. Receivers overhearing the data transmission start to send jam signals over the CD channel. Thereafter, if a receiver does not recognize the data preamble, it would stop sending the jam signal. Hence, the transmitter can conclude a collision when no jam signal is detected on the CD channel, and would then stop transmitting data. WCD is a receiver initiated feedback protocol and requires two transceivers to detect collisions properly. PulseAcc [83] attempts to detect collisions even if the transmitting nodes do not transmit at the same time. As shown in Figure 14, the source of
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4.3.4. Protection of Full-Duplex ongoing Transmissions RFD-MAC [73] exploits jamming to protect an ongoing transmission in full-duplex wireless networks. In RFD-MAC, a transmitter sends a jam signal when it ends transmission and its reception is still going on. The jam prevents neighbor nodes from transmitting, and thus shields the ongoing reception. A similar approach is proposed in [74]. FuMAC [75] is a MAC protocol for full-duplex wireless networks. When the transmitter sends data, the destination should start transmitting data to the source. If the source receives no response from the destination, it assumes that the destination cannot receive correctly and hence aborts transmission. Accordingly, the destination transmits a jam signal when it has no data for the source to prevent the source from aborting the transmission. For fullduplex networks in [76], the primary receiver sends a busy tone to protect against hidden nodes, and 13
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each user has a vector that represents the channel state during the SDJS scheme, and that vector is used to estimate the number of active users. To give an example, User 1’s vector is [1,0,1,1,0,0,0] as illustrated in Figure 15. Finally, users switch back to the normal operation where the estimated number of users can help improve the performance of the network without the need for any control messages.
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data simultaneously transmits data on data channel and pulses of jam signals on control channel. After the receiver gets the header, it sends a CTS pulse on control channel during the first pause of pulses sent by the source. Accordingly, the source can continue sending the rest of the data frame. The sender stops sending data in two cases; whenever it does not get the CTS pulse during the first pause after the header, and when it detects another CTS pulse which means a collision.
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Figure 14: PulseAcc
Figure 15: SDJS Scheme
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4.5. Statistical Estimations The performance of wireless networks relies on various parameters that can be known or unknown a priori. Therefore, there has been an interest in statistically estimating unknown parameters in a wireless network. Then, such estimates are exploited to enhance the performance of the network. A significant example, the main concern of different proposals, is the number of active users in the network. When estimated, number of users can be used to optimize contention; i.e. reducing collisions and idle times to minimal. Without the use of the jamming-based estimation protocols, users are required to periodically exchange information via control or special packets. That would waste bandwidth and lower the performance because of the extra time and energy consumed. Such penalty is due to the transmission of the control packets and the retransmission of these packets in case of collisions. On the other hand, jam signals are short and need not be decoded. Another advantage of using jam signals is that they can be sent simultaneously. In the following, we address some research that take advantage of jam signals in statistical estimation.
Additional work is needed to analyze the length of the SDJS scheme, or to decide the number of jam slots, the length of a slot, and the number of jam signals per user. Also, performance can be optimized by tuning when to transmit the jam signals by each user, and finding if SDJS scheme should be repeated periodically or based on the changing network environment. Finally, it is essential to analyze the effect of SDJS on the network performance. In [85], a protocol is proposed to estimate the number of machines in a large-scale machine-tomachine (M2M) network. Machines send the jam signals on a common control channel in order to allow for parallel transmissions of data and jam signals. The estimation depends on the number of detected jam signals.
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4.5.2. Contention-Based Neighborhood Estimation Similar to SDJS, in contention-based scheme [86, 87] each user transmits a jam signal or listens to the channel during a fixed number of slots. However, only the number of idle slots is utilized in the estimation process. In addition, the start message may specify that only users with some criteria, like a specific level of energy, should participate in sending the jam signals. The authors make use of the probabilities and expected numbers of idle slots, collided jams, and successful jams. However, they show that busy slots, successful or collided jams, cannot be used as they result in two different estimates. Moreover, each
4.5.1. SDJS The SDJS scheme [84] is started with a start packet sent by one of the users as shown in Figure 15. Thereafter, for a fixed number of time slots, each user transmits a jam or listens to the channel, where every user jams the channel at least once. In Figure 15, 1 means a jam is sent, and 0 means a jam is received, by the corresponding user. At the end, 14
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and argues that to achieve agreement the most important packet is the last ACK packet. The problem is if the last ACK packet is disrupted no agreement could be achieved, and worse yet false agreement may happen. To enhance the probability of receiving the last ACK packet in an n-way handshake, the paper proposes encoding it as a jam signal. The encoded jam signal can tolerate other interferences that can corrupt a usual packet. The scheme is extended by the same authors to cover broadcast in [91]. DT-MAC [92] is a dynamic TDMA (Time Division Multiple Access) MAC protocol that provides QoS guarantees for a fully connected network. In DT-MAC, a user sends a one slot jamming signal to indicate the failure of its beacon broadcast. Thus, a jam signal works as a negative acknowledgment in DT-MAC. SMAC [93] provides a reliable broadcast ACK for OFDM wireless networks. Each user sends a jam signal using its assigned subcarrier as an ACK of a correctly received broadcast message. Since jam signal carry no information, the existence of any number jam signals transmitted in parallel is enough to indicate an ACK in broadcast transmissions. In [94], a jam signal indicates the success or collision of a transmitted packet. The jam signal is transmitted after the end of the packet by a time whose duration depends on whether the packet was successfully received or not. Accordingly, a jam is equivalent to negative acknowledgment, and its absence is an acknowledgment. DSS [95] schedules links to different users in an ad-hoc wireless network. The channel is slotted, and a control slot consists of two phases. The source sends a control packet in the first phase. The destination replies with a jam signal during the next phase if the SINR (signal-to-interferencenoise-ratio) requirement cannot be met, and thus, the result is like a negative acknowledgment canceling the scheduling of the link for that slot time. In [96], the source beamforms toward its intended destination, and sends a short jam instead of RTS. All receivers beam-form toward the transmitter whose jam is the strongest, and transmits a short jam instead of CTS. Finally, upon receiving the jam-based CTS, the source directionally transmits the data. Similarly, [97] proposes to use jam signals of one slot duration instead of RTS/CTS handshake for directional antenna wireless networks. In xRDT [62], the transmitter first sends RTS on
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user can only detect the presence or absence of the jam signal, but it is difficult to distinguish between successful and collided jam signals as no data is included in such signals. One disadvantage of the provided protocol is that it works assuming a range of the number of users, i.e number of users is in [nmin to nmax ]. When the actual number is out of range, the estimation’s accuracy lessens, and hence more slots are required wasting more bandwidth and energy. DIP [88] is another protocol that counts idle slots, as a measure of contention level, to statistically estimate the number of nodes in a wireless sensor network. However, although DIP assumes that special signals (jam signals) can be used, users of DIP transmit packets with minimal size instead. The work in [89] provides a multichannel MAC protocol for M2M networks that starts with an estimation phase based on the number of jammed slots. 4.6. Frame Transmission Jamming is used to transmit different types of frames including control, attachment, and data frames.
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4.6.1. Control Packets Control packets, like ACK, CTS, and RTS, increase the overhead specially when transmitted using lower PHY rates. Accordingly, different researchers propose to use shorter jam signals instead to lower the wasted time. BusySiMOn, proposed in [67], aims to minimize the collisions of signaling data in wireless networks. BusySiMOn introduces a two-step procedure for channel reservation. The first is preliminary reservation of channel using two busy tone signals: Busy1 is used to request channel reservation by a source, and Busy2 is sent by its neighbors to confirm the channel reservation. Busy1 has the length of one slot time period (STP), and Busy2 is three STPs long. The advantage of this signaling scheme is that it allows quick channel reservation when compared with the four-way handshake in IEEE 802.11. The used tone signals are short compared to RTS/CTS. Therefore they would mitigate the hidden terminal problem as they provide faster reservation. The second step in the protocol is to send information about the transmission duration and the communicating nodes using RTS/CTS. JAG [90] uses jamming to achieve consensus on certain information between nodes. The paper focuses on an n-way handshake between two nodes
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data transmission by the user who wins the channel access, and 2) attachment for control by all other users. Each receiver is assigned a different subcarrier to send jam signals bounded by the number of data symbols. The pattern of jams is mapped to a given control packet. In the pattern, ”1” indicates the existence of a jam, and ”0” indicates the absence of a jam. For example, the attached packet in Figure 16 is ”101100” for user 2 exploiting subcarrier 4 for attachment, and this pattern may be mapped to an RTS packet. Note that each user is assigned a different subcarrier number, and the assignment process should take place before an attachment is made. To achieve attachment transmissions, a new architecture with different physical components is proposed. First, jam signal generator and detector are designed properly to reduce false alarms and miss detections. Second, interference cancelation is employed to cancel the attached information, the jam signals, and recover the original data. One requirement for interference cancelation to work is synchronization among users. Finally, suitable modulation and demodulation are added in order to attach and decode jam signals accurately. Disadvantages of attachment transmission include the added complexity of the transceiver design (transmitter and receiver). Also, synchronization among users is required to align data transmission and different attachments. In addition, the number of users who may use attachment at a time is restricted as each user is assigned a subcarrier. Only half the number of data subcarriers, for example 24 of the 48 subcarriers in IEEE 802.11a, are allowed for attachment. Attachment is applied for different applications of OFDM-based wireless networks. In [99, 100], users in an infrastructure network attach control packets like RTS frames which are later used by an AP to schedule users’ data transmissions in a TDMA-like protocol with about 200% efficiency improvement. In [101], users attach identifiers to help them learn when to access the channel in a distributed OFDMA network. Attached-RTS MAC [103] solves the exposed terminal problem by having users attach lists of their neighbors, and later use such information to decide whether or not to access the channel in parallel with an ongoing transmission. Two mechanisms are proposed to reduce problems that may arise due to concurrent transmissions. First, dedicated subcarriers are reserved for ACK packets to eliminate collisions among data
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control channel. Then the receiver starts to send a jam signal, serving as CTS to the sender, on the control channel. Thereafter, the transmitter would start to transmit the data frame on data channel. Once the receiver gets the whole data, it sends a jam signal on the control channel to acknowledge the data reception. In DUCHA [64], the destination sends a jam signal on a jamming channel as a negative acknowledgement if the received data is corrupted. The absence of this jam signal is an ACK. REPICK [32] piggybacks ACK with contention information in the frequency domain as jam signals. STRP [98] is a polling protocol that utilizes the capture effect (the ability to correctly receive a stronger packet in the existence of a weaker packet). The AP polls two stations, one active station and one idle station, at the same time using a special control packet. Thereafter, the active station starts to transmit its packet. On the other hand, the idle station replies with a weak jam signal, for a duration that is longer than the packet of the active station, if it has data to transmit. Hence, the AP can receive and decode the data packet using capture effect, and can notice the jam indicating that the idle station should be polled. This removes the unnecessary overhead of polls and provides higher fairness as stations are polled faster when they become active. The jam signals works as a poll-me control packet.
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4.6.2. Attachment Transmission Attachment transmission is an interesting idea that is developed and utilized for different OFDM IEEE 802.11 wireless applications [99, 100, 101, 102, 103, 104, 105, 106, 107]. Attachment enhances the performance by allowing simultaneous transmission of data from one user and one or more control packets from all other users as illustrated in Figure 16.
Figure 16: Attachment Transmission
Each transmission is divided into two types of information that occur at the same time: 1) Normal 16
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receiver. One can expect that such transmission would provide a low throughput or transmission rate. For example, for a payload of 250 bits, there are at least 254 slots without considering headers that include information like transmitter address, receiver address, etc. Also, although parallel transmission is possible, the receiver cannot even know the number of transmitters. Moreover, if a ’1’ is received, the receiver cannot decide if any transmitter actually transmitted a ’0’. Because of this, the authors suggest that jamming should not be used for data transmission but for management tasks; like statistics estimation, solving hidden terminal, and contention resolution described in Section 4.
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and ACK packets. Second, collided users follow a fast retransmission scheme to complete their transmissions. Also, to reduce the cost of attached information, a hash format is used. On the other hand, FAST [108, 104] is proposed to solve both the hidden and exposed terminal problems for full-duplex wireless users. Users attach their IDs announcing their status: transmitting, receiving, or being a victim in case of being a neighbor of the sender or the transmitter. Accordingly, all nodes utilize the attached information to overcome the hidden and exposed terminal problems. In CUTS [105, 106], users attach antenna information in an MU-MIMO infrastructure wireless network. Users contend in frequency domain to send requests attached with antenna information. Consequently, the AP selects a number of successful transmitters to transmit simultaneously.
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4.6.3. Data Transmission In Black Burst transmission [109], the authors propose to send data via jam signals as shown in Figure 17.
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Figure 17: Frame transmission via jam signals
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Each data transmission starts and ends with distinctive frames, SOF (Start of Frame) and EOF (End of Frame), with each of them consisting of two consecutive jam slots. The payload is a series of jam and empty slots. A jam indicates a ’1’ and an empty slot indicates a ’0’ in the payload. The provided scheme requires synchronization to know the beginning of each slot. Hence, an empty slot can be recognized, and minimum and maximum errors of the jam length are found. Accordingly, three types of errors are defined: 1) Too long, or small, jam signals, 2) not recognized jam signals, and 3) receiving a jam that is actually not sent. An experimental evaluation is provided to show that such transmission is possible. The used MAC allows transmitters to send at the same time to one receiver, and thus data received is a combination of all transmitted jam signals; i.e. OR function of transmitted bits. The experiments are limited to wideband wireless network, small ranges, one receiver, up to seven transmitters, and all transmitters are positioned at the same distance from the 17
One of the sources of interference in cellular networks is cochannel interference, which is caused by transmissions in neighboring cells. This type affects most of the users on the cell edge since they are closer to other cells. Busy burst signaling can be used to manage interference and mitigate this problem. Recall that busy burst (BB) is another name for jam signals. BB-OFDMA, a decentralized interference management mechanism for time-slotted OFDMA (OFDMA-TDD) cellular systems like WiMAX, is proposed in [110]. Controlling cochannel interference (CCI) is a major challenge in cellular networks. In OFDMA-TDD, the channel is timeslotted and frequency divided into chunks, where a chunk spans nSC subcarriers (frequency) and nOS symbols (time). A chunk represents a resource unit and can be allocated to a certain user in some cell. Data is transmitted in chunks to users in a downlink (DL) slot, and to the BS in an uplink (UL) slot, where each slot contains NC chunks. A DL slot and a UL slot together constitute a MAC frame. A chunk is identified by (n, k), where n is the frequency index, and k is the slot index. Figure 18 clarifies the terminology. BB-OFDMA divides the channel in a similar fashion, but it uses the last symbol (from the nOS symbols) in each chunk for BB signaling. The operation of BB-OFDMA can be summarized as follows: 1) If a transmitter intends to transmit on chunk (n, k) it must check for BBs in the last symbol in this chunk before transmission. 2) If a BB is detected, the transmitter cannot use this chunk. Using BBs this way can accomplish the following:
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Figure 18: BB OFDMA
There are two problems in a time-synchronization packet exchange in a wireless network. First, the possible loss of packets caused by collisions in wireless environments. Secondly, the variable duration of the packet exchange, due to the random delays introduced in the MAC layer to avoid collisions. BBs, Black Bursts or equivalently jam signals, can solve both problems because they are immune to collisions; therefore, if no collisions can occur there is no need for random delays. MacZ ([113] and [114]) is a MAC protocol that supports reservation-based contention-free medium access and contention-based medium access. To achieve contention-free access, time synchronization is needed. MacZ uses two types of black bursts (short BBs and long BBs) to achieve logicaltime synchronization to some reference point in time. MacZ relies on a set of master nodes, each of which has a unique synchronization sequence (sync-sequence) based on its ID that is composed of short and long BBs. To have sequences of equal lengths, different idle times follow different BBs. The idle time after a long burst (idlelong ) should be long enough for the hardware to process it correctly. However, the idle time after a short burst should compensate for the difference in length, i.e., idleshort = idlelong + Tlong − Tshort , where Tlong and Tshort are the durations of long and short black bursts respectively. In MacZ, a synchronization announcement precedes every synchronization round. The announcement is composed of two short BBs, and is used to notify unsynchronized nodes (which might have just joined the network) of the upcoming synchronization phase. The length of the syncsequence depends on the maximum number of masters. If there are n masters, the sync-sequence will have n bits. The sync-sequence of master j has the bits from 1 to j set to one, and the sequence for master zero is all zeros. The highest bit in a sync-sequence is transmitted first where a 0 is represented by a long BB, and a 1 is represented by a short BB. Therefore, the most dominant sequence (the one that will be heard and decoded by other masters) is the one with more zeros. Note that if a node is in the range of two or more master nodes, it will only hear the most dominant sync-sequence. Each node resets its clock at the end of the first received BB. Therefore, after the first sync-round finishes, all the nodes in one-hop range of at least one master node will be synced to the most dominant sequence. After a defined pause time, every node retransmits the most dominant received syn-
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1. If user X receives chunk (n, k) successfully, it can reserve chunk (n, k+1) with the BS by sending a BB in the UL slot. 2. If a BS in a cell detects a BB in chunk (n, k) from user X who is in a different neighboring cell, it cannot allocate the same chunk to users close to the cell of user X (to reduce CCI). 3. The BS in a cell uses the BB in the DL slot to prevent other users in neighboring cells from using the chunk in the UL slot.
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BB-OFDMA creates an exclusion region around a victim receiver (susceptible to CCI), which reduces the overall system throughput, but enhances the user throughput and system fairness. Other similar schemes for interference management are given in [111, 112].
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4.8. Time Synchronization Time synchronization is an important requirement to enable many networking features and applications like time-slotted medium access. There are two types of time synchronization. First, real-time synchronization, where the clocks in all network nodes need to be synchronized to a real-time clock. This is usually accomplished by using a dedicated time server, to which all nodes can synchronize, or by using GPS receivers on all network nodes. Second, logical-time synchronization, where the clocks in all network nodes need to be synchronized to a certain reference point in time. The value of this reference point may be important, for example, the value may be the local clock of some master node or base station. Alternatively, the value of the reference point may not be important, in such a case, the ordering of events with respect to the reference point is what really matters not the actual time values. This is called tick-synchronization. With the exception of GPS-based synchronization, time synchronization protocols in general depend on a packet exchange between network nodes.
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chronization sequence. Depending on the network diameter, the process repeats until all nodes in the network are synchronized. MacZ also proposes another synchronization method that does not depend on master nodes, and that uses short BBs only. In this method, similar to the previous one, a sync-announcement indicates the beginning of the synchronization process, which in turn, has many rounds depending on the network diameter. However, in this method every node transmits a short BB instead of a sync-sequence. Because of drifts in local clocks, the transmission of the short BB starts at different times. A node synchronizes or resets its timer with the first heard BB and then will rebroadcast the BB in the next sync-round after a known pause time so that the two-hop neighbors can synchronize. The process repeats until all nodes in the network are synchronized. BBS ([115] and [116]) uses only one type of black bursts, and similar to MacZ, BBS introduces a master-based synchronization (BBSm ) and a fully distributed synchronization scheme (BBSd ). In BBS, a bit duration is fixed and should be long enough for the transceiver to switch from receiving to transmitting, send a BB for a duration of dBB (if the bit is 1), and switch back to receiving. Note that if the bit is 0, no BB is sent. BBSm depends on a single master node to synchronize the network and also needs the network to be stabilized, i.e., an initial tick synchronization has been done. Simply, when the nodes are first deployed, they wait for a predefined period of time. If no synchronization messages, BB-encoded data, are received, the node with the fastest clock sends a tick-sync message, a BB transmission, that gets rebroadcasted in a similar fashion to the second sync protocol in MacZ, and this node becomes the master node. As a result, the network will be tick-synchronized with respect to the node with the fastest clock, and there will be a reference point in time known to all network nodes (of course within some maximum offset that can be determined). The master starts the synchronization process by sending a master-tick frame, which consists of a dominant bit (a 1 that is represented by a BB) and an encoded round number, using BBs, initially set to one. Upon receiving the tick frame, the one-hop neighbors wait for one round period (dround ), increment the round value in the frame, and then rebroadcast the master-tick frame. Since the BB transmissions are synchronized and sent in parallel, master-tick frames containing
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the same encoded information that are sent from different nodes can be received by one node in a non-destructive manner. The process is repeated nmaxhops times until all network nodes receive the master-tick frame. Since the round time is fixed and known to all nodes, a node receiving the frame in round X knows that the reference tick happened before (X-1)*dround seconds. The paper also proposes a fully distributes scheme BBSd that is similar to the distributed scheme in MacZ, and a hybrid scheme BBSh that utilizes both BBSm and BBSd . 4.9. Physical Security
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The nature of the wireless medium introduces many challenges to data exchange. If native data units are transmitted as is, they can be picked-up and successfully decoded by any receiver in range. Therefore, guaranteeing data secrecy usually involves using cryptographic techniques to guarantee that only intended receivers can understand the transmitted packet. An alternative approach that lies in the scope of this survey, is to interfere and corrupt the reception of unwanted receivers and eavesdroppers by using jam signals. We categorize jamming-based physical security schemes into nonselective jamming and selective jamming.
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4.9.1. Non-Selective Jamming A cooperative jamming mechanism is proposed in [117] to enhance the physical security in a cooperative relaying scenario. In cooperative relaying, a source S wants to send data to a destination D with the help of a set of relay nodes R. The paper assumes that an illegal eavesdropper E exists in the network. Three scenarios are considered; firstly, if the location of the attacker is known, relay nodes close to E will corporately jam it. Secondly, if the location of E is unknown, D will participate in the cooperative jamming process when S transmits, while S will participate in the jamming process when D receives from R. In this case, signal-processing techniques are used to ensure the reconstruction of data in R and D. Finally, if one of the relay nodes is the eavesdropper (i.e., E ∈ R) the receiver jams the channel while the transmitter is transmitting, and then subtracts the jam signal from the signal received from R. Similar schemes were explored in [118] and [119]. In [118] only one trusted relay node is assumed, while in [119] nodes with multiple antennas are considered. 19
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techniques. This leaves such networks vulnerable to external attacks since little or no security is usually enforced. The work in [123] proposes a selective jamming-based technique to prevent unwanted or externally injected packets from being properly received by devices in an automated home environment. Simply, the idea is to detect such packets and promptly transmit a jam signal to distort the received packets at end devices causing them to be dropped. Nodes can cheat the MAC to achieve better throughput. For example, not conforming to collision resolution, by not increasing the size of the contention window, may enhance the throughput for the cheating node by many folds. Frame-selective jamming can be used to correct the behavior of misbehaving nodes. The work in [124] applies game theory to develop a mechanism to punish nodes that deviate from the standard. The more a node deviates, the more its throughput is degraded. Although not evaluated in their simulation, frameselective jamming is suggested to be used in the paper to degrade the throughput of misbehaving nodes. Jamming is used in [125] to protect Implantable Medical Devices (IMDs) from adversaries who want to program the IMD or retrieve data from it. The scheme uses a guardian device that acts as a proxy for the IMD. Therefore, if a programmer wants to communicate with the IMD he needs to go through the guardian. The guardian and the IMD share a key based on the patient’s ECG (electrocardiogram) signal, and the guardian knows legitimate programmers and their public keys. The IMD works in one of two modes; Normal mode, and emergency mode. In normal mode, all communications go through the guardian. The IMD enters emergency mode when it detects the absence of the guardian, which leads the IMD to think that the patient is in an emergency (thus, the guardian device is removed) and that the programmer (under a doctor supervision) needs to access its data directly. An attacker can try to access the IMD information, or can mislead the IMD to enter emergency mode by jamming the guardian, and then gain direct access to the IMD. The ECG-based shared key can nullify the former attack, but not the latter. If the IMD is forced to enter emergency mode it goes through the following steps:
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The work in [120] deals with a case in which both authorized friendly nodes coexist with unauthorized adversary nodes. The paper proposes a jamming mechanism that allows friendly nodes to communicate, while adversary nodes are jammed. This is accomplished by continuously jamming the channel with a key-controlled jam signal. This jam signal can be understood by authorized friendly nodes, which in turn, regenerate the jam signal locally to remove it from the received signal, and thus, extract the actual data. Since the unauthorized adversary nodes do not know the used key, they cannot regenerate the jam signal to remove it and will always be jammed.
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4.9.2. Selective Jamming Friendly jamming is defined in [121] as using jamming for the good with minimal invasiveness. Minimal invasiveness is required for friendly jamming techniques to coexist with, and benefit, already deployed networks. Friendly jamming is usually frame selective, i.e., it jams certain network frames that satisfy some condition(s). This type of jamming is called reactive jamming because a jamming node needs to receive the frame partially, and depending on the received partial information, decide whether to jam the frame or let it pass. It is obvious that reactive and frame-selective jamming requires strict timing constraints to be able to function properly. The feasibility of friendly jamming is studied in [121]. The research found that friendly jamming may be possible and cost-effective if a large number of jammers collaborate together. The large number of jammers is required for better coverage, to successfully jam unwanted packets. Collaboration is required between jammers to activate only the needed jammers to minimize the impact on legitimate data. Nevertheless, friendly jamming may not be always suitable. The work in [122] evaluates the effectiveness of friendly jamming if an attacker is equipped with multiple antennas. A MIMO-based attack targeting confidentiality is proposed and tested. The attacker can successfully recover confidential messages produced from the data source in the presence of a friendly jammer, which shows that friendly jamming is not always suitable to guarantee confidentiality. Home Automation (HA) network protocols are usually designed to relay small data units (e.g. a single bit can be used to turn a device ON or OFF). Therefore, such protocols are usually simple and do not rely on complicated headers or cryptographic
1. It sends a randomly generated symbol to the programmer (possibly an attacker). 20
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2. It waits for some predefined period of time, and sends another symbol to the programmer. 3. If the programmer is legitimate, and the guardian is removed under trusted supervision, the programmer sends the XOR of the two symbols to the IMD. 4. Upon receiving the correct XORed data, communication can commence between the IMD and the programmer.
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to interrupt all other contending users as shown in Figure 19. Accordingly, users switch to data channel where data transmissions take place.
Frame-selective jamming come into play if the IMD is tricked, and the guardian is still there and hears the first sent symbol. Note that this symbol will only be sent if the IMD does not receive a message from the guardian regarding the required access from the programmer in the first place. Therefore, when the guardian overhears the first symbol it knows that the IMD did not receive its message (maybe because of an attack). The guardian then jams the second symbol to prevent the possible attacker from successfully XORing the correct symbols. In [126] jamming is used to defend against injection attacks. However, unlike [123], where an external device is added to the network to do the jamming, in this paper jamming is part of the protocol (close to [125]). Every data transmission is split into two frames: Data Follows Notification (DFN) frame, and an actual Data frame. Upon receiving a DFN, the target node expects to receive the actual data after a defined Inter-Frame Gap Time. Using DFN and Data enforces the sender to a strict time window since the receiver will not process the data if it was received too late or too early. Moreover, neighboring nodes who hear a DFN are given enough time to authenticate it, and if a fake DFN is detected neighboring nodes can collaborate to jam the injected data packet. WiFire [127] is a system that implements the concept of firewalls in a wireless network. WiFire sends jam signals to block any undesired wireless communication.
Figure 19: Explicit pipelining
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HDCF [129] allows active stations to transmit one after the other without contention. However, when there is a new station, it sends a jam signal to interrupt and join active transmissions. Similarly in a WSN, Rainbow [130] utilizes jamming to allow a node to join a tree topology network. After a node, say A, overhears an invitation message, say from node B, A sends a jam signal, then after some backoff time A sends an application message to join the network. Hence, B becomes the parent of A in the tree topology. A protocol is proposed in [131] to solve alarm storms in WSNs. Alarm storms occur when a large number of sensor nodes respond to an event and start transmitting alarm messages to the base station. This scenario increases collisions, congestion, and energy consumption. The proposed protocol attempts to reduce the number of nodes responding to an event. Each sensor node starts a backoff timer after detecting an event. The sensor node loses contention if a jam signal is received before its timer ends. On the other hand, the node sends a jam signal when its timer ends. Hence, the node sending the last jam wins and would send the alarm message.
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4.10.2. Sense Sharing in Cognitive Wireless Networks SMC-MAC [132] divides the channel access in a cognitive ad-hoc wireless network into four periods. The first period is a period of time that the channel must be sensed idle. The second period is a sense and sharing scheme which consists of a number of slots and each slot is divided into three sub-slots. Each slot corresponds to one of the primary channels. Each user randomly picks a channel, say chi and senses that channel during subslot one in slot i. Thereafter, it shares the results with other secondary users by either sending jams or not. For example, if the remaining subslots in slot i are
4.10. Other Uses In this subsection we summarize a group of schemes that do not belong to any of the previously discussed categories. 4.10.1. Interrupting Some Activity In explicit pipelining [128], users contend on a control channel. The user whose backoof counter reaches zero sends a jam signal for one time slot 21
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CoRe-MAC [136] is a protocol for cooperative constrained wireless relay communications. In CoReMAC, a relay sends a jam signal to acknowledge a packet, to announce availability of a relay, and to block others from transmitting for some period. RTS/CTS+BusyTone [137] also uses three busy tone signals for cooperative communication. However, RTS/CTS+BusyTone attempts to set appropriate timings of different jam signals to enhance the channel utilization and solve the hidden terminal, exposed terminal, and deafness problems. In addition, to select the best relay node, each relay transmits a jam signal with a duration that is proportional to the signal-to-noise (SNR) ratio.
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jammed, the channel was sensed and found to be idle as shown in Figure 20. The third period is the contention period using RTS and CTS. Finally, the winner of contention starts to transmit data in the transmission period. SMC-MAC aims at reducing the overhead seen by sensing all channels without scarifying performance; i.e. each user senses one channel and shares the result with all.
Figure 20: Sense sharing example: channel one is sensed and found idle
4.10.3. Power Control PCMA [133] is a decentralized MAC protocol in which users periodically transmit jam pulses on a control channel. Transmitters first listen to the control channel to determine the minimum power that can be used for a transmission on the data channel. The power of a transmission is controlled in a way that enhances spatial reuse and prevents interfering with an ongoing transmission.
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Figure 21: CTBMA
4.10.4. Cooperative Diversity CTBTMA [134], illustrated in Figure 21, uses three jam channels, BTt , BTr , and BTh , to prevent hidden nodes and to allow users to help each other transmit faster. When no jam channel is active, a source transmits RTS and starts sending a jam on BTt channel. The destination responds with a CTS and activates the BTr channel. Thereafter, the source waits for some time during which any node, that can help make transmission faster, would activate the BTh . CTBTMA allows a helper node to receive data from source and retransmit it to the destination using a rate faster than that would be used by the original source. In CMACORS [135], the relay node sends an in-band jam signal after overhearing both RTS and CTS packets, and only if it could help make the transmission between original source and destination faster.
RO-CMAC [138] is MAC protocol that attempts to select the best relay nod in a cooperative network. A relay node with a higher data rate sends a jam signal earlier than relay nodes with lower data rates. Thereafter, relay nodes with the highest data rate proceed to a contention phase to select a single relay node. RRS-CMAC [139] is similar to ROCMAC.
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4.10.5. Wake-up a Destination in Wireless Sensor Networks With STEM-T [140], a sensor node transmits a jam signal long enough in order for the destination sensor to overhear it. Accordingly, the destination wakes up to receive data from the source. A source node of SCP-MAC [141] also utilizes a jam signal to wake up its intended receiver. ESCDD [142] proposes to send out-of-band jam signals to wake-up cooperating neighbor nodes in a WSN. 4.10.6. Coexistence between Different Wireless Networks CCS [143] suggests using a dedicated ZigBee node to send jam signals at the same time a ZigBee data is being transmitted as illustrated in Figure 22. 22
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5. Summary
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Figure 22: WiFi and ZigBee Coexistence
Table 1 compares the different protocols in terms of three factors. First, using jam signals in-bound or out of bound, where the latter means that jam messages will be sent on a separate control channel. Note that this does not necessarily mean that multiple transceivers were used. In addition, even if multiple transceivers were used, but the jams were sent on the same channel as the data we count this as an in-bound protocol. Second, using fixed length or variable length jam signals. Some protocols divide time into slots and decide to jam these slots depending on a certain criteria, while others will calculate the length of the jam also depending on a certain criteria. A protocol is said to produce a variable length jam if its operation guarantees a continuous jam signal of variable length, which can be produced by consecutive jammed slots or by a single continuous jam signal. Third, using one type of jam messages or multiple types, where a protocol may have predefined jam signals with different lengths, or causes of transmission that are used to indicate different meanings. To clarify, in a certain protocol, if jam signals with different lengths represent different meanings (e.g., RTS, CTS, or ACK) we do not count this as a variable length jam signal, rather we count it as multiple signals. Therefore, to say a protocol uses variable length jams, the meaning of the jam must represent one thing (e.g., priority). Also, if the same jam signal is used to compose different patterns of jams we count this as a single type of jamming. In the table, if multiple jams were used and one type was of variable length, we count the protocol with the ones that use variable length jams. In addition, we point-out whether the protocol was evaluated theoretically, or by simulation, or it was implemented on a real testbed. The table also indicates the usage of jam signals in each of the protocols. Finally, please note that the table only covers base protocols, but not their extensions or variations unless there is a notable difference. From the comparison we note the following:
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Such jamming would allow WiFi users to notice the ZigBee transmissions and yield the channel to the end of the ongoing transmission. A WiFi channel overlaps with four ZigBee channels; assume ZigBee channels are Ch1, Ch2, Ch3, and Ch4. While a ZigBee data is being transmitted on one of the channels (like Ch1), a special node, called a signaler, transmits a jamming signal on one of the other channels (like Ch2). Thus, the WiFi devices senses the WiFi channel to be busy and then would defer accessing the channel. Figure 22 illustrates the use of jamming to allow ZigBee nodes to transmit in the presence of WiFi nodes.
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In [144], the authors analyze the utilization of periodic busy tones to allow coexistence of ZigBee and WiFi networks. They show that periodic jam signals enhance the performance of the ZigBee network.
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4.10.7. Channel Reservation BTMC [145] is a multi-channel MAC protocol that utilizes jamming to reserve a data channel to be used by a pair of nodes. For every data channel there is a corresponding control channel. In addition, a set of hash functions is shared between all network nodes to be used to search for available channels. Nodes available for communication (not sending any information) passively listens to the first available channel returned by the hash functions. The absence of a jam on a control channel indicates the availability of the corresponding data channel. Upon finding an available channel the sender sends an RTS. The receiver replies with a CTS on the data channel and jams the control channel to reserve the corresponding data channel. In OFDM-RR [146], the authors propose a MAC protocol to reduce the interferences in OFDMAbased femtocells. The controller station in a femtocell sends a jam signal on a common channel as long as a transmission is going on. All other overhearing controllers wait for the channel to be free; i.e. no jamming is detected.
• Only a small fraction of the protocols was implemented. Moreover, most of these implementations were for the protocols using single-type and fixed-length jam signals. • Variable length and/or multiple classes of jam signals tend to get used more in QoS schemes to guarantee service differentiation. 23
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• Most of the protocols that address problems in the MAC layer (e.g., Hidden terminal) use out-of-bound signaling.
• All statistical estimators use fixed-length jam signals. • Sending control messages using jam signals mostly involves single-type and fixed-length jam signals.
• Most of the protocols that use out-of-bound signaling use variable length jam signals.
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Protocol IN/OUT F/V S/M TH/SIM/IMP Usage Hiperlan IN VAR M TH QoS and Contention PREMA IN FIX S SIM Contention CONTI IN FIX S SIM Contention K-Round IN FIX S SIM Contention CTP OUT FIX S SIM Contention BPC IN FIX S SIM Contention [17] IN FIX S SIM Contention [18] IN FIX S SIM QoS and Contention Widom IN FIX S IMP Contention CSMA/IC OUT FIX S SIM QoS and Contention [26] IN VAR M SIM Broadcast in VANETs MCBC IN FIX S SIM Contention in FD Back2F IN FIX S IMP Contention in FD FICA IN FIX S IMP QoS and Contention in FD Table 1: IN = In-bound; OUT = Out-of-bound; F = Fixed; V = Variable; S = Single; M = Multiple; TH = Theoretical; SIM = Simulation; IMP = Implementation
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IN/OUT IN OUT IN IN IN IN IN OUT
F/V VAR FIX FIX VAR FIX VAR FIX VAR
S/M S M S S S M M M
TH/SIM/IMP SIM SIM SIM SIM IMP SIM SIM SIM
Usage QoS and Contention QoS and Contention QoS QoS and Contention QoS & Contention QoS and Contention QoS & Contention Preemptive QoS and Contention BTMA OUT VAR S TH Hidden Terminal DBTMA OUT VAR M SIM Hidden Terminal [61] OUT VAR S TH Hidden Terminal xRDT OUT VAR S SIM Hidden Terminal, Deafness, and Control Messages DUCHA OUT VAR M SIM Hidden Terminal, Exposed Terminal, and Control Messages SBA-MAC IN FIX S TH Hidden Terminal BusySiMOn IN FIX M SIM Hidden Terminal and Control Messages ToneDMAC OUT VAR S SIM Deafness DSDMAC OUT FIX M SIM Hidden Terminal and Deafness JMAC IN VAR M SIM Hidden Terminal and erroneous reservation RFD-MAC IN VAR S IMP Full-Duplex Protection FuMAC IN VAR S IMP Full-Duplex Protection [76] IN VAR S SIM Full-Duplex Protection RI-BTMA OUT VAR S TH Collision Detection FAMA-PJ IN FIX S TH Collision Detection WCD OUT VAR S SIM Collision Detection PulseAcc OUT VAR M SIM Collision Detection SDJS IN FIX S SIM Statistical Estimation [86, 87] IN FIX S SIM Statistical Estimation DIP OUT FIX S SIM Statistical Estimation [89] OUT FIX S SIM Statistical Estimation JAG IN FIX S IMP Control Messages SMAC IN FIX S IMP Control Messages Table 1: IN = In-bound; OUT = Out-of-bound; F = Fixed; V = Variable; S = Single; M = Multiple; TH = Theoretical; SIM = Simulation; IMP = Implementation
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Protocol BB BTPS PUMA [45] TOMAC [47] DPCA [49]
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F/V FIX FIX FIX VAR FIX
S/M S S M S S
TH/SIM/IMP SIM SIM SIM SIM IMP
[101, 102]
IN
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AttachedRTS FAST
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[109] [110] MACZ BBS [117] [118] [120] [123] [125] [126] WiFire [128] HDCF
IN IN IN IN IN IN IN IN IN IN IN OUT IN
FIX FIX FIX FIX VAR VAR VAR FIX FIX VAR FIX FIX FIX
S S M S S S S S S S S S S
IMP SIM not evaluated IMP SIM TH IMP IMP IMP IMP IMP SIM SIM
Rainbow PCMA CTBTMA CMAC-OR CoRe-MAC CCS BTMC
IN OUT OUT IN IN OUT IN
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Usage Control Messages Control Messages Control Messages Control Messages Attachment Transmission for Control Messages Attachment Transmission for Contention Resolution Attachment Transmission to solve Exposed Terminal Attachment Transmission to solve Hidden and Exposed Terminal Attachment Transmission for FD Contention Resolution Data Transmission Interference Control Time Synchronization Time Synchronization Non-selective Jamming Non-selective Jamming Non-selective Jamming Selective Jamming Selective Jamming Selective Jamming Selective Jamming Interrupting Contention Interrupting Transmission Schedule Joining Hierarchy Power Control Cooperative Relaying Cooperative Relaying Cooperative Relaying Coexistence Channel Reservation
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Protocol [94] DSS [96] STRP [100, 99]
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Table 1: Summary: IN = In-bound; OUT = Out-of-bound; F = Fixed; V = Variable; S = Single; M = Multiple; TH = Theoretical; SIM = Simulation; IMP = Implementation
6. Discussion
schemes (see sections 4.1.1 and 4.1.2) and statistics estimation protocols (see Section 4.5), the length of a jam signal should be kept to a minimal. The minimal length of each jam signal is given by ([84]):
In this section, we point out some features and we discuss different issues of jamming-based protocols. We also provide some directions for future work. 6.1. Minimal Length of Jamming The minimal length of a jam signal depends on the physical layer, MAC processing, and radio properties. For schemes like time-domain elimination
max(RXtoT Xtime , T XtoRXtime )+ TM ACprocessing + TCCA
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from receiving to transmitting, T XtoRXtime is the time needed to switch from transmitting to receiving, TM ACprocessing is the time needed to process the received signal, and TCCA is the carrier sense time required to detect the signal including the propagation delay of the signal. This is because users in such schemes may send a jam signal or data after an idle period (also after overhearing an earlier jam signal in statistics estimation protocols), and may listen to the channel after transmitting a jam signal. When users are assumed to transmit in parallel as in frequency domain contention resolution, summarized in Section 4.1.3, a form of synchronization is required. However, tight synchronization is not required in WiFi devices; it could be enough to synchronize by observing the medium activities such as detecting preambles of different frames transmitted. When using OFDM modulation, all parallel signals must overlap for at least one FFT window at the receivers. Thus each user should transmit a signal with a minimal time equal to the following value ([32]):
6.3. Fixed and Variable Lengths of Jam Signals Jam signals can be of fixed or variable length. For example, each jam signal is one slot time long in K-Round [12] to resolve contentions, while the AP transmits a jam signal as long as it is receiving data in BTAM [53] to alleviate the hidden terminal problem.
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6.4. In-Band and Out-of-Band Jam Signals
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Jam signals can be sent on the same channel as other frames, in-band signals, or on a separate channel, out-of-band signals. For example, while JMAC [72] uses in-band jam signals to solve erroneous reservation problem, DUCHA [64] uses an out-ofband jamming to solve hidden terminal problem. In addition, data and jam signals can be transmitted simultaneously over the same channel like in attachment transmission, see Section 4.6.2.
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For more details about alignment of parallel transmissions refer to [32, 29, 109, 33, 34, 35]. 6.2. Distinctive Indications of Jam Signals
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Although jam signals carry no information, researchers suggested different methods to provide different indications of these signals depending on some of their properties when used in a system. Some of these properties are: • The length of the jam signal: an example is to send a longer jam to indicate a higher priority like in BB contention [40].
Jamming-based schemes may not be backward compatible with the standards. An example is jamming-based contention schemes, see Section 4.1. Hence, the performance of users not performing jamming could be highly affected. For example, users implementing IEEE 802.11 DCF would see the channel busy most of the time when users following CONTI, a jamming-based MAC protocol discussed in Section 4.1.1, are operating in the same network because of the transmitted jam signals. Accordingly, the performance of DCF users is expected to be extremely degraded. Figure 23 illustrates the problem.
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Figure 23: A network with DCF and CONTI users
• The frequency, or channel, used to transmit the jam signal: an example is to send a jam signal on a specific frequency to indicate a specific user. For example, in REPICK [32], a user sends a jam on a subcarrier j as an acknowledgment to user j.
6.6. Are Jamming-Based Schemes Worth the Pain? Although jamming-based schemes do provide solutions to different problems, they also may incur overheads that could lower the performance of the wireless network. For example, SDJS, see Section 4.5.1, reserves about 64 time slots where jamming can be sent, and then uses the number of jam signals to estimate the number of users in the network.
• Using both length and frequency is also possible like in ToneDMAC [68] which uses both values to indicate who the transmitter is. 27
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To increase accuracy and keep the estimated number correct, SDJS should be repeated. As a result, the performance of the network would be affected. We argue that the effect of jamming-based schemes on different performance measures, like throughput and delay, of the network should be evaluated and compared to non jamming-based solutions. In addition, we argue that it is better to implement a nonjamming based solution if it provides a close performance like that of the corresponding jammingbased scheme. The reasons lie in the fact that jamming adds to the complexity of wireless devices and introduces new types of errors as we discussed in Section 3. Finally, there is a need to consider distinguishing between friendly jamming and possible jamming attacks [147, 148, 149, 150].
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Figure 24: Wideband and narrowband signals
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Jamming is mainly used to solve one problem in a wireless network. A framework, where jamming is used as a solution for more than one problem, requires careful consideration. For example, to provide an efficient contention resolution scheme, to solve the hidden terminal problem, and to estimate the number of nodes at the same time based on jamming requires means to distinguish between different jam signals. 6.8. Generating, Transmitting, Black Bursts
BBs can be used to represent data. Basically, a ’1’ is represented by the presence of a BB, while a ’0’ is represented by the absence of a BB. The ’1’ is called the dominant bit, and the ’0’ is called the recessive bit. A ’1’ can be easily detected if an energy pulse is sensed by the CCA. However, for a ’0’ to be detected the channel must be silent when a transmission is expected, which calls for a timeslotted medium access mechanism for the transmission of BBs. Such medium access mechanisms heavily rely on time synchronization protocols. Clearly, the transmission of BBs is more involved than their reception if they are used to represent data (e.g., as in ID-based contention discussed in Section 4.1). However, we believe time synchronization may not be required in other scenarios that use continuous and variable length jam signals only (e.g. protocols that support QoS discussed in Section 4.2) since no data is encoded in the jam.
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wireless transceivers. The performance of CCA depends on the type of signals used in the network. Unlike wideband signals, narrowband signals have their energy concentrated around the carrier signal, which makes them easy to distinguish from noise, and thus facilitating detection by the CCA as illustrated in Figure 24.
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In the OSI model, upper layers require services from lower layers in the network stack. To the best of our knowledge, jamming the channel is not among the standard provided services. Therefore, to enable MAC (or higher layers if needed) to request jamming the channel, a proper implementation in the PHY layer coupled with a proper interface with the MAC layer are needed. An alternative way to generate BBs might be viable in the MAC layer alone, where special types of frames can be added such that they do not follow the CSMA protocol. That is, such frames will be sent directly over the wireless medium upon request allowing the MAC to directly jam the channel. BBs are thought of as information-less frames that do not carry any kind of information except their length. This property means that it is enough to only detect the energy pulse without the need to decode or understand the content. Therefore, BBs can be received and understood by the Clear Channel Assessment (CCA) [182] mechanism used in all
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6.9. Evaluation of Jamming-Based Approaches
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Most of works are evaluated via simulation assuming that wireless devices are capable of using jamming as required by each work. On the other hand, only a small fraction of the protocols was implemented. Most of the implemented mechanisms, like [109], use the CC2420 radio chip by Texas Instruments, which provides many functionalities to facilitate the transmission and reception of busy tones. This chip allows direct interaction with the physical layer through reading or writing to registers. It allows the programmer to transmit unmodulated carrier signal at will, and to directly sense the channel through the Received Signal Strength Indicator (RSSI). Alternatively, some 28
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of the friendly jamming approaches, like [99], used Software Defined Radios (SDR) in their implementations. 6.10. Some Future Work Directions Statistical estimation can be improved to consider larger and variable number of nodes and improved technologies of future networks. Collection of statistics can be used in IPTV delivery over LTE to count the number of users interested in a certain multimedia content [151]. Also, jamming-based statistical estimations can be applied to new MAC protocols like frequency-domain MAC protocols, see Section 4.1.3, and MAC protocols for D2D (Device to Device) communications [152]. Coexistence and guaranteeing QoS are important in heterogeneous networks, which are the norm in some future technologies such as the Internet of Things (IoT) [153], or M2M networks [154]. These types of networks will contain a huge number of devices using heterogeneous networking technologies and protocols. The nodes in such networks will consist largely from simple low-end devices, although there will be high-end computers too. This will be a suitable environment to apply jamming-based physical security also. In addition, jamming can be utilized to allow coexistence for cognitive radio networks, see [155], where secondary users of different networks may share the same wireless resources. Capture effect [180, 181] provides the capability to correctly receive a stronger packet in the existence of a weaker packet. When a jamming signal overlaps with a normal packet, the receiver radio would synchronize to the strongest signal. Thus, normal packets are lost when overlapping with stronger jam signals. Accordingly, proper selection of energy levels of jam signals is required especially for scenarios, like multihop networks, where jam signals may interfere with normal packets. Capture effect and jamming can be exploited to enhance the performance of a wireless network as in STRP discussed in Section 4.6.1. We think that further work is needed to evaluate and propose jamming-based solutions with capture effect being employed.
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signals to enhance the efficiency of wireless networks. We also classified different research fields where jamming signals are exploited for the good of wireless networks and to solve problems that may arise in wireless networks. In addition, we attempted to categorize different protocols in some of these areas. Finally, we discussed challenges, requirements, advantages, and drawbacks of jamming.
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Haithem Al-Mefleh received the B.S. degree in Computer Engineering from Yarmouk University, Irbid, Jordan in 2000, the M.S. degree in Computer Engineering from Illinois Institute of Technology, Chicago, Illinois, USA in 2004, and the Ph.D. degree in Computer Engineering at Iowa State University, Ames, Iowa, USA in 2009. He joined Yarmouk University, Jordan, in 2009. His research interests include medium access protocols and QoS support for IEEE 802.11 and IEEE 802.16 wireless networks.
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Osameh M. Al-Kofahi received his B.Sc. from Jordan University of Science and Technology (JUST) in 2002, and the Ph.D. degree (with honors) from Iowa State University in 2009, in Electrical and Computer Engineering. He is currently an assistant professor of Computer Engineering at Yarmouk University in Jordan. His research interests are developing network-coding based survivability techniques for multi-hop wireless networks. He was the co-recipient of the best paper award of the IEEE Globecom 2008 Symposium on Ad Hoc and Sensors Networks.
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