- This document is about techniques in which the performance and reliability of a wireless network can be improved. We will discuss the cooperative communication system, how it operates and its security issues. Also, the future of the diversity in wireless networks will be analysed. In cooperative networks, the data is transmitted from sender to receiver via intermediate nodes called relays. The main o bjective is to consider the pros and cons of relaying techniques in order to improve wireless local area networks (WLANs) [1]. Keywords - Relaying ; cooperative communication system; wireless networks I. INTRODUCTION The conventional wireless networks face issues such as interference, signal loss, signal fading and denial of service attacks. The cooperative networking uses cooperation among the involved nodes to achieve significant improvement in terms of the overall system capacity and performance. It is a promisi ng technique which enables efficient utilization of communication resources and mitigates signal fading in wireless networks [2]. Cooperative communication has two phases • Relay Selection [3] • Data Forwarding [4] There are various relay selection mechanisms such as pr oactive relay selection, reactive relay selection , opportunistic relay selection etc. [5] In proactive relay selections , the relays are selected prior to transmission, while in reactive relaying the relays are selected when needed. Reactive relaying is benefi cial when there is failed transmission. In case of opportunistic relay selections, the relays are selected on demand . II. OPERATION Basic mechanism of cooperative communication is shown in figure 1 Figure 1 Cooperation can be used to achieve better data rates. In above figure, we suppose that node 1 is at a larger distance from destination. Since 802.11 rate adaptation allows the stations to adopt different rates based on distance and Signal to Noise Ratio (SNR). So, if node 1 establishes direct link with destination, the data rate will be very low. Conventionally, if an intermediate node with faster bit rate overhears data, it is obliged to drop it. Rather in cooperation, node 1 sends the data to destination via node 2 which is lies i n between. In this way, the data rate is improved and medium is released sooner. [5] Simulation results as well as real implementations show that the cooperative communication system boosts the performance of the networks up to 5 times comparing with the exiti ng technology of IEEE 802.11 [8]. November, 2018 - 2 - Figure 2 - IEEE 802.11 Associ ation sequence Figure 2 shows 802.11 standard process to develop a connection between two wireless devices in a network. A. Specifications of CoopNet In CoopNet, cooperati on among peers compliments traditional client -server communication. Specifically, CoopNet addresses the problem cases of client -server communication. It plays its role only when needed and acts normally when direct links are working just fine. CoopNet, unl ike other peer -to-peer systems, does not assume that helper nodes will be available all the time. For instance, helper nodes may only be willing to cooperate for a few minutes. Hence, network cannot solely depend on peer -to-peer communication. [6] In cooperati ve network, each wireless user is assumed to transmit its own data as well as act as a cooperative agent for another user . One might think this reduces the data rate of nodes. However, the spectral efficiency of each user improves, traffic jams are avoided and hence the overall system performance gets better. III. SECURITY ISSUES Following are some of the security issues associated with cooperative networking and their proposed possible solutions . A. Packet dropping[ 9] One potential security issue is associated with the relay deliberately not forwarding frames received from the source. Here, relay node denies service to the source by simply dropping the packets it receives. [7] November, 2018 - 3 - Figure 3 Packet dropping at relay 3 Consider a wireless mobile netw ork where node 1 is source and node 4 is destination while all the other nodes are acting as relays. In figure above, node 3 is a malicious node and sends a malicious RREP to source while dropping the packets received. Solution: [13] It is then up to the sour ce to realize that this node is malicious and choose another one. If another relay does not exist, it can then transmit directly to the destination, albeit at a lower rate. The source could detect the responsiveness of a rel ay by imposing a timeout, after which if no acknowledgement from destination is received, it would blacklist that relay. B. Spoofing [12] A malicious node may deny service to sender by failing to forward data and spoofing an ACK on behalf of the destination, thereby making the source think that the data was received [15]. Figure 4 - Spoofing attack In Figure 3, a network of laptop computers, which are connected to a switch, is shown. The attaker node disturbs the communication of the other 2 nodes by acting as a malicious relay [18]. Solution: Cooperative networking may use a kind of RTS/CTS scheme to combat this issue [19]. This means that the destination sends the CTS, and is aware that it is an intended recipient of the future frame. Thus, if it does not receive th is frame in an allocated NAV period (due to the fact that the helper didn’t send it and spoofed an ACK to the sender), destination November, 2018 - 4 - node can send a NACK or negative acknowledgement alerting the source that it didn’t receive the frame [20]. C. Payload modification Another issue is a scenario where the relay modifies the payload and then forwards it. The receiver will typically not come to know of this, so it may end up voluntarily replying with privileged information, such as passwords and usernames. This typ e of an attack is po ssible when the change in the payload will not lead to the corruption of the packet, i.e. when no wireless encryption scheme is used, so that no mechanism exists to detect the alteration of the payload. [10] Figure 5 - Payload modificat ion attack [11] Here, node J is acting as relay between A and B yet modifying the payload and forwarding packets in a longer time than is required [14] Solution: Such an attack cannot be easily avoided unless the transmitter and receiver can themselves find that there is an unusually large delay in the received packets, which will be due to the calculation of the CRC etc., at the relay . However, if we implement a protocol which requires the retransmission of the packet by the relay in a SIFS interval. This type of attack will not be possible then unless relay knows the exact key, as SIFS duration is very small to perform any kind of complex calculations and manipulation the packet [16]. D. RTS Attack RTS attack is type of DoS attack where the sender of RTS sends RTS either to flood or to reserve the medium for undue amount of time [17] In IEEE 802.11 standard the Medium Access Control (MAC) layer defines the channel access mechanism. It includes optional RTS/CTC handshake mechanism to avoid collision and hidden node problems. When the sender finds medium free it sends RTS frame after Distributed Inter Frame Space (DIFS) amount of time. DIFS is normally 50 micro seconds, and have considered the same in our evaluation. RTS frame includes 2 Bytes duration field to insert the duration of channel reservation for data transfer. This allows the overhearing nodes to remain in Quiet state for reservation duration. All other nodes will access the channel after expiration of this time. In case of malicious node, the duration field in RTS i s filled to undue amount of time such as maximum allowed time, causing the RTS attack. Figure 6 - Behaviour of RTS attack November, 2018 - 5 - A node with malicious intent can modify the RTS control frame’s set duration operation and replace it with a larger value to pretend that it needs large duration to communicate [21], to held the channel unnecessarily for overdue time and reduce the bandwidth utilization with increase in latency for other communications (Figur e 6) E. Jamming Attack A cooperative commun ication network is particularly vulnerable to malicious attacks in the physical layer. Moreover, jamming is one of the most serious attacks that greatly degrade network performance. It is a physical layer attack where a jammer tries to block the data commu nication between the source and the destination. An energy- constrained relay is able to assist the source to forward the data to the destination even when the jammer tries to block the direct link. In jamming attacks, a jammer attempts to prevent users fro m accessing wireless network resources and reduces network availability by generating interference signals on the channels (Figure 7) . This exhausts the energy of the nodes in the network. In order to tackle the jamming attacks, frequency hopping spread sp ectrum and direct sequence spread spectrum are widely utilized [22]. However, the same sequence can be used by the jammer to attack its target if the hopping sequence is exposed. Thus, the random rendezvous and the uncoordinated frequency hopping is used to saf ely share the hopping sequence. Nevertheless, these tech niques result in the time waste for the communications. Therefore, other secret sharing protocols are proposed such as public key cryptography, certi ﬁcate and authentication protocol but they also cause large overheads [23]. Figure 7: Jamming attack IV. FUTURE OF COOPERATIVE NETWORKING It has been shown that cooperative diversity brings several benefits including better signal quality, reduced transmission pow er, better coverage and h igher capacity [24]. Cooperative diversity has some drawbacks and challenges including resource over -utilization, additional delay, implementation complexity, unavailability of cooperating nodes and security threats [25]. it should be emphasized that the implementa tion of cooperative diversity is shown to be challenging due to the various complexity issues . Reduced -complexity algorithms and protocols are needed to simplify the [ 26] implementation of cooperative diversity in order to be able to harvest the benefits of coo perative diversity. Effective measures and schemes are required to increase users’ motivation for cooperation and to enhance information security and service authenticity in cooperative diversity . The work is currently in progress and we hope to see the co operative diversity based networks in future. It is expected to revolutionize the mobile communications and all the wireless networks [27]. V. GLOSSARY Payload the actual information or message in transmitted data, as opposed to automatically generated metadata. November, 2018 - 6 - CRC A cyclic redundancy check ( CRC ) is an error -detecting code commonly used in digit al networks and storage devices in order to detect accidental changes to raw data. SIFS In IEEE 802.11 networks, SIFS is a short interframe spacing prior to transmissio n of an acknowledgment, a Clear To Send (CTS) frame . ACK The control character used in the Transmission Control Protocol to acknowledge receipt of a packet. RTS / CTS Request to Send / Clear to Send is the optional mechanism used by the 802.11 wireless ne tworking protocol to reduce frame collisions introduced by the hidden node problem.