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Without the need of an infrastructure and being low-cost, auto-managed, flexible and low power consumer, wireless communications is becoming emerging technology. It shows great binder for present and future hi-tech applications. Increasing reliance on wireless communications also brings great challenges to the security measures and other correlated issues. In many cases, the mobility of nodes and infrastructure less topology made wireless network more attractive. But due to the same feature, wireless communication is vulnerable and it is very probable that an intruder catches security measures out. Although the newly introduced corrected security standard, IEEE 802.11i, guarantees extensive security for the wireless environment but it is still premature and does not provide effective measures to protect the wireless networks from confidentiality and integrity threats. Their reliance on authenticity and availability are still a major concern.
Majority of wireless attacks seems to be very simple and in-destructive but it would be danger ineaf to disrupt the whole network communications. This paper provides a survey on security issues, challenges and attacks in wireless communication specially sensor and ad-hoc wireless communications. Our goal here is to probe the principal security issues, challenges and fundamental security requirements of wireless communications and their proposed solutions.
Due to low cost, low power consumption, flexible, no physical infrastructure and easy to deploy, wireless communications have been a popular research topic over the past few years with tremendous growth in the population of wireless users. Many systems are still mapping wire to wireless media. Currently, there are various wireless technologies available for long range applications like cellular mobile, satellite communications, microwave communications and short range applications such as Bluetooth, Infrared (IR), ZigBee, Radio Frequency (RF), Near Field Communication (NFC), and Ultra Wide Band (UWB). These short range wireless technologies are being used in many areas such as wireless local area networks(WLAN), wireless personal area networks(WPANs), wireless body area networks(WBANs) or ad-hoc network. Although wireless communications have several compensations over the conventional wired networks, it is exposed to a range of intrusion attacks. Unlike the wired networks, the wireless networks face unique challenges due to their inherent vulnerability. Any wireless signal is subject to interception, jamming, and false command disruption. The coverage area of wireless communication is limited by the potential of the wireless device used. The device that gives a wide coverage area will be very expensive. Moreover, the more powerful wireless devices will consume more power and produce a high electromagnetic radiation which can be hazardous to human health. In many applications the security aspects are as vital as performance and low energy consumption. Nearly all the proposed solutions concentrate on a specific security problem but pay no attention to others, those which pull off low energy and memory burning up, compromise on the level of security. Thus there is a need for a model which fulfills all these issues with low cost and high security.
Wireless communications suffer with two major types of attackers, inside attacker, outside attacker, an outside attacker has no special access to the network, whereas an inside attacker has access to the encryption keys or other codes used by the network. Like Passive attackers only interested in collecting sensitive data from the wireless network, that compromises the privacy and confidentiality requirements. In contrast the active attacker's goal is to disrupt the function of the network and degrade its performance. Other wireless attacks are DoS attacks, Routing Attack, Black-hole attack, wormhole attack etc.
Issues and Problems
Wireless communication has emerged as a major breakthrough in traditional wired communications. It has changed messy wired world into a clean and flexible atmosphere. According to a well known adage, there is no unmixed good in this world; implementation of wireless network carries numerous performance and security issues. These issues include:
Vulnerabilities and Challenges in current wireless Communications:
The wireless communications survivability relates to wireless communication protection mechanism and robustness against attacks and failure of wireless network elements or communication itself. Some of these issues are as follows:
The Wireless Sensor Network gateway forms a single point of failure for the end-to-end sensor network infrastructure .
After deployment the network, sensor nodes are left unattended which causes serious security concerns .
The existing location tracking solutions have their own limitations in tracing wireless intruders .
Major threat and challenges of wireless communications are still not considered in IEEE 802.11i revised specification 
Ad-hoc Wireless Sensor Networks deployment for monitoring physical environments are still in vulnerable zone .
Current Security Models and Prevailing Security Threats:
Different performance issues of wireless networks operation, administration and management are encountered due to improper security model. Many security schemes don't guard against some prevailing threats. Some of these issues are as follows:
Present security schemes for Wireless Personal Area Networks (WPANs) are immature 
There is no Visualization technique present for wireless communication 
Security model for wired network not necessarily effective for wireless networks .
Compressed Real time Transport Protocol (CRTP) is not suitable for wireless links, which have a very high and variable bit error rate (BER)
Un-satisfactory Security integration scheme for heterogeneous networks .
The current wireless smart home system has range limitation issue .
Major Attacks on Wireless Sensor Net works (WSN):
Wireless Sensor Network (WSN) is an up-and-coming technology that shows great promise for diverse ultramodern applications both for mass public and intelligence. Security in wireless sensor networks is still in its childhood, as little attention has been rendered to this area by the research community, due to this ignorance, WSN still facing numerous issues and challenges. Some of these issues are as follows:
There is no common model to ensure security for each layer in a Wireless Sensor Networks (WSNs) .
Current security Solutions for wireless Sensor networks are not feasible against all Prevailing security threats 
Security issues in wireless sensor network 14].
The current protocols for data link layer and network layers are not adequate for handling various security threats in WSN .
The existing security measures for wireless sensor networks (WSN) are insufficient .
Security Attacks on Ad-hoc Wireless
Truly speaking the most demanding area of wireless networking is ad-hoc wireless networks, but unfortunately it is the most vulnerable. Any intruder can easily get the access of the network and then disrupts the communication. Some of these issues are as follows:
Intrusion Detection Systems (IDS) are ineffective for ad-hoc wireless environment .
Attacks on wireless ad hoc network routing protocols upsets network performance and reliability 
No response method and limitations to handle wormhole attacks in Wireless Ad-hoc Networks (WANs). .
Many existing ad-hoc routing protocols concern only the length of the routers .
Approaches and Methodologies
With the inception of wireless communications, the traditional concept of network elements has shifted from messy physical infrastructure to wireless. LAN, WAN, MAN technologies have evolved as WLAN, Satellite, wireless local loop (WLL) etc. The operations of these networks have been ensured through dedicated control, management and data plane. The main issues for deployment of wireless networks are security attacks, vulnerabilities, battery power and improper security models. The research on security issues and challenges in wireless communication comprises performance implications due to different factors. The effects of these factors or problem areas have been addressed by using different tools, algorithms, models, simulations and design modifications. These sub domains and the approaches or methodologies are discussed in subsequent paragraphs:
Vulnerabilities in Wireless Communications
The Wireless Sensor Network gateway forms a single point of failure for the end-to-end sensor network infrastructure. How strong security model is used, but whenever intruder attacks on WSN-gateway, the whole network operations hampered. The fault endurance of WSN-gateway should be increased to avoid single point failure. A commercial grade WSN is considered and threw a ping-based DDos attack on the WSN-gateway, various computers send ping attack traffic simultaneously to the WSN-gateway.for the entire experiment of 4 hours of DDoS attack, processor exhaustion and sensed data were collected. It is clearly observed that the exhaustion of the computing resources of WSN gateway processor under ping-based DDoS attack traffic. At a load greater than 20% of the ping attack traffic, the WSN gateway processor became 100% busy, which caused the WSN-gateway to stop collection, recording, and reporting of the log sensor data. Result of this experiment stress to increase the fault endurance to avoid single point failure. .
Due to the wireless and unattended deployment nature of sensor networks, there is a risk of unique threats, along with the aim to ensure the confidentiality, integrity and reliability of communication over these networks. A model based on Communal Reputation and Individual Trust (CRIT) in sensor nodes overcomes this problem. In this model, a node judge the reliability or acceptability of a neighbor node through a set of values related to the neighbor node's reliability and reputation. A node monitors their neighboring nodes and ranks the neighbors in terms of a trust vote. Trust table maintained by neighboring nodes determine the communal and individual trustworthiness of nodes. In order to do that a node maintains two tables, a trust table and a reputation table. In trust table a node maintains the trust and un-trust values for all neighbor nodes. Similarly in reputation table the node maintains reputation values for all neighbor nodes. If a node sends a unique message, which is not confirmed by other nodes, then the trustworthiness of the node is under question, and the un-trust value for the node is increased. This message comes to cluster leader, by all other nodes about a specific node, it broadcast the information so that all other nodes ignore the untrustworthy node and it is discarded from the network.  The current location tracking solutions have their own limitations in tracing wireless intruders. The modification for Triangulation method seeks to overcome this limitation. The technique based on two separate databases, the values of these databases consider by two locations, one for inside and other for outside the building. For location tracking using databases, perform the Triangulation method followed by comparing the value with contents of data bases. It will be at a glance that intruder is inside the building or outside, and then looking for nearest values in databases, the corresponding value in column without using Triangulation would be the much precise location of device .
Fig.2: Attackers can access to wireless network 
The IEEE 802.11i modification has concluded to deal with security issues in wireless local area networks but major threats like DoS attacks, insider attacks, and offline guessing attacks are still needed consideration. An improved authentication mechanism can overcome this ignorance. This mechanism adopts an asymmetric cryptography approach to accomplish effective protection in six categories that are discovery phase, authentication and association phase, RADIUS authentication, 4-way handshake, group key handshake, and secure data communication. This included protection for management frames, null data frames and EAPOL frames as well as protection from some basic DoS attacks, Offline guessing attacks and insider attacks. This authentication mechanism is also able to allow stations to configure themselves automatically . Ad-hoc wireless sensor networks deployment for monitoring physical environments, where targets have unforeseen motions, are still in vulnerable zone. For such type of Self Organizing Wireless Sensor Networks (SOWSNs), stat-mesh architecture and mobility scheme provide an efficient monitoring system. The architecture has a base station (BS) nodes and sensor nodes (SN), which combines a mesh of routers to enlarge radio coverage with star endpoints. The Star-Mesh architecture uses multi-hopping to provide multipath routing, using an ad-hoc network based approach. The S.N collects environmental in formations and re-arranging events generated by B.S. The B.S performs actions on receiving events also managing the routes. When a node receives a message for the first time it stores the flooded one and forward the message to all its neighbors. Another received message having the same flood id is erased. Each SN has an initial amount of power so that it can send signals to all nodes within its transmission range. Each SN connected at least one B.S. the BS, with great amount of energy, works as gateway that connects SN to the analysis center. The BSs are fixed and each BS knows exactly its position information. The BSs are assumed to be arbitrarily located. 
Fig.2: The Ad-hoc SOWSN architecture 
Current Security Threats and Range Limitation Issues
Protection mechanism for Bluetooth is that PIN should not entered into the Bluetooth devices for paring in public and only known devices should pair. Encryption technology is essential for unique session key. For ZigBee the source to determine whether a protected or non-protected acknowledge frame is needed, also performing authentication of the source address. Use symmetric key key exchange (SKKE) handshake of a link key between the trust centre and connected devices. For NFC a standard key agreement protocol such as diffie-Hellman based on RSA or Elliptic curve cryptography could be used to establish a shared secret between two devices. This secures the channel . Wireless network visualization technique in presentation mode Visualize the information of access points, mobile devices and linkage using the symbol, colored line, icons etc. in this way more information about network status and performance can be achieve, and will help the network performance and security attacks . A novel method using MAC Spoofing used to avoid any intruder into the wireless communications.MAC address of authenticated user can be used to save any unauthorized access a data base of all authorized client MAC address maintain by organization. If the intrusion detection system finds more than one request of MAC address in the same network, it can be sure that the MAC address has been fraud and can block access to that MAC address temporarily . A modified Enhance Compressed Real-time Transport Protocol (ECRTP) is suitable for wireless point-to-point links, which have a very high and variable Bit Error Rate (BER). In modified ECRTP, the size of header is reduced. In compressed RTP packets occupy only 2 bytes. These bytes may be the UDP check-sum or the compressor inserted header checksum. By sending these checksums only in some packets, the average header size can be reduced . Heterogeneous Network Integration Model yields a security scheme for wireless mesh networks. Each of the heterogeneous wireless networks has formed connection with mesh backbone through the mesh gateway interface. When these networks communicate with the mesh cloud, they pass through the gateway routers of the mesh backbone. The security issues of the boundary between the heterogeneous wireless network and the mesh infrastructure should be dealt intensively. In line with this, when passing through the mesh cloud, each of these heterogeneous networks need the mesh infrastructure to fulfill their own individual security requirements 
Fig.3: Heterogeneous Network Integration Model 
The current wireless home system has range limitation issue. By using IEEE 802.15.4 standard a system is proposed for smart home environment. Main controller is the most important part of the system which will provide interfacing between users and the system. PICI8f452 microcontroller is used as a brain of the main controller for low power consumption CMOS technology's ICs are used. A GSM modem is attached with the controller for SMS. Approximately nine phone numbers can be stored and only these numbers can communicate with main controller for sending and receiving SMS and system resource controller. User can enter into system by entering password. The software consists of programming PICI6LF452 microcontroller using Mikroc compiler from Mikroelectronika. All of this programming is done using C-Language .
Wireless Sensor Networks Issues and Challenges
Many of today's proposed security schemes are based on specific network models. As there is no common model to ensure security for each layer in a network. The proposed holistic approach with respect to security for wireless sensor networks mitigates this issue. The holistic approach has some basic principles like in a given network; security is to be ensured for all the layers of the protocol stack, the cost for ensuring security should not surpass the assessed security risk at a specific time, if there is no physical security ensured for the sensors. The security measure must be able to exhibit a graceful degradation if some of the sensors in the network are compromised. Out of order or captured by the enemy and the security measured should be developed to work in a decentralized fashion. If security is not considered for all of the security layers. There are some efficient security mechanisms working in other layers. By building security layers as in the holistic approach. Protection could be established for the overall network .
Fig.4: Holistic View of Security in wireless sensor networks
Classification and association of security VTA (Vulnerabilities Threats and Attack), is proposed to remodel application-specific WSNs.The proposed scheme has redefined the concepts of vulnerability, threats and attacks with respect to wireless sensor network. On the basis of this differentiation we can check the list of security VTAs. Which can reduce the ambiguity of security information on VTAs.Then; by examining each of VTAs we associate it with a security assessment framework for analysis. 
Table 1. Classification and Association of Security VTAs with
Discrete Security Assessment Framework 
Multitier security architecture is required where each mechanism has different resource requirements Identification of the data type present in sensor network Identify the security threats to the communication for each data type every employing multitier security scheme is tailored to make the most out of the available resources. Use localized algorithm in which only one node heap all others sensors data and then sends this mix data to a sensor node which can communicate network and users . The current protocols for data link layer and network layers are not adequate for handling various security threats in WSN. The proposed scheme is turn to account in the form of two layers, link and network layers. This strategy seep through the jumble attacks layer by layer to diminish the energy cost for processing. The type of attacks the proposed intrusion detection scheme in link layer can easily notice integrity, collision and exhausting attacks, and in network layer it behold selective forwarding, sewage pool, worm, witch and HELLO flood news attacks. This security architecture needed no extra component or hardware . The AWISSENET (Ad-hoc personal area network and Wireless Sensor Secure NETwork) distributed detection system (DIDS) proposed for secure WSNs. The model has plug-in based architecture in order to enable an easy and flexible management of the algorithms running on each node. The local IDS agent is composed of four components. The plug-in manager, data manager, decision model and communication model. Only a subset of the AWISSENET nodes runs intrusion algorithms. AWISSENET cluster can be multi-hop are almost of the same size and the size of a cluster is same magnitude as the number of clusters in network. The AWISSENET DIDS uses timestamps and digests, secrete keys are shared inside each cluster and between the cluster heads. Timestamps are used to determine the freshness of the messages and prevent replay attacks .
Fig. 5: AWISSENET DIDS hybrid architecture: gray nodes are running the IDS; circled ones are cluster heads 
Weaknesses in Wireless Sensor Networks
Through the Intrusion Detection Systems (IDS) mechanism dedicated to the OLSR routing protocol, effectively handle intruder for ad-hoc wireless environment. The proposed system acts on control messages by checking the truthfulness of their content. Nodes operating the OLSR protocol maintain neighborhood information's. All nodes of the network participate in IDS. This solution represent the first line of defense for the OLSR protocol since it mitigates threats exploiting flaws in the OLSR specifications to divert the normal routing operation. A Grouped Black Hole Attack Security Model (GBHASM) mitigates the grouped malicious nodes to broadcast the shortest path through them to source and destination. The model is consisting of two modules; first module has the description about new node connectivity and communications. Server receives request packet from new node. It replies with membership acknowledgement to node and wait for the acceptance. If node doesn't replay within a time limit then the server discards the joining request. Otherwise it sends its information. Received information from new-joining node is added in the database and assigned Node Code (NC) Pkk1 and Pk k2. The second module handles the communication activity within network. After becoming a member of the network, the node send request for shortest path with pkk2 packet. Every node will match pkk1 with pkk2, if its key matches in a given time limit, information will be disclosed; otherwise the time of the packets to live that force it to become meaningless . An effective wormhole attack defense method is proposed to limits the wormhole attacks on wireless ad-hoc network. In this method each new node of ad-hoc network collects information about one hope and two-hope neighbors, in this way the nodes build a neighbor list and share a session key with each neighbor. The identity and MAC address is also present with a packet comes from every node. The next node then checks whether the forwarder is a neighbor. The technique drops the replayed packet, and it broadcasts the exit of the wormhole . A secure routing mechanism called security-aware ad-hoc routing (SAR) not only concern the length of the routers. The security metric is integral part of routing request or RREQ packet and change the forwarding behavior of the nodes receive on RREQ packet with a particular security matrix or trust level. SAR ensures that this node can only forward it if the node itself can provide the required security, otherwise the RREQ is dropped. If an end-to-end path with the required security attributes can be found, a suitable modified RREQ is sent from an intermediate node or the eventual destination. SAR can be implemented based on any on-demand ad-hoc routing protocol with suitable modification .
Future Work and Open Issues
Future research in the area of vulnerabilities in wireless networks would concentrate on communication transmission range and power issues. Wireless sensor network for monitoring physical environment is still in vulnerable zone, due to some issues but transmission range and power are two main open issues.
Future research in the area of security threats and range limitation issues would concentrate on Ultra Wide Band (UWB) frequency range for WPANs which is in still in experimental zone, also when increase the range of wireless devices it consume more energy and create high electromagnetic radiation, which is still an open issue.
As wireless sensors network prone to a limited computational power, short memory, low power supply issues so future research in the area of wireless sensor networks would concentrate on these open issues.
Future research in wireless sensor network would concentrate on the empowerment of sender and more emphasis on memory saving technique, the memory saving technique is still a great challenge and issue for wireless sensor researchers.
In this paper, we looked into the security issues and challenges in wireless communications. We have divided our studies into four sub-domain i.e. security attacks, vulnerability, security models and range limitation issues. Major issues addressed in this paper include the continuity of environment monitoring, limitation and vulnerabilities of sensors networks, the ad-hoc communication scheme, and the security scheme that protects against large number of attacks including DoS, Wormhole attacks, witch, HELLO flood news attacks etc. we also discussed some security model for protection against attacks, these mechanisms still have limitations, which are discussed in this paper. The contribution of this paper is to spell out the current security threats and other correlated issues in wireless communications. In this paper we also discussed the proposed solutions which may offer a new way of thinking towards the solution space.