Abstract – Heterogeneous Wireless Networks is an emerging area. Hidden nodes are a fundamental problem that can potentially affect any heterogeneous wireless network due to the heterogeneous characteristics. The performance may degrade significantly, when use existing methods such as RTS/CTS in the heterogeneous network. In order to overcome the hidden terminal problem in heterogeneous wireless network, the CTS frame can be modified to incorporate the details of the other nodes within the range of the access point. This will give prior information to every transmitting node about the existence of other nodes within the range of the access point. This can also be solved by an agreement between two transmitting nodes by sending packets through an access point. The above idea can be simulated by using NS2. The performance analysis can be done on the basis of overall throughput of the proposed system in various scenarios.
Keywords – heterogeneous wireless networks; hidden terminal problem
Wireless networking is the technology which uses standard network protocols instead of network cabling for communicating with computers. The 802.11 is the widely used wireless standard, which is defined by the Institute of Electrical and Electronic Engineers (IEEE).Wireless network uses an access point or base station for providing connectivity with wireless computers. Each access point or base station has a defined range for maintaining wireless communication between client computer and access point. The actual transmission range varies with the environment.
Wireless networking can be classified into two broad types; homogeneous and heterogeneous networks. A homogeneous sensor network consists of identical nodes in terms of energy, power, capacity, hardware complexity etc, while a heterogeneous network  consists of two or more types of nodes which are heterogeneous in terms of energy, power, capacity, access mechanism etc.
The heterogeneous wireless networks have the following advantages as compared to homogeneous wireless networks. The heterogeneous network increases the reliability, spectrum efficiency, coverage. Reliability can be improved when one particular radio access technology within the heterogeneous network fails to function it may still be possible to maintain a connection by falling back to another radio access technology. Spectrum efficiency is improved by making use of radio access technology which may have few users through the use of??load balancing??across radio access technologies and coverage may be improved because different radio access technologies may fill holes in coverage that any one of the single networks alone would not be able to fill. The main issues that lead to negative effect on heterogeneous network are due to their heterogeneous characteristics. Considering the case where the network may be heterogeneous in terms power levels and thereby causing communication links of varying range. Also due to the interference caused by the high power nodes the throughput of the network may be affected.
The rest portions of the paper divided as follows: section II present the motivation and overview of the proposal, section III reviews the body of related works, section IV briefly describes the proposed concept, section v describes the simulation of proposed concept and finally section V concludes this paper.
II. Motivantion and Overview
The main factors that cause negative effect on wireless networks are low packet delivery ratio and high end -to -end delays. The other factor that causes destructive effect on wireless network is the hidden terminal problem. The nodes can be categorized into two types based on the location and topology of nodes in the network, hidden nodes or visible nodes and non-hidden nodes or visible nodes. Hidden Terminal Problem occurs when more than one transmitter receiver pair shares a channel. In wireless network, nodes cannot hear each other due to their short transmission range. Hidden nodes cannot receive control packets, so packets are transmitted through the visible nodes regardless of any other nodes sending packets. This situation leads to packet loss and collision.
The hidden terminal problem can be illustrated by Figure 1.The given scenario consists of three nodes A, B and C. Node A can hear Node B but not node C, as node C is not in the transmission range of A. Similarly nodes C can hear node B but not node C, as node A is not in the transmission range of C. Nodes which are located in the transmission range of other nodes can easily receive the packets. Thus node B can receive packets from both the nodes A and C. However there will be a chance of collision at node B if both nodes A and C send their packets at the same time and due to this node B cannot receive any packet successfully. According to IEEE 802.11 MAC protocol, the hidden terminal problem can be prevented by exchanging RTS and CTS control packets.
Figure 1: Collision appearance due to hidden node
RTS/CTS??(Request to Send / Clear to Send) is the mechanism used by the??IEEE 802.11??wireless networking protocol to reduce the collisions introduced by the??hidden node problem. In this case, the sender transmits first a short RTS (request to send) control packet. It indicates the total time required to transmit the data and the acknowledgment packet. When the access point receives the RTS packet, it responds by sending a CTS (Clear to send) packet. It includes again the required time for the complete transmission.??The RTS/CTS mechanism informs all stations in the range of the sender and the access point (receiver) about the planned transmission and instructs them not to send for the reserved duration. The RTS and CTS packets are short. So the collision will only last for the duration of the short packet. The following data and ACK packets are transmitted without collision. All stations in the range of the receiver are informed about the transmission and wait until it is finished. Thus the hidden node problem can be avoided.
The RTS/CTS mechanism cannot be used in heterogeneous networks due to the heterogeneous characteristics of the wireless network. The existing mechanism cannot be adopted in the case of heterogeneous network. In order to avoid the hidden terminal problem in heterogeneous networks, the CTS frame will be modified by incorporating the details regarding the neighbor nodes. Normally CTS frame includes the duration, receiver address. Here modify the CTS frame by incorporating the information of neighboring nodes within the transmission range.
III. Related Work
In the wireless networks, the nodes are distributed randomly. Based on their topology and the node’s transmission range, wireless networks include hidden nodes. The hidden node has a negative effect on the throughput and performance of the wireless network. Therefore, researchers try to find mechanisms for preventing collision that occur due to hidden nodes. Some of the collision avoidance mechanisms  utilize control packets and monitor the status of the channel (busy or free).Some other mechanisms are based on hidden node detection and also include mechanisms which store information of the node via a coordinator or access point in order to avoid hidden terminal problem. The mechanisms for avoiding collisions can be categorized as handshake mechanism, busy tone mechanism, routing management mechanism.
Handshaking is the mechanism in wireless communication for avoiding collision in a channel. Wireless communication uses two types of channels such as data channel and control channel. The data channel is used to send and receive data and control channel is used by the control packet for managing connection. The collision of packets can be avoided by this channel division. Multiple Accesses with Collision Avoidance (MACA)  uses two control packets for avoiding collision occurrences. The complete exchange process or handshaking procedure involves four packets such as RTS, CTS, DATA, and ACK. RTS and CTS are exchanged over the control channel before data transmission. For reserving the medium, a sender transmits an RTS packet. After receiving the RTS packet, the destination node sends back a CTS packet. The CTS packet also contains a value related to time that alerts other nodes to hold off from accessing the medium while the station initiates the RTS to transmit its data. At the end of the transfer process, the receiver sends back an ACK packet, which is a confirmation that the data was received. Multiple Access with Collision Avoidance for Wireless (MACAW)  introduces a new control packet. This mechanism uses an RTS/CTS/Data Sending (DS)/DATA/ACK pattern for data transmission. When Node B wants to transmit data to Node A, Node C can hear the RTS. Node C hasn’t any more information about the connection between Nodes A and B because it cannot hear the CTS from Node A. Therefore, if Node C wants to send data to Node B, it must defer this process. In order to avoiding an unnecessary deferral situation, Node B generates a DS packet as an announcement for a successful connection.
Busy tone mechanisms use control packets which are known as Busy Tones (BTs). These types of control packets are sent by the sender or receiver to their neighbors to reserve the channel during the transmission. The protocol Busy Tone Multiple Access (BTMA) is based on the RTS/CTS mechanism  . In BTMA, a base station broadcasts a signal to the other stations to eliminate the effect of hidden nodes when one station uses a channel. In Enhanced Busy Tone Multiple Access (EBTMA) , it uses two channels. The first one is used for transmitting the control packets RTS/CTS/DATA/ACK, while the second one is used for transferring the busy tone signals.
A wireless network can handle the hidden node problem with the mechanisms that manages routing paths. The routing management with coordinator mechanism is proposed in . This mechanism is used for grouping nodes into four phases. The first phase is hidden node situation discovery. The next phase of the proposed mechanism is hidden relationship collection or polling. Third phase is based on the node grouping. And final phase is allocation of bandwidth for each group. Coordinator is responsible for initializing the network formation, and also monitors all channels to determine collisions based on their event’s time. The coordinator wants to collect the information of all nodes after detecting the collision. Based on this information, classify the nodes in different categories. The information gathering about the nodes is known as the polling phase. In this phase the coordinator starts transmission by an aware message. This message forces the nodes to enter the active mode so the coordinator sends the polling message for them in turn and waits for an ACK in response. This process is continued by each node. Thus all nodes have information about their hidden node. They send their information to the coordinator after receiving a report message from it. And the coordinator sends them to update the hidden node graph. Then the coordinator should start a grouping process based on the node’s reports. In order to ensure that no hidden node exists in each group, nodes that are connected in the hidden node graph cannot stand in the same category. After grouping, the coordinators allocate a time interval to each group. And in each time interval, data can be transmitted. It is possible to manage the routing process by allocating the time interval.A coordinator should assign specific bandwidths to each group based on the group size.
There is another mechanism for grouping nodes in an ad hoc network. It is called the Hidden Node Avoidance Mechanism (H-NAMe) .This mechanism does not require hidden node detection. The joining process of nodes is less complex .It also supports multiple clusters of network. It consists of following phases including join requests, neighbor notifications, and neighbor notification reports. In the first phase, Node A wants to avoid collisions. In order to avoid collisions, it sends a broadcast group join request (GJR) message to the head cluster. The head cluster generates the ACK. The ACK that is generated by the head cluster is received by all nodes in the cluster. This ACK message has the following two responsibilities. The first one responds to Node A and confirms the joining process. Also, the ACK helps the neighbors who receive this packet to add Node A to their table. In the second phase, all neighbors who are in Node A’s range should send the neighbor notification message (NNM) to Node A within the timer. It is set via Node A after receiving an ACK from a head cluster. At the end of this step, Node A has the information of bidirectional neighbors who have joined the group. In the third phase, Node A should send a report message about all neighbors to the head cluster. The head cluster acts as a central manager of the cluster and should have detailed knowledge about all groups in the cluster and their members. This knowledge is collected by all report messages from the nodes. And the head cluster should manage the number of groups and is responsible for load balancing and bandwidth allocation. Thus the nodes which are in the cluster choose a routing path that does not involve the hidden node.
IV. Proposed System
The performance of heterogeneous wireless network may degrade due to hidden nodes in the network. The existing handshaking mechanism RTS-CTS cannot be adopted in the case of heterogeneous networks. In order to overcome the hidden terminal problem, CTS frame will be modified. The RTS frame contains five fields, which are: Frame Control, Duration, Receiver Address (RA), Transmitter Address (TA), and Frame Check Sequence (FCS). The CTS frame contains four fields, which are: Frame Control, Duration, Receiver Address (RA), and Frame Check Sequence (FCS).
In order to overcome the hidden node problem the CTS frame can be modified to incorporate the details of the other nodes within the range of the access point. This will give prior information to every transmitting node about the existence of other nodes within the range of the access point. This can be further enhanced to have an agreement between two transmitting nodes by sending packets through an access point.
The Figure 2 illustrates the proposed modified CTS in heterogeneous network. The modified CTS include the details of neighboring nodes. This information can be used by the nodes for further transmission.
Figure 2: Proposed concept in heterogeneous network
The proposed method will overcome the hidden terminal problem in heterogeneous network. Using the above assumptions sample scenarios can be simulated by using NS2.In order to make the network heterogeneous, MAC layer will be modified in terms of their physical layer properties such as transmission range, power, capacity etc. Both the set of nodes will use the same MAC layer in the simulation. But some parameters will be different for them in order to make the MAC layer heterogeneous.
Assume that a three node scenario is required for implementing the mechanism to avoid the hidden terminal problem in heterogeneous network. The source code is modified to accommodate additional fields in the CTS format. The analysis can be on the basis of throughput. The proposed method will shows an improvement in throughput.
Hidden nodes are a fundamental problem that can potentially affect any wireless network. Hidden nodes cannot receive any control packets. The packets are sent to the visible node regardless of any other nodes sending packets, which leads to problems such as collisions and packet loss. A hidden node problem is prevented by exchanging small RTS and CTS control frames in the homogeneous network. But the performance may degrade significantly, while used in the heterogeneous network environment. In order to overcome this problem the CTS frame can be modified to incorporate the details of the other nodes within the range of the access point in heterogeneous networks. The simulation results will show the improvement in throughput and thus it can reduce number of packet loss.
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