A mobile ad hoc network (MANET) has become very popular in a modern society due to its easy and quick deployment with low cost. We can easily access an Internet through mobile ad hoc network in almost everywhere. MANET technology uses IEEE 802.11 standard [1-3]. In a mobile ad hoc network, users exchange their information to their destinations without using an access point (AP) and the channel is shared by all nodes. Sometimes more than one node will try to transmit data concurrently, and collisions will occur. In IEEE 802.11 specification, a medium access control (MAC) protocol is defined to coordinate concurrent transmissions and avoid collisions.
In order to coordinate multiple transmissions at one channel, IEEE 802.11 MAC layer protocol utilizes a four-way handshake to resolve the problem. When a node wants to transmit data to another terminal, sender first transmits a request-to-send (RTS) packet to a receiver. When a receiver receives RTS, it replies back using a clear-to-send (CTS) packet. When a sender receives CTS, it can start to transmit data, and once complete, a receiver transmits back an acknowledgement (ACK) packet to a sender. RTS and CTS packets include duration of data transmission time. Other terminals overhearing RTS or CTS defer their transmissions until the ongoing transmissions finished. CTS is used to avoid collisions occurring at a receiver side, while RTS message is to prevent collisions at a sender side. Terminals transmit their control and data packets at a maximum power level. All current transmissions can avoid collisions through a four-way handshake method.
Although IEEE 802.11 MAC layer protocol defines a four-way handshake to
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coordinate current transmissions without collisions, the problem which we called expose terminal problem may still happen.
Nodes overhearing RTS and CTS defer their transmissions until ongoing transmissions are finished. However, some nodes can transmit data without distrupt ongoing pairs. They just need to select an approximate power to transmit data. For example, nodes A and B are exchanging data as shown in Fig.1-1. . It is obvious nodes D and E cannot transmit data simultaneously to avoid interfering nodes A and B. Since nodes C and E are in RTS and CTS range, they set a network allocation vector (NAV) to wait a channel clear. The dashed circles indicate the maximum transmission ranges, while the dotted ones indicated the ranges of the minimum transmission ranges. Nodes D and E can’t transmit any data to nodes C and F even they won’t distribute the ongoing transmission between nodes A and B. The problems are called expose terminal problems. It is easy to show that the three transmissions can transmit data concurrently when they select their transmission power appropriately. IEEE 802.11 MAC layer protocol can’t dynamically adjust nodes transmission power, even if a position of a sender and a receiver is very close. Expose terminal problems will waste bandwidths and reduce throughputs in a network. In the next section, we introduce a transmission power control method in IEEE 802.11 MAC layer protocol.
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Figure 1 - 1 The expose terminal problem
1.1 Transmission power control in IEEE 802.11 protocol
IEEE 802.11 standard specifies a MAC layer protocol including distributed coordination function (DCF) and point coordination function (PCF). In IEEE 802.11 MAC layer protocol, wireless nodes have two power modes: ongoing transmission and power saving (PS). The power management scheme divides time into beacon intervals. At a beginning of each beacon interval, power saving nodes wake up for a short time period, called announcement traffic indication message (ATIM) window.
In the ATIM window, nodes exchange control frames, called ATIM frames, to inform their power saving counterparts to remain awake until the end of the beacon interval to receive data frames. After ATIM window, all nodes follow a DCF protocol using maximum power to transmit their data frames.
1.2 Power control issues
One of main targets in designing mobile ad hoc networks (MANET) is how to enhance overall networks throughputs while maintaining low energy consumption for packet processing and communications. In power control schemes, there are two
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major issues we often discussed. One is using power control scheme to reduce an energy consumption, and the other one is to reduce interferences to other nodes to increase network throughputs.
First we introduce reducing energy issues. Since IEEE 802.11 MAC layer protocol always uses maximum power to transmit packets, it wastes a lot of energy, and it is based on the carrier sense multiple access with collision avoidance (CSMA/CA) medium access procedure to transmit/receive both control and data frames. We know that CSMA/CA wastes the scarce energy and bandwidth due to frame collisions and lengthens the transmission delay due to waiting backoff time, especially in heavy traffic load. In addition, the IEEE 802.11 power management scheme does not specify how to determine the ATIM window size in a beacon interval. The fixed ATIM window size cannot always accommodate the dynamically changing traffic conditions.
Using power control to reduce the energy consumption has become a popular issue.
Second we introduce increasing network throughputs issues. According to IEEE 802.11 MAC layer protocol, one node overhearing RTS or CTS needs to set NAV and keeps silent to avoid collisions occur. IEEE 802.11 MAC layer protocol uses a maximum transmission power for all nodes to transmit control and data packets, no matter how close a transmitter to its intended receiver. In IEEE 802.11 MAC layer protocol, since nodes always use maximum power to transmit packets, energy is used inefficiently and spatial reuses and throughputs in networks are low. Transmission power control is a technique for increasing the efficiency of space-time utilization in wireless networks. Generally speaking, reducing transmission power results less interferences to nearby transceivers and receivers. Therefore, more transmissions can be activated simultaneously, improving the overall throughputs of the network.
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In this paper, we focus on a reduce interferences to nearby transceivers and receivers to increase an overall network throughputs.
Some literature increases network throughputs by using multiple channels and multiple transceivers. [4-5] use a control channel to communicate channel quality information, and based on channel quality information a terminal can select an appropriate power to transmit data. However, using control channel may add more hardware cost and more difficult to implement. Therefore, recent researches [6-10], use a single channel and a single transceiver instead of multiple channels and multiple transceivers. We will discuss them in the chapter 2. Since using a single channel and a single transceiver can reduce a hardware cost than using multiple channels and multiple transceivers, we propose a power management protocol in a single channel and a single transceiver environment. We modify IEEE 802.11 MAC layer protocol.
Besides, we also consider hidden terminal problems and mobility issues in the chapter 3.
1.3 Objectives
In IEEE 802.11 medium access control layer, nodes can transmit data only in sensing channel idle. As we described before, since nodes in IEEE 802.11 MAC layer protocol always use a maximum power to transmit data, they cannot dynamically adjust their transmission power. One node overhearing RTS and CTS defers its data transmission and avoids collisions. That will cause the expose terminal problem. In IEEE 802.11 MAC layer protocol, it can’t allow concurrent transmissions, and one node wants to transmit data needs to wait a channel clear. Spatial reuses and throughputs are very low. In IEEE 802.11 physical layer (PHY), nodes can correct
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decode packets when signal to noise ratio (SNR) reaches a threshold. We combine the packets decode threshold in PHY and sensing channel clear data transmission in MAC layer. We modify IEEE 802.11 MAC protocol based on SNR threshold in PHY layer.
In order to increase throughputs and spatial reuses, we propose a novel MAC layer protocol based on SNR threshold in PHY layer to increase channels utilization.
We propose a new adaptive transmission power control protocol which can improve the network throughputs significantly using a single channel and a single transceiver.
Specifically, by controlling the transmission power, our protocol can enable several concurrent transmissions without interfering with each other. Since adjust minimum power may cause hidden terminal problems, and most power control protocols doesn’t consider them. In our protocol, we relay CTS to avoid hidden terminal happened.
According to our protocol, nodes can dynamically adjust their transmission power without distributing ongoing pairs. Dissimilar to IEEE 802.11 MAC layer protocol, we allow multiple pairs currently transmit data, thus nodes don’t need to set NAV when sensing channels being busy, and we also don’t need to use extra messages and synchronize data transmission.
1.4 Organization
The organization of this thesis is as follow: In chapter 2, we review related work of transmission power control. Then our protocol will be described in chapter 3. The performance evaluation will be shown and discuss in chapter 4. In the end, chapter 5 will be the conclusion.
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