Tuesday, July 13, 2010

Ns2 in MAC Protocol

Several researchers have proposed simple medications of IEEE 802.11 to incorporate power control. The main idea of these power control schemes is to use different power levels for RTS-CTS and DATA-ACK. Specially, maximum transmit power is used for RTS-CTS, and the minimum required transmit power is used for ATA-ACK transmissions in order to save energy. However, we show that these schemes can degrade network throughput and can result in higher energy consumption than when using IEEE 802.11 without power control. We propose a power control protocol which does not degrade throughput and yields energy saving.

A power control mechanism that can be incorporated into the IEEE 802.11 RTS-CTS handshake. The scheme in allows a node, A, to specify its current transmit power level in the transmitted RTS, and allows receiver node B to include a desired transmit power level in the CTS sent back to A. On receiving the CTS, node A then transmits DATA using the power level specified in the CTS. This scheme allows B to help A choose the appropriate power level, so as to maintain a desired signal-to-noise ratio. A similar protocol is utilized, wherein the RTS and CTS packets are sent at the highest power level, and the DATA and ACK may be sent at a lower power level. We refer to this scheme as the BASIC power control MAC protocol. We found that the BASIC scheme has a shortcoming that can degrade the throughput. Furthermore, the BASIC

Scheme may potentially increase the energy consumption, instead of decreasing it. We elaborate a power-aware routing optimization, determines routes which consume low energy. PARO chooses a cost function based on the transmit power level at each hop on a

Route, to determine a low energy-consuming route between a pair of nodes. PARO also uses a power control MAC protocol similar to BASIC. Several other routing metrics. A power control protocol presented is also similar to the BASIC scheme. It maintains a table for the minimum transmits power necessary to communicate with neighbor nodes. This scheme allows each node to increase or decrease its power level dynamically. However, different power levels among nodes result in asymmetric links, causing col-
lesions’ power control protocol proposed in uses one control channel and multiple data channels. A control channel is used to assign data channels to nodes. An RTS, CTS,

RES (a special packet), and broadcast packets are transmitted through the control channel using the highest transmit power. By an RTS-CTS handshake, source and destination
Nodes decide which channel and what power level to use for data transmissions. On the reception of CTS, the source sends an RES to the destination to reserve a data channel.

Then, DATA and ACK transmissions occur on the reserved data channel using the negotiated power level from the RTS-CTS handshake. Transmit power is controlled according to packet size. The proposed scheme is based on the observation
that reducing transmission power can result in energy savings, but can also result in more errors. A higher bit error rate can lead to increased retransmissions, consuming more

Energy. Thus, the protocol chooses an appropriate transmission power level based on the packet size. An adaptive scheme is also presented in to choose MAC frame
Size based on the channel conditions. IEEE 802.11 may result in unfairness (performance degradation) for nodes which use lower transmission power than their neighbor nodes. Poojary et al. propose a scheme to improve the fairness. COMPOW selects a common power level at all nodes in the network to ensure bi-directional links. Each node runs several routing daemons, each at a different power level. The power level is chosen to be the smallest power level which achieves the same level of network connectivity as the highest power level.