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Inter-Networking Research Group (i-NRG) |
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Inter-Networking Research Group (i-NRG) |
Sensor networks typically refer to large ensembles of interconnected nodes that have processing and communication capabilities, and one or more sensing devices (e.g., thermistors, magnetometers, light detectors). The deployment of such networks is usually done in an ad-hoc manner (e.g., dropping sensors from an aircraft on the field) which implies that sensor-network nodes need to self-organize into a multi-hop wireless network. Many of the large-scale sensor networks of the future will consist of battery-powered sensor nodes whose battery may be difficult to recharge, or that the nodes themselves may be so cheap that recharging them may not be cost effective. A major challenge facing the development and eventual deployment of large-scale sensor networks is the scheduling of transmissions among nodes in a way that (a) is self adaptive to changes in traffic, node state, or connectivity; and (b) prolongs the battery life of each node.
The traffic-adaptive medium access protocol (TRAMA) is introduced for energy-efficient collision-free channel access in wireless sensor networks. TRAMA reduces energy consumption by ensuring that unicast, multicast, and broadcast transmissions have no collisions, and by allowing nodes to switch to a low-power, idle state whenever they are not transmitting or receiving. TRAMA assumes that time is slotted and uses a distributed election scheme based on information about the traffic at each node to determine which node can transmit at a particular time slot. TRAMA avoids the assignment of time slots to nodes with no traffic to send, and also allows nodes to determine when they can become idle and not listen to the channel using traffic information.
The performance of TRAMA is evaluated through extensive simulations using both synthetic- as well as sensor-network scenarios. The results indicate that TRAMA outperforms contention-based protocols (e.g., CSMA, 802.11 and S-MAC) as well as scheduling-based protocols (e.g., NAMA) with significant energy savings. We are currently working on implementing a simplified version of TRAMA on TinyOS platform for Mica2 generation motes.
IEEE 802.11 DCF (CSMA/CA) has become a default standard for media-access in wireless ad hoc networks. For broadcast or multicast transmissions the standard does not provide any reliability and is done by plain carrier-sensing. The performance is severely degraded due to hidden terminal collisions. In this work we propose a scheme that supports reliable multicast / broadcast at the MAC layer. The main challenge is that the probability of collisions of data or control packets in any contention-based scheme increases with the offered load, which degrades channel utilization. Hence, nodes should co-operate within the neighborhood to successfully deliver multicast or broadcast flows, without increasing the channel contention. The multicast group information is passed to the MAC layer by higher layer routing protocol. Each node also maintains a two-hop neighbor information and computes a cover set for the multicast group based on the neighborhood multicast flows to reduce channel contention. Reliability is ensured by doing RTS/CTS/DATA/ACK handshakes between the nodes in the cover set. We are studying the performance of GRAMA by simulations using Qualnet.