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3.1 SIMPLE ENERGY AWARE MAC (SEAMAC) PROTOCOL FOR WIRELESS SENSOR NETWORKS:
A new MAC protocol, SEAMAC has been proposed which works with completely periodic traffic to achieve low energy utilization.
The two main functions of SEAMAC are:
- Achieve low duty cycle for periodic traffic.
- To achieve simple and unique synchronization in the whole network.
For network monitoring applications, it is necessary to sample physical variables periodically. This method gives an advance estimate of how many samples must be taken in a time interval and the specific instants when they are taken. In this way, a lot of energy is conserved by avoiding idle listening between two consecutive samples. Another improvement is that no mechanism is used to wake up the nodes.
Simple and robust synchronization is deployed in SEAMAC. The frequency of synchronization loses is minimizes in the network. Only the base station starts and maintains synchronization making it unique in SEAMAC for the whole network. The operation of other nodes is to broadcast synchronization packets previously generated.
- TIME SCHEDULING IN SEAMAC:
SEAMAC has a lot of advantages including periodic traffic of the entire network. Therefore, a schedule is created in which nodes only wake up when a sample from the environment has to be taken. Periodic listen/sleep schedule that is proposed in protocols like SMAC, TMAC and DMAC is not necessary in SEAMAC.
Another feature that is not essential for SEAMAC as compared to SMAC is that, it does not need a short wake up tone for the transmitting nodes. Since nodes know in advance when to turn on their radios to sample environmental variables. It is also important for packets from nodes to reach their destination which is the base station, in sequence and with low delay. Besides low energy consumption, listening periods in SEAMAC are long enough for all the packets to reach the base station before the lively interval finishes. The base station originates the synchronization packets which would have information about the time left for next listen period (tNLP) and the length of the sleeping period (tSP). The listening period should be noticeably large.
3.1.2 SYNCHRONIZATION IN SEAMAC:
Nodes of SEAMAC turn on their radios for listening to synchronization (sync) packets from the base station; when they are turned on. Since base station is the only node that starts and maintain synchronization and the other nodes only disseminate (spread) synchronization in a multi hop environment; energy is conserved and every node follows only one time schedule.
Sync packet Tx
Sync packet Rx
The time schedule is followed by the base station as soon as it is turned on. The nodes have not yet received any sync packet from the BS, so their radios are kept on until a sync packet is received. In this way, the time schedule is being followed by the nodes. The node1 synchronizes node2 by re-transmitting a sync packet; since node2 cannot directly hear the base station.
3.1.3 ENERGY ANALYSIS:
For energy analysis, a network of (n+1) nodes is considered so that the nodes can hear each other directly. In this circumstance, n nodes are transmitting to the base station, and in response to which received packets are conveyed to storage and analysis proposal.
“The expected energy spent in each node is the sum of the energy consumed in transmitting, receiving, sleeping and sensing states.”
Each term can be expressed as the average power in that state multiplied by the time the node is on that state . Data packets are transmitted in every Tdata time interval. To enable no energy consumption due to idle listening tLP is kept large enough so that all packets generated by the nodes are received by the base station. All packets are considered broadcast packets.
Since the BS does not transmit data packets so every node will receive exactly (n-1) data packets. Only one synchronization packet is received by nodes that are originated in the base station in an interval Tdata, which is the only source of synchronization in SEA-MAC. The energy consumed by a node running SEAMAC is:
- COMPARISON OF SEA-MAC WITH OTHER PROTOCOLS:
By evaluating the energy consumption relation of SEAMAC with the other protocols it has been observed that:
- SEAMAC saves energy by not polling the medium for possible transmissions.
- SEAMAC saves energy by avoiding transmission of tone packets to wake-up nodes.
- Less energy is consumed in synchronization because harmonization is initiated by the base station and one packet is enough to synchronize the entire network.
- SEAMAC spends more time sleeping but since the energy consumed in sleeping time is fewer than that in any other state, it can be measured an unimportant feature.
Due to these reasons it can be said that SEAMAC outperforms the other protocols in the application of monitoring environmental variables. Some protocols attempt to combine the advantages of TDMA scheduling for power saving by periodically wanting sensing nodes become inactive. Furthermore, they are having a fixed duty cycle which is not preferable for an optimal performance for example SMAC.
Further more these protocols follow flat topologies in which inter cluster problems are absent. This topology cannot be implemented in applications where real clusters are to be formed. A significant portion of the nodes will belong to two or more virtual clusters under these protocols so energy consumption would be higher. What happens if the coordinated sleep schedules of two neighboring clusters are opposite?In such cases it is not guaranteed whether the nodes will adopt the different schedules or not. So to solve all the ambiguities it was necessary to study the performance of this scheme with different routing protocols in order to assess its performance of inter cluster communication especially for multi hop messages.
The intention of the project is to reduce latency and to accomplish this; restrictions were made on synchronization of the nodes. However, a significant waste of resources is a trade-off to low latency because the time slots are worn out when data is not sent by any node. So the frames of a large number of nodes are vacant if the nodes are sparsely distributed over a certain area. To overcome the problem of time slot wastage, WPAN format has been realized.
Another major point of consideration is the energy conservation during the switching between a sleep and active states. Because of assigning time slots without any synchronization the nodes have to switch between the active/sleep states many times and during this process, energy sources of nodes are drained off.
Better data delivery is always required during transmission avoiding collision. But this can involve high queuing delays due to the overhead involves in scheduling. This decreases the effective channel access probability for the data transmission and is not preferable because in WSNs the traffic is homogenous across the network.