A wireless sensor network is a communication system which senses and gathers information from a certain area and sends it to where it is required. In such networks the communication system requires networking protocols which are efficient, reliable, scalable and secure.

Fig. 2: Multicast routing reduces message path length and improves efficiency
Fig. 2: Multicast routing reduces message path length and improves efficiency

Fundamental differences in sensor networks which affect protocols are:

Data sink(s). That is, the nature of data sink(s). For example, whether the end user is embedded in sensor network or access points

Sensor mobility. This aspect may influence protocols at the networking level as well as at localisation service

Sensor resources. Such as computing ability

Traffic pattern. For example, whether data is generated continuously during environmental monitoring

The factors to be considered while designing WSN protocols are:
1. Routing decisions to be undertaken considering the importance of energy resources in the network.
2. Communication channels often exist between events and sinks. Since sink nodes are typically used for overall description of the environment rather than explicit readings from the individual sensor devices, the communication in sensor networks is normally referred to as data-centric rather than address-centric, and the data may be aggregated locally rather than collecting all the raw data sent to the sink(s).
3. Sensors have the knowledge of their own location to usefully assess their data. The location information can be utilised for routing purposes.
4. If a sensor network is well connected, the topology control service should be used in conjunction with the normal routing protocols.

Multiple access protocols
When multiple nodes desire to transmit, protocols are needed to avoid collisions and loss of data. In frequency division multiple access (FDMA), different nodes have different carrier frequencies. FDMA also requires additional hardware and intelligence at each node. In code division multiple access (CDMA), a unique code is used by each node to encode its messages. However, this increases complexity of the transmitter and receiver.

In time division multiple access (TDMA), the RF link is divided on time axis with each node given a predetermined time slot it can use for communication. This decreases the sweep rate, but a major advantage is that it can be implemented in software. All nodes require accurate, synchronised clocks for TDMA.

Medium access control protocols
Medium access control (MAC) protocols focus on reducing the idle power consumption by setting the sensing transmitters to sleeping mode as often as possible. MAC protocols have been designed for ad-hoc networks which primarily focus on optimising fairness and throughput efficiency with less emphasis on energy conservation. Some protocols like IEEE 802.11 eliminate the waste caused by colliding packets in WSNs. Some others avoid unnecessary reception of packets by nodes that are unintended destinations. It has been assessed that idle power consumption can be of the same order as that consumed by the transmitter and receiver. Some of the MAC protocols are described below.

S-MAC. This protocol creates a sleep schedule that determines when to activate the receivers and when to put them in sleep mode.

Timeout-MAC (T-MAC). It eliminates idle energy further; instead of allowing the messages to be sent continuously, messages are transmitted in bursts in the beginning of the frame.

DMAC. Since many WSNs have data-gathering trees routed to a single data sink in the direction of packets arriving at a node, DMAC takes advantage of this by staggering the wakeup times for nodes based on their distance from the data sink.

Traffic-adaptive medium access (TRAMA). Aforementioned protocols help in minimising the power consumption by reducing the time that the transmitters remain in idle state, TRAMA attempts to reduce wasted energy consumption caused by packet collisions. Nodes determine their transmitting state using adaptive election algorithm (AEA). In AEA, each node calculates a priority for itself and all two-hop neighbours for the current slot. If a node has the highest priority for that slot and has data to send, it wins that slot and sends the data. If one of its neighbours has the highest priority, it sets itself to the receiving mode. In short, AEP assigns priorities for the unused slots to the nodes needing extra slots.

Sparse topology and energy management (STEM). When data packets are generated, the sensor generating the traffic uses a paging channel (separate from the data channel) to awaken the downstream neighbours.

Storage technology for portable devices
Flash memory has been the most reliable storage technology. However, visible limitations for future Flash cell scaling include power consumption, charge storage requirements of the dielectrics, reliability issues and capacitive coupling between adjacent cells. Efforts are being made to reduce these limitations through system management techniques and fabrication technology (such as high k-dielectrics, nano crystal storage media and Fin-FET). However, there is no doubt that Flash will remain the dominant non-volatile memory technology at least down to 45nm node. Nevertheless, there are new memory technologies available, such as ferro-electric RAM (FeRAM), magnetic RAM (MRAM), ferro-electric polymer RAM (FePRAM), phase change memory (PCM), resistive RAM (RRAM), probe storage, carbon nano-tube memory (CNT) and molecular memory. The most promising are probe storage memories and PCM.



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