XAS analysis

Topology control protocols are a slightly different approach to saving energy than standard routing protocols, as they do not directly operate data forwarding. These protocols run at a lower level of the

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network stack, i.e., just under the network layer. Their objective is to improve the energy efficiency of routing protocols for wireless networks by coordinating the sleep transitions of nodes. Several routing protocols in fact try to enhance network lifetime by reducing the number of data transmis-sions or balancing the transmission power, but neglect idle power consumption. However, several measurements, e.g., in [Ka] [Ste], show that idle power dissipation should not be ignored, as it could be comparable to the transmitting or receiving power. Therefore, in order to optimize energy consumption, nodes should turn off their radios. Topology control protocols exploit redundancy in dense networks in order to put nodes to sleep while maintaining network connectivity. They can be applied to standard routing protocols for ad-hoc networks or for WSNs that do not directly han-dle sleep schedules. Although some of them are designed for wireless ad-hoc networks rather than WSNs, the typically high redundancy of sensor nodes and the need for maximum energy saving make WSNs perhaps the most suitable type of networks for taking advantage of these protocols. In the following sections, a few examples of topology control protocols are given.

7.8.1 Geographic-Adaptive Fidelity (GAF) Protocol

The GAF [Xu] protocol, in order to put nodes into low-power sleep states without excessively increasing the packet loss rate, identifies groups of nodes that are “equivalent” in terms of routing cost and turn off unnecessary nodes. This is achieved by dividing the whole area into virtual grids, small enough that each node in a cell can hear each node from an adjacent cell. Nodes are location-aware, so each sensor obtains its coordinates on the virtual grid from location information. Nodes that belong to the same cell coordinate active and sleep periods, so that at least one node per cell is active and routing fidelity (which requires that in any cell at any one time there is at least one node able to perform routing [Xu]) is maintained.

According to the GAF protocol, nodes can be in three different states: discovery, sleeping, and active. Transitions from one state to another are depicted in Figure ..

In the discovery state nodes are active and exchange discovery messages in order to find nodes within the same cell. Then, after a discovery timeout Td, a node enters the active state. Each node only stays in the active state for a defined time Ta, then moves to the discovery state again. A node in the discovery or active state can go to the sleep state if it finds an equivalent node with higher rank that handles routing. Finally, the sleep period is also limited, so that after a time Tsthe node returns

Sleeping

After Td

Active

After Ta

After Ts

Discovery

Receive discovery msg from high rank

nodes

FIGURE . State transitions of the GAF protocol.

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Power-Efficient Routing in Wireless Sensor Networks 7-35

to the discovery state. In order to balance energy consumption and increase network lifetime, node ranking needs to be conveniently set. The value depends on the state and the residual energy of the nodes. Nodes in the active state always have a higher rank than those in the discovery state, while between nodes in the same state those with a longer expected lifetime have a higher rank.

GAF can run over any ad-hoc routing protocol, such as AODV [Perk], dynamic source routing (DSR) [Joh], TORA [Park], destination-sequenced distance-vector (DSDV) [Perk], and may be used for WSNs as well. Simulations show that with GAF there is no delay increase, while the consumed energy is highly reduced. The downside is that packet loss may slightly increase, as each time a previously active node goes to sleep there is a topology change the above routing protocol has to react to.

7.8.2 Span Protocol

In [Che], another distributed coordination protocol for wireless ad-hoc networks, called Span, is presented. The objective of the Span protocol is to reduce energy consumption without significantly reducing network capacity or the connectivity of a multi-hop network. To achieve this, Span elects in rotation some coordinators that stay awake and actively perform multi-hop data forwarding, while the other nodes remain in power-saving mode and check whether they should become coordinators at regular intervals. Coordinators form a forwarding backbone that should provide as much capacity as the original network.

Each node makes periodic local decisions on whether to be a coordinator or not. Such decisions are based on a coordinator eligibility rule. If a node has two noncoordinator neighbors that cannot communicate with each other either directly or through other coordinators, then it will become a coordinator. In order to avoid contention in elections and to keep the number of coordinators small, nodes wait for a random delay period before sending their announcement message. Then they elect themselves as coordinators only if the eligibility rule still holds after the wait. A convenient selection scheme for the random period is proposed to keep the number of coordinators low and to achieve rotation. This takes two different factors into account, i.e., the number of additional pairs that will be connected if the node becomes a coordinator and its residual energy.

Thisprotocol achieves significant energy saving while maintaining the performance of the upper-layer routing protocol almost unaltered. As compared to GAF [Xu], it has an adaptive but less predictable number of forwarding nodes, as in GAF the grid is fixed and nodes inside the same cell are considered to be equivalent. Besides, Span does not require nodes to know their location. However, Span requires a modification to the lookup mechanism of the routing protocol, as only nodes in the active state have to be considered in choosing the next hop. Finally, the adoption of Span for WSNs could be constrained by the fact that it is designed for the IEEE . PHY and MAC protocol, e.g., it relies upon its ad-hoc power saving functions to buffer packets for sleeping nodes.

7.8.3 Sparse Topology and Energy Management (STEM) Protocol

TheSTEM protocol presented in [Sch] is a topology control protocol specifically designed for WSNs. The assumption of STEM is that nodes in a WSN may spend most of the time only sens-ing the surroundsens-ing environment waitsens-ing for a target event to happen. Thus, unlike other topology management schemes that coordinate the activation of nodes during the transmission phase, STEM optimizes the energy efficiency of nodes during the monitoring state, i.e., when no one is sending data.

STEM exploits the fact that, while waiting for events, the network capacity can be heavily reduced, thus resulting in energy savings.

So all nodes can be asleep when no data transmission is needed. On the other hand, it is important for nodes to be able to wake up neighbors when transmissions occur. The solution proposed by STEM is to keep the duty cycle of nodes very low unless data transmission starts. Nodes keep their radios

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off most of the time, but periodically turn them on for a short time to hear whether some neighbor wants to communicate. A node that wants to communicate switches on its radio and starts sending beacons to the node it wants to wake up. Once the target node receives the beacon, it responds to the initiator node and data transmission starts. If data has to be forwarded further, the same operation is required for the next hop. In order to avoid collisions between data and wake-up beacons, the use of dual radio nodes is suggested, with two different frequency bands for the wake-up plane and the data plane.

In reactive WSNs, the STEM protocol can save more energy than other topology control protocols such as GAF or SPAN. However, it is not suitable for proactive WSNs, in which periodical updates have to be transmitted to the sink. Latency increases with the decreasing duty cycle of nodes, so a trade-off is needed between energy consumption and responsivity. Delay also increases linearly with the number of hops, so in large WSNs very high delays may be experienced.

Finally, it has to be highlighted that, rather than an alternative to other topology control pro-tocols such as GAF or SPAN, the STEM protocol is orthogonal to them, so these approaches may coexist in the same network. As an example, the STEM-GAF combination is proposed in [Sch].

This integration can reduce energy consumption even further.