Adaptive Threshold-Sensitive Energy-Efficient Sensor Network (APTEEN) Protocol

Theauthors of TEEN extend the protocol in [Man] in order to efficiently manage different kinds of queries, implementing both proactive and reactive data transmission. This hybrid routing protocol, called the APTEEN) protocol, defines three different types of queries, i.e.,

• Historical queries, used for the analysis of historical data stored on the BS

• One-time queries, used to give a snapshot of the sensed environment at a defined time

• Persistent queries, used to monitor certain parameters for a defined time interval

Queries are received by WSN nodes and trigger interactions between nodes and the BS (i.e., the sink node). While in historical queries data is already stored in the BS, other queries may be stored or not depending on data criticality. Time-critical data can be fetched by the BS through periodical data transmissions in order to maintain values that are always up-to-date and can be directly sent when needed, while non-time-critical data can be transmitted on demand.

As the BS is able to transmit directly to any node, while nodes are energy-constrained, hierarchi-cal addressing is used. Cluster formation and hierarchihierarchi-cal addressing is the same as in TEEN. Data transmission combines TDMA for intracluster communications and CDMA to limit interference between different clusters. However, a different TDMA schedule for data transmission is proposed (Figure .).

In APTEEN some nodes are assumed always to be in the listening state to receive queries. These nodes must always be active, and they should have a larger time slot than the others, as they may have to transmit both data and queries to their cluster head. Transmission by these nodes is scheduled

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FIGURE . Super-frame comparisons between LEACH, MECH, and APTEEN. (a) LEACH transmission schedule.

(b) MECH transmission and forwarding phases. (c) APTEEN transmission schedule.

after all the data transmissions of low duty-cycle nodes. As in TEEN, forwarding is always to the next-level cluster head, or directly to the sink node for the uppermost-level cluster head. But here the sink node can communicate directly with any node, and hence, in addition to a time slot for communication between cluster head and sink, the TDMA schedule also has a data time slot for transmissions between the sink node and other non-cluster-head nodes.

Regarding energy efficiency, APTEEN performs slightly worse than TEEN, as non-cluster-head nodes that are listening for incoming queries cannot go to sleep. This increased energy consumption is partially balanced by the reactive operating mode. There is a trade-off between energy consump-tion and response time, since the use of periodic transmissions can reduce the response time for the queries (the BS always has updated data, so it does not need to wait for the sensed data), but it also increases energy consumption. Nevertheless, APTEEN improves on LEACH, as it transmits data based on the threshold values, while LEACH transmits data all the time.

7.5.2.6 Power-Efficient Gathering in Sensor Information Systems (PEGASIS) Protocol ThePEGASIS protocol is a LEACH-inspired protocol proposed in [Lin]. PEGASIS is not exactly a cluster-based protocol, as nodes are not explicitly grouped into clusters. PEGASIS is instead a chain-based approach, in which each node only communicates with a close neighbor and takes turns to transmit to the BS, thus reducing the amount of energy spent per round. This approach distributes the energy load evenly among the sensor nodes in the network.

ThePEGASIS protocol is designed for a WSN containing homogeneous and energy-constrained nodes, with no mobility. The BS (sink) is fixed and far away from nodes. The radio model adopted here is the same as the one presented in [Hei], i.e., the first-order radio model. Using this model, energy efficiency can be improved by minimizing the amount of direct transmissions to the sink node. This idea is common to the LEACH protocol, in which clustering is used to reduce both the duty cycle of the nodes and direct transmissions to the BS. A way in which energy efficiency can be further improved is to decrease the number of nodes that perform long-range direct transmissions.

So the basic idea of PEGASIS is to have only one designated node that directly transmits to the BS in each round. This can be achieved with a linear chain-based approach, where sensor nodes form a

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Power-Efficient Routing in Wireless Sensor Networks 7-25

chain, in which gathered data moves from node to node, gets fused, and eventually a designated node will transmit it to the BS. As nodes take turns to transmit to the BS and transmissions are between close neighbors, the average energy spent by each node per round is reduced.

Data fusion, which is performed at each node (except end nodes), decreases the size of the aggre-gated data packet. The designated node that transmits to the BS (called the leader node) changes at each round, so that energy consumption is balanced over the network. Chain setup to minimize the total length is similar to the travelling salesman problem, which is known to be an NP-hard problem, so it is dealt with by a greedy algorithm run either by sensor nodes in a distributed way or by the BS in a centralized way and then broadcast to sensor nodes. In order to build the chain, nodes are assumed to have global knowledge of the WSN topology and location awareness. The proposed greedy algo-rithm for chain setup is started by the node furthest from the BS and then includes the neighbors in the chain. In this way, nodes which are distant from the sink are sure to have close neighbors, so that longer-distance transmission occurring when a node is the leader can be balanced by lower-power transmission when it is not.

PEGASIS divides time into rounds. While the chain remains the same, the leader role is determin-istically rotated at each round, i.e., at round i the leader will be node (i mod N), where N is the total number of nodes. The leader node can start the transmission phase by transmitting a small token to a node (i.e., the first), which will fuse its neighbor data with its own to generate a single packet of the same length, which is then transmitted to its other neighbor (if it has two neighbors).

In the chain shown in Figure ., the leader is node c. The leader starts transmissions by sending a token to the first node in the chain, c, which will pass its data to its neighbor c. Node c fuses the node c data with its own and then transmits the packet to the leader. After node c passes the token to node c, the latter transmits its data to node c. Node c fuses the node c data with its own and then transmits it to the leader. Node c waits to receive data from both neighbors and then fuses its data with its neighbors’ data. Finally, node c transmits one message to the BS. When a node dies, the chain needs to be rebuilt from scratch.

This protocol significantly reduces the overhead and the number of messages as compared with LEACH, so it outperforms LEACH in terms of network lifetime, at least with the model adopted.

In fact, a LEACH network may have many cluster heads that need to transmit directly to the sink node, while in PEGASIS there is only one leader for the whole network. In addition, the number of messages received is also highly reduced, as in PEGASIS the leader only receives two messages, while in LEACH each cluster head receives n messages (where n is the number of nodes belonging to the cluster). This saves energy, as with the first-order radio model [Hei] receiving a message is a costly operation. However, the model does not take idle power consumption into account, which could be as great as the receiving power [Rag]. If idle and sleep power consumption is taken into account, LEACH will probably turn out to be more power-efficient than PEGASIS, as non-cluster-head nodes in LEACH can drastically reduce their duty cycle, while in PEGASIS nonleader nodes must always be active in order to receive the token. In addition, this approach causes a high end-to-end delay for the sensor data, because all the data from the whole WSN has to be gathered and forwarded before a sensed value can reach the sink. Delay grows with a growing network, and a single leader can also become a bottleneck. In the event of a fault, a large part of the network would not be able to forward data and the whole chain would have to be rebuilt, while in LEACH only faults in a cluster head cause data loss, and they are confined within the same cluster.

c0 c1 c2

BS

c3 c4

FIGURE . Chain-based forwarding in PEGASIS.

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7-26 Networked Embedded Systems

ThePEGASIS protocol is extended in [Lin], where not only energy efficiency but also delay is targeted. The authors investigate data gathering schemes that balance the energy and delay costs, as measured by the energy∗delay metric. Two different solutions are proposed, a chain-based binary scheme for sensor networks with CDMA nodes and a chain-based three-level scheme for sensor networks with non-CDMA nodes.

Thetwo approaches share the same basic idea, i.e., achieving multiple parallel communications by different nodes. In order to improve energy and delay performance by simultaneous transmissions, interferences between different communications should be minimized. The first solution proposed to achieve multiple simultaneous communication with low interference is to use CDMA, so CDMA-capable sensor nodes are used. Each pair of nodes can use a distinct code to minimize interference with other transmissions and so a maximum degree of parallelism can be implemented. This means that in an N-node WSN, after the chain has been built as in PEGASIS, N/ nodes can transmit at the same time with a different code (e.g., all the nodes in odd positions can simultaneously transmit to those in even positions). Then, at the second level of the hierarchy, only nodes that were receivers at the first level are considered (i.e., the nodes in even positions). So N/ of this set containing N/

nodes will be senders (i.e., once again the ones in even positions in the second-level set). Then the set of receiving nodes will be considered as the third-level set, and so on, until only one-node set remains. This is the leader node that has to perform direct transmission toward the sink. While in PEGASIS delay is a linear function of the number of WSN nodes N, using this approach delay is a logarithmic function.

However, CDMA may not be applicable for all sensor networks. The authors therefore investigate a second approach to achieve a minimal energy∗delay with non-CDMA nodes. In this case, in order to minimize interferences, only distant nodes should transmit at the same time. A three-level hierarchy for data gathering is therefore proposed, which allows simultaneous transmissions that are far apart so as to minimize interference while achieving a reasonable delay cost.

Simultaneous data transmissions are carefully scheduled taking the position of every node into consideration, so that at each level only the leaders of the previous level participate. The transmission schedule can be calculated once at the beginning, so that all nodes know where to send data in each communication round.

Thisapproach is only based on experimental considerations and is not as efficient as the CDMA-based approach. However, it performs much better than LEACH and PEGASIS in terms of the energy∗delay metric. The energy model adopted here is the first-order radio model, as in LEACH and in PEGASIS, so idle (and sleep) power consumption is not taken into account. Even these exten-sions of the PEGASIS protocol do not directly decrease the duty cycle of nodes. Hence these results may change if the physical and MAC layers feature high idle power consumption.