Energy-efficient forwarding (EEF) focuses on the end-to-end packet delivery process between a sensor and a fixed sink node in the network. In general, the sink node queries the sensor network for some information that is then (or perhaps later after a special event occurred) sent back. Usually the network is large in size such that each node is not able to communicate with the sink directly. Data packets are thus forwarded by intermediate nodes and sent hop-by-hop along a forwarding path, which is based on the reverse tree established by the sink during the querying process. Since the sink is the root of the tree, each node can use that node as its forwarder from which it received the query at first. In doing so, all nodes establish a shortest path in terms of forwarding hops towards the sink.

However, although such hop-based strategies are widely used, they can be quite inefficient, especially in lossy environments. By using a realistic link loss model, we considered different forwarding strategies and proposed two new schemes named single-link and multi-link energy-efficient forwarding (SEEF and MEEF). Both strategies optimize the forwarding paths concerning their energy efficiency and trade off their end-to-end delivery ratio and energy cost. While the SEEF strategy sends a packet to only one neighbor for forwarding, the MEEF strategy exploits the broadcast characteristics of the wireless medium and addresses a packet to several nodes, among them a forwarding node is selected afterwards. By mathe-matical analyses, extensive simulations, and experimental evaluations we contrasted the performance of our approach against a comprehensive framework of different forwarding strategies proposed before.

Single-link energy-efficient forwarding (SEEF) aims at finding the most energy-efficient path in the network that trades off the end-to-end delivery ratio and the energy cost. By examining each of a node's neighbors, the node that maximizes the ratio of delivered information per consumed energy unit is selected as the forwarder. Using this forwarding metric, we take the end-to-end reception rate as well as the energy consumption into account simultaneously.

Therefore, the network sink first initializes the establishment of reverse paths by sending a special routing message. The message contains information regarding the end-to-end delivery ratio and the required energy cost to deliver a packet to the sink. By using this information, each node i is able to calculate its delivery ratio E^r, the energy consumption E^e, and the energy efficiency E^eff for the forwarding path a neighbor j provides. The neighbor that maximizes E^eff is then stored in the routing table of node i as the forwarder, together with information about E^r and E^e. If the energy efficiency could be improved or E^r and E^e have changed with respect to the current forwarder, a new routing message is generated and announced to the node's neighborhood by broadcast.

In contrast to the next forwarding strategy, we called this strategy single-link energy-efficient forwarding. Single-link refers to the fact that a message is always forwarded over a single link to one forwarding node. On the other hand, multi-link forwarding strategies exploits the broadcast characteristics of the wireless medium by using multiple forwarding links.

In the multi-link energy-efficient forwarding (MEEF) strategy, packets may be addressed to more than one forwarding node. The idea is to exploit the broadcast characteristics the wireless communication channel provides. In general, nodes that are not addressed in the header of a packet as the destination temporarily turn their radio off to save energy. However, it might be more efficient if some nodes stay awake and overhear the packet transmission completely.

In case some nodes are not able to receive the packet correctly, retransmitting the entire packet is not necessary if there is another node which could receive it successfully. In order to prevent that the packet is forwarded by multiple nodes, we use a simple polling approach. At first, all potential forwarders are ordered according to their energy efficiency. If the first forwarding node does not acknowledge the packet, we assume the packet got lost. By polling the next node in the list, the second forwarder is informed that it should forward the packet instead. If again no acknowledgement is received, all other nodes in the list are polled. Only if none of them answer, the packet is retransmitted.

Although this approach prevents multi-path forwarding in most of the cases it is still possible that a packet is forwarded by two or even more nodes at the same time. Due to link asymmetries it may happen that a packet is correctly received by the first forwarder but the sender does not get an acknowledgement back. It thus polls the second forwarding node, which in turn starts forwarding the same packet.

Even if such an unintentional multi-path forwarding might be robust against different kinds of network failures, it might be inefficient concerning its energy consumption. However, our approach reduces the impact of multi-path forwarding substantially as packets are forwarded by more than one node only if acknowledgments get lost. Nevertheless, the additional energy costs for this case as well as the energy cost for polling nodes need to be taken into account. Otherwise, a node could not decide reliably if the energy efficiency of its forwarding path can be improved by using additional forwarders.

Concerning the addressing used by multi-link forwarding, the appropriate node creates an ordered set of potential forwarders, the forwarder list, which is added to the packet header. Nodes contained in the forwarder list stay awake during the complete transmission and try to receive the entire packet. All other nodes are assumed to temporally turn their communication radio off like in SEEF. As each additional forwarding node must receive the packet in order to work in a so-called backup mode, more energy is consumed. But at the same time, additional nodes may improve the delivery ratio and thus the overall energy efficiency. Thus, selecting more than one forwarder mainly depends on the packet reception ratios on the forwarding links as well as the end-to-end delivery ratios and energy costs of the selected forwarders.