# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Mechanisms for RFID Active Tags
#### 4.1.2 Zigbee  
Zigbee is one of the main IoT standards for the transmission of small-sized data with low power consumption [115], and for this reason many works have been focused on the analysis its energy efficiency [97; 98]. Most solutions fall into the Resource Allocation and Network categories, as shown in Figure 6.  
**Resource Allocation techniques. ** In a Zigbee network, the majority of the network energy is consumed by the sensor nodes in Transmit and Receive modes. In [116], it is observed that there is the need for considering other sources of energy consumption, using the network lifetime as a metric. Hence, other energy consumption sources are highlighted, such as:  
* Energy consumption of the Power-up mode, for both nodes and routers;
* Energy consumption of reporting data with application acknowledgment requirements, that is, the energy that is spent by the coordinator node or the access point when sending back an ACK for the successful reception of sensor packets. This process only happens when a particular application requires ACK responses, and in this case the process of waiting for ACKs also requires energy, shorting in this way the overall network lifetime;
* Energy consumption of routers, which directly impacts the network lifetime and functioning, since many sensors will not be able to transmit data to the coordinator if a router runs out of battery. In order to make the network more efficient, two resource allocation techniques were also proposed:
* _Energy-saving Access Control_, that employs the use of a control method to shorten the access time of sensor nodes;
* _Energy-saving Sleeping Schedule_, that allows a node to enter sleep mode when an ACK has been received and no other transmissions are needed. **Network techniques. ** Zigbee-based LPSANs, as mentioned in Section 2, mostly use mesh topologies for data exchange [35]. Thus, in order to be bandwidth- and energy-efficient, the usage of proper routing protocols is of paramount importance, and most of network techniques proposed for Zigbee are indeed based on routing. One of the most used routing protocol in Zigbee is a modified version of the Ad Hoc On Demand Distance Vector (AODV), called AODVjr [117]. AODV compares different routing paths by employing a path cost metric. In particular, each link in a path is associated with a link cost, also called cost value [35]. The cost values are summed up to evaluate the cost for the whole path. In a mesh topology, this results in the shortest paths to be usually the optimal paths. AODVjr uses a route discovery technique in order to establish the route for the transmission of the information. The route discovery consists of a source node broadcasting a Route Request (RREQ) for the destination. Every intermediate node that receives the RREQ for the first time, does not obviously have a current route to the destination, and thus has to rebroadcast the RREQ. Then the destination node receiving the RREQ sends back a Route Reply (RREP). Then, RREP back propagates,  
Figure 6: Energy efficiency techniques for Zigbee [116; 117; 118; 119; 120; 121; 122; 123; 124]. and each intermediate node is involved in the final source-destination route. This method reduces the transmission delays, because the route is pre-specified, but at the same time is energy-consuming, because of the RREQ rebroadcasting. To address this shortcoming, the E-AODVjr routing algorithm was also proposed in [117], which takes into account the residual energy of the nodes to determine which of them can be a part of an active route. E-AODVjr requires to monitor the energy consumption of nodes, to be able to estimate the consumption in the next \(\Delta T\) seconds.