# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 2 IoT Technologies
### Low-Power Short-Area Networks (LPSANs)
#### 2.1.2 Zigbee  
The Zigbee technology is based on the IEEE 802.15.4 standard suite, and it is adopted to create personal area networks for IoT applications, such as home automation, medical device data transmissions, and other services requiring low power and bandwidth consumption. Zigbee covers a distance up to one hundred meters in LoS [32], offering data rates up to 250 kbps. It mainly operates in the 2.4 GHz ISM band, but also works at 784 MHz in China, 868 MHz in Europe, and 915 MHz in US and Australia [33]. In the ISM band, a Zigbee system allocates a total of sixteen 2 MHz channels, with a spacing of 5 MHz, using Direct-Sequence Spread Spectrum (DSSS) modulation. In 868 and 915 MHz bands, signals are modulated using BPSK or Offset Quadrature Phase-Shift keying (OQPSK).  
The Zigbee protocol layers are based on the Open System Interconnect (OSI) reference model [34], implementing only the layers that are needed for low-power low-data rate networking. Zigbee standard defines the networking, application and security layers of the protocol [35], and adopts the physical and MAC layer from the IEEE 802.15.4 standard [36]. As shown in Figure 2, Zigbee defines three functional roles [37]:  
* _Coordinator_, that is the most important element of the network based on a tree topology. There is only one coordinator in each network, initiating the network and selecting network configurations;
* _Router_, that acts as an intermediate node, relaying data from and to other devices;
* _End Device_, that is typically battery-operated, and collects information from sensors and transmits it to the coordinator. End devices can only communicate with and send information to the router and the coordinator, and can enter in a sleep mode if there is no information to relay.  
A Zigbee network is composed of two types of nodes, classified based on their processing capabilities:  
* _Full Function Device (FFD)_: a node that can play all functional roles;
* _Reduced Function Device (RFD)_: a node that can only act as end device.  
In terms of network organization, Zigbee mostly uses a mesh topology, where the nodes connect directly to as many other end devices as possible, and cooperate to efficiently routing data in the network, aiming at low latency and high robustness. As reported in the latest specification, named Zigbee 2012, a Zigbee mesh network can provide access to more than 64.000 devices [38].  
Figure 2: Zigbee Network Architecture.  
The communication protocols support both non-beacon and beacon-enabled networks. In the beacon-enabled network, the nodes can transmit information only in time slots that were predetermined before transmission. The coordinator allocates the guaranteed time slots for each device to transmit. The coordinator synchronizes the devices that are transmitting using beacon signals, that tell each device when it is their turn to transmit. In the non-beacon network, the devices do not have guaranteed time slots for transmission, and are not synchronized when transmitting. Non-beacon networks make use of unslotted CSMA/CA. Zigbee can also use a star topology, consisting of one coordinator and many end devices, and in this case it adopts a master-slave network model. The master is the coordinator, which is always a FFD node, while the slaves can either be FFD or RFD devices. In the star topology the nodes can only communicate to the coordinator.