## 4 UAN Classes
### LR-WPAN UANs  
LR-WPAN UANs can be considered the quintessential UAN class, which is currently being deployed in many Smart City initiatives, and for which a majority of manufacturers and providers are offering equipment and solutions. In this UAN class, the sensor nodes use a variant of the IEEE 802.15.4 family [17] or exploit proprietary low-power wireless technologies at the physical and link layers of the protocol stack. IEEE 802.15.4 is the de facto radio interface used for low-power wireless applications. Amendments such as IEEE 802.15.4e and IEEE 802.15.4g provide optimizations that may be useful to overcome issues in urban scenarios such as multipath, narrowband interference, or fading due to obstacles [18, 19].  
In LR-WPAN UANs, the power consumption of the sensor nodes when their radios are in sleep mode is in the order of a few microwatt, whereas when the transceiver is active, nodes consume typically below one hundred milliwatt. The backhaul is an IEEE 802.15.4 multihop backbone composed of nodes that generally exhibit the same hardware characteristics as those of the sensor nodes. In addition, the backhaul nodes themselves can also be used for sensing. These nodes are always prepared to receive or forward data from or to the sensor nodes. The most straightforward solution for providing energy to the backhaul nodes is connecting them permanently to the mains power. To this end, a common solution is to install the backhaul nodes in streetlights. However, some lighting systems are only powered during nighttime. In this case, the backhaul nodes require load circuitry and rechargeable batteries in order to store energy for daytime operation.  
There exist two main protocol architectures that are suitable for LR-WPAN UANs: ZigBee and the IP-based protocol stack for constrained node networks. In the first case, sensor and backhaul nodes implement the ZigBee protocol stack [20], which defines an application layer (APL) including commands and an end-to-end transport sublayer, and a network layer (NWK), which comprises routing and addressing functionality, on top of 802.15.4 (Figure 3a)). The gateway typically translates the ZigBee stack to an IP-based stack for Internet connectivity. The second LR-WPAN UAN type is based on the protocol suite developed by the IETF for constrained node networks (Figure 3b)). In this approach, sensor nodes use IPv6 since it has a vast address space and autoconfiguration capabilities. In order to enable IPv6 on top of IEEE 802.15.4, an adaptation layer called 6 LoWPAN is introduced [21]. Multihop network connectivity is achieved by means of the RPL routing protocol [22]. Finally, an efficient binary protocol called CoAP is used, on top of UDP, at the application layer [23]. Backhaul nodes may support the whole protocol stack, although the CoAP/UDP protocols only need to be used if the backhaul nodes  
Figure 2: Architectures for sensed data collection and actuation in a smart city. (**a**) Architecture of Low-Rate (LR)-WPAN and Wireless LAN (WLAN) UANs; (**b**) Architecture of Mobile Network Operator (MNO) and SIM-less Operator (SO) UANs; (**c**) Architecture of Delay Tolerant Networking (DTN) UANs.  
include sensing or management capabilities. CoAP has been designed to allow easy CoAP to HTTP message mapping, and thus the UAN gateway may effortlessly integrate with HTTP systems.