## 4 UAN Classes
### MNO UANs  
A third class of UANs is based on MNO cellular technology, which has traditionally been used for mobile voice and data communications, as the radio interface for sensor nodes in machine-to-machine (M2M) (In several circles, communication between machines by using cellular technologies has been denoted by M2M.) applications. In fact, the advent of the Short Message Service (SMS) allowed machines to carry out transactions through a cellular network. Later, 2.5G technologies, such as the General Packet Radio Service (GPRS), provided added value to M2M by allowing the use of IP, the lingua franca of current data networks. The performance enhancements offered by subsequent generations of cellular technologies, such as 3G or initial 4G variants are not particularly relevant for M2M, which typically involves short-sized and infrequent data exchanges. Instead, widespread coverage, simplicity, low power consumption and price are much more important attributes, for which 2G/2.5G provides currently the best solution [26]. However, future MNO UANs will benefit from enhancements specifically tailored to M2M communications (see this section). In GPRS-based MNO UANs, in order to set the radio interface in active mode, sensor nodes have to execute procedures for network attachment and communication setup. These procedures may take up to around ten seconds. The sensor nodes consume a few hundred milliwatt in average during these intervals,as well as during data transmission. Therefore, MNO UANs are much more energy-demanding for sensor nodes than the rest of UANs considered in this paper. In this UAN class, the same device acts as both the sensor node and the gateway, which communicates with an MNO base station. The sensor node may use IP-based communication over the cellular link (Figure 3d)). At the application layer, CoAP is a more lightweight solution than HTTP. However, other data transports such as SMS may be used [27]. Figure 3: Protocol stacks and node types for different UAN classes. (**a**) LR-WPAN UAN based on ZigBee; (**b**) LR-WPAN UAN based on IP; (**c**) WLAN UAN; (**d**) MNO UAN; (**e**) DTN UAN. (Note: mesh functionality in WLAN UANs may be present at the link layer, as e.g., in IEEE 802.11s.). Emerging and Future MNO UANs  
The high momentum of the Internet of Things (IoT), and its tremendous growth expectations with forecasts predicting up to 50 billion connected devices by 2020 [28], have significantly impacted on the deployment and/or design of 4G/5G technology. Cellular MNOs and researchers have investigated how Long Term Evolution (LTE) can efficiently support M2M, also known as Machine-Type Communication (MTC) (MTC can be considered a synonym for M2M. which is very popular in the 4G context). It has been shown that, in comparison with other 2G/3G cellular network technologies, LTE (Release 8) provides a similar link budget, and greater capacity, while several proposals for device cost reduction for MTC have been evaluated [29]. On the other hand, LTE-Advanced (Release 10) was designed with particular consideration to MTC, including specific architectural components for MTC communications. Since the number of MTC devices in a cell is expected to be very large, access overload control mechanisms have been proposed [30]. Further work carried out by the 3 GPP (Release 12) comprises solutions for efficient communication of small amounts of data (in terms of signaling overhead and MTC device power consumption) [31,32], reducing MTC device costs to be competitive with GPRS terminals targeting the same market [26], and overload control techniques which are demonstrated to achieve access success probability of almost 100% and average access delay in the order of 30 ms for 5000 MTC devices per cell [33]. A good summary of recent advancements in M2M communications in 4G networks can be found in [34]. Beyond 4G networks, the concern for MTC is even more fundamental.