## 3 NB-IoT Architecture  
An IoT connection differs from a cellular Internet connection. A smartphone, for instance, usually gets information off the Internet in a downlink as large real-time streaming data such as music or a video, but the IoT data are usually exiguous, arriving in short bursts. In contrast to cellular networks, the device generates most of the data, which travels in the uplink. The LTE evolution after the 12 th and 13 th releases has lifted the IoT technology to a new stage, satisfying the data producers in terms of their requirements for extended range, lower costs, and lower power consumption [36]. Narrowband IoT (NB-IoT, or Cat-NB) is standardized to support ultra-low-power end devices in massive IoT applications as an enhanced extension of LTE-M. It is integrated with LTE to add to the deployment flexibility. NB-IoT is specifically tailored so that its carrier is self-contained with the ability to be deployed with a system bandwidth of only 200 kHz. NB-IoT can rapidly grow in the market thanks to its new network software on an existing LTE network. An evaluation of the capacity of NB-IoT indicates that each 200 kHz NB-IoT carrier is able to cover over 200,000 subscribers. This comes with increased coverage of up to 20 dB and a power-saving mode giving 10 years of battery life (Figure 4). It is intended to conveniently manage an increased number of linked tools and to apply sleep algorithms to extend the node battery lifetime. There are 3 deployment modes for NB-IoT, as shown in Figure 5:  
* In the LTE guard band as a specific band;
* Embedded within a normal LTE carrier;
* As a standalone carrier in GSM bands. These types of operations reduce the device complexity for NB-IoT and make it a potential rival to the module costs of the unlicensed LPWAN radio communication technologies. Additionally, it is ideal for in-market applications that have a mature LTE-installed base [37]. In order to thoroughly describe the architecture of the NB-IoT technology, one needs to know some of the general terminology about the LPWAN architecture. The system architecture for radio communication technologies is basically formed by the following:  
* **End devices (EDs)**: These devices are also called nodes, tags, or user equipment (UE), representing the client-side devices that send or receive data. EDs usually refer to the places where the sensing and controlling are happening, such as sensors, detectors, and actuators;
* **Gateway (GW)**: The GW or eNodeB (also called a modem, access point, or base station) has the responsibility to receive or push data between the core network and the connected EDs. The number of end devices can be very high within a gateway. Communication with the core network is enabled via Internet Protocol/IP. More information on the communication protocols is given in Section 5;
* **Network server (NS)**: The NS represents the most intelligent component and is also referred to as the cloud server or serving-GW. Its intelligence is reflected in the responsibility to monitor the GWs and EDs, aggregate the data, and take control over forwarding messages to the corresponding application server;
* **Join server-assign server-home subscriber server**: This joins an ED to the network and controls the ED authentication;
* **Application server-**cloud application-packet data node gateway (**AS**): This is simply the program code that executes on the user side as an interface for the communication  
Figure 4: NB-IoT’s main features. Figure 5: The operation modes of the NB-IoT carrier. with the connected end devices to send or receive data (i.e., the information that the user needs) [23]. The NB-IoT architecture in particular consists of the following entities:  
* The **NB-IoT UE** establishes the wireless connection to the eNodeB over the radio;
* The **eNodeB** is responsible for the access processing with the air interface.