# Digital Twin Conceptual Model within the Context of Internet of Things
## 4 Communication Network Layer  
The communication network layer (CNL) is the second layer of the digital twin model, implemented right above the PSL. The main purpose of the CNL is to effectively transmit/receive the data collected by the sensors (which include the operational and environmental parameters)/actuators, as well as the edge-computing processing results, to the higher layers for further processing and analysis, as will be elaborated in the next section. In other words, the communication layer is essentially the 'bridge' that connects the physical space with the cyber/virtual space. As the virtual space cloud or enterprise services may not necessarily be in the same geographical location as the physical space, the digital twin specifically needs large area wireless networks for its communications systems. These include mobile communications technologies and satellite communication networks. Mobile communication networks have been around for decades, starting with circuit-switched first-generation (1G) network, which allowed voice transmission of speeds up to 2.4 kbps. In contrast to 1G, 2G network stepped up from just voice transmission to further include the facilities of Short Message Service (SMS) and Multimedia Messaging Service (MMS). The 2G network offered data rates of 64 kbps and a bandwidth that ranged between 30 kHz to 200 kHz. Next, the mostly packet-switched 3G wireless technology provided users with broadband-like internet access while on the move. The 3G has data rates of 2 Mbps for stationary users, 384 kbps for low-mobility users, and 144 kbps for high-mobility users, along with a bandwidth of 20 MHz [17]. The digital twin has certain network requirements that must be met in order to be fully functional. As large amounts of real-time big data are being gathered from potentially hundreds of sensors at a time, a high bandwidth and high data rate are required to relay them all at once to the virtual space layer above. Moreover, after all necessary analysis and insights have been performed by software technologies in use, the results must be communicated back to the PSL, for instance, to the actuators, with no perceivable delay. Thus, a successful implementation of the digital twin also demands a very low latency. Another important factor to consider for the digital twin is the network and scalable capacity of the communication technology. In other words, a single base station should be able to hold as many simultaneous connections as possible (networkcapacity), and increasing the hardware components of the communications system should result in a significant improvement in the network (scalable capacity), so that the digital twin can collect as much data as possible and be spread across the entire product or manufacturing process. The aforementioned mobile communication networks are not nearly sufficient in meeting the demands of the digital twin, hence, 4G and (more importantly) 5G networks must be considered for the implementation of the digital twin. The 4G Cellular Network  
The 4G is the first communication technology to be purely packet-switched and provides users with an ultra-broadband experience. Facilities such as multimedia applications, video conferencing, video calls, and high-definition (HD) streaming were achievable with smooth connectivity. The 4G offers data rates up to 100 Mbps for outdoor environments and one Gbps for indoor environments. The 4G networks have a latency of 100 ms; however, the available bandwidth provided by 4G has not changed from the previous generation and is still 20 MHz. The 4G base station systems boast an increased coverage of 31 km. The 4G networks have a scalable capacity of 50 to 500 bits/s/Hz/km\({}^{2}\). In addition, 4G is the first large area wireless network to support vertical handoff (allow a moving user to switch between 4G to other networks and vice versa), whereas all previous generation networks have only supported horizontal handover [17]. The 5G Cellular Network  
The 5G technology is to be completely launched by the year 2020.