# Model Mediation to Overcome Light Limitations--Toward a Secure Tactile Internet System
## 4 Model Mediation  
The model mediation concept appeared with the presence of the bilateral teleoperation systems; mainly to improve transparency and stability [56]. Bilateral teleoperation system is a system, which contains the main components of the tactile Internet system; master robot, slave robot, and a communication network and shares the main features with the tactile Internet system. Figure 2 illustrates the relation between the two systems. In bilateral teleoperation systems, the master robot or the human is able to manipulate, access and interact with a remote environment that contains a slave robot [57]. The system works via a closed loop communication system, where the master device must receive a feedback from the slave device to maintain the communion with an acceptable degree of experience, as illustrated in Figure 3.  
The master device employs haptic devices through which the human can interact. The device transmits haptic data, which contains information about force and acceleration via the communication channel. The slave device is deployed in the remote environment, which interacts with the environment based on the received haptic data. The slave device must send a feedback to the master device contains visual, motion and force information, so as to make the user (operator, human) able to get a certain level of experience [58]. Bilateral teleoperation systems as well as the tactile Internet applications can be classified, based on the interaction level between master and slave devices and the communication latencies, into two main categories; mission-based control systems and direct control systems as illustrated in Figure 4. [37]. Figure 3: General view of the end-to-end system structure for the bilateral teleoperation systems. Figure 2: Tactile internet and bilateral teleoperation systems. In direct control bilateral teleoperation systems, the master device (i.e., human) directly interacts with the remote environment in real time. This can be done by communication position and force signals between master and slave sides in real time. This group of teleoperation systems can be subdivided into two main subgroups; low delay closed loop systems and high delay closed loop systems. The low delay closed loop bilateral teleoperation systems are those with the negligible communication distance between master and slave domains. The propagation latency among communication channel for these systems is very small and can be neglected [59]. The other direct control subgroup is the high delay systems, in which the communication channel introduces high propagation delay due to the large distance between the master and the slave domains. The other category is the mission-based teleoperation system, which is designed for a certain mission that requires the slave robot to be able to work independently. These systems can be divided into fully autonomous and semi-autonomous systems based on the interaction and transferred signals between master and slave sides [60]. The main design attribute of the bilateral teleoperation systems is the end-to-end to latency, which in case of haptic-based bilateral teleoperation systems should be less than or equal to 1 ms to get an acceptable level of experience. This represents the main design challenge of the tactile Internet. The end-to-end latency contains all delays involved in the communication process, in another way the time period from the start of communication process at master robot until the reception of the feedback [51]. Thus, the end-to-end latency mainly affected by the communication distance, and the number of intermediate communication nodes engaged in the communication process. The model-mediated system provides a virtual version of the remote environment at the master side that can interacts with the master device with non-delayed force feedback, which make the haptic-based bilateral systems able to work under different communication network delays [61].