# Model Mediation to Overcome Light Limitations--Toward a Secure Tactile Internet System
## 3 One Millisecond Challenge  
The human senses interaction occurs in real time, only if the communication response is faster than the physiological time constant [49]. Table 2 indicates different physiological time constant for the different human senses [30]. For better understanding the problem of 1 ms, we consider the preliminary example introduced by [PERSON] in [30]. When we move an object on a touch screen, the maximum latency that achieves an unobserved displacement between the moving object and the human finger is 1 ms. This can be interpreted as the 1 ms is the time duration a human finger can move a distance of 1 mm on a touch screen, since the average speed of the human finger on the touch screen is 1 m/s. The 1 mm is the maximum unobserved distance between any two displacements, and larger displacement will be noticeable.  
Signals suffer from different latencies among heterogeneous communication systems. Three main latencies can be defined; end-to-end latency, user plane latency, and control plane latency [50]. The end-to-end latency is the time delay from the start of transmission of a small data packet from the transmitter's application layer until the reception of the data by the receiver's application layer [51]. This represents all signal delays; queuing delay, processing delay, computing delay, transmitting delay, propagation delay, retransmissions delay and reception delay. The round-trip latency includes the previous delays with the feedback delay dedicated by the communication process. The round-trip latency mainly depends on the communication distance and the number of the network nodes involved in the communication process.  
The 1 ms challenge puts high constraints on the communication network design. The current cellular network structure cannot be deployed, because the latest 4G network can achieve a communication with at least 20 ms end-to-end latency [52]. Future 5G cellular system is expected to deploy new technologies such as the software defined networking (SDN), the network function virtualization (NFV) and the mobile edge computing (MEC), which can reduce the communication delay but not to 1 ms [29]. MEC allows the end user to communicate with a cloud unit located at the cellular network edge (i.e., radio access network (RAN)) [53]. This offers all cloud computing capabilities one hop away from the end user, and thus reduces the communication latency. Many other benefits and other deployment scenarios of the MEC-based tactile Internet and 5G systems can be found in [41].  
Due to light limitations; the maximum distance the light can travel in a 1 ms is 300 km, this implies that the distance between the transmitter and the receiver should be at maximum of 150 km. This is under the assumption that there are no other delays, which is a must. Speaking about haptic-based bilateral teleoperation/tactile system, this distance may be proper for minor applications but for the plurality this will be inappropriate and certainly for the tactile Internet. One way to overcome this limitation and break the distance is to build a model mediated system that brings the remote environment near to the user [54]. This can be achieved with the means of artificial intelligence (AI) algorithms that can assist the model mediated systems and develop tactile Internet systems with the required latency constraints. Such systems are referred to as the zero-delay communication systems [55].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Human Sense** & **Time Constant** \\ \hline Muscular interaction & 1 s \\ Auditory interaction & 100 ms \\ Visual interaction & 10 ms \\ Tactile interaction & 1 ms \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physiological time constant of different human senses.