## 4 Current Issues and Challenges of Realising the Tactile Internet
### Round-Trip Time of 1 ms  
To have an option to guarantee a transparent experience, the most challenging factor is to achieve a round-trip time (RTT) of 1 ms [58; 3]. The motion sickness delay that occurs while performing any activity in one place, where another person perceives it in another place is called lag [59]. For haptic communication, various examinations demonstrate that lag will be observable if RTT is greater than 1 ms [60; 61]. If this lag is noticeable, it influences transparency.  
One of the crucial factors that affects the smooth flow of any haptic communication in a network is RTT. However, RTT is significantly affected by queuing and processing delays at the intermediate nodes and packet transmission times. The speed of light is among the most constraining elements for the RTT. Suppose the propagation time taken by the packet to reach a destination is more than 10 ms. Therefore, by considering the 1 ms challenge, the propagation time taken by the packet should be less than 1 ms. With 1 ms time delay constraint, it is workable for short-range communication.  
With the availability of 4G mobile cellular networks, the RTT for short-range communication incurs a 15 ms delay, which cannot accomplish the necessity of 1 ms for a tactile response [60]. Furthermore, due to the evolution of 5G cellular networks, the RTT will incur \(<\)5 ms delay, which supports the necessities of haptic communication [61].  
For haptic data communication, the packet size required should be less than 1500 bytes [56]. Taking all parameters such as transmission and processing delays into consideration, the RTT can be further minimised for smaller packet size data [58]. This is a step that we take closer to achieving haptic data communication with the challenge of 1 ms.  
In addition, the essential point of interest is to execute the concept of the TI which supports infrastructure or architecture with RTT below 1 ms. 5G technology is still being explored and is not available to consumers. To satisfy the conditions of 1 ms, the control server and end-point device should not be placed at distant places as the end-user information needs to be executed immediately, and acknowledgement must be sent to the origin. Therefore, to avoid any delays, the control servers should be profoundly reliable and process all the end-user information.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Challenges** & **Description** & **Potential Enabler and Solution with References** \\ \hline Kinematic Devices & Dealing with Kinematic Sensors and Kinematic actuators & With regard with post-processing time of reading data from sensors, algorithms are needed to be devised which will dynamically select the sensor and read the data. \\ \hline \hline \end{tabular}
\end{table}
Table 3: \(Cont.\)Subsequently, to encounter the design concern for 1 ms RTT, one-way physical-layer transmission is empowered. Thus, the one-way transmission must have a minimum packet size and around the 100 \(\upmu\)s duration. To accomplish this necessity, every packet cannot surpass a 33 \(\upmu\)s packet duration [62]. The explanation for this is the need for additional structural delay that demands encoding, detection, and decoding at the transmitter and receiver ends. This restricts the packet size to less than 0.33 times the desired delay. Using the current LTE cell framework, the modulation used is not a feasible proposition, as the length of one OFDM symbol is nearly 70 \(\upmu\)s long. A total round trip of the cellular physical layer is essential for the TI, possibly with 5G cellular networks.