# Survey on Wireless Technology Trade-Offs for the Industrial Internet of Things
## 4 Wireless Technologies for Industrial Applications
### Long Range Multi-hop Networks
#### 4.2.4 Time Slotted Channel Hopping  
A MAC layer with TSCH can operate on top of both 2.4 GHz- and sub-GHz 802.15.4-based PHY. Assuming up to 4 hops, 802.15.4 networks that operate in unlicensed 2.4 GHz frequency bands can achieve a range of up to 200 m. On the other hand, their sub-GHz counterparts based on 802.15.4 g can reach up to 1 km single-hop range [52].  
In TSCH mode, time is slotted and time slots are grouped in slot frames. Both the time-slot duration and the number of slots in the slot frame are configurable. Slot duration is determined by the time needed to transmit a packet and receive an ACK and typically amounts 6 ms-10 ms. The length of the slot frame is based on the trade-off between energy consumption and throughput. Decreasing the slot frames increases both the energy consumption and the throughput and vice versa. For cyclic communication in a closed loop, a node would need two slots in a slot frame. The cycle could repeat every slot frame, thus the cycle time trades off with the number of nodes in the network. A network of two nodes could exchange packets with \(\sim\)20 ms cycle time but 25 closed loops would increase the minimal cycle time to 500 ms. Considering the best practices regarding the sampling in industrial communications, the minimal cycle times achievable with TSCH in practice increase to 60 ms-80 ms for non-critical traffic and 80 ms-200 ms for a single regulatory loop (2 s-5 s for 25 loops).  
Besides time slotting, TSCH also utilizes channel hopping to diminish the impact of interference and channel fading. Nodes communicate with each other over 16 orthogonal channels and retransmit packets on different channels if initial attempts fail, thereby increasing reliability. Channel hopping scheme could benefit the scalability and the aggregate throughput of the network if all channels were used in a same time slot. However, this benefit is largely limited in industrial networks given that all slave nodes communicate with the same master node in the network, as illustrated in Figure 1. Unless a master node is able to send and receive on multiple channels simultaneously, which is not the default capability, nodes will be forced not to transmit simultaneously, regardless of the channel hopping scheme.  
Furthermore, reliability of 2.4 GHz TSCH networks may be significantly reduced in the presence of interference of co-located Wi-Fi networks, as shown in Reference [53]. When only a few channels in the TSCH matrix are affected by interference, only an increase of the transmission latency due to retransmissions could be expected. However in reality, the probability of eventual packet dropout is no longer negligible because of the quite low default retry limit (4) and the fact that one Wi-Fi channel overlaps many IEEE 802.15.4 channels (four to five). As Reference [53] shows, this results in an increase of packet losses, in turn reducing the reliability. Reliability could be increased by increasing the retry limit, hence trading off the latency and power consumption.  
In conclusion, TSCH networks utilize the schedule on each node to achieve determinism and robustness against channel fading, as well as to conserve energy. That robustness and determinism makes TSCH suitable for class 2-6 industrial applications, assuming it can comply with the range requirement. However, there is a strong trade-off between latency and scalability, given that multiple channels cannot be used in parallel by different slave nodes due to the centralized master node.