# Survey on Wireless Technology Trade-Offs for the Industrial Internet of Things
## 4 Wireless Technologies for Industrial Applications
### Long Range Multi-hop Networks
#### 4.2.3 Bluetooth Low Energy  
Unlike classic Bluetooth which was designed as a point to point wire replacement, BLE provides increased range with 1 Mbps data rates and recently introduced meshing. An experimental study [50] observed the range of around 50 m indoors. The observed outdoors range for non line of sight scenarios was 123 m and 165 m for 0 dBm and 9 dBm transmission gain respectively, versus 490 m and 790 m in line of sight scenarios. Instead of Bluetooth's 1-MHz wide 79 channels, BLE uses 40 channels of 2 MHz. It employs frequency hopping over 37 channels for (bidirectional) communication and 3 for (unidirectional) advertising. Two MAC layer roles are defined for the devices in BLE, master and slave. The master coordinates the medium access using a TDMA-based polling mechanism, in which it periodically polls the slaves in their corresponding _Connection Events (CEs)_ which are uniformly spaced within a configurable periodic interval called a _Connection Interval (CI)_. Frequency hopping is employed in a way that each CE uses a different channel. The standard defines the CI duration as a multiple of 1.25 ms in the range from 7.5 ms to 4 s. Therefore, the minimal cycle time equals the CI in a BLE control loop. Considering that the sample/update rate in an industrial system should be 3-4 times faster than the process time constant for condition monitoring and open loop control and 4-10 times faster for regulatory closed loop control, the minimal cycle times achievable with BLE increase to 22.5 ms-30 ms for non-critical traffic and 30 ms-75 ms for regulatory loops. BLE is theoretically capable of supporting 6 control loops exchanging 47-byte (max. size) packets with the minimal CI. Extending the CI for \(n\) time units of 1.25 ms results in \(S\) control loops, as follows:  
\[S=\left\lfloor\frac{7.5\;\text{ms}+n\cdot 1.25\;\text{ms}+T_{GB}}{2\cdot 0.376\; \text{ms}+0.15\;\text{ms}+T_{GB}}\right\rfloor \tag{1}\]  
where 0.376 ms is the transmission duration of a 47-byte packet at 1 Mbps, 0.15 ms stands for an Inter-Frame Spacing (IFS) and \(T_{GB}\) (in ms) for implementation-specific guard band between two successive CEs. This reasoning is based on the one-to-one mapping between control loops and CEs in a single CI. Besides 25 transmissions in a CI, we must fit \(S\)IFSs between successive transmissions in CEs and S\(-\)1 guard bands between CEs as well.  
Bluetooth 5 provides the options to double the speed to 2 Mbit/s at the expense of range or up to fourfold the range at the expense of data rate and eightfold the data broadcasting capacity of transmissions by increasing the packet lengths. However, BLE's limited range makes it unsuitable for large scale IIoT deployments, as over 1000 basestations would have to be deployed to cover the same area as a single Wi-Fi HaLow basestation. To overcome this issue, Bluetooth introduced meshing. Communication in the Bluetooth Mesh standard in implemented using BLE advertising and scanning on the 3 advertisement channels. The standard uses a flooding mechanism where each node in the network repeats incoming messages, relaying them until the destination. Unlike in normal BLE advertising, Bluetooth Mesh transmissions are not scheduled based on an advertising interval. Instead, the nodes transmit after a random backoff time. In a Bluetooth Mesh network, increasing the number of relaying neighbors from 1 to 10 reduces the 2-hop round-trip time (RTT) from 47 ms to 33 ms for 41-byte packets and 10 ms scanning interval [51]. Increasing the number of hops from 1 to 4 increases the RTT from 22 ms 88 ms. This is because the latency is influenced the most by the random backoff mechanism. However, the absence of a random backoff would increase the collision probability, decreasing the reliability and scalability of Bluetooth Mesh networks which makes it suitable only for classes 4-6 of industrial applications.