# Wireless Mesh Networking: An IoT-Oriented Perspective Survey on Relevant Technologies
## 4 Mesh in Bluetooth/BLE Networks
### Other Solutions  
The Bluetooth Mesh standard has not been designed to support real-time communications over multi-hop mesh networks. To overcome this limitation, in [41] the authors describe the Multi-hop Real-time BLE (MRT-BLE): a real-time protocol built on top of the BLE standard, which is able to implement mesh networks with BLE devices. The proposed MRT-BLE protocol is connection-oriented, since the connection mode is generally characterized by a higher throughput with respect to a connection-less approach. Moreover, the connection-oriented model allows to employ 37 channels for hopping, instead of the 3 that are available in the connection-less one. The core feature of the proposed MRT-BLE protocol resides in the fact that the network is split into a set of sub-networks coordinated by a master node. Sub-networks are linked by a device that, acting as a bridge, behaves both as a slave and a master node. To show the feasibility of the proposed protocol the authors implement it on the X-NUCLEO-IDB05A1 devices produced by STMicroelectronics and provide a performance analysis. Figure 4: Examples of optional features within the Bluetooth Mesh standard and protocol stack. (**a**) Examples of optional features within the Bluetooth Mesh standard; (**b**) BLE protocol stack. As the SIG Bluetooth Mesh specifications is recent, in recent years chip vendors and researchers made efforts to develop mesh protocols for BLE devices. In 2014, a proprietary flood-based mesh protocol for BLE, denoted as CSRmesh, was released by CSR Inc. [32]. The CSRmesh protocol is highly scalable (support up to 65535 nodes) and has a measured propagation delay between nodes shorter than 15 ms. In [32], the packet delivery ratio of different setups using CSRmesh protocol is investigated. The presented results show that the flooding approach, even if simpler than route-based approaches, is not the optimal solution in traffic intensive applications. Another option to enable mesh networking with BLE devices is the nRF OpenMesh [42], a flooding mesh protocol for BLE, developed by Nordic Semiconductor Inc. nRF OpenMesh is based on the Trickle algorithm [43] that, considering the node density and value update frequency as metrics, is able to dynamically control the rebroadcasting behavior of mesh nodes. An Opportunistic Routing (OR)-based mesh protocol, denoted as BLEmesh, was also proposed in [30], before the release of the SIG standard. In the BLEmesh approach, a source node \(S\) broadcasts messages to a set of nodes, selected in an opportunistic way, among those which successfully received the packets. With respect to conventional routing protocols for wireless mesh networks, the OR-based ones requires to broadcast less packets to accomplish the transmission, which also results in a minor power consumption. As previously described, the Bluetooth Mesh standard uses the advertising communication mode and is based on flooding mechanisms--we remark that the vast majority of proprietary BLE mesh protocols make use of the advertising approach. Despite that, in [44] the authors try to compare the performance of flooding and connection-oriented networking considering the Packet Delivery Ratio (PDR), end-to-end latency, and power consumption as metrics. The conclusion is not definitive, as the obtained results show that none of the mesh BLE approaches can offer optimal performance for all metrics simultaneously. Therefore, the right choice is led by the specific application requirements. In [44], a hybrid BLE mesh approach, denoted as Bluetooth Now paradigm, is proposed, allowing able to use both advertising and connection modes. More in detail, in Bluetooth Now-based networks, nodes runs in connected mode to send periodic and non-time-sensitive data to the sink, while switching to flooding to deliver sporadic and urgent data.