Smart home solutions based on SIG MESH

The core system of smart home is an intelligent detection and control system based on wireless communication transmission system, and wired communication technology is used as a partial supplement in some scenarios. Smart home, as the name suggests, is an intelligent home system. In recent years, technological progress and industry development have achieved a lot of success in intelligence, and many applications have also hit the pain points, such as smart lighting systems, smart energy management systems, security systems, etc.; but currently these systems are mostly run independently. The reason is that in addition to the fact that there are no unicorn-type companies in this field, a big reason is that there is no technical solution for smooth access and coexistence of multiple systems. The MESH V1.0 technology (hereinafter referred to as SIG MESH) launched by Bluetooth SIG at the end of 2017 is a technology that provides solutions to the current market situation. Regardless of whether this technology can be popularized in smart home systems in the future, its original intention is indeed so. This article will interpret the smart home solution provided by SIG MESH.

SIG MESH is divided according to the normal network hierarchy, from top to bottom, Model Layer, Foundation Model Layer, Access Layer, Upper Transport Layer, Lower Transport Layer, Network Layer, Bearer Layer. SIG MESH is still based on BLE technology, and the connection is completed through Bearer Layer and BLE core; the application interface provided by the top layer of SIG MESH is implemented through Model Layer. For different applications, SIG has models for reference, such as Light Model for lighting systems, and also provides expandable space. Users can define their own models to complete various applications. The network structure of SIG MESH is shown in Figure 1:

Figure 1: SIG MESH network hierarchy

As a complete MESH communication protocol, the most important part is the security mechanism, including the security authentication of devices joining the system and the security mechanism of network communication. The security part runs through the SIG MESH protocol. There are five main security algorithms, including an elliptic algorithm and four key generation and authentication algorithms.

The ellipse algorithm can be referred to section 5.4.3.1 of the MESH protocol, which is described as follows

Figure 2: Description of the ellipse algorithm

As an algorithm for authenticating the joined network devices in the SIG MESH protocol, the elliptic algorithm provides a good encryption algorithm. The elliptic algorithm is a medium-complexity algorithm that can be implemented by most MCUs. This is commendable, unlike Homekit, which has too high requirements for a series of encryption and decryption algorithms, and also requires Apple's own chips to cooperate, which undoubtedly greatly raises the entry threshold of Homekit. After all, consumers will still choose low-cost products under the premise of meeting their needs.

The four key encryption and decryption algorithms of the MESH protocol are all based on AES. As long as the encryption and decryption features of version 4.0 or above are supported, the four encryption and decryption algorithms required by the MESH protocol can be well supported.

Smart home systems cover many devices, including lighting, security, white appliances, curtains, door locks, etc. Can all these things be installed in one network? The existing technology cannot do this because different devices have different properties and the applications between different devices are also very different. The Bluetooth SIG MESH protocol is indeed very advanced and solves the problem of smart homes through two main features.

The first major feature of SIG MESH is to solve the coexistence of devices with different attributes in the same MESH network. MESH protocol is not essentially a low-power protocol. MESH node devices work in the listening state most of the time, which consumes a lot of power. Many smart home devices, such as door locks, sensor node devices, etc., are battery-powered and have relatively high requirements for low power consumption. In order to solve the coexistence of devices with different attributes, the MESH protocol has designed a "friend" node device attribute. The friend node is a node with low power requirements. It can be in sleep state most of the time. It communicates with other device nodes through "friendship". The sending and receiving information of the friend node can be saved in other devices. After the friend node enters the sending and receiving state, it can request cached received data from the node device it is bound to or send data to the device node. Figure 3 shows a typical SIG MESH network topology with low-power node devices.

Figure 3: MESH network topology with low-power nodes

In the network shown in FIG3 , I, J, K, L, and M are all low-power devices. They receive, send, and cache data through O devices and P devices. O devices and P devices are node devices that support friendship in the MESH network.

The second main feature of SIG MESH is that data from multiple MESH network devices can be forwarded to each other and the independence and reliability of the networks can be guaranteed. There are several different networks in a smart home, such as smart lighting systems, air quality detection systems, etc. These networks have their own characteristics, but due to the wireless communication technology used by these node devices and the limitations of node locations, nodes may not be able to communicate directly.

Consider a smart home system shown in Figure 4. The system includes 16 lighting devices distributed in the bedroom, bathroom, living room and dining room. These lights can be controlled individually and combined to achieve various scenarios; including 4 air quality detectors (A1~A4) distributed in four locations to monitor indoor air quality; including 4 curtain controllers (C1~C4) to control the opening and closing of four windows according to indoor air quality; including 1 home data concentrator (D1) to record the working status of all devices. If these expectations are to be achieved, several problems must be solved:

1. The lighting devices need to form a network so that they can communicate with each other and be controlled in groups;

2. The air quality detector needs to communicate with the home data concentrator to collect air quality information to the data concentrator, that is, A1~A4 needs to communicate with D1;

3. The data concentrator needs to send commands to the curtain controller for control, that is, D1 needs to communicate with C1~C4.

Figure 4: Smart home system

Problem 1 is relatively easy to solve. The current intelligent lighting system can solve the problem of single control and group control of lights through MESH technology. Problem 2 is more difficult at present because A1~A4 are scattered everywhere. If they communicate directly with D1 devices, it will put forward high requirements on the wireless communication performance and low power consumption performance of A1~A4 devices. However, if A devices can communicate with lights, and then communicate with D1 through the MESH network of lights, it will be much simpler. Problem 3 faces the same problem as 2, and the solution is also the same. The MESH network of lights can perfectly solve the problem. Then there is the system solution shown in the figure below:

A3 uses light 5 to complete the communication with D1;

D1 completes the communication with C3 with the help of lamp 1;

A1 uses lights 16, 13, 12, 3 to complete the communication with D1;

C1 communicates with D1 via lights 16, 13, 12, and 3.

This network data transmission mode can be implemented by the data flow shown in FIG5 .

Figure 5: Smart home system MESH network data flow

So how to implement the above system in the MESH V1.0 protocol? We must go back to the MESH network layering shown in Figure 1. This network structure provides upper-layer interfaces for multiple applications, while the bottom layer provides the ability to forward different network data packets.

The Model layer is the top user interface, which can access models of various devices, including the official Bluetooth model and user-defined models. Different models correspond to different behaviors. In the system of Figure 5, there are three different devices at the same time, so three different models can be defined, including the light model, the air detector model, and the curtain controller model. In addition to their different behaviors, these three models can also set their own encryption keys, so that data packets of different devices can borrow other devices to forward data while ensuring the independence and security of the network where the devices are located.

According to the various device requirements of the smart home system, relevant configurations at other layers (Foundation Model Layer, AccessLayer, etc.) can complete the functional support for the various data packet forwarding required by the Model Layer.