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Control the home with a wireless network

Posted: 16 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:wireless? home nets? mesh? z-wave? zensys?

Recently, wireless home-control products such as light switches, thermostats, blinds/drapes and appliance controls have reached the market. To have a true mass-market for these products, it is important to have a low-cost technology that is easy to install and operate. This requires a lightweight system that, from the end-user or installer perspective, is easy to install and requires no ongoing network management. The network must be a self-organized mesh network that ensures error-free communication and, in the case of malfunction, uses self-healing mechanisms to re-establish a reliable network.

To support a full home-control system, the technology must support horizontal applications, enabling different product types from various vendors to communicate with each other and use each others' functionalities. To reach low cost points, the RF platform must be highly integrated, manufactured in low-cost processes and the associated software protocol must be lightweight.

From a product developer's perspective, it is important that the development and manufacturing of products based on the technology is simple. The physical modules must have a small form factor that enables easy integration into new and existing home-control products.

Main requirements

When designing home-control technology, three main requirements must be considered: ease-of-use, reliability and low cost.

When designing this technology for the mass-market, it's important that an average homeowner or a semi-skilled installer can easily install the system. The technology must provide simple intuitive installation that requires no network management by the user. Finally, the technology must support horizontal applications, enabling different product types from various vendors to seamlessly communicate with each other and use each other's features.

Robust and reliable RF communication is crucial to allow the home-control system to handle sensitive operations. For example, if the homeowner instructs the central door-locking application to lock and arm the alarm system, he must be assured that the instruction is registered and executed.

Furthermore, since RF operates on a shared medium and is sensitive to changes in the environment, protocol algorithms must be applied to make the RF link as reliable as a wired system. The implementation of this robustness includes features such as frame acknowledgment, collision avoidance, random back off algorithms, retransmission and routing to achieve reliable links and full home-network coverage.

To have a true mass-market technology, the physical wireless platform must be low-cost. The right trade-offs between technology choice and cost must be made without compromising the reliability of the network.

Control protocol

The home-control protocol must address the required network traffic pattern while supporting network flexibility, reliability and ease-of-use. A home-control network is characterized by relatively few nodes (20 to 200) within a 150m? to 600m? area where each node communicates relatively infrequently (e.g. every 5 to 15mins).

A typical communication consists of 4-6bytes of payload (i.e. turn on, set dim level, read temperature, read door status etc.). Also, most home-control applications have relaxed latency requirements of 200ms or more. The infrequent traffic, in conjunction with latency requirements, is served with a 9.6Kbps network bandwidth.

The network typically consists of a complex mix of AC-powered nodes, battery-operated nodes, fixed-position nodes and moving nodes. All nodes must communicate with each other seamlessly. The network behavior of these node types typically requires too many resources to support them all in one protocol stack. Z-Wave technology, in particular, supports the full range of nodes and potentially bridges them to other technologies.

In Z-Wave technology, nodes are divided into three fundamental typescontrollers, routing slaves and slavesbased on their communication behavior. All types work seamlessly together and can be mixed in any combination. Z-Wave supports moving battery-powered devices, such as handheld remotes and moving sensors, within each node type. For controller-node types, the portable controller protocol stack supports dynamic changes in position. For routing slave node types, the routing slave stack supports the rediscovery of moving nodes within the overall network topology. The controller node type contains self-organization management functionalities that simplify the network's installation and operation.

The home-control technology must handle battery-operated nodes with great power efficiency to provide 10 or more years of operation on two AAA batteries. Thus, it is important that the protocol provides an efficient wakeup sequence, such as powering up based on cyclic wakeup timers, transmitting the frame and returning to sleep mode.

In a medium-sized home, two nodes that must communicate may be beyond direct communication range. The system therefore needs to support a mesh network structure that enables the two nodes to use other nodes for routing. The mesh network also serves as the basis for self-healing functionalities. RF links vary over time due to their strong correlation to the physical environment. For example, when a door opens or closes, furniture moves, or there are simply many people moving about, RF links may fail because the environment is changing. In these situations, self-healing mechanisms will automatically reroute the message through other nodes until the message reaches the destination node.

Ease-of-use

A typical home-control network is installed and managed by the homeowner. This imposes a strong ease-of-use requirement on the network protocol. Four fundamental elements must be addressed from an ease-of-use viewpoint: easy installation, zero network management, self-organization and product interoperability.

The main challenge for easy network installation is to balance the requirements for easy network-joining and the requirements for easy identification of the installed devices. A number of different network-joining philosophies exist, ranging from full plug-and-play to manual processes with serial number typing. Most of these philosophies have shortcomings in real life due to the limited user interface on the typical home-control product with one or two actuators and indicators.

Full plug-and-play installation has severe identification problems in the installation process where many devices are installed at the same time. The manual process burdens the user with an input and/or validation process, which is impossible in many simple systems where the user interface is minimal.

The local installation is suited for small low-cost systems which are installed by the homeowner or an installer. The basic philosophy of the Z-Wave local installation process is that the user activates the node and the controller to install a new product. The activation can be simultaneous or skewed and it can be initiated once or for all new nodes, depending on the installation scenario. The new product sends out a request to join the network, which is acknowledged by a controller by assigning the node an ID. Finally the new node reports back its neighbor list to the controller enabling it to have full network-topology information.

The central installation is ideal for complex home-control systems with many different products and applications that are installed by a professional. The basic philosophy of the central installation process is that the Z-Wave technology enables any controller in the system to include new products in coordination with the SIS node. The SIS is typically implemented in a PC or equivalent intelligent device, allowing the installer to have full remote control and monitoring all steps in the process.

The central challenge in network management is the fact that the homeowner generally doesn't fully comprehend that the product he has installed is part of a mesh network. It's therefore important that there's no need for network management in a typical installation.

Interoperability, cost

A central challenge in product interoperability is to balance full interoperability with the vendor's requirement to be able to differentiate in the market. Furthermore, the interoperability requirement should reasonably match end-user expectations. The average user doesn't expect that all functionalities are identical in two products. However, he will expect that all basic functionality is the same or at least behaves logically.

Interoperability is the basis for creating complete home-control systems in which different applications from different vendors work together. Product interoperability requires standardization on two levels: command level, where all commands that can be transferred between nodes must be standardized; device level, where all products must be a member of a device class that defines which of the commands are mandatory, recommended and optional.

This structure allows products to be interoperable with their basic functionality. In Z-Wave, interoperability is guaranteed using the appropriate device class specification and by a certification program. The device class specification governs standardization on command and device level for all home-control products. The work is carried out in the Z-Wave Alliance ensuring that all relevant market inputs from Z-Wave partners are injected into the device classes.

To have a home-control hardware platform that is highly reliable, the platform must use leading-edge technologies throughout the design process, from initial chip design to final product design. This includes wafer technology, layout and assembly methodologies, and production test.

A minimal RF platform consists of an RF transceiver and front-end, a microprocessor, memory and a system.

Bringing the overall product cost down implies the ability to deploy the same RF module in a wide range of products. Using connectors or solder bumps adds to the overall cost. A good alternative is to implement castellation notches, which are plated indentations on the side of the PCB. Castellation notches are suitable for soldering the RF module to the application PCB in a standard reflow soldering process together with the other surface mounted components. The same module can be hand-soldered if required.

When developing and manufacturing a low-cost RF platform, testing the individual components and the RF module is key because testing time comprises a significant amount of the module's cost. Having a single chip reduces the die test to one die per module, and few external digital and RF components can be tested using simple test equipment. Comprehensive self-test circuitry can minimize wafer test time.

Niels Johansen, VP of R&D

Thomas Jorgensen, Hardware Manager of R&D

Zensys A/S




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