Colin Faulkner, NXP Semiconductors
Electric lighting has been around since the first commercial incandescent lamps were sold in 1880. Since then, we have seen little widely adopted innovation - the introduction of the fluorescent in 1938, dimmer switches and a certain amount of simple networked control systems such as DALI for commercial lighting. These systems have been limited by the control capabilities of the lamps themselves, cost of installation and the lack of a key driver to initiate widespread innovation. However, we now have a combination of key factors that make truly "smart" lighting systems a practical reality. From an economic perspective, lighting represents one of the largest opportunities for saving energy and reducing C02 emissions. With more than 12 billion bulbs sold each year, lighting can represent 25% of energy usage in the home. With Governments and utility companies recognizing the need for energy reduction and with many potential solutions such as upgrading to smarter grid infrastructure taking may years to implement, one of the quickest ways to reduce energy is through improved efficiency. For example, a 30% saving in street lighting energy for a city the size of Paris could save 13,000 tons of CO2 emissions and about $2.5M per year in cost. Improved controls can also lead to a significant enhancement of the user experience. A truly smart lighting system can be programmed to different "scenes" to suit the occupant's activity - when watching TV, any lamps close to the TV can be turned off and the rest of the room dimmed to suit requirements, when going to sleep, a single button press can switch off all the lights in the house. There are several reasons why the timing is now right for systems of this complexity to be realized. There are now standards that exist that can provide a control system for the whole of the house - ZigBee LightLink and JenNet-IP are described in this paper and broadly conform to the architecture shown in Figure 1 below. The rapid advances in LED lamp technology now provide acceptable light quality in a package that have an operating life of up to 50,000 hours. With no regular replacement cycle now needed, it becomes cost effective to include a wireless control chip in each lamp. Contributing to this is the inexorable downward trend of chip pricing, such that a single chip wireless control device is approaching the sub $1 price point. The availability of the core technology is just one contributor. Any sophisticated control system requires a fairly flexible and powerful user interface to allow the user to set up the scenes he requires. This is readily available in most households now through the Smartphone. This provides a powerful remote control capability with the ability to use easy-to-use graphical techniques to program and control the lighting systems. These three factors between them enable true smart lighting systems to be developed now. Requirements and Architecture There are a number of top-level requirements that have to be met by any domestic lighting control system. Ease of use is perhaps the most important - consumers like the convenience of a simple switch, so anything that adds more complexity into the system must be presented in an easy to use, intuitive fashion. Using remote controls or switches for very simple functionality and Smartphones for more complex functions helps to manage the complexity of operation. One of the most important performance parameters is the latency between requesting an action and it happening. Consumers expect their lighting to respond almost instantaneously. There are 3 key elements to the functionality that needs to be provided to meet these requirements; Commissioning, Networking and Control. Commissioning allows devices to join the network easily but also securely. Consumers could easily get very confused if the lamp they have just installed joins their neighbors network! The commissioning process must also allow for the easy manipulation of lamps into groups and scenes. Networking is key to ensuring that the system works reliably. There are likely to be many 10s of lamps in a typical house, each of which will form a node in the network. Combining these in a network means that the communications between lamps is always being managed in the background. For example, if one of the lamps fails or becomes disconnected, the networking stack will ensure that connections to all the other devices in the network are maintained. Control functionality needs to be able to form groups of lamps that can all be controlled together, to provide ways of setting up scenes like watching TV, like the example discussed earlier, and then to enable on/off, dimming and perhaps color control for all lamps. In order to react to the networking and control systems, a smart lamp is obviously more complex than a standard lamp. Many of the modern lamp technologies such as Compact Fluorescent or LED already use silicon chips to provide the drive and dimming capability. The truly smart light simply adds a wireless microcontroller and, perhaps a separate power supply, a relatively small cost overhead compared with the standard lamp. Now let us examine two systems for providing this smart lighting functionality. ZigBee LightLink In ZigBee LightLink, based on the well-known ZigBee mesh-networking stack, all individual lamps act as repeaters. Messages destined for a distant part of the network can discover a route by hopping from lamp to lamp until they reach the desired destination. If an individual lamp fails, then a new route would be discovered using a different combination of lamps. This approach works well for lighting systems where there is usually a fairly high density of nodes. LightLink provides two main innovations compared with other ZigBee profiles. Firstly it avoids the need for a dedicated coordinator by using a remote control device to establish the network and manage the security and uses a simple system of commissioning, known as "Touchlinking". Touchlinking involves bringing the remote control close to the lamp being commissioned. The remote control sends out a scan request and the lamp responds. Multiple lamps could respond to this command, but the remote control will select only the one with the highest signal strength. Once the identity of the lamp being commissioned is established, the remote control can share the network security key and accept it into the network. It can also assign the lamp into one or more groups to enable the setting of scenes. Once multiple lamps are commissioned into the network and are members of groups, they need to be individually controlled to allow the setting of scenes. In order to do this, it is necessary to be able to address lamps individually to set them to their role in the scene. This is done by means of an "identify" command, which enables the remote control to toggle through all the attached devices to find the appropriate one. It can then be set to the desired color or dim level. By working through a number of lamps in this way, a scene can be built up. The individual properties of the lamps are defined using standard ZigBee "Clusters". These include properties such as on/off, level control, color control etc. By setting these values, the lamps are controlled by the remote control. Other devices available in the system include sensors which allow lighting to be automatically controlled, for example by an occupancy sensor and a control bridge which enables control of the system to be exerted from outside the LightLink network, for example from a WiFi connected Smartphone.
JenNet-IP In many respects, the operation of JenNet-IP is very similar. The major difference is that JenNet-IP uses Internet Protocol (v6) packets throughout the entire system. This means that every device on the network has it's own IPv6 address and can, in principle, be directly controlled from anywhere on the Internet. In order to realize this capability, JenNet-IP uses a number of protocols from the Internet Engineering Task Force (IETF). These are standardized worldwide and are highly familiar to the engineering community. The network stack architecture is described in Figure 2 below. JenNet-IP uses the same IEEE802.15.4 MAC and PHY layers as ZigBee. The networking layer uses a robust tree network layer known as JenNet. In order to maintain the robustness of the system, this layer includes a self repair mechanism. If an individual routing node breaks or loses power, any nodes below it in the tree will try and rejoin the network, restoring communications. The 6LoWPAN layer implements an IETF standard for enabling IPv6 packets to fit into the maximum 128 bytes length of the IEEE802.15.4 standard. It does this in two ways; firstly by providing a means to fragment any long packets into a number of segments which can be sent individually across the network and reassembled at the destination and secondly by providing compression of the IP addresses. Standard IP and UDP provide the transport layers such that the wireless system looks like a normal UDP interface to the outside world. One of the benefits of this approach is that an IP packet destined for a particular IP address will always make its way there without the need for translation into a different format. Provided the end device knows how to interpret the packet, it will result in the desired action. What is missing from this picture so far is the application layer; the mechanism that defines exactly how a device will behave when it receives a packet. The IETF have defined a structure called Simple Network Management Protocol (SNMP) but this is too unwieldy for use on simple devices with limited resources. JenNet-IP takes the same basic principle of SNMP and adapts it for use with low power wireless nodes. The basic approach is to use a technique known as Management Information Base (MIB). This defines device characteristics in a number of small database entries, which can then be interrogated or modified using simple self-explanatory commands like "SET" and "GET". This is a powerful technique that allows complex behaviors to be built up for any type of device. Examples for a lamp will include "ON/OFF", "DIM LEVEL", "GROUP MEMBERSHIP" etc. Using this approach, it is possible for a remote internet connected controller such as a Smartphone to send the relevant "SET" commands directly to a lamp. Devices within the network such as remote controls or light switches can also be added to provide local control. Groups and scenes also have addresses assigned to them in the same way as individual lamps. Lamps know which groups they are members of and the characteristics to set up in scenes and simply respond to those addresses as well as their own individual ones. In this way, JenNet-IP can set up similar lighting control functions to ZigBee LightLink, and indeed, can be easily expanded to cover the wider home automation or building automation functionality using the same basic approach. To summarize, Smart Lighting is a reality right now with the advent of low cost silicon, long life LED lamps and smart controllers in the form of smart phones. There has been a huge amount of effort invested in the development of new wireless standards such as ZigBee LightLink and JenNet-IP that will allow consumers to experience new control functionality and reduced energy bills. With these capabilities in place, the future of Smart Lighting is starting right now. NXP Semiconductor