When developing comprehensive business case models for advanced metering infrastructure (AMI) systems, utilities require that technology supporting two-way, ‘always on’ connectivity is provided, that the system must extend beyond just metering endpoints to include utility specific devices – such as whole house disconnect and transformer monitoring – and that it allows access to home area network (HAN) devices. Utilities have also identified the benefits of open technology and are always consciously trying to use existing or planned infrastructure such as Internet Protocol (IP)-based backbone networks across multiple applications.
As detailed below, the new open standards IEEE 802.15.4 and ZigBee, which provide a robust wireless meshing-capable environment, can serve as an effective component in systems for AMI. The wide variety of previously selected systems range from adaptations of proprietary networks initially optimised for meter reading or other purposes, to implementations that are principally IP, often coupled with an Internet service deployment. Many systems are hybrids, for example including an IP backbone for meter data back-haul, and there are significant efforts to include or adopt ‘open’ protocols.
Generally speaking, ZigBee is a specification for a suite of high level application and communication protocols using small, low power, digital radios based on the IEEE 802.15.4 standard for wireless personal area networks (WPANs). The ZigBee Alliance, the official industry working group, has a membership of over 200 companies developing products around the ZigBee specification. ZigBee protocols are intended for use in embedded applications requiring low data rates and/or low power consumption. As applied to AMI, ZigBee offers potential for two-way access to metering points – electric,
water and gas – and the possibility of utility connection to more consumer oriented products such as thermostats, in-home displays and appliance
Technical details of ZigBee applications, networks and IEEE 802.15.4
The ZigBee Alliance was created to enable a reliable wireless network that is simple to implement and use, allowing cost effective and low power products based on an open global standard optimised for monitoring and control products. To achieve this goal the Alliance leverages the powerful IEEE
802.15.4 physical radio open standard as the basis of the wireless
communication link. ZigBee first expands on this by defining the upper level network and security levels and then adds software application layers. The result, which is represented in Figure 1, where individual applications can
choose to be ‘public’ or ‘private,’ is described below.
IEEE 802.15.4 MAC and PH Y radio level
The IEEE 802.15.4 standard defines a simple but powerfulwireless protocol and specifies the low level physical (PHY) and medium access control (MAC) networking layers. The specifications for the PHY encapsulate how to establish the RF link and how to assess the quality of the received signal.
The packet size – 128 bytes – and addressing scheme for the network nodes are also defined. The MAC layer further defines how nodes transmit over the channel, especially collision avoidance related to more than one node trying to communicate simultaneously.
IEEE 802.15.4 radios operate in unlicensed bands worldwide at 2.4 GHz (global), 915 Mhz (Americas) and 868 Mhz (Europe). Raw data throughput rates of 250 kb can be achieved at 2.4 GHz (16 channels), 40 kb at 915 Mhz (10channels) and 20 kb at 868 Mhz. The IEEE 802.15.4 radios are designed to coexist with other equipment in the 2.4 GHz spectrum as its 16 channels do not overlap with IEEE 802.11. x (WiFi) devices. Due to its global acceptance, the 2.4 GHz frequency has become the most widely utilised technology by ZigBee IC chip vendors.
Advanced network control and organisation of ZigBee
The ZigBee specification utilises the IEEE 802.15.4 standard and adds advanced logical network capabilities, security, and application support. The ZigBee network handles star, mesh and cluster tree network topologies enabling robust large area wireless network coverage. There are three types of ZigBee devices, ZigBee coordinator (ZC), ZigBee router (ZR), and ZigBee end device (ZED). The ZC is responsible for forming a ZigBee network and handing security. The ZC can also act as a trust centre and can determine what nodes are allowed to join its network. Both ZC and ZR devices should be line powered as they function as routers to relay messages to other devices on the network, and are considered full function ZigBee devices. A ZED, also known as a reduced function device, is the simplest type of ZigBee node and is typically a very simple or battery powered device. A ZED is not responsible for routing messages, and can only communicate with a parent ZC or ZR node, so it can sleep to extend battery life.
To form a ZigBee network the ZC first conducts an energy scan of all channels and selects the channel with the least RF interference. It then selects a unique identifier (PAN ID) not in use by any other networks in range. The coordinator announces that a network is available and ZR devices are then able to attempt to join this newly formed network. ZigBee also has provisions which allow the network to change its operating channel should unacceptable RF interference occur.
ZigBee mesh networking
Mesh networking has recently received significant attention as a configuration that has especially reliable wireless links, even in harsh RF environments. The mesh also affords the ability for devices to communicate over a much greater range than their individual radios by relaying messages to each other. A ZigBee mesh network consists of one coordinator device and many router devices creating a completely self-forming and selfhealing WPAN. The meshing capabilities mean that messages can pass from one router to another through multiple paths, and if a particular node becomes unavailable network traffic will be automatically routed around it. Each router is capable of determining the best next hop to relay messages to a destination other than its own. This routing table is dynamic, continuously adapting to changing network conditions and ensuring that all messages will reach their intended destination. ZigBee also implements full message acknowledgement and automatic retries to further improve network reliability.
Applications and devices under ZigBee
A ZigBee application profile is a set of definitions for a common set of devices, which allows for interoperable equipment. Within the ZigBee specification there are provisions for specific public application profiles, but applications with private profiles are also possible. Each profile defines the specific rules for forming and joining a network, including security level, types of devices supported, and network topology. By designing an appropriate profile and making it public, it becomes possible for devices to ‘interoperate,’ that is to cooperate on the same network. The initial release of the ZigBee specification 1.0 defines a Home Controls Lighting profile. Additional profiles such as Home Automation, Commercial Building Automation, and Industrial Plant Monitoring are under development.
As an alternative, ZigBee adopters can create private application profiles to meet the needs of their specific products or to provide a development platform. These products and profiles can provide guidance to evolve a public AMR profile. Further, since ZigBee thermostats and appliance control devices are currently available, with proper cooperation connection to load management components is within easy reach.
IN THE FLESH
Standards require implementations and tools, and ZigBee has moved to this point with multiple vendors providing silicon and full featured software development tools. At least two such companies provide multiple configurations of the hardware, which is appropriate given the variety of devices that are expected, from extremely simple light switches to full featured home automation systems. Commercially available in-home devices are currently being sold in markets that are not specific to AMI. As is typical of such full featured standards, reference designs are provided to speed product development, and standard modules are provided in addition to component level support. Additional information and resources are listed at www.zigbee.org
HOW ZIGBEE COMES TOGETHER FOR AMI
As shown in Figure 2 one basic approach has a mesh of ZigBee electric meters connecting to a coordinator that has a back-haul, for example an IP-enabled device. The relationship between coordinators and nodes is a balance between coverage, cost and reliability. While mesh networks, including ZigBee, may have large numbers of nodes (theoretically 65,000 for ZigBee), issues of redundancy and latency speak to having smaller ‘cells’ with a moderate number of coordinators. The basic mesh can then be expanded with battery-powered nodes for gas and water metering, and by connection to in-home devices.
Figures 3a and 3b show implementations of an electric meter and an IP-based coordinator establishing that the hardware component of ZigBee is appropriate for metering. Since the coordinator can be a more powerful device, it can, for example, provide ‘data mirroring’ to reduce latency, and could in fact be a device that also provides additional utility application features, such as line monitoring.
WHY OPEN? WHY ZIGBEE?
Given an almost over-abundance of solutions in the metering community, what does ZigBee offer?
In the most general terms, open or commonly accepted standards have the potential of producing value for customers by allowing the work of many companies to contribute to a common goal, and for the economies of scale to drive prices lower with the more widespread use of the technology. Since standards must appeal to a large group and cover a broad spectrum of applications, initial efforts to produce a standard can be slow. ZigBee has the advantage of being deep into the development and acceptance process, with the ‘incubation period’ close to completion.
Specifically, utilities developing business cases for AMI will benefit from the adoption of standards-based technology in the following ways:
- Flexibility, especially of vendor selection due to interoperability
- Decreased cost provided by larger economies of scale
- Product quality improvements, again amplified because of the contributions from many application areas instead of one.
Perhaps of equal importance, for situations where wireless (mesh) communication is implemented the adoption of a standard system for the radio and interoperability allows the AMI professional to concentrate effort on the ‘application’ end of the problem, crafting the most effective solutions for
the industry. Of course, open or proprietary, any system has to prove its
mettle by successful implementations and deployments. ZigBee and 802.15.4 are in a position for this challenge.