Zigbee

Zigbee

                                         

INTRODUCTION

The field of wireless communications has been in existence since the first humans learned to communicate. In early days of civilization humans would transmit notices of important events, such as enemy invasions or royal births, through the sounding of horns or the lighting of fires. While simple messages could be effectively transmitted in this manner, in order to communicate over long distances the manpower expense was great, since watchtowers had to be built within sight of each other and continually manned, and the number of messages was small.  It was not until the 1800’s that wireless communications became what we know it as today. Now we are able to use radio frequencies to communicate information over long distances (think of the Cassini mission to Saturn), we can send voice or video at rates of more than hundreds of megabits per second, and the associated technology has become so inexpensive that many people are able to afford a mobile phone in order to be in constant contact with others.

ZigBee is an established set of specifications for wireless personal area networking (WPAN), i.e.  digital radio connections between computers and related devices. WPAN Low Rate or ZigBee provides specifications for devices that have low data rates, consume very low power and are thus characterized by long battery life. ZigBee makes possible completely networked homes where all devices are able to communicate and be controlled by a single unit. The ZigBee Alliance, the standards body which defines ZigBee, also publishes application profiles that allow multiple OEM vendors to create interoperable products. The current list of application profiles either published or in the works are:

• Home Automation 

• ZigBee Smart Energy 

• Telecommunication Applications 

• Personal Home 

The relationship between IEEE 802.14.4 and  ZigBee is similar to that between IEEE 802.11 and  the Wi-Fi Alliance. For non-commercial purposes, the ZigBee specification is available free to the general public. An entry level membership in the ZigBee Alliance, called Adopter, costs US$ 3500 annually and provides  access to the as-yet unpublished specifications and permission to create products for market using the specifications. ZigBee is one of the global standards of communication protocol formulated by the relevant task force under the IEEE 802.14 working group. The fourth in the series, WPAN Low Rate/ZigBee is the newest and provides specifications for devices that have low data rates, consume very low power and are thus characterized by long battery life. Other standards like Bluetooth and IrDA address high data rate applications such as voice, video and LAN communications. ZigBee devices are actively limited to a throughrate of 250Kbps, compared to Bluetooth's much larger pipeline of 1Mbps, operating on the 2.4 GHz ISM band, which is available throughout most of the world.In the consumer market ZigBee is being explored for everything from linking low-power household devices such as smoke alarms to a central housing control unit, to centralized light controls.

The specified maximum range of operation for ZigBee devices is 250 feet (76m), substantially further than that used by Bluetooth capable devices, although security concerns raised over "sniping" Bluetooth devices remotely, may prove to hold true for ZigBee devices as well. Due to its low power output, ZigBee devices can sustain themselves on a small battery for many months, or even years, making them ideal for install-and-forget purposes, such as most small household systems. Predictions of ZigBee installation for the future, most based on the explosive use of ZigBee in automated household tasks in China, look to a near future when upwards of sixty ZigBee devices may be found in an average American home, all communicating with one another freely and regulating common tasks seamlessly. The ZigBee Alliance has been set up as “an association of companies working together to enable reliable, cost-effective, low-power, wirelessly networked, monitoring and control products based on an open global standard”. Once a manufacturer enrolls in this Alliance for a fee, he can have access to the standard and implement it in his products in the form of ZigBee chipsets that would be built into the end devices. Philips, Motorola, Intel, HP are all members of the Alliance . The goal is “to provide the consumer with ultimate flexibility, mobility, and ease of use by building wireless intelligence and capabilities into every day devices. ZigBee technology will be embedded in a wide range of products and applications across consumer, commercial, industrial and government markets worldwide. For the first time, companies will have a standards based wireless platform optimized for the unique needs of remote monitoring and control applications, including simplicity, reliability, low-cost and low-power”.

The target networks encompass a wide range of devices with low data rates in the Industrial, Scientific and Medical (ISM) radio bands, with building-automation controls like intruder/fire alarms, thermostats and remote (wireless) switches, video/audio remote controls likely to be the most popular applications. So far sensor and control devices have been marketed as proprietary items for want of a standard. With acceptance and implementation of ZigBee, interoperability will be enabled in multi-purpose, self-organizing mesh networks.

History:

ZigBee-style networks began to be conceived around 1999, when many installers realized that both Wi-Fi and Bluetooth were going to be unsuitable for many applications. In particular, many engineers saw a need for self-organizing ad-hoc digital radio networks.The real need for mesh has been cast in doubt since that, in particular as mesh is largely absent in the market.

ZigBee 2007 is fully backward compatible with ZigBee 2006 devices: A ZigBee 2007 device may join and operate on a ZigBee 2006 network and vice versa. Due to differences in routing options, ZigBee PRO devices must become non-routing ZigBee End-Devices (ZEDs) on a ZigBee 2006 network, the same as for ZigBee 2006 devices on a ZigBee 2007 network must become ZEDs on a ZigBee PRO network. The applications running on those devices work the same, regardless of the stack profile beneath them.

The ZigBee 1.0 specification was ratified on 14 December 2004 and is available to members of the ZigBee Alliance. Most recently, the ZigBee 2007 specification was posted on 30 October 2007. The first ZigBee Application Profile, Home Automation, was announced 2 November 2007.

ZIGBEE OVERVIEW:

The name ZigBee is said to come from the domestic honeybee which uses a zig-zag type of dance to communicate important information to other hive members. This communication dance (the "ZigBee Principle") is what engineers are trying to emulate with this protocol  a bunch of separate and simple organisms that join together to tackle complex tasks.

ZigBee is a low-power wireless communications technology and international standard protocol for the next-generation wireless network, reducing data size and allowing for lower-cost network construction with simplified protocol and limited functionality. ZigBee uses the PHY and MAC layers defined by IEEE® 802.14.4, which is the short-distance wireless communication standard for 2.4 GHz band. ZigBee comprises the ZigBee platform specifications and ZigBee profiles defined by the ZigBee Alliance.

Network topologies

IEEE 802.14.4 can manage two types of networks, i.e., star topology or the peer-topeer topology. Both the topologies are illustrated in Figure 2.1. In ZigBee, these two topologies can be combined to build so-called mesh networks.

Star network formation

The first FFD that is activated may establish its own network and become a Personal Area Network (PAN) coordinator. Then both FFD and RFD devices can connect to the PAN coordinator. All networks within the radio sphere of influence must have a unique PAN identity. All nodes in a PAN must talk to the PAN Coordinator.

Peer-to-Peer network formation

In the peer-to-peer topology there is also a PAN coordinator, but it differs from the star topology in that any device can communicate with any other device as long as they are in the range of one another. The peer-to-peer topology allows more complex network formations to be implemented, such as the mesh topology.

ZIGBEE KEY FEATURES:

1. Low Power-The benefits of simple, cost-effective, low-power wireless connectivity that ZigBee technology provides address a variety of markets, including industrial and home monitoring, control and automation, as well as health care diagnostics. Free scale provides all the building blocks used in a complete ZigBee-compliant platform solution: the RF transceiver, MAC and ZigBee software, microcontrollers and sensors. The development hardware and reference designs provide developers with the tools they need to easily and quickly implement these building blocks. One solution, one provider—built, tested, compatible and ready for integration.

2.  Robust-802.14.4 provides a robust foundation for ZigBee, ensuring a reliable solution in noisy environments. Features such as energy detection, clear channel assessment and channel selection help the device pick the best possible channel, avoiding other wireless networks such as Wi-Fi®. Message acknowledgement helps to ensure that the data was delivered to its destination. Finally, multiple levels of security ensure that the network and data remain intact and secure.

3. Mesh Networking-The ability to cover large areas with routers is one of the key features of ZigBee that helps differentiate itself from other technologies. Mesh networking can extend the range of the network through routing, while self healing increases the reliability of the network by re-routing a message in case of a node failure

4. Interoperability-The ZigBee Alliance helps ensure interoperability between vendors by creating testing and certification programs for ZigBee devices.  Users can be assured the devices that go through certification testing and use the ZigBee logo will work with other devices based on the same applications.  This provides end customers with the customers with peace of mind while creating brand awareness of products with the ZigBee logo.

ZIGBEE CHARACTERISTICS

The focus of network applications under the IEEE 802.14.4 / ZigBee standard include the features of low power consumption, needed for only two major modes (Tx/Rx or Sleep), high density of nodes per network, low costs and simple implementation.

These features are enabled by the following characteristics,

• 2.4GHz and 868/915 MHz dual PHY modes. This represents three license-free bands: 2.4-2.4835 GHz, 868-870 MHz and 902-928 MHz. The number of channels allotted to each frequency band is fixed at sixteen (numbered 11-26), one (numbered 0) and ten (numbered 1-10) respectively. The higher frequency band is applicable worldwide, and the lower band in the areas of North America, Europe, Australia and New Zealand .

• Low power consumption, with battery life ranging from months to years. Considering the number of devices with remotes in use at present, it is easy to see that more numbers of batteries need to be provisioned every so often, entailing regular (as well as timely), recurring expenditure. In the ZigBee standard, longer battery life is achievable by either of two means: continuous network connection and slow but sure battery drain, or intermittent connection and even slower battery drain.

•Maximum data rates allowed for each of these frequency bands are fixed as 250 kbps @2.4 GHz, 40 kbps @ 915 MHz, and 20 kbps @868 MHz.

•High throughput and low latency for low duty cycle applications (<0.1%)

•Channel access using Carrier Sense Multiple Access with Collision Avoidance (CSMA - CA)

•Addressing space of up to 64 bit IEEE address devices, 65,535 networks.

•50m typical range 

•Fully reliable “hand-shaked” data transfer protocol. 

•Different topologies as illustrated below: star, peer-to-peer, mesh .

TRAFFIC TYPES:

ZigBee/IEEE 802.14.4 addresses three typical traffic types. IEEE 802.14.4 MAC can accommodate all the types.

1.Data is periodic. The application dictates the rate, and the sensor activates, checks for data and deactivates.

2.Data is intermittent. The application, or other stimulus, determines the rate, as in the case of say smoke detectors. The device needs to connect to the network only when communication is necessitated. This type enables optimum saving on energy.

3.Data is repetitive, and the rate is fixed a priori. Depending on allotted time slots, called GTS (Guaranteed time slot), devices operate for fixed durations.

ZigBee employs either of two modes, beacon or non-beacon to enable the to-and-fro data traffic. Beacon mode is used when the coordinator runs on batteries and thus offers maximum power savings, whereas the non-beacon mode finds favour when the coordinator is mains-powered. 

In the beacon mode, a device watches out for the coordinator's beacon that gets transmitted at periodically, locks on and looks for messages addressed to it. If message transmission is complete, the coordinator dictates a schedule for the next beacon so that the device ‘goes to sleep'; in fact, the coordinator itself switches to sleep mode.  While using the beacon mode, all the devices in a mesh network know when to communicate with each other. In this mode, necessarily, the timing circuits have to be quite accurate, or wake up sooner to be sure not to miss the beacon. This in turn means an increase in power consumption by the coordinator's receiver, entailing an optimal increase in costs.

The non-beacon mode will be included in a system where devices are ‘asleep' nearly always, as in smoke detectors and burglar alarms. The devices wake up and confirm their continued presence in the network at random intervals.  On detection of activity, the sensors ‘spring to attention', as it were, and transmit to the everwaiting coordinator's receiver (since it is mainspowered). However, there is the remotest of chances that a sensor finds the channel busy, in which case the receiver unfortunately would ‘miss a call'. 

   

ARCHITECHTURE:

ZigBee stack architecture follows the standard Open Systems Interconnection (OSI) reference model, ZigBee's protocol stack is structured in layers. The first two layers, physical (PHY) and media access (MAC), are defined by the IEEE 802.14.4 standard.

The layers above them are defined by the ZigBee Alliance.           

        The model has five layers namely

•           Physical (PHY) layer                          

•           Media access control (MAC) layer

•           Network (NWK) and security layers  

•           Application framework

•           Application profiles

PHYSICAL LAYER:

ZigBee-compliant products operate in unlicensed bands worldwide, including 2.4GHz (global), 902 to 928MHz (Americas), and 868MHz (Europe). Raw data throughput rates of 250Kbps can be achieved at 2.4GHz (16 channels), 40Kbps at 915MHz (10 channels), and 20Kbps at 868MHz (1 channel). The transmission distance is expected to range from 10 to 75m, depending on power output and environmental characteristics. Like Wi-Fi, Zigbee uses direct-sequence spread spectrum in the 2.4GHz band, with offset-quardrature phase-shift keying modulation. Channel width is 2MHz with 5MHzchannel spacing. The 868 and 900MHz bands also use direct-sequence spread spectrum but with binary-phase-shift keying modulation.

MEDIA ACCESS CONTROL LAYER:

The media access control (MAC) layer was designed to allow multiple topologies without       complexity. The power management operation doesn't require multiple modes of operation. The MAC allows a reduced functionality device (RFD) that needn't have flash nor large amounts of ROM or RAM. The MAC was designed to handle large numbers of devices without requiring them to be "parked".

The MAC provides network association and disassociation, has an optional superframe structure with beacons for time synchronization, and a guaranteed time-slot mechanism for high-priority communications.

Frame structure: Figure illustrates the four basic frame types defined in 802.14.4: data, ACK, MAC command, and beacon.

The data frame provides a payload of up to 104 bytes. The frame is numbered to ensure that all packets are tracked. A frame-check sequence ensures that packets are received without error. This frame structure improves reliability in difficult conditions. Another important structure for 802.14.4 is the acknowledgment (ACK) frame. It provides feedback from the receiver to the sender confirming that the packet was received without error. The device takes advantage of specified "quiet time" between frames to send a short packet immediately after the data-packet transmission.

A MAC command frame provides the mechanism for remote control and configuration of client nodes. A centralized network manager uses MAC to configure individual clients' command frames no matter how large the network. Finally, the beacon frame wakes up client devices, which listen for their address and go back to sleep if they don't receive it. Beacons are important for mesh and cluster-tree networks to keep all the nodes synchronized without requiring those nodes to consume precious battery energy by listening for long periods of time.

NETWORK LAYER

Zigbee’s self-forming and self-healing mesh-network architecture lets data and control messages pass from one node to another by multiple paths. This feature extends the network range and improves data reliability. It may also be used to build large, geographically dispersed networks with smaller networks linked to form a 'cluster-tree' network.  

The NWK layer supports multiple network topologies including star, cluster tree, and mesh, all of which are shown in Figure

In a star topology, one of the FFD-type devices assumes the role of network coordinator and is responsible for initiating and maintaining the devices on the network. All other devices, known as end devices, directly communicate with the coordinator. In a mesh topology, the ZigBee coordinator is responsible for starting the network and for choosing key network parameters, but the network may be extended through the use of ZigBee routers. The routing algorithm uses a request-response protocol to eliminate sub-optimal routing. Ultimate network size can reach 264 nodes (more than we'll probably need). Using local addressing, you can configure simple networks of more than 65,000 (216) nodes, thereby reducing address overhead.

SECURITY LAYER:

Security and data integrity are key benefits of the ZigBee technology. ZigBee leverages the security model of the IEEE 802.14.4 MAC sub-layer which specifies four security services: access control—the device maintains a list of trusted devices within the network

•           data encryption, which uses symmetric key 128-bit advanced encryption standard

•           frame integrity to protect data from being modified by parties without cryptographic keys

•           sequential freshness to reject data frames that have been replayed—the network controller compares the freshness value with the last known value from the device and rejects it if the freshness value has not been updated to a new value.

COMPARISION BETWEEN THE WIRELESS STANDARDS

APPLICATIONS

            ZigBee is well suited for a wide range of control uses in just about any market.  The Alliance has focused its standards development efforts around the commercial, residential, energy, consumer and industrial sectors.  It has developed global standards for energy management and efficiency, home and building automation, health care and fitness, telecom and consumer electronics. Here are just a few examples of what our standards control:

• Demand Response
• Advanced Metering Infrastructure
• Automatic Meter Reading
• Lighting controls
• HVAC control
• Heating control
• Wireless smoke and CO detectors
• Home security
• Blind, drapery and shade controls
• Medical sensing and monitoring
• Remote control of home entertainment systems
• Indoor location sensing
• Advertising on mobile devices

CONCLUSION:

There are many wireless monitoring and control applications for industrial and home markets which require longer battery life, lower data rates and less complexity than available from existing wireless standards like Bluetooth and Wi-Fi. So, there was a need for a standard based, interoperable wireless technology that addresses the unique needs of low data rate wireless control and sensor based networks. In this regard, zigbee was poised to become the global control/sensor network standard.

Zigbee promises to put wireless sensors in everything from factory automation systems to home security systems to consumer electronics. Zigbee is a new standard that still needs to pass through the circles or rigorous technology critics and establish its own place in the industry. The next zigbee challenge will be devising the proposed extension to the 802.14.4 standard,’4a’ which could be based on ultra-wideband (UWB).