01-05-2010, 07:44 PM
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1. Introduction
ZigBee is a communication standard that provides a short-range coast effective networking capability. It has bee developed with the emphasis on low-coast battery powered application such as building automation industrial and commercial control etc. Zigbee has been introduced by the IEEE and the zigbee alliance to provide a first general standard for these applications. The IEEE is the Institute of Electrical and Electronics Engineers. They are a non-profit organization dedicated to furthering technology involving electronics and electronic devices. The 802 group is the section of the IEEE involved in network operations and technologies, including mid-sized networks and local networks. Group 15 deals specifically with wireless networking technologies, and includes the now ubiquitous 802.15.1 working group, which is also known as Bluetooth.
The name "ZigBee" is derived from the erratic zigging patterns many bees make between flowers when collecting pollen. This is evocative of the invisible webs of connections existing in a fully wireless environment. The standard itself is regulated by a group known as the ZigBee Alliance, with over 150 members worldwide.
While Bluetooth focuses on connectivity between large packet user devices, such as laptops, phones, and major peripherals, ZigBee is designed to provide highly efficient connectivity between small packet devices. As a result of its simplified operations, which are one to two full orders of magnitude less complex than a comparable Bluetooth device, pricing for ZigBee devices is extremely competitive, with full nodes available for a fraction of the cost of a Bluetooth node.
ZigBee devices are actively limited to a through-rate of 250Kbps, compared to Bluetoothâ„¢s much larger pipeline of 1Mbps, operating on the 2.4 GHz ISM band, which is available throughout most of the world.
ZigBee has been developed to meet the growing demand for capable wireless networking between numerous low-power devices. In industry ZigBee is being used for next generation automated manufacturing, with small transmitters in every device on the floor, allowing for communication between devices to a central computer. This new level of communication permits finely-tuned remote monitoring and manipulation. 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.
2. What is wireless technology
Wireless networks are becoming more pervasive, accelerated by new wireless communications technologies, inexpensive wireless equipment, and broader Internet access availability. These networks are transforming the way people use computers and other personal electronics devices at work, home, and when traveling.
There are many wireless communications technologies that can be differentiated by frequency, bandwidth, range, and applications. In this white paper, we survey these technologies, which can be broadly organized into the four categories depicted in Figure below. These categories range from wireless wide area networks (WWANs), which cover the widest geographic area, to wireless personal area networks (WPANs), which cover less than 10 meters.
Bluetooth wireless technology is the prevalent WPAN technology today. It operates in the 2.4-GHz unlicensed frequency band. Figure 2 shows its evolution from version 1.1 at a data rate of 1 Mbps to version 1.2, which improves the signaling and frequency band coexistence mechanisms. The 3-Mbps Bluetooth 2.0+ Enhanced Data Rate (EDR) was ratified in November 2004 and products are beginning to appear on the market.
Over the next three years, WPAN applications that require higher data rates may adopt the emerging high bandwidth Ultrawideband (UWB) technology. UWB provides high bandwidth by transmitting at very low power across a broad frequency spectrum. The UWB physical interface (or PHY) specificationâ€802.15.3a†is under development in the IEEE, and a competing specification is under development by an industry working group called the Multi Band Orthogonal Frequency Division Multiplexing (OFDM) Alliance (MBOA). Initial UWB products with data rates of 100-480 Mbps are anticipated in early 2006. Future versions are expected to have data rates of up to 1 Gbps. Failure to resolve the issue of competing standards may stall the market opportunity for UWB technology. In addition, although the U.S. Federal Communications Commission (FCC) has approved a large amount of spectrum for UWB in the U.S., there are regulatory and regional policy issues outside the U.S.
An additional wireless technology that fits roughly in the WPAN categoryâ€ZigBee (802.15.4)â€is optimized for low-bandwidth niche applications such as instrumentation and home automation. Zigbee is not depicted in Figure 2 because it is unlikely that it will be deployed outside these specialized applications.
Wireless Local Area Networks
(WLANs)
In contrast to WPANs, WLANs provide robust wireless network connectivity over a local area of approximately100 meters between the access point and associated clients. Today's WLANs are based on the IEEE 802.11 standard and are referred to as Wi-Fi networks. 802.11b was the first commercially successful WLAN technology. It operates in the 2.4-GHz frequency band at 11Mbps. By implementing a different data transmission method, data rates were increased to 54 Mbps in 2003with 802.11g in the 2.4-GHz band and 802.11a in the 5-Ghz band. Today, dual-band Wi-Fi access points and client network adapters that support various combinations of 802.11a, b, and g are common. Highly integrated, single-chip solutions that are smaller and require less power have enabled new designs and applications.
In addition, new standards address Wi-Fi network security. Wi-Fi Protected Access (WPA) and 802.11i (or WPA2) focus on user authentication and encryption. WPA2 employs next-generation Advanced Encryption Security (AES) encryption. A component of WPA and WPA2â€the IEEE 802.1X standardâ€provides a port-level authentication framework. Finally, the upcoming 802.11e standard addresses quality of service (QoS). QoS enables the prioritization of latency-sensitive applications such as voice and multimedia. The Wi-Fi Alliance, an industry group responsible for certification and interoperability testing, has developed the Wi-Fi Multi- media (WMM) test specification to certify product compliance with the 802.11e standard.
The next-generation WLAN standard is IEEE 802.11n, which is currently being defined. 802.11n will be backward-compatible with 802.11a, b, and g, and will provide data rates in excess of 100 Mbps. The 802.11n performance increases stem from new Multiple-Input, Multiple-Output (MIMO) radio technology, wider radio frequency (RF) channels, and improvements to the protocol stack. MIMO enables higher data rates by increasing the number of radios and antennas in a wireless device. 802.11n is scheduled for IEEE ratification in mid-2006. Dell is leading an initiative in the Wi-Fi Alliance to launch a product certification program concurrent with ratification of the IEEE 802.11n standard.
Wireless Metro Area Networks
(WMANs)
A WMAN is a wireless communications network that covers a large geographic area such as a city or suburb. Traditionally, long-distance wireless technologies providing T1 or T3 data rates have been proprietary owned and operated by major telephone companies, in dependent local exchange carriers (ILECs), and other providers to page link remote sites or large campuses. The IEEE has standardized a new set of WMAN technologies that operate in licensed and license exempt frequency bands. The best known of these technologies†IEEE 802.16d or WiMaxâ€will operate in the 2- to11-GHz frequency range. (In the U.S., it will operate in the 2.5-, 3.5-, and 5.8-GHz frequency bands.) Its maximum data rate when operated within line of sight and under ideal conditions is 70 Mbps over 50 kilometers. Initial deployments will require an external antenna at the customer premises. A mobile versionâ€802.16eâ€is planned in 2007. It is not yet clear when (or whether) telecommunications and Internet service providers will broadly deploy the required infrastructure to support either the fixed or mobile versions of Wi-Max. However, it is widely expected that WiMax deployments will lever age existing and emerging tower infrastructures and installations.
Wireless Wide Area Networks
(WWANs)
WWANs are digital cellular networks used for mobile phone and data service and operated by carriers such as Cingular Wireless, Vodafone, and Verizon Wireless WWANs provide connectivity over a wide geographical area, but, until recently, data rates have been relatively lowâ€115 Kbpsâ€compared to other more localized wireless technologies.
Two WWAN technologies Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA)â€dominate WWAN deployments worldwide. These two technologies are expected to evolve on parallel paths for the foreseeable future.pe standardized early on GSM. Today, GSM and its associated wireless data capability, General Packet Radio Service (GPRS) and next-generation Enhanced Data GSM Evolution (EDGE), have about two thirds of the worldwide market. These technologies have been deployed in North America, Europe, and Asia. Next-generation EDGE boosts GPRS data rates by 3“4 times. Other GSM operators, especially those that have acquired new 3G frequency spectrum, are commercializing Wideband CDMA (WCDMA), which is expected to have data rates of 2 Mbps. An extension called High-Speed Downlink Packet Access (HSDPA) is expected to be deployed starting in 2006. HSDPA will further increase these data rates to 3.6 Mbps and beyond.
CDMA technology dominates in the U.S. The CDMA2000 WWAN technology has seen strong deployments in North America, Japan, Korea, and China. The CDMA2000 single-carrier radio transmission technology (1xRTT) version has been widely deployed. The next-generation 1xEvolution-Data Optimized (1xEV-DO) is currently being aggressively deployed by Verizon Wire- less and Sprint PCS in the U.S. and will support a data rate of 2.4 Mbps. Carriers will build on EV-DO with version A of the specification, which will support even higher data rates and Voice over Internet Protocol (VoIP) calls.
3. Need of zigbee
Wireless sensor networking is one of the most exciting technology markets today. They say that over the next five to ten years, wireless sensors will have a significant impact on almost all major industries as well as our home lives. Broadly, this technology market includes application segments such as automated meter reading, home automation, building automation, container security/tracking, and many others.
Although products that span these application segments are diverse and different in how they operate and what they do, their requirements from a wireless communication technology are very similar. For example, these applications generally require low data rates and are battery powered.
The main motivations for migrating these products to wireless
communications are three-fold:
1. Installation cost “ The cost of running wires in a typical building automation project in an existing facility can be as high as 80% of the total project cost
2. Maintenance “ It is easier to configure a hot-water heater controller with a hand-held remote than a keypad in the closet.
3. New markets “ Eliminating the wire opens new markets that were previously unavailable to wired products.
In the wireless worlds of WiFi and BlueTooth, market growth was fueled by standards development that ultimately brought down the cost of the technology and ensured excellent value to the user. In that spirit, a number of companies forged an alliance to create a wireless standard for the embedded wireless market space, also called personal area networking (PAN); this standard is now called Zigbee. The list of promoting members is prominent and includes names like Honeywell, Phillips, Motorola, Freescale, Invensys, and many others.
Technically, Zigbee is a protocol standard that defines network, security, and application framework protocol software. Zigbee is designed to work on top of the IEEE 802.15.4 PHY/MAC layer standard. The IEEE 802.15.4 standard was ratified in May of 2003; to our knowledge the Zigbee standard is not at the time of this writing ratified, though we understand that it is very close.
4. What is Zigbee / 802.15.4
The IEEE 802.15.4 standard defines the PHY and MAC layers, which are used by Zigbee.
PHY description
Three frequency bands are specified, though an implementation need only operate on one of the three, the bands are:
868 MHz - for European applications
902-928 MHz - for North American applications
2.450 GHz -for world wide application
In all bands, the modulation scheme is direct sequence spread spectrum. In the
868 and 902-928 MHz bands, the transmitter is modulated using BPSK. In the
2.450 GHz band, the transmitter is modulated using offset-QPSK, which is more bandwidth efficient than BPSK.
Direct sequence spread spectrum is a technique that essentially spreads the narrow band of data over a much broader bandwidth by using a pseudo-random chipping sequence. This process provides gain at the receiver because of the correlating effect of de-spreading the data. The amount of gain is determined by the ratio of the chipping rate to the data rate. The higher the ratio, the higher the gain. This gain also provides proportional rejection of on channel interference. As the wanted signal is correlated and de-spread, the interferer is spread, increasing the level of the wanted signal and decreasing the level of the interfering carrier. The amount of rejection is determined by the spreading gain.
In the 2.450 GHz band, an 802.15.4 radio spreads the data using an 8 bit chipping sequence. Actually, the chipping sequence is 32 bits, but the data being spread is actually 4 bits, thus the 8:1 chipping ratio. The process gain in dB is calculated by multiplying ten times the log of the chipping ratio; in this case the gain is 9dB. Receiver sensitivity is specified at “85dBm; adjacent channel rejection is 0dB minimum.
In the 868 and 902-928 MHz band, an 802.15.4 radio spreads the data using a 15 bit chipping sequence. In this case, chipping ratio is 15 and spreading gain is
12dB. Receiver sensitivity is specified at -92dBm; adjacent channel rejection is 0dB minimum.
Zigbee/802.15.4 Specifications by Band
868 MHz 902-928 MHz 2.450 GHz
Data Rate 20 kbps 40 kbps 250kbps
# channels 1 10 16
TX Power -3dBm -3dBm -3dBm
RX Sensitivity -92dBm -92dBm -85dBm
Link Budget 89dB 89dB 82dB
Adjacent channel
rejection
Alternate
channel rejection
0dB 0dB 0dB
30dB 30dB 30dB
MAC Description
The 802.15.4 specification defines a very complicated MAC layer, and I will not attempt to give a detailed explanation here.802.15.4 defines two classes of implementations: full function devices (FFD) and reduced function devices (RFD).
An FFD can operate in three modes serving as a PAN coordinator, a coordinator, or a device. FFDs contain all of the features of 802.15.4 and can talk to both RFDs and FFDs. A PAN coordinator is the primary controller of the network, and it must be a FFD. There can be only one PAN controller per network. A PAN controller is required for an 802.15.4 network. A coordinator is a FFd that provides synchronization Services by transmitting beacons.
A RFD can operate only as a device. RFDs contain a subset of the features of
802.15.4 and are intended to be high-volume, low cost devices. They can be duty-cycled to reduce power consumption. RFD devices can talk only to FFDs. This means that RFDs have no routing capability, so they must be on the perimeters of a mesh network. A device is a simple end-point. A device can be a RFD or FFD. Conceptually, each network would have one FFD that acted as the PAN coordinator and several more FFDs that formed the mesh network. The majority of the nodes in the network would be low-cost RFDs. The number and position of FFDs in the network would determine the coverage of the network.
The illustration on the above shows an example Zigbee network configuration. There is one PAN coordinator, six FFD devices, and nine RFD devices. The actual mesh network is formed by the FFD devices and the PAN coordinator. The RFD devices form a point to multipoint network with FFD devices that are in range Node 8 is not connected to the network. Although it is in range of nodes 7 and 9, it cannot connect to them because all three are RFD devices. An additional FFD device would be required to connect node 8 to the network.
There in lies an inherent limitation of the Zigbee model. The number of FFD devices in the network determines the coverage area; the more FFD devices, the larger the coverage area. It is probable, given the current 802.15.4 specification, that a real-world application of Zigbee would require a high ratio of FFD devices to RFD devices to attain the required coverage, which will adversely affect the pricing model.
This also has implications in system deployment. The primary factor driving the market need is lower installation cost . Using the example just given, it is easy to see how the installation will be complicated. If a device (node 8) is installed in a location that is not in range of an FFD device, it will not be connected to the network. The installer would then be required to place an additional FFD device to serve as an intermediate router. This would have to be done by trial and error, increasing both labor and materials cost.
If this all sounds complicated, that is because it is. The 802.15.4 specification alone consumes 670 printed pages. A typical implementation requires nearly 32K of flash, and that is just for the MAC layer.
The Zigbee specification is likely to be just as large and the software implementation requires another 32K or more of flash memory.
The important aspects of the 802.15.4 standard are listed below :
82-89 dB page link budget
0 dB adjacent channel rejection
10 channels @ 900 MHz, 16 channels at 2.450 GHz MHz
40kbps @ 900 MHz, 250 kbps @ 2.450 GHz
RFD devices are not a part of the mesh network
Every network requires a PAN coordinator
The coverage area is determined by both the 802.15.4 page link budget and the number of FFD devices deployed
5. Benefits of zigbee
In all of its uses, ZigBee offers four inherent characteristics that are highly beneficial:
Low cost
The typical ZigBee radio is extremely cost-effective. Chipset prices can be as low as $12 each in quantities as few as 100 pieces (while the 802.15.4 and ZigBee stacks are typically included in this cost, crystals and other discrete components are not). Design-in modules fall in the neighborhood of $25 in similar quantities. This pricing provides an economic justification for extending wireless networking to even the simplest of devices.
Range and obstruction issues avoidance:
ZigBee routers double as input devices and repeaters to create a form of mesh network. If two network points are unable to communicate as intended, transmission is dynamically routed from the blocked node to a router with a clear path to the dataâ„¢s destination. This happens automatically, so that communications continue even when a page link fails unexpectedly. The use of low-cost routers can also extend the networkâ„¢s effective reach; when the distance between the base station and a remote node exceeds the deviceâ„¢s range, an intermediate node or nodes can relay transmission, eliminating the need for separate repeaters
Multi-source products
As an open standard, ZigBee provides customers with the ability to choose among vendors. ZigBee Alliance working groups define interoperability profiles to which ZigBee-certified devices must adhere, and certified radio will interoperate with any other ZigBee-certified radio adhering to the same profile, promoting compatibility and the associated competition that allows the end users to choose the best device for each particular network node, regardless of manufacturer.
Low power consumption
Basic ZigBee radios operate at 1 mW RF power, and can sleep when not involved in transmission (higher RF power ZigBee radios for applications needing greater range also provide the sleep function). As this makes battery-powered radios more practical than ever, wireless devices are free to be placed without power cable runs in addition to eliminating data cable runs.
6. Technology behind zigbee
IEEE 802.15.4
The IEEE workgroup 802.15.4 has standardized the PHY and MAC, layer within WPAN (Wireless Personal Area Network) area. The primary goal for the working group within IEEE was to define the standard to meet requirements on low complexity, low cost and low power consumption.
802.15.4 MAC
The MAC sublayer provides interfaces towards the PHY and higher (Zigbee) layers. The MAC is responsible for several functions, such as: generation of acknowledgment frames, association, disassociation, security control, and some optional services such as: beacon generation and guaranteed time slot management. One aim when defining the MAC was to make the scheduling engine simple, thus improving the overall power consumption.
Channel Access
For all types of deployed networks the Carrier Sense Multiple Access Collision Avoidance (CSMA-CA) protocol is used. This method is useful to avoid unnecessary collision over the radio channel. CSMA-CA is used for all traffic except beacons, ACK frames and transmissions within a GTS.
Beacon signaling
Only a FFD has the capability of generating beacon frames in a true peer-to-peer network topology there can only be FFDs operating. One exception is when a RFD operates as a peripheral device without routing capability.
Guaranteed Time Slot (GTS)
By using GTS scheduling at the MAC level two important attributes can be achieved over the channel.
Low latency: For specific applications sensitive for delays such as alarms, PC mice, lamp switches or QoS differentiation based on application software. It is possible to reach response times down to 15 msec in GTS mode.
Bandwidth allocation:
Useful for applications that requires to generate a known data traffic rate. The higher layer protocol is responsible for assigning the traffic rate and the MAC level will grant the request.
802.15.4 PHY
The physical layer provides the interface to the wireless medium and is responsible for low-level control for page link quality, energy detection, activation/deactivation of the radio transceiver etc. The PHY in 802.15.4 has three different modes of operation depending of geographically region.
The IEEE 802.15.4 PHY uses Direct Sequence Spread Spectrum (DSSS) as the transmission distances ranges from 10-100 meter (approx), depending on output power, radio environment and antenna solution.
Network Topologies
In general there exist three different network topologies:
¢ Star
¢ Cluster Tree
¢ Mesh
In this mode the PAN Coordinator is responsible for updating routing tables, transmit beacons, maintaining synchronization, routing of messages between nodes etc. within the network.
Antenna Considerations
The antenna design is often very crucial for the overall product. One requirement is often to minimize the size of the antenna, which is needed in many embedded applications for short range wireless communications. However, a small antenna may be inefficient unless attention is paid to the design of both the antenna and its placement in the product.
Choice of antenna
There are several design choices in the case of an internal antenna.
The obvious and cheapest choice is to use a substrate antenna, by using a piece of circuit board trace. One disadvantage with this solution is the relative resistive loss and the possible cross interference with components on the board and nearby ground planes.
Another choice is wire antennas where the antenna can be placed away from the board, which improves the performance. One drawback is that this solution may require tuning, due to mechanical differences and size variations. A third choice is to use ceramic antennas which offers smaller physical size than the above mentioned solutions but with a substantially higher cost.
ZigBee Alliance
The ZigBee alliance was founded in order to develop a common standard for Short Range Devices (SRD) with focus on low power consumption, ad-hoc behavior, low latency, self configurable radio nodes. The ZigBee alliance is today an association with over 50 member companies. The main focus within ZigBee is to define the routing mechanisms that together with IEEE802.15.4 will form the total solution.
General Characteristics
¢ Data rates from 20 to 250 kbps
¢ Star Topology/Peer to Peer (mesh)
¢ 255 devices / network
¢ CSMA-CA Access Scheme
¢ Enumeration (new node) ~ 15-30 ms
¢ Dual PHY (868/915 MHz, 2.4 GHz), DSSS
¢ Range: 10-100 m
¢ Low Duty Cycle (< 0 .1%, TX)
¢ Low Power Cons, +1 year battery life time
¢ Complexity: 4 -32 kB (protocol)
7. Applications
Water level sensing
Zigbee can be installed in remote location where conventional GSM modems would be out of their network coverage area, such as inside water tanks. Zigbee transceiver can be hermetically sealed with batteries and co-located with sensors. Each transceiver transmits periodically to another unit installed above ground. A GSM modem transmits the data to base.
In building control
Zigbee-enabled switches and lights can be reduce installation costs in new building by eliminating the need to route light control through the walls, and remove the need to call in qualified electrician when switches need to be relocated. Thermostats and air-conditioning placed anywhere, free of any wiring constraints.
8. Conclusion
The Zigbee solution will be available soon. It will not hit the $10 price point until 2009. The 802.15.4 radio specification has a very poor page link budget; 89dB. A Zigbee based solution is not scalable; it will not work reliably with only two end-points separated by the length of a house. It is complicated and requires a significant learning curve from the engineer and significant resources from the protocol controller. The customer must form three supplier relationships; the chip vendor, the software vendor, and the microcontroller vendor.
In addition to cost, reliability, and scalability, Zigbee purports to offer other advantages over proprietary solutions such as Interoperability, Vendor independence and Common platform.
There are three separate frequency bands specified for Zigbee. If one manufacturer of heating controls chooses the 900 MHz band, and another chooses the 2.4 GHz band, the products will not operate together. Additionally, it is likely that IC vendors will add proprietary features to their 802.15.4 implementations in an effort to differentiate their product; if the OEM uses these proprietary features, the benefit of interoperability will be negated. In the end, the only way to guarantee interoperability using Zigbee is to design only 2.4GHz products using only Zigbee standard features.
9. References
Chipcon, CC2420 2.4 GHz IEEE 802.15.4 RF Transceiver Data Sheet
On World, October 2004, Wireless AMR and submetering: A market dynamics study on fixed wireless technologies
Electronic Design, The Zigbee buzz is growing: New low- power wireless standard opens powerful possibilities
On World, Wireless Sensor Networks: Mass Market
Opportunities
IEEE, 802.15.4 Part 15.4: Wireless Medium Access control
(MAC) & Physical (PHY) Layer Specifications for Low Rate Wireless Personal
Area Networks
ZigBee Open House Oslo.htm
Zigbee Alliance -- Home Page.htm
Zigbee Tutorial.htm
please read http://studentbank.in/report-zigbee-netw...ull-report and http://studentbank.in/report-seminars-re...ss-network for getting more information of zigbee technology
1. Introduction
ZigBee is a communication standard that provides a short-range coast effective networking capability. It has bee developed with the emphasis on low-coast battery powered application such as building automation industrial and commercial control etc. Zigbee has been introduced by the IEEE and the zigbee alliance to provide a first general standard for these applications. The IEEE is the Institute of Electrical and Electronics Engineers. They are a non-profit organization dedicated to furthering technology involving electronics and electronic devices. The 802 group is the section of the IEEE involved in network operations and technologies, including mid-sized networks and local networks. Group 15 deals specifically with wireless networking technologies, and includes the now ubiquitous 802.15.1 working group, which is also known as Bluetooth.
The name "ZigBee" is derived from the erratic zigging patterns many bees make between flowers when collecting pollen. This is evocative of the invisible webs of connections existing in a fully wireless environment. The standard itself is regulated by a group known as the ZigBee Alliance, with over 150 members worldwide.
While Bluetooth focuses on connectivity between large packet user devices, such as laptops, phones, and major peripherals, ZigBee is designed to provide highly efficient connectivity between small packet devices. As a result of its simplified operations, which are one to two full orders of magnitude less complex than a comparable Bluetooth device, pricing for ZigBee devices is extremely competitive, with full nodes available for a fraction of the cost of a Bluetooth node.
ZigBee devices are actively limited to a through-rate of 250Kbps, compared to Bluetoothâ„¢s much larger pipeline of 1Mbps, operating on the 2.4 GHz ISM band, which is available throughout most of the world.
ZigBee has been developed to meet the growing demand for capable wireless networking between numerous low-power devices. In industry ZigBee is being used for next generation automated manufacturing, with small transmitters in every device on the floor, allowing for communication between devices to a central computer. This new level of communication permits finely-tuned remote monitoring and manipulation. 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.
2. What is wireless technology
Wireless networks are becoming more pervasive, accelerated by new wireless communications technologies, inexpensive wireless equipment, and broader Internet access availability. These networks are transforming the way people use computers and other personal electronics devices at work, home, and when traveling.
There are many wireless communications technologies that can be differentiated by frequency, bandwidth, range, and applications. In this white paper, we survey these technologies, which can be broadly organized into the four categories depicted in Figure below. These categories range from wireless wide area networks (WWANs), which cover the widest geographic area, to wireless personal area networks (WPANs), which cover less than 10 meters.
Bluetooth wireless technology is the prevalent WPAN technology today. It operates in the 2.4-GHz unlicensed frequency band. Figure 2 shows its evolution from version 1.1 at a data rate of 1 Mbps to version 1.2, which improves the signaling and frequency band coexistence mechanisms. The 3-Mbps Bluetooth 2.0+ Enhanced Data Rate (EDR) was ratified in November 2004 and products are beginning to appear on the market.
Over the next three years, WPAN applications that require higher data rates may adopt the emerging high bandwidth Ultrawideband (UWB) technology. UWB provides high bandwidth by transmitting at very low power across a broad frequency spectrum. The UWB physical interface (or PHY) specificationâ€802.15.3a†is under development in the IEEE, and a competing specification is under development by an industry working group called the Multi Band Orthogonal Frequency Division Multiplexing (OFDM) Alliance (MBOA). Initial UWB products with data rates of 100-480 Mbps are anticipated in early 2006. Future versions are expected to have data rates of up to 1 Gbps. Failure to resolve the issue of competing standards may stall the market opportunity for UWB technology. In addition, although the U.S. Federal Communications Commission (FCC) has approved a large amount of spectrum for UWB in the U.S., there are regulatory and regional policy issues outside the U.S.
An additional wireless technology that fits roughly in the WPAN categoryâ€ZigBee (802.15.4)â€is optimized for low-bandwidth niche applications such as instrumentation and home automation. Zigbee is not depicted in Figure 2 because it is unlikely that it will be deployed outside these specialized applications.
Wireless Local Area Networks
(WLANs)
In contrast to WPANs, WLANs provide robust wireless network connectivity over a local area of approximately100 meters between the access point and associated clients. Today's WLANs are based on the IEEE 802.11 standard and are referred to as Wi-Fi networks. 802.11b was the first commercially successful WLAN technology. It operates in the 2.4-GHz frequency band at 11Mbps. By implementing a different data transmission method, data rates were increased to 54 Mbps in 2003with 802.11g in the 2.4-GHz band and 802.11a in the 5-Ghz band. Today, dual-band Wi-Fi access points and client network adapters that support various combinations of 802.11a, b, and g are common. Highly integrated, single-chip solutions that are smaller and require less power have enabled new designs and applications.
In addition, new standards address Wi-Fi network security. Wi-Fi Protected Access (WPA) and 802.11i (or WPA2) focus on user authentication and encryption. WPA2 employs next-generation Advanced Encryption Security (AES) encryption. A component of WPA and WPA2â€the IEEE 802.1X standardâ€provides a port-level authentication framework. Finally, the upcoming 802.11e standard addresses quality of service (QoS). QoS enables the prioritization of latency-sensitive applications such as voice and multimedia. The Wi-Fi Alliance, an industry group responsible for certification and interoperability testing, has developed the Wi-Fi Multi- media (WMM) test specification to certify product compliance with the 802.11e standard.
The next-generation WLAN standard is IEEE 802.11n, which is currently being defined. 802.11n will be backward-compatible with 802.11a, b, and g, and will provide data rates in excess of 100 Mbps. The 802.11n performance increases stem from new Multiple-Input, Multiple-Output (MIMO) radio technology, wider radio frequency (RF) channels, and improvements to the protocol stack. MIMO enables higher data rates by increasing the number of radios and antennas in a wireless device. 802.11n is scheduled for IEEE ratification in mid-2006. Dell is leading an initiative in the Wi-Fi Alliance to launch a product certification program concurrent with ratification of the IEEE 802.11n standard.
Wireless Metro Area Networks
(WMANs)
A WMAN is a wireless communications network that covers a large geographic area such as a city or suburb. Traditionally, long-distance wireless technologies providing T1 or T3 data rates have been proprietary owned and operated by major telephone companies, in dependent local exchange carriers (ILECs), and other providers to page link remote sites or large campuses. The IEEE has standardized a new set of WMAN technologies that operate in licensed and license exempt frequency bands. The best known of these technologies†IEEE 802.16d or WiMaxâ€will operate in the 2- to11-GHz frequency range. (In the U.S., it will operate in the 2.5-, 3.5-, and 5.8-GHz frequency bands.) Its maximum data rate when operated within line of sight and under ideal conditions is 70 Mbps over 50 kilometers. Initial deployments will require an external antenna at the customer premises. A mobile versionâ€802.16eâ€is planned in 2007. It is not yet clear when (or whether) telecommunications and Internet service providers will broadly deploy the required infrastructure to support either the fixed or mobile versions of Wi-Max. However, it is widely expected that WiMax deployments will lever age existing and emerging tower infrastructures and installations.
Wireless Wide Area Networks
(WWANs)
WWANs are digital cellular networks used for mobile phone and data service and operated by carriers such as Cingular Wireless, Vodafone, and Verizon Wireless WWANs provide connectivity over a wide geographical area, but, until recently, data rates have been relatively lowâ€115 Kbpsâ€compared to other more localized wireless technologies.
Two WWAN technologies Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA)â€dominate WWAN deployments worldwide. These two technologies are expected to evolve on parallel paths for the foreseeable future.pe standardized early on GSM. Today, GSM and its associated wireless data capability, General Packet Radio Service (GPRS) and next-generation Enhanced Data GSM Evolution (EDGE), have about two thirds of the worldwide market. These technologies have been deployed in North America, Europe, and Asia. Next-generation EDGE boosts GPRS data rates by 3“4 times. Other GSM operators, especially those that have acquired new 3G frequency spectrum, are commercializing Wideband CDMA (WCDMA), which is expected to have data rates of 2 Mbps. An extension called High-Speed Downlink Packet Access (HSDPA) is expected to be deployed starting in 2006. HSDPA will further increase these data rates to 3.6 Mbps and beyond.
CDMA technology dominates in the U.S. The CDMA2000 WWAN technology has seen strong deployments in North America, Japan, Korea, and China. The CDMA2000 single-carrier radio transmission technology (1xRTT) version has been widely deployed. The next-generation 1xEvolution-Data Optimized (1xEV-DO) is currently being aggressively deployed by Verizon Wire- less and Sprint PCS in the U.S. and will support a data rate of 2.4 Mbps. Carriers will build on EV-DO with version A of the specification, which will support even higher data rates and Voice over Internet Protocol (VoIP) calls.
3. Need of zigbee
Wireless sensor networking is one of the most exciting technology markets today. They say that over the next five to ten years, wireless sensors will have a significant impact on almost all major industries as well as our home lives. Broadly, this technology market includes application segments such as automated meter reading, home automation, building automation, container security/tracking, and many others.
Although products that span these application segments are diverse and different in how they operate and what they do, their requirements from a wireless communication technology are very similar. For example, these applications generally require low data rates and are battery powered.
The main motivations for migrating these products to wireless
communications are three-fold:
1. Installation cost “ The cost of running wires in a typical building automation project in an existing facility can be as high as 80% of the total project cost
2. Maintenance “ It is easier to configure a hot-water heater controller with a hand-held remote than a keypad in the closet.
3. New markets “ Eliminating the wire opens new markets that were previously unavailable to wired products.
In the wireless worlds of WiFi and BlueTooth, market growth was fueled by standards development that ultimately brought down the cost of the technology and ensured excellent value to the user. In that spirit, a number of companies forged an alliance to create a wireless standard for the embedded wireless market space, also called personal area networking (PAN); this standard is now called Zigbee. The list of promoting members is prominent and includes names like Honeywell, Phillips, Motorola, Freescale, Invensys, and many others.
Technically, Zigbee is a protocol standard that defines network, security, and application framework protocol software. Zigbee is designed to work on top of the IEEE 802.15.4 PHY/MAC layer standard. The IEEE 802.15.4 standard was ratified in May of 2003; to our knowledge the Zigbee standard is not at the time of this writing ratified, though we understand that it is very close.
4. What is Zigbee / 802.15.4
The IEEE 802.15.4 standard defines the PHY and MAC layers, which are used by Zigbee.
PHY description
Three frequency bands are specified, though an implementation need only operate on one of the three, the bands are:
868 MHz - for European applications
902-928 MHz - for North American applications
2.450 GHz -for world wide application
In all bands, the modulation scheme is direct sequence spread spectrum. In the
868 and 902-928 MHz bands, the transmitter is modulated using BPSK. In the
2.450 GHz band, the transmitter is modulated using offset-QPSK, which is more bandwidth efficient than BPSK.
Direct sequence spread spectrum is a technique that essentially spreads the narrow band of data over a much broader bandwidth by using a pseudo-random chipping sequence. This process provides gain at the receiver because of the correlating effect of de-spreading the data. The amount of gain is determined by the ratio of the chipping rate to the data rate. The higher the ratio, the higher the gain. This gain also provides proportional rejection of on channel interference. As the wanted signal is correlated and de-spread, the interferer is spread, increasing the level of the wanted signal and decreasing the level of the interfering carrier. The amount of rejection is determined by the spreading gain.
In the 2.450 GHz band, an 802.15.4 radio spreads the data using an 8 bit chipping sequence. Actually, the chipping sequence is 32 bits, but the data being spread is actually 4 bits, thus the 8:1 chipping ratio. The process gain in dB is calculated by multiplying ten times the log of the chipping ratio; in this case the gain is 9dB. Receiver sensitivity is specified at “85dBm; adjacent channel rejection is 0dB minimum.
In the 868 and 902-928 MHz band, an 802.15.4 radio spreads the data using a 15 bit chipping sequence. In this case, chipping ratio is 15 and spreading gain is
12dB. Receiver sensitivity is specified at -92dBm; adjacent channel rejection is 0dB minimum.
Zigbee/802.15.4 Specifications by Band
868 MHz 902-928 MHz 2.450 GHz
Data Rate 20 kbps 40 kbps 250kbps
# channels 1 10 16
TX Power -3dBm -3dBm -3dBm
RX Sensitivity -92dBm -92dBm -85dBm
Link Budget 89dB 89dB 82dB
Adjacent channel
rejection
Alternate
channel rejection
0dB 0dB 0dB
30dB 30dB 30dB
MAC Description
The 802.15.4 specification defines a very complicated MAC layer, and I will not attempt to give a detailed explanation here.802.15.4 defines two classes of implementations: full function devices (FFD) and reduced function devices (RFD).
An FFD can operate in three modes serving as a PAN coordinator, a coordinator, or a device. FFDs contain all of the features of 802.15.4 and can talk to both RFDs and FFDs. A PAN coordinator is the primary controller of the network, and it must be a FFD. There can be only one PAN controller per network. A PAN controller is required for an 802.15.4 network. A coordinator is a FFd that provides synchronization Services by transmitting beacons.
A RFD can operate only as a device. RFDs contain a subset of the features of
802.15.4 and are intended to be high-volume, low cost devices. They can be duty-cycled to reduce power consumption. RFD devices can talk only to FFDs. This means that RFDs have no routing capability, so they must be on the perimeters of a mesh network. A device is a simple end-point. A device can be a RFD or FFD. Conceptually, each network would have one FFD that acted as the PAN coordinator and several more FFDs that formed the mesh network. The majority of the nodes in the network would be low-cost RFDs. The number and position of FFDs in the network would determine the coverage of the network.
The illustration on the above shows an example Zigbee network configuration. There is one PAN coordinator, six FFD devices, and nine RFD devices. The actual mesh network is formed by the FFD devices and the PAN coordinator. The RFD devices form a point to multipoint network with FFD devices that are in range Node 8 is not connected to the network. Although it is in range of nodes 7 and 9, it cannot connect to them because all three are RFD devices. An additional FFD device would be required to connect node 8 to the network.
There in lies an inherent limitation of the Zigbee model. The number of FFD devices in the network determines the coverage area; the more FFD devices, the larger the coverage area. It is probable, given the current 802.15.4 specification, that a real-world application of Zigbee would require a high ratio of FFD devices to RFD devices to attain the required coverage, which will adversely affect the pricing model.
This also has implications in system deployment. The primary factor driving the market need is lower installation cost . Using the example just given, it is easy to see how the installation will be complicated. If a device (node 8) is installed in a location that is not in range of an FFD device, it will not be connected to the network. The installer would then be required to place an additional FFD device to serve as an intermediate router. This would have to be done by trial and error, increasing both labor and materials cost.
If this all sounds complicated, that is because it is. The 802.15.4 specification alone consumes 670 printed pages. A typical implementation requires nearly 32K of flash, and that is just for the MAC layer.
The Zigbee specification is likely to be just as large and the software implementation requires another 32K or more of flash memory.
The important aspects of the 802.15.4 standard are listed below :
82-89 dB page link budget
0 dB adjacent channel rejection
10 channels @ 900 MHz, 16 channels at 2.450 GHz MHz
40kbps @ 900 MHz, 250 kbps @ 2.450 GHz
RFD devices are not a part of the mesh network
Every network requires a PAN coordinator
The coverage area is determined by both the 802.15.4 page link budget and the number of FFD devices deployed
5. Benefits of zigbee
In all of its uses, ZigBee offers four inherent characteristics that are highly beneficial:
Low cost
The typical ZigBee radio is extremely cost-effective. Chipset prices can be as low as $12 each in quantities as few as 100 pieces (while the 802.15.4 and ZigBee stacks are typically included in this cost, crystals and other discrete components are not). Design-in modules fall in the neighborhood of $25 in similar quantities. This pricing provides an economic justification for extending wireless networking to even the simplest of devices.
Range and obstruction issues avoidance:
ZigBee routers double as input devices and repeaters to create a form of mesh network. If two network points are unable to communicate as intended, transmission is dynamically routed from the blocked node to a router with a clear path to the dataâ„¢s destination. This happens automatically, so that communications continue even when a page link fails unexpectedly. The use of low-cost routers can also extend the networkâ„¢s effective reach; when the distance between the base station and a remote node exceeds the deviceâ„¢s range, an intermediate node or nodes can relay transmission, eliminating the need for separate repeaters
Multi-source products
As an open standard, ZigBee provides customers with the ability to choose among vendors. ZigBee Alliance working groups define interoperability profiles to which ZigBee-certified devices must adhere, and certified radio will interoperate with any other ZigBee-certified radio adhering to the same profile, promoting compatibility and the associated competition that allows the end users to choose the best device for each particular network node, regardless of manufacturer.
Low power consumption
Basic ZigBee radios operate at 1 mW RF power, and can sleep when not involved in transmission (higher RF power ZigBee radios for applications needing greater range also provide the sleep function). As this makes battery-powered radios more practical than ever, wireless devices are free to be placed without power cable runs in addition to eliminating data cable runs.
6. Technology behind zigbee
IEEE 802.15.4
The IEEE workgroup 802.15.4 has standardized the PHY and MAC, layer within WPAN (Wireless Personal Area Network) area. The primary goal for the working group within IEEE was to define the standard to meet requirements on low complexity, low cost and low power consumption.
802.15.4 MAC
The MAC sublayer provides interfaces towards the PHY and higher (Zigbee) layers. The MAC is responsible for several functions, such as: generation of acknowledgment frames, association, disassociation, security control, and some optional services such as: beacon generation and guaranteed time slot management. One aim when defining the MAC was to make the scheduling engine simple, thus improving the overall power consumption.
Channel Access
For all types of deployed networks the Carrier Sense Multiple Access Collision Avoidance (CSMA-CA) protocol is used. This method is useful to avoid unnecessary collision over the radio channel. CSMA-CA is used for all traffic except beacons, ACK frames and transmissions within a GTS.
Beacon signaling
Only a FFD has the capability of generating beacon frames in a true peer-to-peer network topology there can only be FFDs operating. One exception is when a RFD operates as a peripheral device without routing capability.
Guaranteed Time Slot (GTS)
By using GTS scheduling at the MAC level two important attributes can be achieved over the channel.
Low latency: For specific applications sensitive for delays such as alarms, PC mice, lamp switches or QoS differentiation based on application software. It is possible to reach response times down to 15 msec in GTS mode.
Bandwidth allocation:
Useful for applications that requires to generate a known data traffic rate. The higher layer protocol is responsible for assigning the traffic rate and the MAC level will grant the request.
802.15.4 PHY
The physical layer provides the interface to the wireless medium and is responsible for low-level control for page link quality, energy detection, activation/deactivation of the radio transceiver etc. The PHY in 802.15.4 has three different modes of operation depending of geographically region.
The IEEE 802.15.4 PHY uses Direct Sequence Spread Spectrum (DSSS) as the transmission distances ranges from 10-100 meter (approx), depending on output power, radio environment and antenna solution.
Network Topologies
In general there exist three different network topologies:
¢ Star
¢ Cluster Tree
¢ Mesh
In this mode the PAN Coordinator is responsible for updating routing tables, transmit beacons, maintaining synchronization, routing of messages between nodes etc. within the network.
Antenna Considerations
The antenna design is often very crucial for the overall product. One requirement is often to minimize the size of the antenna, which is needed in many embedded applications for short range wireless communications. However, a small antenna may be inefficient unless attention is paid to the design of both the antenna and its placement in the product.
Choice of antenna
There are several design choices in the case of an internal antenna.
The obvious and cheapest choice is to use a substrate antenna, by using a piece of circuit board trace. One disadvantage with this solution is the relative resistive loss and the possible cross interference with components on the board and nearby ground planes.
Another choice is wire antennas where the antenna can be placed away from the board, which improves the performance. One drawback is that this solution may require tuning, due to mechanical differences and size variations. A third choice is to use ceramic antennas which offers smaller physical size than the above mentioned solutions but with a substantially higher cost.
ZigBee Alliance
The ZigBee alliance was founded in order to develop a common standard for Short Range Devices (SRD) with focus on low power consumption, ad-hoc behavior, low latency, self configurable radio nodes. The ZigBee alliance is today an association with over 50 member companies. The main focus within ZigBee is to define the routing mechanisms that together with IEEE802.15.4 will form the total solution.
General Characteristics
¢ Data rates from 20 to 250 kbps
¢ Star Topology/Peer to Peer (mesh)
¢ 255 devices / network
¢ CSMA-CA Access Scheme
¢ Enumeration (new node) ~ 15-30 ms
¢ Dual PHY (868/915 MHz, 2.4 GHz), DSSS
¢ Range: 10-100 m
¢ Low Duty Cycle (< 0 .1%, TX)
¢ Low Power Cons, +1 year battery life time
¢ Complexity: 4 -32 kB (protocol)
7. Applications
Water level sensing
Zigbee can be installed in remote location where conventional GSM modems would be out of their network coverage area, such as inside water tanks. Zigbee transceiver can be hermetically sealed with batteries and co-located with sensors. Each transceiver transmits periodically to another unit installed above ground. A GSM modem transmits the data to base.
In building control
Zigbee-enabled switches and lights can be reduce installation costs in new building by eliminating the need to route light control through the walls, and remove the need to call in qualified electrician when switches need to be relocated. Thermostats and air-conditioning placed anywhere, free of any wiring constraints.
8. Conclusion
The Zigbee solution will be available soon. It will not hit the $10 price point until 2009. The 802.15.4 radio specification has a very poor page link budget; 89dB. A Zigbee based solution is not scalable; it will not work reliably with only two end-points separated by the length of a house. It is complicated and requires a significant learning curve from the engineer and significant resources from the protocol controller. The customer must form three supplier relationships; the chip vendor, the software vendor, and the microcontroller vendor.
In addition to cost, reliability, and scalability, Zigbee purports to offer other advantages over proprietary solutions such as Interoperability, Vendor independence and Common platform.
There are three separate frequency bands specified for Zigbee. If one manufacturer of heating controls chooses the 900 MHz band, and another chooses the 2.4 GHz band, the products will not operate together. Additionally, it is likely that IC vendors will add proprietary features to their 802.15.4 implementations in an effort to differentiate their product; if the OEM uses these proprietary features, the benefit of interoperability will be negated. In the end, the only way to guarantee interoperability using Zigbee is to design only 2.4GHz products using only Zigbee standard features.
9. References
Chipcon, CC2420 2.4 GHz IEEE 802.15.4 RF Transceiver Data Sheet
On World, October 2004, Wireless AMR and submetering: A market dynamics study on fixed wireless technologies
Electronic Design, The Zigbee buzz is growing: New low- power wireless standard opens powerful possibilities
On World, Wireless Sensor Networks: Mass Market
Opportunities
IEEE, 802.15.4 Part 15.4: Wireless Medium Access control
(MAC) & Physical (PHY) Layer Specifications for Low Rate Wireless Personal
Area Networks
ZigBee Open House Oslo.htm
Zigbee Alliance -- Home Page.htm
Zigbee Tutorial.htm
please read http://studentbank.in/report-zigbee-netw...ull-report and http://studentbank.in/report-seminars-re...ss-network for getting more information of zigbee technology
