Zigbee: Overview and Analysis

Zigbee is a Ad-hoc networking technology for LR-WPAN,based on IEEE 802.15.4 standard that defines the PHY and Mac Layers for Zigbee. Intended for 2.45 Ghz , 868 MHz and 915 MHz Band. Low in cost, complexity & power consumption as compared to competing technologies. Intended to network inexpensive

Devices. Data rates touch 250Kbps for 2.45Ghz ,40 Kbps 915Mhz and 20Kbps for 868Mhz band.

Origin Of Name ZigBee

The domestic honeybee, a colonial insect, lives in a hive that contains a queen, a few male drones, and thousands of worker bees. The survival, success, and future of the colony is dependent upon continuous communication of vital information between every member of the colony. The technique that honey bees use to communicate new-found food sources to other members of the colony is referred to as the ZigBee Principle. Using this silent, but powerful communication system, whereby the bee dances in a zig-zag pattern, she is able to share information such as the location, distance, and direction of a newly discovered food source to her fellow colony members. Instinctively implementing the ZigBee Principle, bees around the world industriously sustain productive hives and foster future generations of colony members.

Zigbee Architecture

There are three areas of architectural responsibility in Zigbee engineering effort. They are The physical and MAC layers take full advantage of the physical radio specified by IEEE 802.15.4. The 802.15.4 specification describes a peer-to-peer radio using Direct Sequence Spread Spectrum. The specification also calls out the data rates, channelization, and modulation techniques to be employed.

The Zigbee Alliance specifies the logical network, security, and application software, which are implemented in a firmware stack. It is the Zigbee

stack that creates the mesh networking capability. Each microcontroller/RF chip combination requires its own Zigbee stack due to the differences in microcontrollers and RF chips. Typically, the Zigbee stack is included with either the microcontroller or RF chip. The stack may belong to the chip vendor, be provided by the chip vendor from a third party source, or be provided by a third party

source for a specific microcontroller/RF chip combination. The application layer is defined by profiles, of which there are two types: public

profiles are those certified by the Zigbee Alliance for interoperability purposes,

and private profiles are for use in closed systems.

A word about the Zigbee Alliance: The following discussion includes options that require access to intellectual property available only to members of the Zigbee Alliance. There are three types of membership; all companies that plan to release products incorporating Zigbee technology must become at least adopting members, an entry-level membership that provides such benefits as access to specifications and developer conferences/workshops.

ZigBee/IEEE 802.15.4 – General Characteristics:

Dual PHY (2.4GHz and 868/915 MHz)

Data rates of 250 kbps (@2.4 GHz), 40 kbps (@ 915 MHz), and 20 kbps (@868 MHz)

Optimized for low duty-cycle applications (<0.1%)

CSMA-CA channel access Yields high throughput and low latency for low duty cycle devices like sensors and controls

Low power (battery life multi-month to years)

Multiple topologies: star, peer-to-peer, mesh

Addressing space of up to:

– 18,450,000,000,000,000,000 devices (64 bit IEEEaddress)- 65,535 networks

Optional guaranteed time slot for applications requiring low latency

Fully hand-shaked protocol for transfer reliability

Range: 50m typical (5-500m based on environment)

Use Case Scenario:

It is 4:00 a.m. on a farm in Iowa. Sensors distributed throughout the fields report the moisture content in the soil and humidity of the air. The staff on the farm uses this data to decide where and when to water for optimum effect. The information also serves as an early warning system for environmental issues such as frost. Precious resources are used more efficiently and productivity increases. The sensors distributed in the field are interconnected in a “mesh” network. If a sensor node goes down, the network is self-healing; the nodes are able to connect with one another dynamically, finding another route to stay connected within the network.

Network Topologies:

It support three types of topologies. They are:

1) Star topology

2) Mesh topology

3) Cluster tree topology

Star Topology:

In the star topology, the communication is established between devices and a single central controller, called the PAN coordinator. The PAN coordinator may be mains powered while the devices will most likely be battery powered. Applications that benefit from this topology include home automation, personal computer (PC) peripherals, toys and games. After an FFD is activated for the first time, it may establish its own network and become the PAN coordinator. Each start network chooses a PAN identifier, which is not currently used by any other network within the radio sphere of influence. This allows each star network to operate independently.

Mesh Network:

A key component of the ZigBee protocol is the ability to support mesh networks. In a mesh network, nodes are interconnected with other nodes so that at least two pathways connect each node. Connections between nodes are dynamically updated.

In some cases, a partial mesh network is established with some of the nodes only connected to one other node.

Mesh networks are decentralized in nature; each node is self-routing and able to connect to other nodes as needed. The characteristics of mesh topology and ad-hoc routing provide greater stability in changing conditions or failure at single nodes

Cluster-tree Topology:

Cluster-tree network is a special case of a peer-to-peer network in which most devices are FFDs and an RFD may connect to a cluster-tree network as a leave node at the end of a branch. Any of the FFD can act as a coordinator and provide synchronization services to other devices and coordinators. Only one of these coordinators however is the PAN coordinator. The PAN coordinator forms the first cluster by establishing itself as the cluster head CLH) with a cluster identifier (CID) of zero, choosing an unused PAN identifier, and broadcasting beacon frames to neighboring devices. A candidate device receiving a beacon frame may request to join the network at the CLH. If the PAN coordinator permits the device to join, it will add this new device as a child device in its neighbor list. The advantage of this clustered structure is the increased coverage area at the cost of increased message latency.

SUCCESS FACTORS:

Zigbee is a low cost wireless technology, data type support, ease of installation, reliable data transfer, short range operation and is has reasonable battery life. Zigbee operates on unlicensed band and in unrestricted geographical use global implementation.

IEEE 802.15.4 protocol features:

Master/slave topology

Automatic network configuration

Dynamic slave device addressing

Virtual peer-to-peer links (pairing)

Full handshaking for packet transfers

Power management features

Up to 254 (+ master) network nodes

CSMA-CA channel access mechanism

15ms frame structure

TDMA slots can be allocated

28kbps & 250kbps data throughput

Service discovery

Low impact internet capability

ZigBee Applications:

Zigbee networks handle multiple traffic types with their own unique characteristics, including periodic data, intermittent data, and repetitive low latency data. The characteristics of each are as follows:

• Periodic data – application defined rate (e.g. wireless sensor or meter). Data is typically

handled using a beaconing system whereby the sensor wakes up at a set time and checks for the

beacon from the PAN coordinator, it then requests to joint the network. If the coordinator accepts

it, data is passed by the sensor before it goes to sleep again. This capability provides for very low

duty cycles.

• Intermittent data – either application or external stimulus defined rate (e.g. Wireless light

switch). Data can be handled in a beaconless system or disconnected. In disconnected operation,

the device will only attach to the network when communications is required thus saving

considerable energy.

• Repetitive low latency data – Allocations of time slots. (e.g. medical alerts and security systems). These applications may use the guaranteed time slot (GTS) capability when timeliness and critical data passage is required. GTS is a method of QoS that allows each device a specific duration of time as defined by the PAN coordinator in the Superframe to do whatever it requires without contention or latency. ZigBee networks are primarily intended for low duty cycle sensor networks (<1%). A new network node may be recognized and associated in about 30 ms. Waking up a sleeping node, accessing a channel and transmitting data takes about 15 ms respectively. ZigBee applications benefit from the ability to quickly attach information, detach, and go to deep sleep. These

procedures occur much faster than with a Bluetooth technology. Some examples where short-range, low-data, cheap wireless networks can be used are:

Automatic Meter Reading provides the usage statistics for Power Management and Energy Conservation whether it is electric, natural gas, water or other utilities.

Controlling the environment in HVAC systems. Lighting, temperature and other building controls help save utility usage and maintenance costs. Wireless monitoring and control systems remove expensive installation costs where wiring is difficult, extensive or part of a retrofit design.

ZigBee network can help in collecting the information necessary for an effective Inventory and Logistics Management. In fleet management, vehicles can automatically transmit logged information or receive updates when inside the fleet yard. Various control and automation scenarios are possible both for homes and industries

using cheap wireless communication including security systems and access control.

ZigBee enables broad-based deployment of wireless networks with low-cost, low-power solutions. It provides the ability to run for years on inexpensive batteries for a host of monitoring applications: Lighting controls, AMR (Automatic Meter Reading), smoke and CO detectors, wireless telemetry, HVAC control, heating control, home security, Environmental controls, drapery and shade controls, etc.

OUR IMPLEMENTATION-

WIRELESS KEYBOARD

Block Diagram

Our implementation is divided into two sections: Transmitter and Receiver.

Transmitter section:

Transmitter Section consists of following:

1. Keyboard as input device

2. PS2 connector

3. Uniboard

4. Xbee

Keyboard is connected to Uniboard through PS2 Connector.When a key is pressed the data is transmitted to UART0 at every falling edge of the clock pulse.

PS2 Connector’s data line is given to a port pin of ATMEGA128 and clock pin to

the externel INT pin.

Uniboard has the ATMega 128 MCU. It has two UARTs UART0,1. UART1 is directly

connected to serial port via IC MAX232.

Xbee is connected to the UART0 interface. MCU transmits data to UART0 and

hence forwarded to Xbee.

Receiver section:

Receiver section comprises of following:

1. Zigbee

2. Uniboard

3. PC (gtkterm) as o/p device.

Zigbee receives the data transmitted from zigbee on transmitter side using wireless communication and send it to UART0.

Uniboard provides same functionality as Tx. When a frame is received at UART0, it is processed to extract the required data from the whole frame and decode data to its equivalent ASCII char .

The character could be then either transmitted to UART1 via MAX232 and simultaneously display it to gtkterm or could be processed as user wants it to be.

Conclusion:

The ZigBee Standard enables the broad-based deployment of reliable wireless networks with low complexity, low cost solutions and provides the ability for a product to run for years on inexpensive primary batteries (for a typical monitoring application). It is also, of course, capable of inexpensively supporting robust mesh networking technologies ZigBee is all set to provide the consumers with ultimate flexibility, mobility, and ease of use by building wireless intelligence and capabilities into every day devices. The mission of the ZigBee Working Group is to bring about the existence of a broad range of interoperable consumer devices by establishing open industry specifications for unlicensed, untethered peripheral, control and entertainment devices requiring the lowest cost and lowest power consumption communications between compliant devices anywhere in and around the home.