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This standard defines a communication layer at level 2 in the OSI (Open System Interconnection) model.
It used Direct Sequence Spread Spectrum (DSSS) to modulate the information before being sent to the physical layer. This causes less interference in the frequency bands used and improves the Signal to Noise Ratio (SNR) in the receiver due to the fact that is easier to detect and decode the message which is being sent by the transmitter. There are different DSSS modulations depending on the hardware physical limits of the circuit and number of symbols which can be processed at a given time. Once a node has joined to the network can send information to its brothers through the routers which are always awake waiting for the packets. The concept "Mesh Network" relays in the Ad hoc communications, also called peer to peer (P2P). The proliferation of wireless communication devices in daily surroundings shows no sign of stagnation. For the last few years, we've witnessed a great expansion of remote control devices in our day-to-day life. To interact with all these remotely controlled devices, we'll need to put them under a single standardized control interface that can interconnect into a network, specifically a HAN or home-area network. Right now, the more remotely controlled devices we install in our homes, the more remotes we accumulate. Some modern IR remotes enable you to control multiple devices by "learning" transmitting codes.
A HAN can solve both problems because it doesn't need line-of-sight communication and because a single remote (or other type of control unit) can command many devices. Of the few attempts to establish a standard for home networking that would control various home appliances, the X-10 protocol is one of the oldest. In the last few years, new wireless local area networks (WLANs) such as Wi-Fi and Bluetooth became available. If we take a look at the type of data that circulates within a network of sensors and actuators, we may find that most of it is small packets that control devices or obtain their status. A home network should also support different configurations, such as a star or mesh network, to effectively cover a household area of 30 to 70 meters. ZigBee is a home-area network designed specifically to replace the proliferation of individual remote controls. The ZigBee Alliance will also serve as the official test and certification group for ZigBee devices. A MAC command frame provides the mechanism for remote control and configuration of client nodes.
Finally, the beacon frame wakes up client devices, which listen for their address and go back to sleep if they don't receive it. The reduced function device (RFD) carries limited (as specified by the standard) functionality to lower cost and complexity. Ultra-low power consumption is how ZigBee technology promotes a long lifetime for devices with nonrechargeable batteries.
The non-beacon mode is a simple, traditional multiple-access system used in simple peer and near-peer networks. Beacon mode is a mechanism for controlling power consumption in extended networks such as cluster tree or mesh. Non-beacon mode is typically used for security systems where client units, such as intrusion sensors, motion detectors, and glass-break detectors, sleep 99.999% of the time.
This timing requirement may have an impact on the cost of the timing circuit in each end device. The actual security implementation is specified by the implementer using a standardized toolbox of ZigBee security software.
The NWK layer associates or dissociates devices using the network coordinator, implements security, and routes frames to their intended destination.
The NWK layer supports multiple network topologies including star, cluster tree, and mesh, all of which are shown in Figure 3.
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. 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 General Operation Framework (GOF) is a glue layer between applications and rest of the protocol stack.
Actual application profiles are defined in the individual profiles of the IEEE's working groups. As Figure 4 shows, a typical ZigBee-enabled device includes a radio frequency integrated circuit (RF IC) with a partially implemented PHY layer connected to a low-power, low-voltage 8-bit microcontroller with peripherals, connected to an application sensor or actuators. Mikhail Galeev is a senior engineer at Motorola with seven years experience in firmware design for embedded systems. The functions of the Coordinator, which usually remains in the receptive mode, encompass network set-up, beacon transmission, node management, storage of node information and message routing between nodes.
The network node, however, is meant to save energy (and so ‘sleeps' for long periods) and its functions include searching for network availability, data transfer, checks for pending data and queries for data from the coordinator. For the sake of simplicity without jeopardising robustness, this particular IEEE standard defines a quartet frame structure and a super-frame structure used optionally only by the coordinator. These frame structures and the coordinator's super-frame structure play critical roles in security of data and integrity in transmission. All protocol layers contribute headers and footers to the frame structure, such that the total overheads for each data packet range are from 15 octets (for short addresses) to 31 octets (for 64-bit addresses). In the world of wireless communications, the name ZigBee is becoming as readily recognised as Wi-Fi or Bluetooth.
Like those technologies, ZigBee defines a short-range wireless networking capability that spans relevant layers of the OSI 7-layer network communications model, from the physical layer through to the application layer. ZigBee has been developed by the ZigBee alliance to provide an open-source wireless networking standard aimed at monitoring, control and sensor applications, where responsiveness is more important for real-time operation than high bandwidth. It does this in a way that avoids the complexities of other WPAN standards, like Bluetooth, to deliver a low-latency solution; allowing devices to remain in a low-power sleep mode most of the time while still being able to respond quickly when required – exactly what is needed for control and monitoring applications. A particular benefit of the higher frequency is that it allows more integrated designs with fewer external components. The ubiquity of such highly integrated but also low power and low cost solutions clearly satisfies the primary objective of the ZigBee Alliance, enabling equipment manufacturers to develop interoperable products for its target markets in building automation, smart energy, lighting, health care and consumer electronics. Mesh networks allow devices to communicate directly with one another or through a neighbour.
A typical mesh network, as illustrated in figure 2, may comprise of both fixed and mobile nodes along with a gateway node (to connect to the main system controller or a computer network).
The mobile nodes might be portable instruments or perhaps body-worn devices used for healthcare monitoring but in either case an application where the ZigBee transceiver is serving a regular data communications function. Assuming the transmit and receive antennas are located together the range or distance (D) from antenna to object can be determined by measuring the running time (t) taken by the signal to travel from antenna to object and back again, given that the propagation of radio waves is the same as the speed of light (c). Secondly, the requirement is to measure the distance to a specific node on the network not simply the range of an unidentified object.
The drawback to this approach is that the RSSI performance of each fixed node requires calibration at various distances and through different mediums like walls and glass that affect signal attenuation compared to free space.
A design approach, which is more akin to radar in measuring propagation delay, is what chip supplier Atmel refers to as ‘Active Reflector Technology’, described below.
Passive ranging can measure distance based on the running time of a reflected pulse (as above) or by measuring the phase difference between the transmitted and received signal. Instead, the active system shown on the right decouples the transmitting and receiving operations between the node initiating the ranging process (device A) and the node that is acting as a reflector (device B). This is illustrated by figure 4, where the initiator steps through a set of frequencies where ?f is the frequency step (say around 1MHz). Reversing the roles of the initiator and reflector allows these measurements to be repeated with transmission in the opposite direction, improving accuracy by averaging results and cancelling out any non-symmetric errors. Implementing all this firstly requires a ZigBee transceiver that provides hardware support for phase measurement. Unlike radar, ZigBee antennas are not usually directional so location based on range and bearing from a single position is not possible. ZigBee provides a standardised solution for short-range wireless communications in control and monitoring applications, which require near real time performance but relatively low-bandwidth.
ZigBee was not originally designed to provide the functionality for locating mobile nodes and yet, with hardware support for phase measurement available in some transceivers and the implementation of extensions to the software stack, this has become possible as demonstrated by the hardware and software solutions.

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Its main purpose is to let the communication between two devices.It was created by the Institute of Electrical and Electronics Engineers (IEEE), entity which main task is to set standards so that technological developments can count with a common platform of rules to be set over.
Its main purpose is to create a network topology (hierarchy) to let a number of devices communicate among them and to set extra communication features such as authentication, encryption, association and in the upper layer application services.
Basically, each bit of information to be transmitted is modulated into 4 different signals (another kind of bits alphabet), this process causes the total information to be transmitted to occupy a larger bandwidth but it uses a lower spectral power density for each signal. This systems uses a centralized node (PAN coordinator) which gives slots of time to each node so that any knows when they have to transmit.
It means the transceiver can be sleeping most of the time (up to 99% on average) while the receiving and sending tasks can be set to take just a small part of the devices' energy.
It is being developed by the Systems and Automation Society (ISA) and tries to be an standard for this kind of projects. One example is the Digi's own mesh protocol (DigiMesh), which is though to be a completely distributed mesh protocol where all the nodes can sleep and route their brother's packets. The first thing a node (route or end device) which want to join the network has to do is to ask to the coordinator for a network address (16b), as part of the association process. When the router gets the packet and the destination is in its radio of signal, the router first looks if the destination end device is awake or slept. To create a completely mesh network such as the one showed in the image below all the nodes have to have the same role, all of them have to be "end devices + routers" so that they can route their brothers information and sleep when no action is required (saving energy).
It is also obvious that no single technology is capable of satisfying all the needs of the vast number of applications within private or industrial environments. Five years ago, infrared (IR) remotes for the television were the only such devices in our homes. Devices such as TVs, garage door openers, and light and fan controls predominantly support one-way, point-to-point control. But because the range for IR control is limited by line of sight, they're used predominantly for home entertainment control.
It was introduced in 1978 for the Sears Home Control System and the Radio Shack Plug'n Power System. A binary 1 is represented by a 1ms burst of 120KHz at the zero-cross point and binary 0 by the absence of 120KHz. It enables plug-and-play operation with any home appliance and doesn't require special knowledge to configure and operate a home network. The effective data transfer rate is 60bps, too slow for any meaningful data communication between nodes. For many applications, such as wireless smoke and CO2 detectors or wireless home security, the device mostly stays in deep-sleep mode and only sends a short burst of information if a trigger event occurs. ZigBee was created to satisfy the market's need for a cost-effective, standards-based wireless network that supports low data rates, low power consumption, security, and reliability. ZigBee is the only standards-based technology that addresses the needs of most remote monitoring and control and sensory network applications. Following the standard Open Systems Interconnection (OSI) reference model, ZigBee's protocol stack is structured in layers. It provides feedback from the receiver to the sender confirming that the packet was received without error. A centralized network manager uses MAC to configure individual clients' command frames no matter how large the network.
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. For a non"beacon network, a standard ALOHA CSMA-CA (carrier-sense medium-access with collision avoidance) communicates with positive acknowledgement for successfully received packets. The address field within the MAC can contain both source and destination address information (needed for peer-to-peer operation). It's the most sophisticated of the three types and requires the most memory and computing power. Beacons are used to synchronize the network devices, identify the HAN, and describe the structure of the superframe. It operates like a two-way radio network, where each client is autonomous and can initiate a conversation at will, but could interfere with others unintentionally. Remote units wake up on a regular, yet random, basis to announce their continued presence in the network. Client units listen for the network coordinator's beacon (broadcast at intervals between 0.015 and 252s). Longer intervals of sleep mean that the timer must be more accurate or turn on earlier to make sure that the beacon is heard, both of which will increase receiver power consumption. In addition, the NWK layer of the network coordinator is responsible for starting a new network and assigning an address to newly associated devices. For network-edge devices with limited capabilities, the stack requires about 4Kb of the memory.
Predictions for the future of ZigBee-enabled devices are a popular topic for numerous market-research firms.
Backed by IEEE, ZigBee has the potential to unify methods of data communication for sensors, actuators, appliances, and asset-tracking devices. He holds a BS in applied physics from Rostov State University, Russia, and an MSEE from the University of South Alabama, Mobile.
This interval is determined a priori and the coordinator thus enables sixteen time slots of identical width between beacons so that channel access is contention-less. Hence there are products available today from many manufacturers, which combine both a radio transceiver and a microcontroller in a single chip solution. One such capability is ranging, which can be used to locate mobile nodes within a ZigBee mesh network. A key benefit of such decentralised operation is the ability to dynamically reconfigure data pathways to take account of changing situations or difficult conditions e.g. When used for location purposes, the fixed nodes (sometimes called ‘anchor’ nodes) provide the reference points for locating the mobile nodes, in much the same way as satellites in GPS systems. However the concept of ranging and location using ZigBee introduces the possibility of this being its sole purpose in tagging inventory or even people, particularly within a building where GPS performance can be a problem.
Actually herein lies a solution to both these issues – by addressing the target node, it can be directly involved in the ranging process.
The latter method is only valid if the wavelength of the transmitted signal is at least twice the distance to be measured (to allow for the return path).
The reflector then measures the phase of the fractional part of the wavelength of each received frequency relative to a local reference, from which it is possible to calculate the required phase difference as ?? = ?2 – ?1 and hence the distance D. It then requires software extensions to the ZigBee protocol that enable the ranging function with an API that allows dedicated ranging frames to be transmitted in place of normal data or MAC frames.
Location, or localisation as it is sometimes called, is however a natural extrapolation of the ranging process.
It also enables the flexibility and resilience of mesh networking with the ability to add or remove nodes as required and to tolerate node failures.
This opens up a whole new field of applications for ZigBee in security for asset tagging or access control.
Electronics Weekly is at the heart of the electronics industry and is reaching an audience of more than 120,000 people each month. Here the digital information units (bits) are managed and organized to become electromagnetic impulses (waves) on the lower level, the physical one. If the energy found higher of a specific level the node the transceiver waits during a random time (including in an interval) and tries again.
All the information in the network is routed using this address and not the 64b MAC address. In the first case the router sends the packet to the end device, however if it is sleeping, the router will bufferize the packet untill the end device node gets awake and ask for news to the router.
They have to be able to discover each other and send broadcast messages to all the brothers ("hello!, is there anybody out there?").
In the quest for multimedia-capable data rates, the need of low cost, low power radio solutions for fairly simple applications was, until recently, not covered by a suitable standardised technology. Now I quickly run out of fingers as I count the devices and appliances I can control remotely in my house. This article will introduce you to ZigBee—how it works and how it may be more appropriate than simply accumulating more remotes.

The network consists of transmitter units, receiver units, and bidirectional units that can receive and transmit X-10 commands.
Wireless cameras for remote monitoring are an example of how to employ those technologies in home automation and control areas. The transmission distance is expected to range from 10 to 75m, depending on power output and environmental characteristics. This dual address information is used in mesh networks to prevent a single point of failure within the network. Additional memory and computing power make it ideal for network router functions or it could be used in network-edge devices (where the network touches the real world).
For most of the time, a slave device is in deep-sleep mode and wakes up only for a fraction of a second to confirm its presence in the network. The beacon intervals are set by the network coordinator and vary from 15ms to over 4 minutes. Here, the two-way radio network has a central dispatcher that manages the channel and arranges the calls. When an event occurs, the sensor wakes up instantly and transmits the alert ("Somebody's on the front porch"). Longer sleep intervals also mean the timer must improve the quality of the timing oscillator circuit (which increases cost) or control the maximum period of time between beacons to not exceed 252s, keeping oscillator circuit costs low. It includes subaddressing and addressing modes and device descriptions, such as type of device, power source, sleep modes, and coordinators. Currently, a ZigBee chip set costs about $7, but that price should fall to $2 after market acceptance. But as with any crystal ball reading, the results of those analyses are subject to interpretation. It offers a means to build a reliable but affordable network backbone that takes advantage of battery-operated devices with a low data rate and a low duty cycle.
Passive ranging, as used in conventional radar systems, works on the principle of a transmitted radio wave pulse being reflected by a target object.
One method for doing this utilises the received signal strength indicator (RSSI) feature that many ZigBee radios provide. Range measurements from several fixed nodes can be used to calculate the location of a mobile ZigBee node either in a single plane (2D) or within a 3-dimensional environment. With the ATLOC Location Evaluation Kit, it is possible to demonstrate the use of the AT86RF233 device in location applications, using software from partner company Agilion for coordinate calculation.
As a first step a node must to send to the PAN coordinator a GTS request message, as response the coordinador will send a beacon message containing the slot allocated and the number os slots assigned. This number will only increase as more devices are controlled or monitored from a distance.
Because most remotely controlled devices are proprietary and not standardized among manufacturers, even those remotes used for the same function (like turning on and off lights) are not interchangeable with similar remotes from different manufacturers. Receiving units work as remote control power switches to control home appliances or as remote control dimmers for lamps.
For any power appliances, the X-10 transmission looks like noise and is subject to removal by the power line filters. The alliance is working closely with the IEEE to ensure an integrated, complete, and interoperable network for the market. Like Wi-Fi, Zigbee uses direct-sequence spread spectrum in the 2.4GHz band, with offset-quadrature phase-shift keying modulation.
The superframe is set up by the network coordinator to transmit beacons at predetermined intervals (multiples of 15.38ms, up to 252s) and provides 16 equal-width time slots between beacons for contention-free channel access in each time slot. For example, the transition from sleep mode to data transition is around 15ms and new slave enumeration typically takes just 30ms. The network coordinator, powered from the main source, has its receiver on all the time and can therefore wait to hear from each of these stations.
If no messages are pending, the client returns to sleep, awaking on a schedule specified by the coordinator. Using local addressing, you can configure simple networks of more than 65,000 (216) nodes, thereby reducing address overhead. It includes switching and dimming load controllers, corresponding remote-control devices, and occupancy and light sensors.
The network coordinator may require extra RAM for a node devices database and for transaction and pairing tables. ZigBee can be used in many applications, from industrial automation, utility metering, and building control to even toys. Nonetheless, the coordinator provides as many as seven GTS (guaranteed time slots) for every beacon interval to ensure better quality. Waspmote and SquidBee use the CSMA-CA method due to the fact that it is inherent to each mote and does not depend on a central server. In other words, you'll have as many separate remote control units as you have devices to control. The transmitter unit is typically a normally-open switch that sends a predefined X-10 command if the switch is closed. Reliability and security issues rule out the use of the X-10 network for critical household applications like remote control of an entry door.
Since the network coordinator has an "infinite" source of power it can allow clients to sleep for unlimited periods of time, enabling them to save power.
There area some special frames like the ACK packets which doesn't  require this method to be performed.
They can be sleeping for minutes or hours and wake up all at the same time to perform an Adhoc communication creating a mesh network just when really needed.
In addition, the compliant system implementations will enable long battery life by using the power-saving features at the physical (PHY), media control access (MAC) and network layers specified by this standard. The X-10 commands enable you to change the status of the appliance unit (turn it on or off) or to control the status of a lamp unit (on, off, dim, bright). The 868 and 900MHz bands also use direct-sequence spread spectrum but with binary-phase-shift keying modulation. However, the network coordinator can dedicate up to seven guaranteed time slots for noncontention based or low-latency delivery.
This follows from the number of remote controlled devices (or devices that may be connected wirelessly) in the average household.
However, the network coordinator can dedicate up to seven guaranteed time slots per beacon interval for quality of service. Such a compact footprint enables you to run Zigbee on a simple 8-bit microcontroller such as an HC08- or 8051-based processor core. This cost-effective and easy-to-use home network potentially creates a whole new ecosystem of interconnected home appliances, light and climate control systems, and security and sensor subnetworks.
The applications of this Wireless Personal Area Network (WPAN) technology ranges from industrial monitoring and control, home automation and sensor networks to gaming, medical and automotive solutions. The air interface is direct sequence spread spectrum (DSSS) using BPSK for 868 MHz and 915 MHz, and O-QPSK for 2.4 GHz.
The operational power-saving features include low duty cycle operation together with strict power management and low transmission overhead. The implementation of standard-compliant radios-on-chip is mainly governed by the PHY specification. At the PHY level, which is the main topic of this discussion, low power silicon devices can be implemented by exploiting relatively low and controllable output power, constant-envelope modulation, and relatively relaxed channel spacing and blocking requirements. EVM is the scalar distance between the end points of the two phasor representing the ideal and actual measured chip positions.
Expressed another way, it is the residual noise and distortion remaining after an ideal version of the signal has been stripped away. His work experience includes research and development of terrestrial cellular and satellite mobile phones, and design of RF CMOS circuits and systems for wireless communications.

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