Zigbee Interview Questions & Answers

  1. Question 1. What Is The Zigbee Alliance?

    Answer :

    The ZigBee Alliance is a non-profit industry consortium of leading semiconductor manufacturers, technology providers, OEMs and end-users worldwide. Members aim at defining a global specification for interoperable, cost-effective, low-power wireless applications based on the IEEE 802.15.4 standard. Current membership is about 200 and includes both heavyweights (such as Siemens and Texas Instruments) and small startups.

  2. Question 2. What Is The Goal Of The Zigbee Alliance?

    Answer :

    The goal of the ZigBee Alliance is to create an open specification defining mesh and tree network topologies with interoperable application profiles for wireless control systems. Its focus is clearly on standards-based, low-cost, low-power, and low-data rates applications. Means to certify products are also within the scope of the ZigBee Alliance.

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  4. Question 3. Who Is Supporting The Zigbee Alliance Today?

    Answer :

    • Industry leaders worldwide have committed to providing ZigBee-compliant products and solutions. Member companies include 13 promoters (BM Group, Ember, Free scale, Honeywell, Huawei, Mitsubishi Electric, Motorola, Philips, Samsung, Schneider Electric, Siemens, STMicroelectronics, and Texas Instruments) and participants that comprise semiconductor manufacturers, wireless intellectual property providers and OEMs.
    • Until recently the ZigBee Alliance used to be dominated by semiconductor manufacturers and technology providers. Since 2006 large OEMs like Siemens and Schneider Electric have joined the Board of Directors. They are expected to bring to the Alliance a more application-centric focus, which will be instrumental in achieving the promise of product and vendor interoperability.
    • Although the ZigBee Alliance used to be so far very US-centric, membership has significantly grown in Europe and Asia. Supporting companies are now spread all over the world.
  5. Question 4. What Are The Typical Applications Promoted By The Zigbee Alliance?

    Answer :

    Since its inception ZigBee technology has been designed as a general-purpose low-data rate, low-power wireless solution. Contrary to competing technologies (such as Z-Wave, which focuses on home control), ZigBee has a very wide application scope. Typical examples include home automation (lighting, heating, closures, security, access to set-top boxes), building automation (lighting, HVAC, smoke detection, access control), industrial monitoring, automatic meter reading, environmental data collection, and medical sensing.

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  7. Question 5. Which Zigbee Alliance Members Are Active In Residential And Building Automation?

    Answer :

    Control4, Danfoss, Eaton, Grundfos, Hitachi, Honeywell, Hubbell, Invensys, Johnson Controls, Legrand, Marlin Controls, Mitsubishi Electric, Nice, Niko, Philips, Schneider Electric, Siemens, Trane, Urmet Domus, Vantage Controls, Viconics, and Yamatake.

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  9. Question 6. Which Zigbee Alliance Members Are Active In Industrial Automation?

    Answer :

    Crane, Eaton, Grundfos, Hitachi, Honeywell, Invensys, Legrand, Mitsubishi Electric, Omron, Schneider Electric, Siemens, Yamatake, and Yokogawa.

  10. Question 7. Which Zigbee Alliance Members Are Active In Automated Metering?

    Answer :

    Holley Metering, Itron, and Schneider Electric.

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  12. Question 8. Are There Several Zigbee Versions?

    Answer :

    Currently, the public revision of the specification is ZigBee 1.0 (dated December 2004). It includes the network layer, the application layer, and the ‘Home Controls, Lighting’ (HCL) application profile (which will be superseded by a new profile in the next revision). ZigBee 1.0 does not include any commissioning recommendations, nor does it specify any particular stack profile since only tree addressing is defined.

    The next public revision of ZigBee is 1.1, which should be available by end 2006. It will include advanced features allowing to depart from current limitations imposed by tree addressing and centralized binding. New application profiles, along with corresponding commissioning frameworks, will also be released.

    Some vendors, such as Ember, provide ZigBee stacks that are ahead of the current specification. Advanced features are then often brought to the ZigBee Alliance to be included in the next official revision.

  13. Question 9. What Are The Various Zigbee Stack Profiles?

    Answer :

    ZigBee 1.1 will include two stack profiles:

    • ‘ZigBee’ (formerly ‘Home Controls’) targets simpler, smaller networks that typically operate in a residential environment. Addressing is performed in a tree fashion, security implementation is fairly simple, and application bindings take place in the coordinator in a centralized manner. The corresponding specification is now complete.
    • ‘ZigBee Pro’ (formerly ‘Commercial, Industrial and Institutional’) targets larger and more sophisticated networks. Addressing and routing are more scalable, security is more robust, and advanced features such as multicast are included. Also, ‘ZigBee Pro’ aims at providing minimal reliance on the coordinator through distributed application bindings. The corresponding specification should be complete by end 2006.
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  15. Question 10. What Are The Various Zigbee Application Profiles?

    Answer :

    ZigBee 1.0 includes the ‘Home Controls, Lighting’ (HCL) application profile, which provides basic definitions for simple residential lighting applications.

    ZigBee 1.1 will include additional application profiles:

    • ‘Home Automation’ (HA) replaces and expands HCL. It relies on ‘ZigBee’ stack profile and defines a set of devices for use in home environments: switches, thermostats, window shades, radiators, etc. The corresponding specification is almost complete.
    • ‘Commercial Building Automation’ (CBA) targets large building systems and relies on ‘ZigBee Pro’ stack profile. The specification includes device descriptions for lighting and HVAC management, for instance. It should be available by end 2006.
    • ‘Industrial Plant Monitoring’ (IPM) includes device definitions for sensors and actuators used in industrial control: temperature, pressure, infrared, etc. The corresponding specification should be complete by end 2006. 
    • Other ongoing initiatives cover additional applications within the scope of ZigBee:
    • ‘Wireless Sensor Applications’ (WSA) will provide features for decreasing power consumption in router devices and allow them to be in sleeping mode.
    • ‘Telecom Applications’ (TA) will address new scenarios around the use of mobile phones in residential or tertiary environments.
    • ‘Automatic Meter Reading’ (AMR) targets metering applications but is today on hold.
  16. Question 11. What Is The Zigbee Commissioning Framework (zcf)?

    Answer :

    The purpose of the ZigBee Commissioning Framework (ZCF) is to specify standardized ways of forming and configuring ZigBee networks. This includes the definition of general commissioning modes (fully automatic, with or without commissioning tool) and consistent procedures to activate ZigBee devices. The ZCF will be available in revision 1.1 and comes together with HA and CBA application profiles.

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  18. Question 12. What Are The Various Zigbee Certification Mechanisms?

    Answer :

    The ZigBee Alliance has defined three certification levels:

    • The ZigBee Compliant Platform (ZCP) certification is available today for hardware and software technology providers. So far 13 vendors have had their ZigBee platform certified.
    • The ZigBee Network Capable (ZNC) certification targets products that make use of a private ZigBee application profile in addition to a compliant platform. The policy is being designed to allow easy self-certification and the use of a logo.
    • The ZigBee Certified Product level provides interoperability with other vendors since it requires the use of a public ZigBee application profile in addition to a compliant platform. A specific test plan comes along each public application profile released by the ZigBee Alliance.
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  20. Question 13. Is It Possible To Deploy Zigbee Networks In Sub-ghz Bands?

    Answer :

    Yes, it is. ZigBee can use IEEE 802.15.4 physical interfaces transmitting at 868 MHz or 915 MHz. Data rates are reduced to 20 kbps and 40 kbps, respectively. The rest of the protocol stack is the same as in ZigBee implementations at 2.4 GHz.

  21. Question 14. What Is The Typical Communication Range Of Zigbee?

    Answer :

    Range depends on a number of factors including the surrounding environment, transceivers’ characteristics, antenna design, output power, etc. Some of these parameters can be optimized to increase the range in specific propagation conditions. Using today’s off-the shelf ZigBee radios at a nominal output power of 0 dBm, typical values are 10 to 50 m indoor and about 100 m outdoor, more in case of perfect line-of-sight.

  22. Question 15. What Is The Typical Battery Lifetime Of Zigbee End Devices?

    Answer :

    Battery lifetime is a function of many parameters such as battery type, capacity, duty cycling and end-use application. Different radios and micro-controllers feature different levels of power consumption, which ultimately affect battery lifetime. Today’s off-the shelf ZigBee components operating under low duty cycles allow for typical lifetimes comprised between 5 and 10 years. Applications like automatic meter reading that require even sparser transmissions may exceed 10 years.

    Further improvements in power consumption are foreseen with the increasing availability of single-chip ZigBee solutions (integrating the radio and the micro-controller on the same chip).

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  24. Question 16. Is It Possible To Have Battery-powered Zigbee Routers?

    Answer :

    ZigBee routers are today assumed to be mains-powered. The low-power router functionality is expected to be part of the ZigBee specification in the near future. This initiative is driven by the ‘Wireless Sensor Applications’ (WSA) work group.

  25. Question 17. Does The Zigbee Coordinator Represent A Single Point Of Failure?

    Answer :

    All ZigBee networks require a coordinator, which is responsible for network formation. In ZigBee 1.0 and in ‘ZigBee’ stack profile, the coordinator additionally hosts the binding table and this way becomes a weak point. Mechanisms to deal with coordinator breakdowns (such as duplication of the binding table) are implementation issues and out of the scope of revision 1.0.

    ZigBee 1.1 will host mechanisms to deal with coordinator failures and manage decentralized binding tables. The coordinator will not be any more at risk of becoming a single point of failure.

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  27. Question 18. How Is Addressing Performed In Zigbee?

    Answer :

    In ZigBee 1.0 and in ‘ZigBee’ stack profile, nodes are logically organized as a tree. Routers branch out in a tree-like manner from the coordinator, with end devices typically being sleepy devices. Addressing is performed hierarchically by following the tree structure.

    In ‘ZigBee Pro’ stacks profile, there is no logical tree structure. Network addresses are randomly attributed, with potential address conflicts resolved subsequently.

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  29. Question 19. How Is Routing Performed In Zigbee?

    Answer :

    In ZigBee 1.0 and in ‘ZigBee’ stack profile, routing is initially performed deterministically, along the branches of the tree structure. Tree routing means that resulting routes are sometimes indirect and assumes that the network topology is static. As a second step, table routing can be used to shortcut the tree and discover new routes.

    In ‘ZigBee Pro’ stack profile, only table routing is allowed. Along a route, each intermediate node uses its own routing table to forward the packet to the next node, until the packet reaches its destination. For a particular destination, each node stores the next hop information in its routing table.

  30. Question 20. What Is The Zigbee Cluster Library (zcl)?

    Answer :

    The ZigBee Cluster Library (ZCL) is a significant addition to revision 1.1. In ZigBee, a cluster is a message or collection of messages pertaining to a given application domain. Some devices (such as on/off switches) have the same definition and functionality whatever application profile is used. The idea behind creating the ZCL was to provide cluster reusability by abstracting clusters across several application domains and placing them in a library organized according to functional domains (e.g., lighting, closures, HVAC).

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  32. Question 21. Is Multipath Fading A Chief Concern For Zigbee?

    Answer :

    Multipath fading impacts all wireless transmissions. Resulting radio signals reach the receiving antenna by several paths, leading to signal interference and phase shifts that can cause errors and affect the link quality. This phenomenon tends to be overlooked in mesh networks since data packets can travel through alternative routing paths in case of communication failure. While multipath fading may not be a concern for densely meshed ZigBee networks, it surely is for single point-to-point links.

  33. Question 22. Are Zigbee Networks Scalable?

    Answer :

    ZigBee addressing scheme can support up to 65’535 nodes per coordinator, and multiple coordinators can be linked together to increase the overall size. This limit obviously is theoretical and does not guarantee proper operation of such a large network. Scalability will highly depend on the application requirements in terms of density, traffic load and acceptable latency.

    With little experimental material available for very large networks, it is however safe to assume that such big networks will be adequately organized into geographical clusters. Building on the ongoing ZigBee Alliance’s work regarding IP gateways, one can also imagine linking several ZigBee networks through high-speed IP backbones that already exist in many environments.

  34. Question 23. Are Zigbee Networks Secure?

    Answer :

    The ZigBee specification includes provisions for security and data integrity based on access control lists, packet freshness timers and 128-bit AES encryption. It is fairly safe to expect that these are adequate for residential and most commercial and industrial applications.

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  36. Question 24. Does Zigbee Have A Transport Layer?

    Answer :

    No, the ZigBee specification does not define a transport layer. Since most ZigBee data exchanges target simple and small messages, the question of ensuring end-to-end data delivery has been left to the product vendor. The latter can choose to include that capability as part of its application.

    Some stack vendors, such as Ember, provide an additional transport layer that guarantees reliable end-to-end messaging and simplifies application development. This feature is however not specified in the standard and will therefore be used in combination with private application profiles.

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  38. Question 25. Will Products Based On Different Zigbee Stack Profiles Be Interoperable?

    Answer :

    No, they will not. ‘ZigBee’ and ‘ZigBee Pro’ stack profiles do not rely on the same addressing and routing schemes, which prevent interoperability.

  39. Question 26. Is Zigbee A Competitor For Bluetooth?

    Answer :

    ZigBee and Bluetooth have been developed to serve different application spaces. Bluetooth targets higher data rates and better QoS to transport richer media (typically voice signals). ZigBee conveys much simpler messages across more scalable networks that require very low duty cycling without close synchronization.

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  41. Question 27. Is Zigbee A Competitor For Wifi (ieee 802.11b/g)?

    Answer :

    ZigBee and WiFi clearly address very distinct application requirements. Cost and power consumption patterns of WiFi cannot fit wireless control needs, whereas ZigBee bandwidth is by far too low to transmit large data flows.

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  43. Question 28. Is Zigbee A Competitor For Z-wave?

    Answer :

    ZigBee competes with Z-Wave in the home automation space. As a technology focused on the home control segment exclusively, Z-Wave has been faster than ZigBee in bringing its protocol stack to the market. While Z-Wave is a single-source technology, ZigBee enjoys multi-sourcing and relies on an IEEE standard. In addition, ZigBee provides higher data rates and operates in both 2.4 GHz and sub-GHz unlicensed bands.

  44. Question 29. Are There Yet Zigbee Products On The Market?

    Answer :

    Yes, there are. Some implement an actual ZigBee stack while others feature an ZigBee ready platform like Ember Net.

    Examples in the residential space include Control4 (lighting), Eaton (home automation), Golden Power Manufacturing (sprinklers and thermostats), Hawking Technology (home gateways), Kalirel (heating), Mija (fire extinguishers), Nice (shutters), and TSC Systems (home automation).

    Examples in the commercial space include Mija (fire extinguishers), Philips (lighting), Siemens (building automation), and TAC (building automation).

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