ZigBee and IEEE 802
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Transcript of ZigBee and IEEE 802
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ZigBee Technology
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Abstract
In todays modern world technological advancements are improving and developing
at a very fast pace. In this scenario the world is also facing a bigger crisis and that is power
crisis. The world today is in intense need of technologies that can save power and yet
enhance the efficiency. At this juncture ZigBee technology presents tremendous
opportunities design almost infinite life batteries.
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INTRODUCTION
ZigBee technology is a low data rate, low power consumption, low cost, wireless
networking protocol targeted towards automation and remote control applications. IEEE
802.15.4 committee started working on a low data rate standard a short while later. Then the
ZigBee Alliance and the IEEE decided to join forces and ZigBee is the commercial name for
this technology. ZigBee is expected to provide low cost and low power connectivity for
equipment that needs battery life as long as several months to several years but does not
require data transfer rates as high as those enabled by Bluetooth. In addition, ZigBee can be
implemented in mesh networks larger than is possible with Bluetooth. ZigBee compliant
wireless devices are expected to transmit 10-75 meters, depending on the RF environment
and the power output consumption required for a given application, and will operate in the
unlicensed RF worldwide(2.4GHz global, 915MHz Americas or 868 MHz Europe). The datarate is 250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz. IEEE and ZigBee
Alliance have been working closely to specify the entire protocol stack. IEEE 802.15.4
focuses on the specification of the lower two layers of the protocol(physical and data link
layer). On the other hand, ZigBee Alliance aims to provide the upper layers of the protocol
stack (from network to the application layer) for interoperable data networking, security
services and a range of wireless home and building control solutions, provide interoperability
compliance testing, marketing of the standard, advanced engineering for the evolution of the
standard. This will assure consumers to buy products from different manufacturers with
confidence that the products will work together.
IEEE 802.15.4 is now detailing the specification of PHY and MAC by offering
building blocks for different types of networking known as star, mesh, and cluster tree.Network routing schemes are designed to ensure power conservation, and low latency
through guaranteed time slots. A unique feature of ZigBee network layer is communication
redundancy eliminating singlepoint of failure in mesh networks. Key features of PHY
include energy and link quality detection, clear channel assessment for improved coexistence
with other wireless networks.
The name ZigBee is said to come from the domestic honeybee which uses a zig-zag
type of dance to communicate important information to other hive members. This
communication dance (the "ZigBee Principle") is what engineers are trying to emulate
with this protocol _ a bunch of separate and simple organisms that join together to
tackle complex tasks.
Components of WPANA ZigBee system consists of several components. The most basic is the device. A device can
be a full-function device (FFD) or reduced-function device (RFD). A network shall include at
least one FFD, operating as the PAN coordinator.
The Full Function Device (FFD) supports all IEEE 802.15.4 functions and features
specified by the standard. It can function as a network coordinator. 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).
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The Reduced Function Device (RFD) carries limited (as specified by the standard)
functionality to lower cost and complexity. It is generally found in network-edge
devices. The RFD can be used where extremely low power consumption is a
necessity.
Fig1 Zigbee network model
Network Topologies
Star TopologyIn 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.
Peer-to-peer TopologyIn peer-to-peer topology, there is also one PAN coordinator. In contrast to star topology, any
device can communicate with any other device as long as they are in range of one another. A
peer-to-peer network can be ad hoc, self-organizing and self-healing. Applications such as
industrial control and monitoring, wireless sensor networks, asset and inventory tracking
would benefit from such a topology. It also allows multiple hops to route messages from any
device to any other device in the network. It can provide reliability by multipath routing.
Cluster-tree TopologyCluster-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 neighbouring 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 neighbour list. The newly joined
device will add the CLH as its parent in its neighbour list and begin transmitting periodicbeacons such that other candidate devices may then join the network at that device. Once
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application or network requirements are met, the PAN coordinator may instruct a device to
become the CLH of a new cluster adjacent to the first one. The advantage of this clustered
structure is the increased coverage area at the cost of increased message latency.
Fig 2 different type of network topologies
ZigBee Architecture
ZigBee Stack System Requirements
Full protocol stack
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Network Layer
The responsibilities of the ZigBee NWK layer include:
Starting a network: The ability to successfully establish a new network.
Joining and leaving a network: The ability to gain membership (join) or
relinquish membership (leave) a network.
Configuring a new device: The ability to sufficiently configure the stack for
operation as required.
Addressing: The ability of a ZigBee coordinator to assign addresses to devices
joining the network.
Synchronization within a network: The ability for a device to achieve
synchronization with another device either through tracking beacons or by
polling. Security: applying security to outgoing frames and removing security to
terminating frames
Routing: routing frames to their intended destinations.
The network layer builds upon the IEEE 802.15.4 MACs features to allow
extensibility of coverage. Additional clusters can be added; networks can be
consolidated or split up.
Application layer
The ZigBee application layer consists of the APS sub-layer, the ZDO and the
manufacturer-defined application objects. The responsibilities of the APS sub layer include
maintaining tables for binding, which is the ability to match two devices together based on
their services and their needs, and forwarding messages between bound devices. Another
responsibility of the APS sub-layer is discovery, which is the ability to determine which other
devices are operating in the personal operating space of a device. The responsibilities of the
ZDO include defining the role of the device within the network (e.g., ZigBee coordinator or
end device), initiating and/or responding to binding requests and establishing a secure
relationship between network devices. The manufacturer-defined application objects
implement the actual applications according to the ZigBee-defined application descriptions.
PHYSICAL LayersThe PHY layer defines the physical and electrical characteristics of the network. The basic
task of the PHY layer is data transmission and reception. At the physical/electrical level, this
involves modulation and spreading techniques that map bits of information in such a way as
to allow them to travel through the air. Specifications for receiver sensitivity and transmit
output power are in the PHY layer.
The PHY layer is also responsible for the following tasks:
Enable/disable the radio transceiver
Link quality indication (LQI) for received packets
Energy detection (ED) within the current channel
Clear channel assessment (CCA)
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MAC Layer
The MAC layer defines how multiple 802.15.4 radios operating in the same area will share
the airwaves. This includes coordinating transceiver access to the shared radio link and the
scheduling and routing of data frames. There are network association and disassociation
functions embedded in the MAC layer. These functions support the self-configuration and
peer-to-peer communication features of a ZigBee network.
The MAC layer is responsible for the following tasks:
Beacon generation if device is a coordinator
Implementing carrier sense multiple access with collision avoidance (CSMA-CA)
Handling guaranteed time slot (GTS) mechanism
Data transfer services for upper layers
IEEE802.15.4 MAC is flexible enough to handle each of these types.
Periodic data can be handled using the beaconing system whereby thesensor will wake up for the beacon, check for any messages and then go
back to sleep.
Intermittent data can be handled either in a beaconless system or in adisconnected fashion. In a disconnected operation the device will only
attach to the network when it needs to communicate saving significant
energy.
Low latency applications may choose to the guaranteed time slot (GTS)option. GTS is a method of QoS in that it allows each device a specific
duration of time each Super frame to do whatever it wishes to do without
contention or latency.
Non-beacon network communication Beacon network communication
Fig 3 MAC Data Service Diagrams
Frame Structure
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The frame structures have been designed to keep the complexity to a minimum
while at the same time making them sufficiently robust for transmission on a
noisy channel. Each successive protocol layer adds to the structure with layer specific
headers and footers.
The IEEE 802.15.4 MAC defines four frame structures:
A beacon frame, used by a coordinator to transmit beacons. A data frame, used for all transfers of data. An acknowledgment frame, used for confirming successful frame reception. A MAC command frame, used for handling all MAC peer entity control
transfers.
SecurityWhen security of MAC layer frames is desired, ZigBee uses MAC layer security to
secure MAC command, beacon, and acknowledgement frames. ZigBee may secure messages
transmitted over a single hop using secured MAC data frames, but for multi-hop messaging
ZigBee relies upon upper layers (such as the NWK layer) for security. The MAC layer usesthe Advanced Encryption Standard (AES) as its core cryptographic algorithm and describes a
variety of security suites that use the AES algorithm. These suites can protect the
confidentiality, integrity, and authenticity of MAC frames. The MAC layer does the security
processing, but the upper layers, which set up the keys and determine the security levels to
use, control this processing. When the MAC layer transmits (receives) a frame with security
enabled, it looks at the destination (source) of the frame, retrieves the key associated with that
destination (source), and then uses this key to process the frame according to the security
suite designated for the key being used. Each key is associated with a single security suite
and the MAC frame header has a bit that specifies whether security for a frame is enabled or
disabled.
When transmitting a frame, if integrity is required, the MAC header and payload dataare used in calculations to create a Message Integrity Code (MIC) consisting of 4, 8, or 16
octets. The MIC is right appended to the MAC payload. If confidentiality is required, the
MAC frame payload is also left appended with frame and sequence counts (data used to form
a nonce). The nonce is used when encrypting the payload and also ensures freshness to
prevent replay attacks. Upon receipt of a frame, if a MIC is present, it is verified and if the
payload is encrypted, it is decrypted. Sending devices will increase the frame count with
every message sent and receiving devices will keep track of the last received count from each
sending device. If a message with an old count is detected, it is flagged with a security error.
The MAC layer security suites are based on three modes of operation. Encryption at the
MAC layer is done using AES in Counter (CTR) mode and integrity is done using AES in
Cipher Block Chaining (CBCMAC) mode. A combination of encryption and integrity is doneusing a mixture of CTR and CBC- MAC modes called the CCM mode.
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Properties of 802.15.4
802.15.4 defines operation in three license-free industrial scientific medical (ISM) frequency
bands. Below is a table that summarizes the properties of IEEE 802.15.4 in two of the ISM
frequency bands: 915 MHz and 2.4 GHz.
Zigbee features
(1) Low power consumption, simply implemented.
Users expect batteries to last many months to years! Consider that a typicalsingle family house has about 6 smoke/CO detectors. If the batteries for
each one only lasted six months, the home owner would be replacing
batteries every month.
Bluetooth has many different modes and states depending upon latencyand power requirements such as sniff, park, hold, active, etc.; ZigBee/IEEE
802.15.4 has active (transmit/receive) or sleep. Application software needs
to focus on the application, not on which power mode is optimum for each
aspect of operation.
Even mains powered equipment needs to be conscious of energy. Consider a futurehome with 100 wireless control/sensor devices.
(2) Low cost (device, installation, maintenance)Low cost to the users means low device cost, low installation cost and low
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maintenance. ZigBee devices allow batteries to last up to years using
primary cells (low cost) without any chargers (low cost and easy
installation). ZigBees simplicity allows for inherent configuration and
redundancy of network devices provides low maintenance.
(3) High density of nodes per networkZigBees use of the IEEE 802.15.4 PHY and MAC allows networks to
handle any number of devices. This attribute is critical for massive sensor
arrays and control networks.
(4) Simple protocol, global implementation
ZigBees protocol code stack is estimated to be about 1/4th of Bluetooths or
802.11s. Simplicity is essential to cost, interoperability, and maintenance.
The IEEE 802.15.4 PHY adopted by ZigBee has been designed for the 868
MHz band in Europe, the 915 MHz band in N America, Australia, etc; and
the 2.4 GHz band is now recognized to be a global band accepted in almost
all countries.
ZigBee Applications
Home and office automation
Industrial automation
Medical monitoring
Low-power sensors
HVAC control
Plus many other control and monitoring uses
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Comparison of wireless standards
The demand for wireless solutions continues to grow and with it new standards have come
forward and other existing standards have strengthened their position in the marketplace. This
section compares three popular wireless standards being used today and lists some of the
design considerations that differentiate them.
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Whether ZigBee and Bluetooth are competitors or complements?
Bluetooth seems best suited for:
Synchronization of cell phone to PDA Hands-free audio PDA to printerWhile ZigBee is better suited for:
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Controls Sensors Lots of devices Low duty cycle Small data packets Long battery life is critical
Bluetooth Battery Drain
Packet length can affect battery drain. Typically the shorter the packet the
quicker the device can go to sleep. Bluetooth is a slotted protocol.
Communication can occur in either: 625 S, 1875 S, or 3125 S slots.
The following graph showing effective data rate was based upon the
transmissions speeds stated in Bluetooth v1.1 and IEEE 802.15.4 draft 18,
using the 250 kb/s rate. The general trend is that at larger packet sizes the
effective data rate approaches the raw data rate. The peaks for the Bluetooth rate are a result
of the three slot sizes, when a packet becomes too big for one slot it must increment to thenext slot even though it doesnt fill the whole slot allocation.
ZigBee Battery DrainA typical scenario for sensors and control devices is to remain connected to the network.
We use connected to mean that the device periodically listens for incoming packets. In this
manner the devices behaviour may be altered or at least checked to verify correctness.
Timing Considerations
ZigBee devices can quickly attach, exchange information, detach, and then
go to deep sleep to achieve a very long battery life. Bluetooth devices
require about ~100X the energy for this operation.
Power Considerations
ZigBee
2+ years from normal batteries Designed to optimize slave power requirements
Bluetooth
Power model as a mobile phone (regular daily charging) Designed to maximize ad-hoc functionality
Comparison Summary
ZigBee and Bluetooth are two solutions for two different application areas. The differences are from their approach to their desired application. Bluetooth has
addressed a voice application by embodying a fast frequency hopping system with a
master slave protocol. ZigBee has addressed sensors, controls, and other short
message applications by embodying a direct sequence system with a star or peer to
peer protocols.
Minor changes to Bluetooth or ZigBee wont change their inherent behaviour orcharacteristics. The different behaviours come from architectural differences.
Conclusion
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It is likely that ZigBee will increasingly play an important role in the future of
computer and communication technology. In terms of its low cost and less power
consumption. In upcoming days ZigBee is used for several application, it has that much level
of security (AES). Bluetooth and ZigBee not at all a competitors both are designed for
different purpose.
References
ZigBee Technology: Wireless Control that Simply works by Patrick Kinney ZigBee Specifications v1.0 Designing with 802.15.4 and ZigBee, Presentation Slides, available on ZigBee.org
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ZigBee Tutorial,http://www.tutorial-reports.com/wireless/zigbee IEEE 802.15.4 Specification
http://www.tutorial-reports.com/wireless/zigbeehttp://www.tutorial-reports.com/wireless/zigbeehttp://www.tutorial-reports.com/wireless/zigbeehttp://www.tutorial-reports.com/wireless/zigbee