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Wireless Hart Standard Of Wireless Sensor Network

Discussion in 'WirelessHART' started by Homaei, Nov 10, 2013.

  1. Homaei

    Homaei Administrator Staff Member

    Wireless HART - Introduction
    • HART technology has been serving the process automation industry for the last 20 years.
    • The HART communication protocol is an open standard, mast er-slave token passing network protocol, where devices are connected over 4- 20 mA analog loop.
    • WirelessHART is a key part of the HART field communications protocol revision 7.1.
    • It adds Wireless capability with existing HART devices, commands and tools.
    • It is compatible with existing HART devices and applications
    • It is a secure and robust mesh networking technology operating in the 2.4 GHz ISM radio band.
    • It uses IEEE 802.15.4 compatible DSSS radios with channel hopping on a packet by packet basis.
    • It uses TDMA technology to arbitrate and co-ordinate communications between the network devices.
    • It is designed to be simple, reliable and useful.
    • All Wireless HART devices shall support Device Description Language (DDL)
    WirelessHART Stack: Physical Layer
    
    • It is based on IEEE 802.15.4-2006 with a data rate of 250Kbps and operating frequency of 2400 – 2483.5 Mhz.
    • Modulation used is O-QPSK with Direct Sequence Spread Spectrum (DSSS).
    • The transmit power is 10 dBm (nominal) adjustable in discrete steps.
    • The Physical layer PDU is IEEE 802.15.4-2006 compliant with a maximum payload of 127 bytes.

    WirelessHART Stack: Data-Link Layer

    • Data link layer is responsible for the secure, reliable, error free communication of data between HART compatible devices.
    • The MAC PDU is IEEE 802.15.4 compliant. 
    • For collision free communication, WirelessHART uses TDMA and channel hopping.
    • TDMA uses a time slot where communication between devices occur.
    • A series of time slots ( 100 timeslots per second ) form a TDMA Superframe.
    • Superframes are repeated continuously and one Super frame is always enabled while additional Superframes can be enabled or disabled.Super frame length is fixed when it is active. Its length can be modified when inactive and different Superframes may have lengths that differ from each other.
    • Acknowledgement packets include timing information to continuously synchronize TDMA operation across the network.
    WirelessHART Stack: Network Layer
    • This layer provides routing and end-to-end security. Two types of routing are supported – Graph routing & Source routing. All devices must support source & graph routing.
    • In Graph routing, each device maintains a graph table with a list of graph ids. When the device receives a packet containing a graph id, information in the graph table is used to decide the next hop.
    • In source routing, unlike graph routing, the routing device needs no extra information, as the sending device embeds the entire route in the packet as an ordered list of nodes.
    • Network statistics are maintained and routinely published
    • Reports discovered neighbors, disconnected and dropped neighbors
    WirelessHART Stack: Transport Layer
    • This is a thin layer in WirelessHART, which ensures reliable data transmission.
    • It manages sessions for end to end communication with corresponding devices.
    • Data sets automatically segmented at source device and re-assembled at destination
    WirelessHART Stack: Application Layer

    • WirelessHART uses the standard HART application layer, which is command based. Universal, Common Practice, Device Family and Wireless commands are specified.
    • Universal commands – This is a collection of commands that must be supported by all WirelessHART devices.
    • Common Practice commands - These apply to a wide range of devices. Implementation is optional.
    • Device Specific Commands – These commands are specified by the manufacturer according to the needs of the field device. Implementation is optional.
    • Wireless commands – A collection of commands to support WirelessHART products. All devices supporting Wireless HART must implement these commands.
    Wireless HART Security

    • Utilizes AES-128 block ciphers with symmetric keys
    • Supports multiple Key architecture
      • Separate Join Key per device
      • Network key to authenticate Data-Link PDUs
      • Session key between end-point devices
    • Key management is not part of WirelessHART stack. It is part of plant policy.



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    Referenced by: http://www.wsnlab.org
    Author: Mohammah Hossein Homaei
    Wireless Sensor Networks Laboratory of Iran
    www.wsnlab.org

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