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Chapter 4: IEEE 802.15.4 Based Wireless Sensor Network Design for Smart Grid Communications

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Title: Chapter 4: IEEE 802.15.4 Based Wireless Sensor Network Design for Smart Grid Communications


1
Chapter 4 IEEE 802.15.4 Based Wireless Sensor
Network Design for Smart Grid Communications
HANDBOOK ON GREEN INFORMATION AND COMMUNICATION
SYSTEMS
  • Chun-Hao Lo and Nirwan Ansari
  • Advanced Networking Laboratory
  • Department of Electrical Computer Engineering
  • New Jersey Institute of Technology
  • Newark, New Jersey, USA

2
Agenda
  • Wireless sensor networks (WSN) and associated
    applications supported in Smart Grid
    Communications
  • A comparison of IEEE 802.15.4 and Power Line
    Communications technologies
  • An introduction of IEEE 802.15 Task Groups,
    particularly the IEEE 802.15.4g Task Group (TG4g)
    in developing the Smart Utility
    Network/Neighborhood (SUN) design
  • Discussion of studies and challenges in IEEE
    802.15.4 LR-WPAN with respect to network design
    in PHY/MAC layers, fairness, routing, and
    security/privacy issues
  • Conclusions

3
WSNs in Smart Grid Communications (1/4)
  • Wireless Sensor Networks (WSNs) are deployed
    throughout the electric power system from
    generation, transmission, distribution, to
    end-use sectors
  • Applications equipment sensing and monitoring,
    fault diagnosis, meter reading, etc.
  • Components Supervisory Control and Data
    Acquisition and Energy Management Systems
    (SCADA/EMS), Phasor Management Units and Phasor
    Data Concentrators (PMU/PDC), Advanced Metering
    Infrastructure (AMI), and a wide range of Remote
    Terminal Units (RTUs), etc.

4
WSNs in Smart Grid Communications (2/4)
5
WSNs in Smart Grid Communications (3/4)
  • Five major domains
  • Traditional power plants, transformers and
    substations control, Distributed Energy Resources
    (DERs), power lines monitoring, and demand-side
    customers

6
WSNs in Smart Grid Communications (4/4)
  • Types of sensors
  • chemical, electrical, environmental, pressure,
    smart appliances sensors, smart meters, etc.
  • Different classes of sensor data to meet
    different latency requirements, e.g., voltage and
    frequency control (lt 100ms), smart metering (gt
    1s)
  • Collected data may be shared and reused for
    multiple applications (Ref. 9)
  • Challenges modification to data packet headers
    may be required data may not carry sufficient
    information for some specific applications
  • Developments Advanced sensors and associated
    sensor data management

7
IEEE 802.15.4 vs. PLC Technology
  • IEEE 802.15.4 (Ref. 1718)
  • Fast deployment, low implementation cost, low
    complexity, low energy consumption
  • Matured technology used in various applications
    and tailored by popular working groups Alliances,
    e.g., ZigBee, WirelessHART, ISA100
  • Power line Communications (PLC) (Ref.
    21258)
  • Another viable approach that utilizes existing
    power line cables as the communications medium
    for data transmission
  • Shortcomings 1) high bit error rates (due to
    noisy power line, e.g., motors, power supplies),
    2) limited capacity (attributed to the number of
    concurrent network users and applications
    concurrently being used), 3) high signal
    attenuation (dependent of geographical
    locations), 4) phase change between indoor and
    outdoor environments, and 5) disconnected
    communications due to opened circuits

8
IEEE 802.15 Task Groups
9
PHY specifications in IEEE 802.15.4a, b, c, and d
  • The legacy IEEE 802.15.4 standards adopt BPSK,
    ASK, O-QPSK modulations, support data rates 20,
    40, 100, 250 kbps, and operate in 868/915 MHz and
    2.4 GHz frequency bands
  • The standards only specify PHY and MAC layers and
    leave the upper layers to be designed by the
    application designers

10
IEEE 802.15.4g Task Group (TG4g)
  • TG4g specifies Smart Utility Network/Neighborhood
    (SUN) development by tackling a number of
    technical challenges in communications systems
    for the utility operators, especially the
    interference and coexistence issues
  • TG4g amends the legacy IEEE 802.15.4 standard for
    SUN PHYs while IEEE 802.15.4e is tailored for SUN
    MAC (Ref. 59)
  • Three major SUN PHYs are proposed
    multi-rate/multi-regional frequency shift keying
    (MR-FSK), multi-rate orthogonal frequency
    division multiplexing (MR-OFDM), and multi-rate
    offset quadrature phase shift keying (MR-OQPSK)
    (Ref. 59)
  • Bands allocated in domains/countries for SUN are
    470510 MHz (China), 863870 MHz (Europe),
    902928 MHz (United States), 950958 MHz (Japan),
    and 2.42.4835 GHz (worldwide)
  • Keys utilization of sub-GHz frequency bands
    (i.e., license-exempt bands below 1 GHz), and
    development of multi-PHY management (MPM) (Ref.
    60)

11
Other Approaches
  • Other techniques have been proposed to enhance
    the network performance in smart grid
    communications from the PHY perspective
  • Multi-channel access (Ref. 14)
  • WiFi features adoption (Ref. 15)
  • Cognitive radio (Ref. 16)
  • TV White Space (Ref. 59)

12
IEEE 802.15.4 Studies and Challenges (1/4)
  • LR-WPAN generally employs TDMA with CSMA-CA, and
    adopts DSSS for various modulation schemes
  • Network variables and metrics in LR-WPAN design
    are predominantly based on topology control and
    traffic engineering
  • Network size
  • Node placement
  • Data packet size
  • Traffic loads

13
IEEE 802.15.4 Studies and Challenges (2/4)
  • The network performance of LR-WPAN is determined
    by several key factors
  • Frequency of wireless medium contention
  • Successful data delivery ratio collisions from
    hidden node transmission, congestions from heavy
    traffic loads, and packet losses and drops from
    wireless deterioration and buffer overflow
  • Latency unnecessary delayed transmission from
    the exposed node problem, a clumsy increase in
    MAC CSMA backoff periods, and inflexible routing
    design
  • Energy depletion rate affected by the duty-cycle
    arrangement as well as data aggregation and
    fusion mechanisms.

14
IEEE 802.15.4 Studies and Challenges (3/4)
  • Wireless impairments such as background noise,
    signal attenuation, path loss, multipath/fading,
    and interference are also found in LR-WPAN
  • Several measurements and parameters specified in
    LR-WPAN PHY/MAC are principal attributes to the
    network performance and design
  • Receiver energy detection (ED) within the current
    channel
  • Link quality indicator (LQI) for received packets
    and channel frequency selection
  • Clear channel assessment (CCA) for CSMA-CA
  • NB the number of times that CSMA-CA is required
    to backoff
  • BE a backoff exponent that is used to calculate
    the backoff period
  • CW the contention window length

15
IEEE 802.15.4 Studies and Challenges (4/4)
  • The PHY payload in IEEE 802.15.4 is limited to
    127 bytes the application payload (useful
    information) is reduced to 60 bytes 80 bytes
    after an inclusion of control bits. Since the
    ratio of overhead to data payload is considerably
    large, one needs to determine
  • How to use bandwidths in LR-WPAN efficiently?
  • How to manage packet size with useful data to
    achieve low delay and low packet-loss rate during
    transmission? (Ref. 38)
  • Two types of data packet collision can also be
    found in LR-WPAN (RTS/CTS is not supported in
    IEEE 802.15.4)
  • Collision due to regular medium contention
  • Collision due to hidden node problem (Ref.
    343537)
  • Exposed node problem can occur in LR-WPAN as well
    (Ref. 36)

16
IEEE 802.15.4 Superframe structure (1/2)
CAP Contention access period CFP Contention
free period BSD Base slot duration SD
Superframe duration BSFD Base superframe
duration NSFS Number of superframe slots BI
Beacon interval SO Superframe order BO Beacon
order
  • Two operation modes
  • beacon-enabled (B-E) with slotted CSMA-CA mode,
    and beaconless (BL, i.e., beacon-disabled) with
    unslotted CSMA-CA mode
  • In the B-E mode, the superframe is bounded by two
    consecutive beacons, and constructed by the
    active and inactive parts
  • The active portion is divided into 16 equal time
    slots that comprises CAP and CFP, which defines
    GTS
  • Up to 7 GTSs can be allocated by a WPAN
    coordinator and each GTS may occupy more than one
    slot period (i.e., 1 BSD)

17
IEEE 802.15.4 Superframe structure (2/2)
CAP Contention access period CFP Contention
free period BSD Base slot duration SD
Superframe duration BSFD Base superframe
duration NSFS Number of superframe slots BI
Beacon interval SO Superframe order BO Beacon
order
  • GTS allocation and management specify starting
    slot, length, direction (i.e., transmit or
    receive), and associated node address. Each GTS
    is allocated first come first serve and released
    when it is not required
  • Slot boundary rule a node begins to transmit on
    the next available slot boundary when the channel
    is idle. Otherwise, it allocates the boundary of
    the next backoff slot before it goes into the
    backoff stage. If the time between the next
    available backoff slot and the end of the active
    period is not long enough for a node to complete
    its transmission, it may have to wait until the
    arrival of the next superframe

18
Network design for IEEE802.15.4-based WSN (1/9)
  • A number of principal research issues in IEEE
    802.15.4 are categorized into four areas PHY/MAC
    layers, fairness, routing, and security
  • Analysis in PHY/MAC under different network
    environments is grouped into B-E and BL studies
  • In B-E study, CAP/CFP and BO/SO are examined
  • In both studies, ED/LQI, CCA, CC/HNC/ENP, and
    NB/BE/CW are investigated

19
Network design for IEEE802.15.4-based WSN (2/9)
  • CAP and CFP (with GTS) Management
  • QoS consideration in data transmission specified
    in smart grid applications, e.g., GTSs are
    allocated to nodes with mission-critical data
  • The positions of CAP and CFP are swapped
    (modification to the standard is required) in
    order to grant the retransmission attempt of GTS
    to proceed in CAP of the same superframe upon a
    failed transmission in GTS (Ref. 42)
  • Analysis of GTS request drop due to possible
    collisions in CAP when BO is considerably small
    (Ref. 43)
  • Two-traffic class is proposed to allow nodes with
    higher-priority data to transmit by assigning
    CW1 (Ref. 44)

20
Network design for IEEE802.15.4-based WSN (3/9)
  • SO and BO Measurement
  • Consideration of the need for power saving on
    each node at the cost of transmission latency,
    i.e., SOBO (100 duty cycle) if a node is not
    power-constrained
  • Analysis of end-to-end delay and packet loss by
    studying the packet inter-arrival time and the
    ratio of BO to SO (Ref. 45)
  • Tradeoff between latency and energy consumption
    under the same duty cycle, which can be
    constructed by different combination sets (Ref.
    46), e.g., both BO3/SO2 and BO11/SO10 cases
    have 50 duty cycle

21
Network design for IEEE802.15.4-based WSN (4/9)
  • ED and LQI Assessment (Ref. 47)
  • Determination of ED and LQI to identify the radio
    condition
  • While using LQI and RSSI (or ED) metrics for a
    number of field tests in real-world power
    delivery and distribution systems, several
    conclusions are made 1) the background noise
    (varied in temperature and time) is higher for
    the indoor than the outdoor environment 2)
    channel 26 in IEEE 802.15.4 is not influenced by
    IEEE 802.11b interference and 3) LQI is a good
    estimator when the signal is found below and
    close to the sensitivity threshold, i.e., -94
    dBm otherwise, RSSI (or ED) is recommended
  •  RSSI Received signal strength indicator

22
Network design for IEEE802.15.4-based WSN (5/9)
  • CCA Analysis
  • Determination of whether a specific radio channel
    is busy or idle prior to the data transmission
  • Collision may occur during the receive-to-transmit
    (Rx-to-Tx and vice versa) turnaround time even
    if a channel was initially detected as idle (Ref.
    48)
  • An adaptive MAC engine containing a collection of
    preset optimal protocols for different network
    conditions is proposed to avoid time spent on
    restarting the design process each time (Ref.
    49)

23
Network design for IEEE802.15.4-based WSN (6/9)
  • NB, BE, and CW Examination (Ref. 50)
  • NB and BE parameters can be directly affected in
    consequence of CCA, which is related to CW
    assignment
  • Under light or medium traffic condition,
    increasing the BE value seems to bring down the
    probability of packet loss, however, at the cost
    of increased latency
  • Under heavy traffic condition, adjusting BE
    becomes insignificant to improve network
    performance

24
Network design for IEEE802.15.4-based WSN (7/9)
  • Fairness
  • An adaptive GTS allocation scheme is proposed to
    determine the success of GTS requests and the
    present traffic-level state of a node (Ref. 51)
  • A node generating heavy or more recent data
    traffic is likely to have a higher probability of
    staying in a higher priority state
  • A node staying in a higher-level state with
    temporary transmission interruption will slightly
    be demoted to a lower state. On the other hand, a
    node in a lower-level state can be promoted to a
    higher state if a consecutive success of GTS
    requests is achieved

25
Network design for IEEE802.15.4-based WSN (8/9)
  • Routing Arrangement
  • While the standard does not specify
    network/transport layer, various routing
    protocols based on AODV have been proposed (Ref.
    525354)
  • A routing strategy based on OLSR that responds to
    the requirements specified in power generation
    industry is also proposed (Ref. 10)
  • A hybrid routing scheme unifying flat and
    hierarchical multi-hop algorithms with respect to
    power consumption is also proposed (Ref. 33)
  • New integrated routing techniques in supporting
    IPv6 via 6LoWPAN need to be developed (Ref. 55)

26
Network design for IEEE802.15.4-based WSN (9/9)
  • Security and Privacy
  • Owing to the low computation capability and high
    overhead constraints, limit of number of access
    control list (ACL) entries and lack of group
    keying are identified (Ref. 56)
  • Security architecture for smart grid WSNs
    specifying security standards and
    testing/evaluation for both hardware and software
    need to be developed (Ref. 13)
  • Privacy in smart grid communications is
    comparable to patients' medical records in home
    and hospital
  • Elliptic curve cryptography adopted in healthcare
    WSN is proven to be lightweight computationally
    and uses smaller key sizes for obtaining the same
    security level as compared to RSA (Ref. 57)

27
A Summary of Network design and challenges in
IEEE802.15.4-based WSN
28
Conclusions (1/2)
  • Smart grid applications with different bandwidth
    and latency requirements can be provisioned in
    HR-WPAN (IEEE 802.15.3 based) and LR-WPAN (IEEE
    802.15.4 based), which require further
    investigations for smart grid communications
    (improvement to legacy IEEE 802.15.4)
  • Design of data prioritization related to specific
    applications and QoS requirements
  • Adequacy of control (i.e., overheads) and data
    packet size (including commands)
  • Schemes for multi-PHY management
  • Assessment of communications link quality
  • Innovation of MAC medium contention
  • Flexibility of routing mechanisms
  • Fairness issues upon adopted schemes
  • Security/privacy models for protecting data and
    associated transmission

29
Conclusions (2/2)
  • Proposed techniques to alleviate interference and
    coexistence problems by utilizing spectrums more
    effectively and efficiently, e.g., operating
    frequency bands below 1 GHz and developing
    multi-PHY management (specified in IEEE
    802.15.4g), and adopting TV White space, WiFi
    features, multi-channel access, as well as
    cognitive radio in the legacy IEEE 802.15.4
    standard (spectrum use efficiency)
  • Complementary strategy of combining IEEE 802.15.4
    with PLC technologies should be considered to
    provision sensor applications in various smart
    grid domains (interoperability)
  • Innovative mechanisms and models of integrating
    IP and other technologies with WSNs need to be
    developed to facilitate smart grid communications
    and management (integration)

30
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