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

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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

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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

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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.

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WSNs in Smart Grid Communications (2/4)
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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

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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

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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

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IEEE 802.15 Task Groups
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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

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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)

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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)

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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

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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.

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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

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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)

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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)

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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

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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

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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)

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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

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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

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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)

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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

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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

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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)

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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)

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A Summary of Network design and challenges in
IEEE802.15.4-based WSN
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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

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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)

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Thanks for your attention!