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CSE 122/222C ; WES 269�IEEE 802.15.4

Pat Pannuto, UC San Diego

ppannuto@ucsd.edu

CSE 122/222C ; WES 269 [WI25]

CC BY-NC-ND Pat Pannuto – Content developed in coordination with Branden Ghena and Brad Campbell

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IEEE 802.15.4 Goals

  • Introduction to 802.15.4

​

  • Overview of physical layer details

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  • Exploration of link layer
    • Network topologies
    • Communication structure
    • Access control
    • Packet structure

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References

  • 802.15.4 Specification [2006]
    • “Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (WPANs)”

​

Other helpful references:

​

​

​

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Outline

  • Overview

​

  • Physical Layer

​

  • Link Layer

​

  • Packet Structure

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Comparison of networks

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Data

Throughput

Range

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Comparison of networks

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Bluetooth

WiFi

Cellular

BLE

802.15.4

Data

Throughput

Range

LPWANs

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Comparison of networks

7

Bluetooth

WiFi

Cellular

BLE

802.15.4

Data

Throughput

Range

There are some missing qualities here.��Why be closer to the origin?

LPWANs

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Comparison of networks

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Bluetooth

WiFi

Cellular

BLE

802.15.4

Data

Throughput

Range

Lower Power &�Lower Cost

Higher Power &�Higher Cost

LPWANs

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IEEE 802….

  • Anyone heard “Eight-Oh-Two Dot” ?
    • Where?
    • What is it?

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IEEE 802

  • Network standards for variable-sized packets
    • Ethernet
    • WiFi
    • WPANs

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  • E.g. not networks that send periodic constant-sized packets

​

  • Specify PHY Layer and Link Layer [MAC+LLC]

​

  • Another example standard:
    • IEEE 754: Floating Point

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IEEE 802.15

  • Wireless Personal-Area Networks (WPAN)
    • All the things within the workspace of a person
    • Conceptually smaller domain that the Local Area Network
    • Realistically about the same thing as a LAN (or really a WLAN)

​

  • Formerly included a Bluetooth spec
    • Bluetooth SIG took over governance

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802.15.4 (LR-WPANs) Overview�“Low-Rate Wireless Personal Area Networks”

  • Goals
    • “The IEEE 802.15 TG4 was chartered to investigate a low data rate solution with multi-month to multi-year battery life and very low complexity.” [TG4]
  • Applications
    • “Potential applications are sensors, interactive toys, smart badges, remote controls, and home automation.” [TG4]
    • Ultimately home automation, industrial control/monitoring, vehicular sensing, agriculture; really most M2M sensor applications you might imagine

​

  • Other contemporary technologies
    • WiFi 802.11b and Bluetooth Classic
      • Too complex in specification and overachieving in capability

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CC BY-NC-ND Pat Pannuto – Content developed in coordination with Branden Ghena and Brad Campbell

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IEEE 802.15.4

  • Low-Rate Wireless PAN
    • 250 kbps, ~100 m range
    • Radio hardware available with low-power and low-cost

​

  • Specification: 2003
    • Also 2006, 2007 [UWB!], 2009, 2011, 2015, and 2020 revisions [and frankly probably others]
      • Mostly various added capabilities such as extra PHY layers
      • Also define optional security, scheduling, and larger frame sizes

​

  • We’ll mostly work off of the 2006 version
    • Thread is based on 2006 version
    • Zigbee is based on the original 2003 version
    • Roughly 200 pages of meaningful specification (100 of appendices)
      • Compare to 3000 pages of Bluetooth/BLE

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Outline

  • Overview

​

  • Physical Layer

​

  • Link Layer

​

  • Packet Structure

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802.15.4 Physical Layers

  • Multiple options of physical layers are supported
    • We’ll focus on 2.4 GHz (2400 MHz)

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Physical Layer

  • O-QPSK modulation
    • Offset Quadrature Phase-Shift Keying
    • Twice the data rate of BPSK for same BER
    • Cost: most complicated design of receivers
      • Which is pretty minimal with all the transistors we’ve got
      • Plus the ability to reuse previous designs
    • 4 bits per symbol

​

  • Symbols versus bits
    • A symbol is the unit of data transfer for a modulated signal
      • Does not necessarily correspond 1:1 with bits
    • The rate of symbols per second is a baudrate

​

  • 802.15.4 bit rate at 2.4 GHz: 2000 chips/s, which is 250 kbps, which is 62.5 kBaud

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802.15.4 Modulation (@2.4 GHz fc)�O-QPSK with half-sine shaping is MSK!

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802.15.4 Modulation (@2.4 GHz fc)�O-QPSK with half-sine shaping is MSK!

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Input bit stream

Broken into 4-bit symbols

Each symbol maps to a 32-bit pseudo-noise code (PN-code) or sometimes pseudo-random sequence

Each bit of the PN code is called a chip

Each chip encodes half a sine wine

Chips alternate in-phase and quadrature

Quadrature component is offset π/2

I and Q half-sines are baseband, which are mixed with the carrier

I and Q carriers are combined to create the final on-air signal

Signal is MSK, which is a special, optimal case of FSK!

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CC BY-NC-ND Pat Pannuto – Content developed in coordination with Branden Ghena and Brad Campbell

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802.15.4 Modulation (@2.4 GHz fc)�O-QPSK with half-sine shaping is MSK!

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Input bit stream

Broken into 4-bit symbols

Each symbol maps to a 32-bit pseudo-noise code (PN-code) or sometimes pseudo-random sequence

Each bit of the PN code is called a chip

Each chip encodes half a sine wine

Chips alternate in-phase and quadrature

Quadrature component is offset π/2

I and Q half-sines are baseband, which are mixed with the carrier

I and Q carriers are combined to create the final on-air signal

Signal is MSK, which is a special, optimal case of FSK!

Final detail:

​

This shows a fb :: fc ratio of 1 :: 10 so you can see the impact on the carrier. In reality, it’s closer to 1 :: 1200 (2,000 chips / s :: 2,400,000 Hz)

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O-QPSK results in continuous wave

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Standard BPSK

O-QPSK (MSK)

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The magic of I and Q channels are that we get two dimensions

  • This is called a “constellation diagram”
    • We’ll talk about these more with cellular

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Constellation Diagrams give ‘at-a-glance’ understanding of modulation schemes

  • Constellation diagrams for On-Off-Keying (OOK), Frequency Shift Keying (FSK)?
    • And what does that tell us about how the two modulation schemes compare?

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I

Q

802.15.4

( MSK )

BLE ?

( [G]FSK ) ?

I

Q

OOK ?

( ASK ) ?

Obligatory EE Disclaimer

​

Many FSK frontends are implemented via IQ modulation internally…

0

1

0

1

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Why do we map symbols to chips?

  • We took the 4 bits we want to send…

… and sent 32 bits instead??

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  • Why?

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Direct Sequence Spread Spectrum (DSSS)

  • Increases the signal bandwidth of a transmission beyond information bandwidth
    • Send sequences of chips, which are a translation of one symbol to a pattern of many bits
    • Chips are transmitted much faster than symbols, essentially increasing the data rate

​

  • Enables better interference avoidance
    • Received bits are correlated against codes to see which is most likely
    • 802.15.4 tolerates 13-15 bit flips (almost half!)

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DSSS example

  • Data sent is 101
    • Code is longer than data, so we replicate bits
    • Data is recoverable, even with noise

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https://circuitcellar.com/research-design-hub/dsss-in-a-nutshell/

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Show me the money:�What is the actual bit rate of 802.15.4 (2.4 GHz)?

  • Chip rate: 2000 kchips/sec

​

  • ”Bit rate” is the term for rate of meaningful digital bits over the PHY
    • i.e. link layer bits

​

    • (n.b., sometimes also called “data rate”, but sometimes people use “data rate” for goodput; bit rate is unambiguous)

​

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802.15.4 RF channels

  • 27 channels across three bands
  • 5 MHz channel separation at 2.4 GHz
    • Compare to 2 MHz for BLE
    • (or to 1 MHz for BT Classic)

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Regional bands

  • Different RF bands have different regional availability

​

  • Also have different rules
    • 915 MHz: 400 ms dwell time
    • 868 MHz: 1% duty cycle

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Bringing it back together—what does all this mean for communication in practice?

  • Transmit power
    • Typical: 0 dBm

​

  • Receiver sensitivity
    • nRF52840 802.15.4: -100 dBm
      • Compare to BLE sensitivity of -95 dBm

​

    • Minimum acceptable per-spec: -85 dBm
    • Circa-2006 radios (CC2420): -95 dBm

​

  • Which has longer range, 802.15.4 or BLE? Why?
    • 802.15.4, for our boards with +5 dBm more margin; lower bit rate plays into this

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Outline

  • Overview

​

  • Physical Layer

​

  • Link Layer

​

  • Packet Structure

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802.15.4 network topologies

  • Only specifies PHY and MAC, but has use cases in mind

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Star and Tree topologies

  • PAN Coordinator
    • Receives and relays all messages
    • Most capable and power-intensive
  • Coordinators (a.k.a. Routers)
    • Control “clusters”
    • Receives and relays to its children
    • Communicates up to parent coordinator
  • End Devices
    • Only communicate with single�parent coordinator
    • Least capable and power intensive

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Break + Mesh networks

  • Most devices are capable of communicating with multiple neighbors

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  • What are advantages of mesh?

​

​

​

​

  • What are disadvantages of mesh?

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Mesh networks

  • Most devices are capable of communicating with multiple neighbors

​

  • What are advantages of mesh?
    • Devices can communicate over longer distances
    • Device failures less likely to collapse the entire network
  • What are disadvantages of mesh?
    • Some nodes have to spend more energy communicating
    • Network protocol becomes more complicated to manage routing

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Reminder: CSMA/CA�Carrier Sense Multiple Access with Collision Avoidance

0. Set wait range to [0, short)

  1. First, wait a random amount (collision avoidance part)
  2. Then, listen and determine if anyone is transmitting (carrier sense part)
    • If idle, you can transmit
    • If busy, increase wait time min/max, and repeat step 1

​

  • Can be combined with notion of slotting
    • Synchronize to slots (smaller than transmit times)
    • Wait for a number of slots
    • Listen for idle slots

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Modes of operation

  • Beacon-enabled PAN
    • Slotted CSMA/CA
    • Structured communication patterns
    • Optionally with some TDMA scheduled slots

​

  • Non-beacon-enabled PAN
    • Unslotted CSMA/CA
    • No particular structure for communication
      • Could be defined by other specifications, like Thread or Zigbee

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Beacon-enabled superframe structure

  • Beacons occur periodically [15 ms – 245 seconds]
    • Devices must listen to each beacon

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  • Contention Access Period
    • Slotted CSMA/CA synchronized by beacon start time�
  • Inactive Period
    • No communication occurring. Assumes sleepy devices

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Contention Access Period

Beacon

Inactive Period

Beacon

…

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Guaranteed Time Slots (GTS)

  • PAN Coordinator may create a Contention Free Period with Guaranteed Time Slots
    • TDMA schedule assigned to specific devices
    • Slots eat up part of the Contention Access Period
    • No CSMA/CA within a slot

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Contention Access Period

Beacon

Inactive Period

Beacon

Guaranteed Time Slots

…

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Handling tree-based topologies

  • All coordinators listen to beacon from PAN coordinator
    • And can participate in that contention period

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  • Send their own beacons to child devices during inactive period
    • Children participate in that contention period

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Non-beacon-enabled PAN

  • Same idea, just no beacons
    • Which removes synchronization benefit (and slotted CSMA/CA)
    • Also removes beacon listening cost
      • Devices only need to check for activity before transmitting
    • Still need an algorithm to determine when it should receive data
      • All the time is a huge energy drain
      • Algorithms can get complicated here
      • Does BLE mechanism of listen-after-send apply?

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Contention Access Period

…

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Non-beacon-enabled PAN

  • Same idea, just no beacons
    • Which removes synchronization benefit (and slotted CSMA/CA)
    • Also removes beacon listening cost
      • Devices only need to check for activity before transmitting
    • Still need an algorithm to determine when it should receive data
      • All the time is a huge energy drain
      • Algorithms can get complicated here
      • Does BLE mechanism of listen-after-send apply?
        • Only if sending to a high-power device, not among equals

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Contention Access Period

…

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Receiving messages

  1. Listen during entire contention period
    • Can receive direct messages from any other device
    • Can immediately respond to messages as well

​

  • Request messages from Coordinator
    • Make all communication go through Coordinator
    • Send a request-for-data packet to coordinator to get information
    • Coordinator can include list of devices with pending data in beacon

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  • More complicated listening algorithms are possible

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Clear Channel Assessment (CCA)

  • The “listen” part of CSMA/CA
  • Variety of implementations are acceptable

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  1. Energy above threshold?
    • Energy for 8 symbol durations above threshold (RSSI)
  2. Carrier present?
    • Valid 802.15.4 carrier signal
  3. Energy AND/OR Carrier

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Slotted CSMA/CA operation

  • Have data to send
  • Wait for next backoff slot (synchronized from beacon)
  • Wait for 0-7 backoff slots (slot is 20 symbol durations: 320 us)
  • Listen for two empty slots
    • Idle: Transmit
    • Occupied: wait 0-15 backoff slots and repeat
      • Next time: 0-31 backoff slots and repeat
      • Next time: 0-31 backoff slots and repeat (upper limit configurable)
      • Next time: 0-31 backoff slots and repeat
      • Next time: 0-31 backoff slots and repeat
      • Timeout

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Unslotted CSMA/CA operation

  • Have data to send
  • Wait for next backoff slot (synchronized from beacon)
  • Wait for 0-7 backoff slots (slot is 20 symbol durations: 320 us)
  • Listen for two empty slots
    • Idle: Transmit
    • Occupied: wait 0-15 backoff slots and repeat
      • Next time: 0-31 backoff slots and repeat
      • Next time: 0-31 backoff slots and repeat (upper limit configurable)
      • Next time: 0-31 backoff slots and repeat
      • Next time: 0-31 backoff slots and repeat
      • Timeout

​

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Break + Question

  • What are benefits/costs of using or not using beacons?

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Break + Question

  • What are benefits/costs of using or not using beacons?
    • Beacons
      • Enable energy savings by designating period with radios off
      • Enable structured communication like Guaranteed Slots
      • Require some central coordinator within range of all devices
      • Tradeoff in inactive period:
        • communication latency vs beacon-listening costs

​

    • No beacons
      • Enable all devices to be identical (no coordinator needed)
      • Require custom communication scheme
        • Could be better or worse for various qualities… (always-on radios?)

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Outline

  • Overview

​

  • Physical Layer

​

  • Link Layer

​

  • Packet Structure

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Base packet format

  • Synchronization
    • Preamble: four bytes of zeros
    • Start-of-Packet: 0xA7
  • PHY Header
    • One field: length 0-127
    • Why still 8 bits?

​

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Base packet format

  • Synchronization
    • Preamble: four bytes of zeros
    • Start-of-Packet: 0xA7
  • PHY Header
    • One field: length 0-127
    • Why still 8 bits? Because computers depend on bytes

​

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MAC frame format

  • Sequence number
    • 8-bit monotonically increasing
  • Addressing fields
    • PAN and addresses
    • Varies based on frame type

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  • Frame payload
    • Depends on frame type
  • Frame check sequence
    • 16-bit CRC
  • Frame control
    • Header

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Frame control

  • Frame type
    • Type of payload included
  • Security enabled
    • Packet is encrypted
    • (extra 0-14 byte header)
  • Frame pending
    • Fragmented packet

​

​

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  • Acknowledgement required
  • PAN ID compression
    • No PAN ID if intra-network
  • Addressing modes
    • Which fields to expect

​

​

​

Why no length field?

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Frame control

  • Frame type
    • Type of payload included
  • Security enabled
    • Packet is encrypted
    • (extra 0-14 byte header)
  • Frame pending
    • Fragmented packet

​

​

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  • Acknowledgement required
  • PAN ID compression
    • No PAN ID if intra-network
  • Addressing modes
    • Which fields to expect

​

​

​

Why no length field?

Already in prior header

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Frame types - Beacon

  • Beacon
    • Information about the communication�structure of this network
    • Sent in response to requests from scanning devices
    • Sent periodically at start of Superframes (if in use)
      • Sent without CSMA/CA

​

  • MAC Header
    • Source address only, broadcast to everyone

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  • Packet contents
    • Superframe details, including Guaranteed Time Slots (if any)
    • Pending addresses lists devices for which Coordinator has data

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Frame types - Data

  • Data
    • Data from higher-layer protocols

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  • MAC Header
    • Source and/or Destination addresses as necessary

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  • Packet Contents
    • Whatever bytes are desired (122 bytes – address sizes)
    • May be fragmented across packets

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Frame types – MAC Command

  • MAC Command
    • Various commands for supporting link layer
      • Join/leave network
      • Change coordinator within network
      • Request data from coordinator
      • Request Guaranteed Time Slot

​

  • MAC Header
    • Source and/or Destination addresses as necessary

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Frame types - Acknowledgement

  • Acknowledgement
    • Acknowledges a Data or MAC Command packet
      • Don’t send ack’s for beacons or other acknowledgements
    • What happens if an acknowledgement isn’t received?
      • Packet will be re-transmitted

​

  • MAC Header Contents
    • Repeats Sequence Number of acknowledged packet
    • No Source or Destination addresses (short packet)

​

  • Sent TIFS after the packet it is acknowledging (immediately)

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Quick Analysis: Maximum goodput?

  • Assume best possible case for data transmission
    • 122 Bytes per packet
      • At 250 kbps -> 3.904 ms
    • Plus Inter-frame spacing of 40 symbols
      • At 62.5 kBaud -> 0.640 ms

​

    • 122 Bytes / 4.544 ms -> 214 kbps
      • Compare to BLE advertisements: 9.92 kbps
      • Compare to BLE connections: 520 kbps

58

CSE 122/222C ; WES 269 [WI25]

CC BY-NC-ND Pat Pannuto – Content developed in coordination with Branden Ghena and Brad Campbell

59 of 59

Next time: Meshing and Low Power MACs

59

CSE 122/222C ; WES 269 [WI25]

CC BY-NC-ND Pat Pannuto – Content developed in coordination with Branden Ghena and Brad Campbell