Introduction IoT Connectivity (Part I)

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Que. Which communication protocols are important for IoT?

  • IEEE 802.15.4
  • ZigBee
  • 6LoWPAN
  • Wireless HART
  • Z-Wave
  • ISA 100
  • Bluetooth
  • NFC
  • RFID


Que. IEEE 802.15.4 and its features?
  • This standard provides a framework meant for lower layers (Media Access Control and PHY or Physical) for a wireless personal area network (WPAN).
  • PHY defines frequency band, transmission power, and modulation scheme of the link.
  • MAC defines issues such as medium access and flow control (frames).
  • This standard is used for low power, low cost (manufacturing and operation), and low speed communication between neighboring devices (< ~75m).
Features:

  • This standard utilizes DSSS (direct sequence spread spectrum) coding scheme to transmit information.
  • DSSS uses phase shift keying modulation to encode information. BPSK - 868/915 MHz, data transmission rate 20/40 kbps respectively.
  • OQPSK - 2.4 GHz, data transmission rate 250 kbps.
  • DSSS scheme makes the standard highly tolerant to noise and interference and thereby improving link reliability.
  • The preferable nature of transmission is line of sight (LOS).
  • The standard range of transmission - 10 to 75m.
  • The transmission of data uses CSMA-CA (carrier sense multiple access with collision avoidance) scheme.
  • Transmissions occur in infrequent short packets for duty cycle (<1 %), thus reducing consumption of power.
  • Star network topology and peer-to-peer network topology is included.

Que. ZigBee and its features.
  • Provides a framework for medium-range communication in IoT connectivity.
  • Defines PHY (Physical) and MAC (Media Access Control) layers enabling interoperability between multiple devices at low-data rates.
  • Operates at 3 frequencies –
  • 868 MHz (1 channel using data transmission rate up to 20 kbps)
  • 902-928 MHz (10 channels using data transmission rate of 40 kbps)
  • 2.4 GHz (16 channels using data transmission rate of 250 kbps).
Features:
  • The lower frequency bands use BPSK.
  • For the 2.4 GHz band, OQPSK is used.
  • The data transfer takes place in 128 bytes packet size.
  • The maximum allowed payload is 104 bytes.
  • The nature of transmission is line of sight (LOS).
  • Standard range of transmission – up to 70m.
  • Relaying of packets allow transmission over greater distances.
  • Provides low power consumption (around 1mW per Zigbee module) and better efficiency due to adaptable duty cycle.
  • Low data rates (20 - 250 kbit/s).
  • Low coverage radio (10 -100 m).
  • Networking topologies include star, peer-to-peer, or cluster- tree (hybrid), mesh being the popular.
  • The Zigbee protocol defines three types of nodes:
  • Coordinators - Initializing, maintaining and controlling the network. There is one and only one per network.
  • Routers - Connected to the coordinator or other routers. Have zero or more children nodes. Contribute in multi hop routing.
  • End devices - Do not contribute in routing.
  • Star topology has no router, one coordinator, and zero or more end devices.
  • In mesh and tree topologies, one coordinator maintains several routers and end devices.
  • Each cluster in a cluster-tree network involves a coordinator through several leaf nodes.
  • Coordinators are linked to parent coordinator that initiates the entire network.
  • ZigBee standard comes in two variants:
  • ZigBee
  • ZigBee Pro - offers scalability, security, and improved performance
  • utilizing many-to-one routing scheme.


Que. 6LoWPAN and its features.
  • 6LoWPAN is IPv6 over Low-Power Wireless Personal Area Networks.
  • It optimizes IPv6 packet transmission in low power and lossy network (LLN) such as IEEE 802.15.4.
  • Operates at 2 frequencies:
  • 2400–2483.5 MHz (worldwide)
  • 902–929 MHz (North America)
  • It uses 802.15.4 standard in unslotted CSMA/CA mode.
Features:
  • 6LowPAN converts the data format to be fit with the IEEE 802.15.4 lower layer system.
  • IPv6 involves MTU (maximum transmission unit) of 1280 bytes in length, while the IEEE 802.15.4 packet size is 127 bytes.
  • Hence a supplementary adaptation layer is introduced between MAC and network layer that provide:
  • Packet fragmentation & packet reassembly
  • Compression of header
  • Routing of data link layer.
  • Fragmentation is required to fit the intact IPv6 packet into a distinct IEEE 802.15.4 frame ( >~106 bytes).
  • The fragmentation header allows 2048 bytes packet size with fragmentation.
  • Using fragmentation and reassembly, 128-byte IPv6 frames are transmitted over IEEE 802.15.4 radio channel into several smaller segments.
  • Every fragment includes a header.
  • Header compression reduces the transmission overhead and allows efficient transmission of payload.
  • IPv6 addresses are compressed in 6LoWPAN:
  • 8-byte UDP header
  • 40-byte IPv6 header
  • Stateless auto configuration allows any device to create the IPv6 address automatically devoid of external dealing using a DHCP server.
  • Data link layer routing is classified into two schemes:
  • mesh-under - utilizes link layer address to forward data packets.
  • route-over - utilizes network layer IP address.
  • Provides link layer security (AES-128) from IEEE 802.15.4 such as authentication of link and encryption.

Que. Wireless HART and its features.
  • WirelessHART is based on HART (Highway Addressable Remote Transducer).
  • It is the first international industrial wireless standard (IEC 62591), based upon the standard IEEE 802.15.4.
  • Functions in the 2.4GHz ISM band using data rate of up to 250 kb/s.
  • 11 to 26 channels are supported, with a gap of 5MHz between two adjacent channels.
  • The same channel can’t be used consecutively.
Features:
  • Exploits IEEE 802.15.4 accustomed DSSS coding scheme.
  • A Wireless HART node follows channel hopping every time it sends a packet.
  • Modulation technique used is offset quadrature phase shift keying (OQPSK).
  • Transmission Power is around 10dBm (adjustable in discrete steps).
  • Maximum payload allowed is 127 bytes.
  • It employs TDMA (time division multiple access) that allots distinct time slots of 10ms of each transmission.
  • TDMA technology is used to provide collision free and deterministic communication.
  • A sequence of 100 consecutive time slots per second is grouped into a super frame.
  • Slot sizes and the super frame length are fixed.
  • The devices support multiple super frames with differing numbers of timeslots.
  • At least one super frame is always enabled while additional super frames are enabled or disabled according to demand of bandwidth.
  • For any message, communication occurs in the alloted timeslot and frequency channel.
  • Supports both star and mesh topologies.




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