IEEE 802.15.4 Low-Rate Wireless PAN (LR-WPAN)

Wireless Sensor Network Standards IEEE 802.15.4 Low-Rate Wireless PAN ZigBee 6LoWPAN IEEE 1451 standards for connecting smart transducers to networks

Wireless Sensor Network Standards 1: Web Services Architecture Wireless Sensor Network Standards End developer applications, designed using application profiles ZA1 ZA2 … IA1 IA2 IAn Application interface designed using general profile API Transport ZigBee NWK 6LowPAN Topology management, MAC management, routing, discovery protocol, security management 802.2 LLC MAC (SSCS) Channel access, PAN maintenance, reliable data transport IEEE 802.15.4 MAC (CPS) Transmission & reception on the physical radio channel IEEE 802.15.4 PHY

802.15.4 with Five Key Words Very low cost Very low power consumption Low complexity Low rate Short range 4

Basic Radio Characteristics

802.15.4 Applications Space Home Networking Automotive Networks Industrial Networks Interactive Toys Remote Metering 6

High-Level Characteristics 7

802.15.4 Architecture 8

Device Classes Full function device (FFD) Any topology Network coordinator capable Talks to any other device Reduced function device (RFD) Limited to star topology Cannot become a network coordinator Talks only to a network coordinator Very simple implementation 9 9

Network Topology Star Point to point Cluster tree PAN Coordinator Full function device Reduced function device Cluster tree Point to point

LR-WPAN: Data Rate DSSS Tx range: 10 ~ 75 m at 0 dBm (1 mW) Band Symbol rate Modulation Bit rate channels 868 MHz 20 Ksps BPSK 20 Kbps 1 915 MHz 40 Ksps 40 Kbps 10 2.4 GHz 62.5 Ksps O-QPSK 250 Kbps 16

MAC Features Generating network beacons if the device is a coordinator Synchronizing to beacons PAN association, disassociation Optional acknowledged frame delivery Employing the CSMA/CA for channel access mechanism Guaranteed time slot management MAC management has 35 primitives RFD has 24 primitives cf. 131 primitives of 802.15.1 / Bluetooth 12

1: Web Services Architecture Superframe Structure For some applications requiring dedicated bandwidth to achieve low latencies A superframe is divided in 16 time slots CAP: Slotted CSMA-CA channel access (beacon-enabled network) Unslotted or standard CSMA-CA in networks (non beacon-enabled network) CFP: Optionally, contention-free access using Guaranteed Time Slots (GTSs) in beacon-enabled netrwork aBaseSuperframeDuration = 60 symbols/slot * 16 slots = 960 symbols 15.36 ms at 250 kbps, 24 ms at 40 kbs, 48 ms at 20 kbps BO (Beacon Order) How often the PNC transmits a beacon, 0 ≤ BO ≤ 14 (15.36 ms ~ 251.65824 sec) 15 if non beacon

Unslotted CSMA-CA Backoff periods of a device not related to that of any other device Therefore, synchronization is not required CCA – Clear Channel Assessment to check if channel is busy or idle 14

Slotted CSMA-CA Backoff period boundaries aligned by the periodic beacon transmission It also implies that they are aligned with superframe slot boundaries (for GTS) as Slot = n * aUnitBackoffPeriod 15

Inter-frame Spacing Short frame: frame size <= aMaxSIFSFrameSize Long frame: otherwise

MAC addressing All devices have IEEE addresses (64 bits) Short addresses (16 bits) can be allocated Addressing modes PAN identifier (16 bits)+ device identifier (16/64 bits) Beacon frame: no destination address

General Frame Format

General MAC Frame Format Frame control field Destination in Beacon frame Beacon frame Data frame Acknowledgement frame MAC command frame source PAN id is skipped

1: Web Services Architecture Data Frame format Provides up to 104 byte data payload capacity Data sequence numbering to ensure that all packets are tracked Robust frame structure improves reception in difficult conditions Frame Check Sequence (FCS) ensures that packets received are without error

Acknowledgement Frame Format 1: Web Services Architecture Acknowledgement Frame Format Provides active feedback from receiver to sender that packet was received without error Short packet that takes advantage of standards-specified “quiet time” immediately after data packet transmission

MAC Command Frame Format 1: Web Services Architecture MAC Command Frame Format Mechanism for remote control/configuration of client nodes Allows a centralized network manager to configure individual clients no matter how large the network

1: Web Services Architecture Beacon Frame format Client devices can wake up only when a beacon is to be broadcast, listen for their address, and if not heard, return to sleep Beacons are important for mesh and cluster tree networks to keep all of the nodes synchronized without requiring nodes to consume precious battery energy listening for long periods of time Minimum beacon PPDU length = 136 bits / 250 Kbps = 544 μsec

MAC Data Primitives

Data Transfer: no-beacon mode Device  Coordinator Coordinator  Device Indirect transmission

Data Transfer: Beacon Mode Device  Coordinator Coordinator  Device

Management Service Access to the PIB Association / disassociation GTS allocation Message pending Node notification Network scanning/start Network synchronization/search

MAC Management Primitives 1: Web Services Architecture MAC Management Primitives Access to the PIB Association / disassociation GTS allocation Message pending Node notification Network scanning/start Network synchronization/search

Association

Disassociation

Data Polling No data pending at the coordinator

ED SCAN When a prospective PAN coordinator to select a channel Measure peak energy in each requested channel Discard every frame received while scanning Return energy levels

Active Scan When FFD wants to locate any coordinator within POS A prospective coordinator selects PAN ID Prior to device association Receive beacon frames only macPANId = 0xffff Send beacon request command Destination PAN ID = 0xffff Return PAN descriptors

Passive Scan No beacon request command Device to prior to association Receive beacon frames only macPANId = 0xffff

Orphan Scan Device attempts to relocate its coordinator For each channel, send orphan notification command Dest PAN id, dest short addr = 0xffff Only the original coordinator will reply Receive coordinator realignment command frame only

Differences from 802.11 WLAN Simpler PHY Simpler MAC One Tx rate per channel Low Tx power Simpler MAC No virtual carrier-sense No worry about hidden nodes No RTS/CTS & No fragmentation No continuous CCA Relaxed timing requirement Extensive power saving features

Power Save Mechanisms Going to sleep state as often as possible by utilizing: Inactive mode in superframes Backoff periods when macRxOnWhenIdle is reset. GTS for other devices Extracting pending messages from coordinator Using data request command Message pending indicated in beacon frames 37

LR-WPAN: Low Duty Cycle Beacon interval (max) 960 symbols * 214 = 15,728,640 symbols At 250 Kbps, (min) 15.36 msec ~ (max) 251.65824 sec (over 4 min) Beacon duty cycle 544 μsec / 251.65824 sec = 0.000216% (lowest possible) Non-beacon mode is also possible Example: 0.1% duty cycle 10 mW active, 10 μW standby → 19.99 μW average power AAA battery with capacity of 750mAh, regulated to 1V Battery life: 37,519 hours ≈ 4.28 years

LR-WPAN: Imperfect Time Bases εTbeacon TC εTbeacon [Guti03] εRX Tbeacon εRX Tbeacon Uncertainty due to imperfect receiver time base “Ideal” beacon reception time receiver εTX Tbeacon εTX Tbeacon “Ideal” beacon transmission time Uncertainty due to imperfect transmitter time base transmitter

LR-WPAN: Duty Cycle vs. Cost Lowest possible duty cycle of a receiver is (2ε·Tbeacon + TC) / Tbeacon Duty cycle is limited by the time base tolerance ε No matter how long Tbeacon is made IEEE 802.15.4 is designed to support Time base tolerance as great as ±40 ppm (note) lowest duty cycle = 2.16 ppm Use of inexpensive reference crystals Lower duty cycle requires more stable time base Increases the cost of time base

1: Web Services Architecture IEEE 802.15.4a Scope and Description: Develop an alternate physical layer (PHY) for data communication with high precision ranging / location capability (1 meter accuracy and better) high aggregate throughput and ultra low power scalability to data rates longer range lower power consumption and cost. The alternate PHY is an (optional) amendment to the current IEEE 802.15.4-2003 LR-WPAN standard. 802.15.4a became an official Task Group in March 2004; with its committee work tracing back to November 2002. Current Status The baseline is two optional PHYs UWB Impulse Radio (operating in unlicensed UWB spectrum) Chirp Spread Spectrum (operating in unlicensed 2.4GHz spectrum) The UWB Impulse Radio will be able to deliver communications and high precision ranging.

1: Web Services Architecture IEEE 802.15.4b Scope and Description Resolve ambiguities, provide corrections, removing unnecessary complexity, and define enhancements to the current IEEE 802.15.4-2003 standard. The revised standard will be backward compatible. Enhancements support for distributing a shared time-base Support for group addressing Extensions of the 2.4GHz derivative modulation Yields higher data rates at the lower frequency bands Support of Beacon-Enabled Cluster Tree network. IEEE802.15.4 does not support while 15.4b does Protection of broadcast and multicast frames possible Easier setup of protection parameters possible Possibility to vary protection per frame, using a single key Optimization of storage for keying material