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

1. IEEE 802.15.4 Low-Rate Wireless PAN (LR-WPAN) 1

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

3. 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

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

5. Basic Radio Characteristics

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

7. High-Level Characteristics

8. 802.15.4 Architecture

9. 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

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

11. LR-WPAN: Data Rate • DSSS • Tx range: 10 ~ 75 mat 0 dBm (1 mW)

12. 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

13. 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

14. 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

15. 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

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

17. 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

18. General Frame Format

19. 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

20. 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

21. 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

22. 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

23. 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

24. MAC Data Primitives

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

26. Data Transfer: Beacon Mode Device  Coordinator Coordinator  Device

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

28. MAC Management Primitives • Access to the PIB • Association / disassociation • GTS allocation • Message pending • Node notification • Network scanning/start • Network synchronization/search

29. Association

30. Disassociation

31. Data Polling No data pending at the coordinator Data pending at the coordinator

32. 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

33. 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

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

35. 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

36. Differences from 802.11 WLAN • Simpler PHY • 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

37. 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

38. 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

39. 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

40. 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

41. 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.

42. 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