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The IEEE 802.15.4 standard

The IEEE 802.15.4 standard . Credits to: Yao Liang (IUPUI, Indianapolis USA). Wireless Simplified Stack. 802.15.4. Principal options and difficulties. Medium access in wireless networks is difficult mainly because of Impossible (or very difficult) to send and receive at the same time

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The IEEE 802.15.4 standard

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  1. The IEEE 802.15.4 standard Credits to: Yao Liang (IUPUI, Indianapolis USA)

  2. Wireless Simplified Stack 802.15.4

  3. Principal options and difficulties • Medium access in wireless networks is difficult mainly because of • Impossible (or very difficult) to send and receive at the same time • Interference situation at receiver is what counts for transmission success, but can be very different from what sender can observe • High error rates compound the issues • Requirement • As usual: high throughput, low overhead, low error rates, … • Additionally: energy-efficient, handle switched off devices!

  4. Requirements for energy-efficient MAC protocols • Recall • Transmissions are costly • Receiving about as expensive as transmitting • Idling can be cheaper but is still expensive • Energy problems • Collisions and high BERs – wasted effort when two packets collide or corrupted packet • Overhearing – waste effort in receiving a packet destined for another node • Idle listening – sitting idly and trying to receive when nobody is sending • Protocol overhead • Always nice: Low complexity solution

  5. Schedule- vs. contention-based MACs • Schedule-based MAC • A schedule exists, regulating which participant may use which resource at which time (TDMA component) • Schedule can be fixed or computed on demand • Usually: mixed – difference fixed/on demand is one of time scales • Usually, collisions, overhearing, idle listening no issues • Needed: time synchronization! • Contention-based MAC • Risk of colliding packets is deliberately taken • Hope: coordination overhead can be saved, resulting in overall improved efficiency • Mechanisms to handle/reduce probability/impact of collisions required • Usually, randomization used somehow

  6. www.IEEE802.org Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [IEEE 802.15.4 Tutorial] Date Submitted: [4 January, 2003] Source: [Jose Gutierrez] Company: [Eaton Corporation] Address: [4201 North 27th Street, Milwaukee WI. 53216] Voice:[(414) 449-6525], FAX: [(414) 449-6131], E-Mail:[josegutierrez@eaton.com] Re: [IEEE 802.15.4 Overview; Doc. IEEE 802.15-01/358r0, TG4-Overview; Doc IEEE 802.15-01/509r0] Abstract: [This presentation provides a tutorial on the 802.15.4 draft standard.] Purpose: [] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

  7. 802.15.4 Applications Space • Home Networking • Automotive Networks • Industrial Networks • Interactive Toys • Remote Metering

  8. 802.15.4 Applications Topology Cable replacement - Last meter connectivity Virtual Wire Wireless Hub Stick-On Sensor Mobility Ease of installation

  9. Some needs in the sensor networks Thousands of sensors in a small space  Wireless but wireless implies Low Power! and low power implies Limited Range. Of course all of these is viable if a Low Cost transceiver is required

  10. Solution: LR-WPAN Technology! By means of IEEE 802.15.4

  11. 802.15.4 General Characteristics • Data rates of 250 kb/s, 40 kb/s and 20 kb/s. • Star or Peer-to-Peer operation. • Support for low latency devices. • CSMA-CA/TDMA channel access. • Dynamic device addressing. • Fully handshaked protocol for transfer reliability. • Low power consumption. • Frequency Bands of Operation • 16 channels in the 2.4GHz ISM band • 10 channels in the 915MHz ISM band • 1 channel in the European 868MHz band.

  12. 802.15.4 Architecture Upper Layers Other LLC IEEE 802.2 LLC IEEE 802.15.4 MAC IEEE 802.15.4 IEEE 802.15.4 868/915 MHz 2400 MHz PHY PHY

  13. IEEE 802.15.4 PHY Overview Operating Frequency Bands Channel 0 Channels 1-10 2 MHz 868MHz / 915MHz PHY 868.3 MHz 902 MHz 928 MHz 2.4 GHz PHY Channels 11-26 5 MHz 2.4 GHz 2.4835 GHz

  14. IEEE 802.15.4 PHY Packet Structure • PHY Packet Fields • Preamble (32 bits) – synchronization • Start of Packet Delimiter (8 bits) • PHY Header (8 bits) – PSDU length • PSDU (0 to 1016 bits) – Data field Start of Packet Delimiter PHY Header PHY Service Data Unit (PSDU) Preamble 6 Octets 0-127 Octets

  15. IEEE 802.15.4: PHY Layer Output signal Input Bit Symbol to chip Bit to symbol Modulation 16 channels, 5 MHz each

  16. IEEE 802.15.4 PHY Primitives • PHY Data Service • PD-DATA – exchange data packets between MAC and PHY • PHY Management Service • PLME-CCA – clear channel assessment • PLME-ED - energy detection • PLME-GET / -SET– retrieve/set PHY PIB parameters • PLME-TRX-ENABLE – enable/disable transceiver

  17. IEEE 802.15.4 MAC Overview Design Drivers • Extremely low cost • Ease of implementation • Reliable data transfer • Short range operation • Very low power consumption Simple but flexible protocol

  18. IEEE 802.15.4 MAC Overview Typical Network Topologies

  19. IEEE 802.15.4 MAC Overview 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

  20. IEEE 802.15.4 MAC Overview Star Topology PAN Coordinator Master/slave Communications flow Full function device Reduced function device

  21. IEEE 802.15.4 MAC Overview Peer-Peer Topology Cluster tree Point to point Full function device Communications flow

  22. IEEE 802.15.4 MAC Overview Combined Topology Clustered stars - for example, cluster nodes exist between rooms of a hotel and each room has a star network for control. Communications flow Full function device Reduced function device

  23. IEEE 802.15.4 MAC Overview Addressing • All devices have IEEE addresses • Short addresses can be allocated • Addressing modes: • Network + device identifier (star) • Source/destination identifier (peer-peer)

  24. IEEE 802.15.4 MAC Overview General Frame Structure • 4 Types of MAC Frames: • Data Frame • Beacon Frame • Acknowledgment Frame • MAC Command Frame

  25. IEEE 802.15.4 MAC Overview Optional Superframe Structure GTS 2 GTS 1 Contention Access Period Contention Free Period 15ms * 2n where 0  n  14 Transmitted by network coordinator. Contains network information, frame structure and notification of pending node messages. Network beacon Beacon extension period Space reserved for beacon growth due to pending node messages Contention period Access by any node using CSMA-CA Guaranteed Time Slot Reserved for nodes requiring guaranteed bandwidth [n = 0].

  26. IEEE 802.15.4 MAC Overview Traffic Types • Periodic data • Application defined rate (e.g. sensors) • Intermittent data • Application/external stimulus defined rate (e.g. light switch) • Repetitive low latency data • Allocation of time slots (e.g. mouse)

  27. IEEE 802.15.4 MAC Overview MAC Data Service Recipient MAC Originator MAC MCPS-DATA.request Channel access Data frame Originator Recipient Acknowledgement (if requested) MCPS-DATA.indication MCPS-DATA.confirm

  28. IEEE 802.15.4 PHY Overview MAC Primitives • MAC Data Service • MCPS-DATA – exchange data packets between MAC and PHY • MAC Management Service • MLME-ASSOCIATE/DISASSOCIATE – network association • MLME-SYNC / SYNC-LOSS - device synchronization • MLME-SCAN - scan radio channels • MLME-GET / -SET– retrieve/set MAC PIB parameters • MLME-START / BEACON-NOTIFY – beacon management • MLME-POLL - beaconless synchronization • MLME-GTS - GTS management • MLME-ORPHAN - orphan device management • MLME-RX-ENABLE - enabling/disabling of radio system

  29. A numerical example • Adopting beacon-enabled networks; • Data transfer protocols (e.g. towards PANC); • Maximum bandwidth 250 kb/s = 62.5 ksym/s (16-ary coding, 1sym = 4 bits); • Maximum number of GTS (Guaranteed Time Slots) = 7. AP= 16 (slots) * 960 (baseslotduration) * 2SOцs

  30. Transfer of large data sets (1) • Suppose you want to transmit 1 picture (P2P), use the lowest resolution (80 * 64 pel): is 1.6 kBytes • Maximum MAC MSDU (payload) is 102 bytes, i.e. 16 MAC frames each resulting in 132 bytes = 264 sym at the PHY layer; • The average amount of time to transmit the data in CSMA is (BE=2, default) w/o taking into account traffic and different sources of overheads: • 16 * [(1.5 (avg BT) * BP) + 2 * SP (CCA) + 264 ] sym/ 62.5 Ksym/s = 80 ms (optimistic); • In free access: • 16 * 264 sym / 62.5 Ksym/s = 68 ms (but pay attention at the reservation cost).

  31. Transfer of large data sets (2) • To fit the transmission into 6 slots of CAP we have to use SO = 4: • 960 цs * 24 * 6 > 80 ms; • If we want to use the GTSs: • we have an overhead of 1 superframe + minimum CAP (440 symbols) = 16 * 960 * 24 цs + 7 ms = 100 ms (maximum) !!!!!

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