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PART I: IEEE 802.15.4, a novel MAC/Phy layer for the Zigbee stack

PART I: IEEE 802.15.4, a novel MAC/Phy layer for the Zigbee stack. A.G. Ruzzelli. Adaptive Information Cluster (AIC) Group, University College Dublin, Ireland. Summary. Wireless Sensor networks (WSNs) Generality Prototypes Application Requirements Zigbee generality

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PART I: IEEE 802.15.4, a novel MAC/Phy layer for the Zigbee stack

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  1. PART I: IEEE 802.15.4, a novel MAC/Phy layer for the Zigbee stack A.G. Ruzzelli Adaptive Information Cluster (AIC) Group, University College Dublin, Ireland.

  2. Summary • Wireless Sensor networks (WSNs) • Generality • Prototypes • Application Requirements • Zigbee generality • Overview of the stack • The components • The primitives • Zigbee NWK layer • The NWK layer architecture and services • The address assignment • The AODV protocol • IEEE 802.15.4 • Generality • Superframe structure • Transmission modes • Association phase • Conclusion

  3. Energy-Efficient Wireless Sensor Networks (WSNs) • A large number of tiny wireless devices to sense the environment: • Sensor nodes • Few more powerful devices to collect the data: • Gateways (or sinks or PAN coordinators) PDA, laptop, PC etc.

  4. Some WSN applications environmental data collection: temperature light, humidity, pressure, solar radiation. Wind Response Of Golden Gate Bridge • Remote area monitoring • Object location • Industry machinery monitoring • Disaster prevention • Wireless medical systems Monitoring nesting patterns of Storm Petrels. medical systems

  5. Wireless sensor characteristics WSN manager • Sensors are of : • Low cost • Low processing capability •  System strength based on sensor collaboration • Large scale networks • Multihop communication • Sensors are battery operated for long unattended period: •  Saving energy is a primary objective

  6. WSN issues • Large number of nodes Scalability issues • High dynamic condition (number and position of nodes might change) Network Reactivity and Self-organization • Power management The network needs to be connected as long as possible • System reliability The wireless signal needs to cope with interference Coordination among node communication • Node synchronization (clock skew and offset) To avoid sending to a sleeping node • Robustness Subject to environmental variability (harsh condition) Complex interoperability of network devices

  7. Sensor node prototypes Mica2 mote Tyndall sensor Eyes node prototype Philips sand nodes

  8. General sensor node architecture: Sensing devices Application Data interpolation Sensing coverage Localization Cross layer interaction Routing MAC Physical Antenna • Any layer try to achieve the task using the smallest amount of energy possible

  9. The need of the Zigbee standard • An exponential increase of the interest on WSNs • No communication systems that addressed: • Energy efficiency; • Low cost devices; • Low data rate per node; • Very low duty cycle; • Scalability (e.g. issues with Bluetooth); • WSN proprietary systems cause interoperability problems

  10. Stack Reference Model End developer applications, designed using application profiles ZA1 ZA2 … ZAn IA1 IAn Application interface designed using general profile API UDP ZigBee NWK IP 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 LLC= logical link control SSCS = Service specific convergence sublayer

  11. Protocol Stack Features • 8-bit microcontroller • Full protocol stack <32 KB • Simple node-only stack ~4KB • Coordinators require extra RAM • Node device database • Transaction table • Table of neighbours APPLICATION Customer APPLICATION INTERFACE NETWORK LAYER DATA LINK LAYER ZigBee Alliance MAC LAYER MAC LAYER IEEE PHY LAYER Silicon ZigBee Stack Application

  12. Z-NWK layer: The components Zigbee coordinator (ZC) • Only one ZC present in the network • Initiates the network formation (PAN ID, channel stack etc.) • It acts as PAN coordinator with FFD capability • It can act as a router to other nodes • It acts as interface between the user and the network

  13. Z-NWK layer: The components Zigbee router (ZR) • Responsible for tree/mesh packet routing • Associates/disassociates node to the network • Coordinates communication to children nodes • It is in RX mode when idle • (no sleep mode implemeted) • Maintains a table of neighbours

  14. Z-NWK layer: The components Zigbee end device (ZED) • It has reduced functionalities • It has reduced duty cycle regulated by the parent ZR • It can talk with the parent ZR only • It cannot associate other nodes

  15. Zigbee primitives and services Upper layer Request Confirm Indication Response Lower layer • Zigbee primitives are used to communicate between layers • 4 primitive types are present: • Request/confirm • Indication/response • Layers communicate through the entitities of the Service Access Point (SAP) e.g. NLDE-SAP = network layer data entity-SAP

  16. Architecture of the Z-NWK layer • ZigBee Device Types • Stack Profile, Network Rules • Network Management and Addressing • Message Routing • Route Discovery and Maintenance • Security

  17. Network formation modalities PAN coordinator Coordinator FFD End device RFD Star topology Mesh topology Tree topology

  18. NWK Layer services • Layer management entity LME • allow requesting services and interfacing to other layers • Layer data entity LDE • Allow transmitting data SAP= Service access point

  19. Network Initiation by ZCoordinator • NLME_NETWORK_DISCOVERY.request • Performs an Active Scan • Looks for other ZigBee networks on the channel • Selects a compatible network Stack Profile

  20. Network Association: ZR & ZED • NLME_JOIN.request • Selects the highest acceptable router • Link Quality, with capacity • Associates with the router • Allocated an address on the network • Device authenticates with network • NLME_START_ROUTER.request • Updates Beacon Payload • Depth, Capacity • Starts a router • Updates Association Permit Status

  21. Transmitting data • NLDE-DATA.request • Used by NHL for all data transmissions • Uni-casts and broadcasts • Accepts the following parameters • Destination Address • Radius • Discover Route • NLDE-DATA.indication • Reports the receipt of a data transmission • Includes the following parameters • Source Address

  22. IEEE addressing • IEEE provides unique long address of 64bits for nodes that uses 802.15.4 • Long addresses cause high data overhead if used for node communication • Communication relies on not-unique short address of 16bits (65536 devices) • Short adrresses are forged by the Zigbee address assignment procedure

  23. Zigbee Tree-structure address assignment Router (FFD) at depth d+1 Cskip(d) = [1 + Cm-Rm-Cm*Rm^(Lm-d-1)]/(1-Rm) N-th end device (RFD) An = Aparent + Cskip(d)Rm+n Note: In order to assign addresses, it is necessary to know a priorimaxDepth, maxRouter numbers and maxNumbChildren

  24. Ad-Hoc on demand vector (AODV) routing Route discovery • Find or update route between specific source and destination • Started if no active route present in routing table • Broadcast routing request (RREQ) packets • Generates routing table entries for hops to source • Endpoint router responds with Routing response (RREP) packet • Routes generated for hops to destination • Routing table entry generated in source device

  25. The ADOV protocol • Route discovery • A routing table is required if a route already exists 2 1 3 5 2 1 4 RREQ RREP picture taken from “ZigBee” presentation by Jan Dohl et al.

  26. THE IEEE 802.15.4 • Defined by the IEEE for low-rate, wireless personal area networks (WPANs). • Defines the physical layer “Phy” and the medium access control layer “MAC”. • low-power spread spectrum signal at:

  27. 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

  28. Concurrent channel allocation • An example of Frequency Channel allocation for device classes IEEE 802.11b channel in North America and Europe Bluetooth cannels IEEE 802.11b channel in Europe 2480 2401 2402 2403 2481 2482 2483 2400 picture taken from ZigBee Specifications v1.0

  29. IEEE 802.15.4: PHY layer 2400MHz Band specs • 4 Bits per symbol • DSSS with 32 Bit chips • O-QPSK modulation • Sine halfwave impulses Medium Bit to Symbol Symbol to Chip QPSK Mod. Binary Data picture taken from IEEE 802.15.4 Specification

  30. PHY layer contd. • General specs and services • Error Vector Magnitude (EVM) < 35% • -3dBm minimum transmit power (500µW) • Receiver Energy Detection (ED) • Link Quality Indication (LQI) • Use ED & LQI to reduce TX-power • Clear Channel Assessment (CCA) with 3 modes • Energy above threshold • Carrier sense only • Carrier sense with energy above threshold

  31. Device types • In conformity with Zigbee devices, IEEE802.15.4 are of 3 types: • PAN coordinator • Act as network initiator • Only one allowed in the network • Full functional devices FFDs • That have all access control functionalities implemented (channel scan, beacon transmission, association etc.) • Reduced functional devices RFDs • That can only talk to the FFD that associated them

  32. IEEE 802.15.4: MAC layer • Managing PANs • Channel scanning (ED, active, passive, orphan) • PAN ID conflict detection and resolution (in progress) • Starting a PAN • Sending beacons • Device discovery • Device association/disassociation • Synchronization (beacon mode) • Orphaned device realignment

  33. Beacon/nonbeacon-enable modes • Beacon-enabled mode: • Beacons are broadcasted periodically by the FDD • Beacons do not employ CSMA prior transmission • Beacons contain info related to superframe length • and GTS allocation details • ACK is optional • Nonbeacon-enabled mode: • The MAC reduces to a simple unslotted CSMA-CA • No Superframe • No GTS • ACK is optional

  34. The superframe structure • Becons, transmitted by FFDs, contain a superframe specification

  35. IEEE 802.15.4 association phase Coordinator FFD RFD RFD: Broadcast Beacon request FFD: Superframe spec. RFD: Association req.. FFD: ACK with seq#. FFD: Broadcast standard timezone packet FFD: Broadcast standard data packet RFD: Data request FFD: ACK with seq#. FFD: Association response with short ID. RFD: ACK with seq#.

  36. The IEEE802.15.4 chip • IEEE802.15.4 is coded onto the chip CC2420 (partially hard coded) • Zigbee licence must be bought separately • Zigbee compliancy might be lost if some change to the code is made  NOT very suitable for research purposes

  37. End of PART I

  38. PART II: MERLIN over IEEE 802.15.4: routing capabilities without Zigbee A.G. Ruzzelli Adaptive Information Cluster (AIC) Group, University College Dublin, Ireland.

  39. Network formation by the IEEE 802.15.4 MAC One PAN coordinator Zero or more coordinators Zero or more end devices First device starts the network as PAN coordinator A new device can detect all coordinators (both the PAN coordinator and coordinators) A device can join the network by associating with any coordinator in range After joining a device can volunteer as coordinator PAN coordinator Coordinator End device

  40. Step 1: Starting a new network Device starts network scan (MLME_SCAN) Detects no network Starts new network as PAN coordinator (MLME_START with PANCoordinator=TRUE) If PANCoord then other devices in range can discover device 1 by means of a network scan PAN coordinator Coordinator FFD End device RFD 1

  41. Step 2: Second device joins the network Device 2 starts network scan (MLME_SCAN) Detects PAN coordinator device 1 Sends association request to device 1 (MLME_ASSOCIATE) Node2 is now and End device  Other devices cannot discover device 2 by means of a network scan PAN coordinator Coordinator FFD End device RFD 1 2 range

  42. Step 3: Device 2 becomes a coordinator Device 2 starts serving as a coordinator of the existing network (MLME_START with PANCoordinator=FALSE, PANId & channel parameters are ignored) Node2 is now Other devices in range can now discover device 2 by means of a network scan PAN coordinator Coordinator FFD End device RFD 1 2

  43. Step 4: Device 3 joins the network Device 3 starts network scan (MLME_SCAN) Detects coordinator device 2(assuming device 1 is not in range of device 3) Sends association request to device 2 (MLME_ASSOCIATE) Note: Other devices cannot discover device 3 by means of a network scan PAN coordinator Coordinator FFD End device RFD 1 2 3 range

  44. Step 5: Device 4 joins the network Device 4 starts network scan (MLME_SCAN) Detects two coordinators: device 1 and device 2(assuming device 1 and device 2 are in range of device 4) Sends association request to device 1 (MLME_ASSOCIATE)(alternatively it could join the network also through device 2) Note: Other devices cannot discover device 4 by means of a network scan PAN coordinator Coordinator FFD End device RFD 1 4 range 2 3

  45. Step 6: Device 4 becomes a coordinator Device 4 starts serving as a coordinator of the existing network (MLME_START with PANCoordinator=FALSE, PANId & channel parameters are ignored) Note: Other devices in range can now discover device 4 by means of a network scan PAN coordinator Coordinator FFD End device RFD 1 4 2 3

  46. Other devices can join in the same way IEEE 802.15.4 allows only direct (single hop) communication between two devices that are in range of each other. IEEE 802.15.4 leaves it to the higher layers to define how network-wide unique short MAC addresses are assigned by coordinators. Extended MAC addresses can be used instead of short addresses  High packet overhead PAN coordinator Coordinator FFD End device RFD 5 6 1 4 2 range 7 8 3

  47. Other devices can join in the same way A networking protocol (e.g. ZigBee) on top of IEEE 802.15.4 is required to allow communication between nodes that are not in range of each other by routing of packets via intermediate nodes (multi hop). ZigBee defines how short NWK addresses are assigned to devices. The short NWK addresses are used also as short MAC addresses. PAN coordinator Coordinator FFD End device 5 6 1 4 2 range 7 8 3

  48. Issue 1: The hidden association problem The IEEE 802.15.4 does NOT provide coordination between coordinators End devices (RFDs) can talk to its coordinator only  packet collisions might occur 1) Eg. Node9 transmitting to node2 might generate collision at node8 that is receiving from node11. 2) Eg. Either node10 and node7 transmission might prevent correct neighbouring node reception PAN coordinator Coordinator FFD End device RFD 5 6 1 4 9 2 range 7 8 10 3 11

  49. Issue 2: Beacons are weak Beacons are more prone to collide as transmitted without CSMA If a beacon collides then no children RFD devices can transmit or receive. 1 4 9 Beacon 2 7 Beacon Beacon 8 10 3 Tx 11

  50. Question PAN coordinator Coordinator FFD End device RFD • Q.1 How can we avoid packet collisions? • A.1 By using RTS/CTS/ACK • Cons1 We lose the 802.15.4 compliancy • Cons2: Results show a very long delay when associated to low node duty cycle 1 9 2 8 RTS CTS ACK 11

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