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Time Slotted, Channel Hopping MAC

This document proposes extensions for IEEE802.15.4 MAC to provide industrial-grade reliability and robustness in wireless personal area networks (WPANs).

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Time Slotted, Channel Hopping MAC

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Time Slotted, Channel Hopping MAC] Date Submitted: [1 Sep, 2008] Source: [Kris Pister, Chol Su Kang, Kuor Hsin Chang, Rick Enns, Clint Powell, José A. Gutierrez, Ludwig Winkel] Companies [Dust Networks, Freescale, Emerson, Siemens AG] Address: [30695 Huntwood Avenue, Hayward, CA 94544 USA;890 N. McCarthy Blvd, Suite 120, Milpitas, CA 95035 USA; 8000 West Florissant Avenue St. Louis, Missouri 63136 USA; Siemensallee 74, Karlsruhe, Germany] Voice: [+1 (510) 400-2900, +1 (408) 904-2705, +1 (650) 327-9708, +1 (480) 413-5413, +1 (314) 553-2667,+49 (721) 595-6098] E-Mail: [ kpister@dustnetworks.com, ckang@dustnetworks.com, Kuor-Hsin.Chang@freescale.com, enns@stanfordalumni.org, clinton.powell@freescale.com, Jose.Gutierrez@emerson.com,ludwig.winkel@siemens.com ] Re: [n/a] Abstract: [This document proposes extensions for IEEE802.15.4 MAC] Purpose: [This document is a response to the Call For Proposal, IEEE P802.15-08-373-01-0043] 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. Kris Pister et al.

  2. Time Slotted, Channel Hopping MAC (TSCH) Kris Pister – UC Berkeley/Dust Networks Chol Su Kang - Dust Networks Kuor Hsin Chang - Freescale Rick Enns - Consultant Clinton Powell - Freescale José A. Gutierrez – Emerson Ludwig Winkel – Siemens September, 2008 Kris Pister et al.

  3. Target Applications Industrial and commercial applications with a particular focus on: • Equipment and process monitoring • Non-critical control • Diagnostics/predictive maintenance • Asset management Kris Pister et al.

  4. Requirements • Industrial-Grade Reliability and robustness in the presence of multipath, path obstructions and interference • Industrial and commercial environments • Sustained operation in the presence of non-standards based communications systems • Long operational life for battery powered devices (> 5 years) • Co-existence • Flexible and scale-able • Easy wireless network deployment and maintenance Kris Pister et al.

  5. TSCH- Accepted, Proven & Practical • Time Slotted, Channel Hopping (TSCH) technology is the basis for the wireless network of two industrial standards • HART Foundation (www.hartcomm2.org - over 200 companies worldwide): WirelessHART- published 9/07 • ISA (www.isa.org – over 30,000 members worldwide): ISA100 Committee, ISA100.11a working group- in working group draft • TSCH has been implemented by multiple companies on multiple 2.4 GHz IEEE std. 802.15.4 platforms Kris Pister et al.

  6. Timeslot Access CCA: RX startup, listen, RX->TX Slot Frame Cycle Unallocated Slot Allocated Slot Tx Transmit Packet: Preamble, SFD, Headers, Payload, CRC RX startup or TX->RX RX ACK Rx RX startup RX packet Verify MIC Calculate ACK MIC Transmit ACK RX/TX turnaround timeslot TX/RX packet TX/RX ACK Devices are configured with a slotframe and timeslots to communicate with each other. Kris Pister et al.

  7. Timeslot Basics All devices in the same network synchronize slotframes All timeslots are contained within a slotframe cycle Timeslots repeat in time: the slotframe period Device-to-device communication within a timeslot includes packet Tx/Rx & ACK Tx/Rx Configurable option for CCA before transmit in timeslots Kris Pister et al.

  8. Timeslot Operation In Devices Devices use timeslots to: • Schedule when they wakeup to transmit or listen • Keep time synchronized • Specification on time difference tolerances • Time synchronization mechanisms • Time the sequence of operations • Allow the source and destination to set their frequency channel • Listening for a packet • Sending a packet • Listening for an ACK • Generating an ACK • Synchronizes channel hops • Provide time to higher layers Kris Pister et al.

  9. Sample Timeslot Processing Kris Pister et al.

  10. Link Types • Dedicated Link – assigned to one device for transmission and to one or more devices for reception • A dedicated broadcast link is assigned to all devices for reception • Shared Link – assigned to more than one device for transmission • ACK failures detect collisions • A slot based back-off algorithm resolves collisions Kris Pister et al.

  11. Sample Shared Link Processing Kris Pister et al.

  12. Channel Hopping Slot n+2 Slot n Slot n-2 Slot n-1 Slot n+1 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 802.15.4 Channels • Combined with timeslot access to enhance reliability Kris Pister et al.

  13. Channel Hopping • Mitigate Channel Impairments • Channel hopping adds frequency diversity to mitigate the effects of interference and multipath fading • Increase Network Capacity • One timeslot can be used by multiple links at the same time Kris Pister et al.

  14. Link = (Timeslot , Channel Offset) D B One Slot Time Chan. offset A BA CA DA C BA BC E F BE BF • The two links from B to A are dedicated • D and C share a link for transmitting to A • The shared link does not collide with the dedicated links Kris Pister et al.

  15. Channel Hopping Time BA (ch 15) BA (ch25) BA (ch18) CA DA CA DA CA DA Channel Offset BA BA BA BC BC BC BE BF BE BF BE BF ASN= N*4 N*4+1 N*4+2 N*4+3 (N+1)*4 CycleN+2 Cycle N+1 Cycle N • Each link rotates through k available channels over k cycles. • Ch # = Chan Hopping Seq. Table ( ( ASN + Channel Offset) % Number_of_Channels ) • Blacklisting can be defined globally and locally. Kris Pister et al.

  16. Timeslot Timing Offsets T1 T2 T4 T3 RX ACK Transmitter prepare to receive CCA TX Packet TsRxAckDelay TsCCAOffset AWT TsTxOffset process packet, prepare to ack RX Packet TX ACK Receiver prepare to receive TsRxOffset PWT TsTxAckDelay R1 R2 R3 End of timeslot Start of timeslot = transmitting packet Timeslot with Acknowledged Transmission = receiver on = receiving packet PWT = TsPacketWaitTime AWT = TsAckWaitTime Kris Pister et al.

  17. Timeslot Timing Offsets (Cont’d) T1 T2 T4 T3 Idle receive Transmitter prepare to receive CCA TX Packet TsRxAckDelay TsCCAOffset AWT TsTxOffset RX Packet Receiver prepare to receive process packet, decide not to ack TsRxOffset PWT R1 R2 End of timeslot Start of timeslot = transmitting packet = receiver on Timeslot with Unacknowledged Transmission = receiving packet PWT = TsPacketWaitTime AWT = TsAckWaitTime Kris Pister et al.

  18. Timeslot Timing Offsets (Cont’d) T1 T2 Transmitter no ack expected CCA TX Packet TsCCAOffset TsTxOffset RX Packet Receiver process packet, decide not to ack prepare to receive TsRxOffset PWT R1 R2 End of timeslot Start of timeslot = transmitting packet = receiver on Timeslot with Unacknowledged Broadcast = receiving packet PWT = TsPacketWaitTime AWT = TsAckWaitTime Kris Pister et al.

  19. Timeslot Timing Offsets (Cont’d) Transmitter idle Receiver prepare to receive idle Idle rx TsRxOffset PWT R1 R2 End of timeslot Start of timeslot Timeslot with Idle Receive = receiver on PWT = TsPacketWaitTime Kris Pister et al.

  20. Time Synchronization TCCA TCCA TCCA TCCA Transmit Packet: Preamble, SFD, Headers, Payload, FCS TProcessing TACK Transmit Packet: Preamble, SFD, Headers, Payload, FCS TProcessing TACK Transmit Packet: Preamble, SFD, Headers, Payload, FCS TProcessing TACK Tg Tg Tg Tg Transmit Packet: Preamble, SFD, Headers, Payload, FCS Early Perfect Late Tcomm = TTXPacket+TProcessing+TACK Timeslot Period TProcessing includes the processing of FCS and MIC validation as well as FCS and MIC generation for ACK. It’s the time from the last bit of the packet to the first bit of the preamble of the ACK. Kris Pister et al.

  21. Time Synchronization (Cont’d) Kris Pister et al.

  22. Time Synchronization Acknowledgement-based Synchronization Transmitter node sends a packet, timing at the start symbol. Receiver timestamps the actual timing of the reception of start symbol Receiver calculates TimeAdj = Expected Timing – Actual measured Timing Receiver informs the sender TimeAdj Transmitter adjusts its clock by TimeAdj Kris Pister et al.

  23. Time Synchronization (Cont’d) Received Packet-based Synchronization Receiver timestamps the actual timing of the reception of start symbol Receiver calculates TimeAdj = TimeExpected (expected arrival time) – Actual timing Receiver adjusts its own clock by TimeAdj A node can be synchronized to more than one parent (i.e. timing reference nodes) Kris Pister et al.

  24. Non-conflicting Timeslot assignment • Devices with multiple radios can be given one or more offsets. • Devices can be given one or more slots in a particular slotframe. • Devices with management ability can be given a block of (slot,offset)s slot Chan. offset Kris Pister et al.

  25. Non-conflicting timeslot assignment Multiple slotframes with different lengths can operate at the same time. 4 cycles of the 250ms slotframe are shown, along with a 1000ms slot frame There are never collisions if the 1000ms slot frame uses only the empty slots of the 250 ms slot frame 250ms 250ms 250ms 250ms 1,000ms Kris Pister et al.

  26. Added MAC PAN Service Primitives Kris Pister et al.

  27. SET-SLOTFRAME • Request (Device Management  TSCH MAC) • Add, delete, or change a slotframe • Parameters: slotframe Id, operation, slotframe size, channel page, channel map, active flag • Confirm (TSCH MAC  Device Management) • Reports the results of SET-SLOTFRAME request command • Parameters: slotframe Id, status Kris Pister et al.

  28. SET-LINK • Request (Device Management  TSCH MAC) • Add, change, or delete a link • Parameters1: operation type=ADD or CHANGE, link handle, frame Id, timeslot, channel offset, link options, link type, node addresses • Parameters2: operation type=DELETE, link handle • Confirm (TSCH MAC  Device Management) • Indicates the result of add, change or delete link command • Parameters: status, link handle Kris Pister et al.

  29. TSCH-MODE • Request (Device Management  TSCH MAC) • Puts the MAC to TSCH mode of operation • Parameters: none • Confirm (TSCH MAC  Device Management) • Reports the result of the TSCH-MODE request • Parameters: status Kris Pister et al.

  30. LISTEN • Request (Device Management  TSCH MAC) • Request the MAC to search for a network • Parameters: channel page, 802.15.4 channel, duration • Confirm (TSCH MAC  Device Management) • Reports when the MAC completes the listen operation • Parameters: status • Indication (TSCH MAC  Device Management) • Indicates that the MAC received an ADVERTISEMENT packet while listening • Parameters: link quality, PAN ID, channel map, join priority, slotframes, links in each slotframe (these parameters, except link quality, are received in ADVERTISEMENT packet) Kris Pister et al.

  31. New TX Option in Existing Primitive MCPS.DATA.request Primitive • In TSCH Mode, the Next Higher Layer (NHL) may provide TSCH MAC a list of links. The NHL may choose the links the MSPDU may be transmitted on. The TSCH MAC selects the next available link from the list. Kris Pister et al.

  32. Examples of TSCH Capability Data collection 100 pkt/s per access point channel using 10 ms slots* 1600 pkt/s (16*100) network capacity with no spatial reuse of frequency Radio duty cycle (power consumption) Near theoretical limit for networks with moderate to high traffic ~0.02% for very low traffic networks Latency 10ms / PDR (Packet Delivery Rate) per hop: best case Statistical, but well modeled * 10 ms slots are an example – the standard can define a range of slot sizes that can be selected for use Kris Pister et al.

  33. Built-In Flexibility Trade performance and power Sample & reporting rate Latency Reliability Throughput High bandwidth connections Tradeoffs can vary with Time Location Events Use power intelligently if you’ve got it Highest performance with powered infrastructure Kris Pister et al.

  34. TSCH Summary • Proven technology- aligns with several industrial wireless standards • Meets the requirement for commercial and industrial monitor and process control applications • Extends the capabilities of the existing IEEE 802.15.4-2006 MAC Kris Pister et al.

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