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Wakeup mechanism of WBAN

Wakeup mechanism of WBAN. Bin Zhen, Huan-bang Li and Ryuji Kohno National Institute of Information and Communications Technology (NICT). Motivation and problem. Dynamic duty cycle of BAN device Medical event and medical implant: <0.1% Stream: >20%

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Wakeup mechanism of WBAN

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  1. Wakeup mechanism of WBAN Bin Zhen, Huan-bang Li and Ryuji Kohno National Institute of Information and Communications Technology (NICT) Zhen, Li and Kohno

  2. Motivation and problem • Dynamic duty cycle of BAN device • Medical event and medical implant: <0.1% • Stream: >20% • Both BAN coordinator and device have limited battery budget • Coordinator cannot be assumed to be always ready as it was in 802.15.4 • Coordinator may have more resource than devices • When and how to wakeup an inactive device with the least power consumption? • for both device and coordinator • re-association of orphan node • creation of piconet Zhen, Li and Kohno

  3. MAC clock • All BAN devices have a MAC clock, which synchronizes to the clock at piconet coordinator • The clock may provide by hardware or software • Running of the clock should be low power consumption • Devices obtain the clock information from coordinator when joining a piconet • Not an application layer clock • Why a MAC clock? • TinyOS defined a clock event • To enable optimization of MAC protocol with clock Zhen, Li and Kohno

  4. BAN day • Given the concept of MAC clock, the time can be divided into “BAN day” • A “BAN day” consists of N BAN superframes • In a “BAN day”, some BAN superframes are active and some superframes are inactive bspf_3 bspf_0 bspf_1 bspf_2 bspf_N-1 bspf_0 ……. time A “BAN day” The first BAN superframe in a “BAN day” Inactive BAN superframe Active BAN superframe Zhen, Li and Kohno

  5. BAN day (cont.) • MAC clock provides coarse synchronization between devices and coordinator • Not bit-wise synchronization for communication • MAC clock can be refreshed in the active BAN superframes to combat clock drift bspf_i bspf_i+1 bspf_N-1 bspf_i+2 ……. time active period inactive period Active BAN superframe Inactive BAN superframe Zhen, Li and Kohno

  6. BAN superframe • BAN superframe is a time duration • It is different from the superframe defined in 802.15.4 • In a BAN superframe, BAN devices action can be one of the following three actions • Tx/Rv interaction • Tx or Rv only • Sleeping • BAN superframe • Active superframe • Beacon-enabled superframe (wakeup point) • Non-beacon superframe • Inactive superframe • Detail structure of BAN superframe will be defined in the future Zhen, Li and Kohno

  7. 15ms * 2SO where 0  SO  14 15ms * 2BO where SO ≤ BO ≤ 14 BAN superframe (cont.) SO = Superframe order BO = Beacon order beacon CAP CFP Inactive Period 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 802.15.4 superframe Active BAN superframe Inactive BAN superframe Active BAN superframe Zhen, Li and Kohno

  8. BAN size scalability • The beacon mode of 802.15.4 has been criticized for 16 slots and 7 GTS • In a “BAN day”, a GTS is labeled by MAC clock and GTS number. • This enables circuit-switch like GTS • No need GTS allocation in beacon bspf_i bspf_i+1 S1 S2 CFP CAP Zhen, Li and Kohno

  9. Wakeup point • Wakeup point is the first time slot in an active BAN superframe where beacon is broadcasted • Functions same as 802.15.4 • Bit-wise synchronization, pending data, network management • To refresh MAC clock • Coordinator shall broadcast beacon in all wakeup points Wakeup point beacon Time slot …… …… Active BAN superframe Inactive BAN superframe Coordinator: transmit listen/transmit Zhen, Li and Kohno

  10. Wakeup point listening • Listening to the wakeup points is distributed decided by device • A device may skip one or more wakeup points • The decision may be based on clock accuracy, battery power, traffic, QoS, etc. bspf_k bspf_k+4 bspf_N-1 ……. coordinator beacon beacon Device A listen listen Device B sleep sleep Inactive BAN superframe Wakeup point BAN superframe Zhen, Li and Kohno

  11. Placement of wakeup point BAN superframe • Placement of wakeup point BAN superframe is controlled by coordinator in a “BAN day” • Wakeup point should be period and random • Random wakeup point is good for security • Wakeup point should be adaptive to traffic pattern, duty cycle, number of device, and QoS requirements. • In principle, the lower the duty cycle, the fewer the wakeup point. • Devices get the placement of wakeup point BAN superframe when joining a piconet • Coordinator may change it after piconet is created Zhen, Li and Kohno

  12. Placement of wakeup point BAN superframe (cont.) • Example conditions • (k-rnd()) mod 2i=0, i=0,1,2,… • (k-rnd()) mod 3i=0, i=0,1,2,… • Random algorithm is TBD k mod 22 bspf_k 25% duty cycle bspf_N-1 ……. 33.3% duty cycle k mod 31 ……. time Wakeup point BAN superframe Inactive BAN superframe Zhen, Li and Kohno

  13. Why clock in MAC layer ? • Clock in application layer • Absolute global time • Multi-hop support • Independent of PHY and MAC • Complex synchronization algorithms • FTSP, TSPN, LTS, Tiny-sync, mini-sync, RBS • More resource requirement in power and computation • Clock in MAC layer • Relative local time in a “BAN day” • Single hop • Clock is part of MAC and can be used to optimize PHY and MAC design for BAN purpose • e.g. low power wakeup, QoS support, network size scalability • Low complexity system • Broadcast based synchronization Zhen, Li and Kohno

  14. BAN superframe vs 15.4 superframe • BAN superframe is a unit of time duration to define “BAN day” • More flexible to allocate active and inactive period with the help of MAC clock • The active period of 15.4 can be only at the beginning of a superframe • Beacon may be not necessary in an active BAN superframe • for very low duty cycle devices • Beacon listening in wakeup point is optional • Labeled superframe • Time stamp of MAC clock in beacon packet • Future work • BAN superframe definition Zhen, Li and Kohno

  15. BAN superframe vs non-beacon mode • Given MAC clock, BAN is a semi-synchronized piconet • It becomes a beacon-enabled network after wakeup, which make it easy for QoS control for both medical and non-medical applications • In the asynchronous networks (non-beacon mode of 802.15.4), coordinator must always be ready. • It is unrealistic for wearable coordinator of BAN • Additional power for clock maintenance • A clock in application layer need maintenance also Zhen, Li and Kohno

  16. Conclusion • Concept of MAC clock and “BAN day” • Clock unit: superframe • Semi-synchronized network • Coarse time synchronization of device through MAC clock • Scheduled wakeup points • Central-controlled wakeup point BAN superframe • Distributed beacon listening in wakeup point • Benefits of MA clock • Power saving of both coordinator and devices • Optimization PHY and MAC with the help of clock • Network size scalability issue of BAN • Future works • BAN Superframe definition Zhen, Li and Kohno

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