1. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 1 Wakeup mechanism of WBAN Bin Zhen, Huan-bang Li and Ryuji Kohno National Institute of Information and Communications Technology (NICT)

2. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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

3. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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

4. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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_N-1 ……. Active BAN superframe Inactive BAN superframe time bspf_0bspf_1bspf_2 bspf_3 bspf_0 A “BAN day” The first BAN superframe in a “BAN day”

5. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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_i+2 bspf_N-1 ……. Inactive BAN superframe time Active BAN superframe active period inactive period

6. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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

7. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 7 BAN superframe (cont.) 15ms * 2 BO where SO ≤ BO ≤ 14 15ms * 2 SO where 0  SO  14 SO = Superframe order BO = Beacon order Inactive Period 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 beacon CAPCFP Active BAN superframe Inactive BAN superframe 802.15.4 superframe Active BAN superframe

8. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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 CFP S1 bspf_i bspf_i+1 S2 CAP

9. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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 beacon Time slot …… Active BAN superframe Wakeup point Inactive BAN superframe …… Coordinator:listen/transmittransmit

10. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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_N-1 ……. Device A listen Device B sleep listen sleep bspf_k+4 coordinator beacon Wakeup point BAN superframe Inactive BAN superframe

11. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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

12. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 12 Placement of wakeup point BAN superframe (cont.) Example conditions –(k-rnd()) mod 2 i =0, i=0,1,2,… –(k-rnd()) mod 3 i =0, i=0,1,2,… –Random algorithm is TBD Inactive BAN superframe time bspf_k bspf_N-1 ……. 25% duty cycle ……. 33.3% duty cycle k mod 2 2 k mod 3 1 Wakeup point BAN superframe

13. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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

14. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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

15. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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

16. doc: IEEE 802.15-08-0352-00-0006 May/2008 Zhen, Li and KohnoSlide 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