MBStar: A Real-time Communication Protocol for Wireless Body Area Networks Xiuming Zhu, Song Han, Pei-Chi Huang, Al Mok Deji Chen

23 rd Euromicro Conference on Real-Time Systems Agenda Background and Related Works MBStar Design MBStar Implementation –Link Scheduling Problem and Solution Experiments

23 rd Euromicro Conference on Real-Time Systems Background and Related Work

23 rd Euromicro Conference on Real-Time Systems Typical Bandwidth and data size Requirements High frequency and small data packet

23 rd Euromicro Conference on Real-Time Systems Our Solution: MBStar Real-time Delivery. TDMA MAC layer is adopted Higher Frequency. Highest data rate is 400Hz Reliable. Channel Hopping and Channel blacklist are utilized Secure. Public/private key mechanism for provisioning before join and AES for encrypting health data after join Star Topology. One access point, several devices

23 rd Euromicro Conference on Real-Time Systems MBStar in Detail

23 rd Euromicro Conference on Real-Time Systems MBStar Architecture Secure Application Layer TDMA MAC Layer DSSS Radio There is no network layer because it is only start topology and routing is not needed here Security module is put in the Application Layer because encryption/decryption involves high computational cost and MAC time slot may be too short

23 rd Euromicro Conference on Real-Time Systems MBStar MAC Time Slot Definition Transmitting Receiving Maximum MAC Payload is only 24 bytes Time slot length is very stringent!!!

23 rd Euromicro Conference on Real-Time Systems MBStar Implementation

23 rd Euromicro Conference on Real-Time Systems Hardware Platform Freescale MC1322x Series –Cheap: ~5$ – ratio

23 rd Euromicro Conference on Real-Time Systems Software Architecture Queues are used in non-time-critical processing No queue between MAC and PHY to ensure quick handling Encryption of sensor data can be done before MAC layer transmission

23 rd Euromicro Conference on Real-Time Systems MBStar Link Scheduling Problem

23 rd Euromicro Conference on Real-Time Systems Link Scheduling: Many To One Scenario … Devices n devices

23 rd Euromicro Conference on Real-Time Systems Link Scheduling: Many To One Scenario … Devices One Access point

23 rd Euromicro Conference on Real-Time Systems Link Scheduling: Many To One Scenario … Devices Access point can only talk to one device at one time slot (except broadcast)

23 rd Euromicro Conference on Real-Time Systems Link Scheduling: Many To One Scenario … Devices How to schedule them?

23 rd Euromicro Conference on Real-Time Systems Common solution: global hyper-period schedule Each device is configured with a global hyper-period schedule and only one device is sending at each slot. However… –We cannot expect every device to have enough memory or computing capacity –Each join will involve updating the global hyper-period schedule for all devices. However, the packet size must be small (24 bytes payload) by the limit (32 microseconds per byte). The configuration loop will be very time-consuming and thus not scalable.

23 rd Euromicro Conference on Real-Time Systems Our solution: local schedule Gateway can keep the global hyper-period while each device only keeps its own, shorter schedule for one period. ‘Collisions’ may happen: two devices send at the same slot (but on different channels). However, gateway knows which packet it should ‘pick up’ at the time slot. No Physical Collision The ‘unsuccessful’ device resends its data in the next time slot and gateway will pick up the retransmission.

23 rd Euromicro Conference on Real-Time Systems Example Time slot 0 Listening Both Sending Channel 11 Channel 15 GW only listens on Channel 11

23 rd Euromicro Conference on Real-Time Systems Example Time slot 0 Channel 11 Channel 15 GW only sends back ACK to Device 1 Both Waiting ACK Sending ACK Only Device 1 gets its ACK

23 rd Euromicro Conference on Real-Time Systems Example Time slot 1 Channel 16 GW then listens on Channel 16 Listening Device 2 retransmits its data Device 1 stops

23 rd Euromicro Conference on Real-Time Systems Example Time slot 1 Channel 16 GW then sends back ACK to Device 2 Sending ACK Device 2 gets its ACK

23 rd Euromicro Conference on Real-Time Systems How to make a schedule at GW? MBStar offset-free scheduling problem –Set of periodic tasks : (T i, O i ), period T i, offset O i –Utilization U = –retran_number (average retransmission number) : number of retransmissions divided by the number of successful transmissions –Add a new task Objective is to find a pair (T new,O new ) subject to (1) T new [minT new, maxT new ] (2) Maximize U (3) Minimize retran_number, subject to (1) and (2)

23 rd Euromicro Conference on Real-Time Systems Solution Step 1 : T new is set to be the smallest value in [minT new, maxT new ] satisfying U < 1.0 Step 2 : O new is set to be the value in [0,gcd{T new, lcm{T 1,…,T n }}) that minimizes retran_number.

23 rd Euromicro Conference on Real-Time Systems Theorem IV.1: Given an offset-free MBStar task set, EDF is optimal. Proof of Optimality –EDF achieves the highest utilization ratio EDF is optimal for unit-time periodic task sets in discrete time slot model

23 rd Euromicro Conference on Real-Time Systems Theorem IV.1: Given an offset-free MBStar task set, EDF is optimal. Proof of Optimality –EDF achieves the highest utilization ratio –EDF achieves the smallest retran_number We need to prove this

23 rd Euromicro Conference on Real-Time Systems Optimality Proof: Step 1 Lemma IV.1 (Generalized Chinese Remainder Theorem) Let T 1, T 2, …, T n be positive integers. Let P be the least common multiple of T 1, T 2, …, T n and let a,O 1,…, O n be any integers. There exists one integer t which satisfies the conditions (1) provided that (2) No such integer exists if condition (2) fails. Lemma IV.1 means there exists a non-zero number of ‘collisions’ between devices if condition (2) is satisfied.

23 rd Euromicro Conference on Real-Time Systems Optimality Proof: Step 2 Lemma IV.2: If n devices collide at the same time, at least n * (n-1)/2 retransmissions will be generated. –Proof: Suppose n devices collide at time slot t. It takes at least n time slots for the access point to receive all packets. At time slot t+k, n-k devices will resend. Total retransmissions = … + (n-1) = n * (n-1)/2 Lemma IV.2 shows that the minimum retransmission number is a function of number of colliding devices and is independent of the scheduling algorithm

23 rd Euromicro Conference on Real-Time Systems Optimality Proof: Step 3 Lemma IV.3: For any given task set, the smallest retran_number is achieved by any scheduling algorithm that does not idle tasks unnecessarily. Same Tasks Different schedulers same retran_nums

23 rd Euromicro Conference on Real-Time Systems Choosing O new Lemma makes choosing T new easily, but how about O new ? Same Periods, different offsets, different retran_nums

23 rd Euromicro Conference on Real-Time Systems Solution for choosing O new We do not have analytical solution We have to choose solution by search. –Fortunately, Goosen has shown that the search range is [0,gcd{T new,lcm{T 1,…,T n }})

23 rd Euromicro Conference on Real-Time Systems How about physical packet loss? Re-schedule –If a packet is corrupted by noise, the sender will not get ACK and must retry. The access point knows the sender will retry and it can re-schedule another ‘pickup’ before packet expiration. Corrupted Collided Time Slot

23 rd Euromicro Conference on Real-Time Systems Experiments

23 rd Euromicro Conference on Real-Time Systems Testbed

23 rd Euromicro Conference on Real-Time Systems Normal Office Environment Total packet loss ratio for one hour is 4.65*10 -5.

23 rd Euromicro Conference on Real-Time Systems Noisy Environment A noisy maker is created to randomly jam a channel for one minute. With channel blacklist and reschedule, the loss ratio is 10 times lower w/o channel blacklist and re-schedulere-schedule w/ channel blacklist and re-schedulere-schedule Total packets Missed packets Loss ratio

23 rd Euromicro Conference on Real-Time Systems Coexistence with Bluetooth Calling with Bluetooth earplug Distance adjustment should be recommended when there is Bluetooth earplug working around

23 rd Euromicro Conference on Real-Time Systems Coexistence with Wi-Fi With channel hopping and blacklist, the loss ratio is seven times lower

23 rd Euromicro Conference on Real-Time Systems Summary MBStar is a light-weight, secure, reliable wireless protocol for WBAN Use local period instead of global hyper- period schedule to reduce the resource requirements for sensors Real implementation proves practicality

23 rd Euromicro Conference on Real-Time Systems Future Work Provisioning over the air Application to a real body medical system

23 rd Euromicro Conference on Real-Time Systems Thank you & QA