RF Wakeup Sensor – On-Demand Wakeup for Zero Idle Listening and Zero Sleep Delay.
Ubiquitous Networks Wakeup Scheduling Lynn Choi Korea University.
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Transcript of Ubiquitous Networks Wakeup Scheduling Lynn Choi Korea University.
Ubiquitous Networks
Wakeup Scheduling
Lynn Choi
Korea University
Motivation
Most of WSN applications have real-time constraints
Sensors in battlefield to detect odorless biochemical weapons
Disaster monitoring applicationsForest fire alarm, volcano monitoring, seismometer
Real-time target tracking
Intrusion detection
Emergency health application
Traffic coordination
Existing MAC protocols focus on low energy consumption
But, how about the communication latency required for real-time applications?
Sleep delay A packet can traverse at most a single hop during each wakeup period
DMAC: Synchronous Skewed Wakeup
“An Adaptive Energy-Efficient and Low-Latency MAC for Data Gathering in Wireless Sensor Networks”
Krishnamachari and Raghavendra (at USC), IPDPS 2004.
DMAC calls this staggered wakeupSkew the wakeup period of each node in the path from a source node to a sink node
Assume the tree topology starting from the sink as a root
The wakeup schedule of each node is determined by the level of the node in the tree
Node A
Node B
Node C Tx
Tx
Tx
Rx
Rx
Rx
Tx
Tx
Tx
Rx
Rx
Rx
Sleep
Sleep
Sleep
Wakeup Patterns
“Wakeup Scheduling in Wireless Sensor Networks”Keshavarzian, Lee (at Stanford), Venkatraman, MobiHoc 2006.
Fully Synchronized Wakeup Pattern (SMAC)
All the nodes wake up at the same time
Delay = (#hops – 0.5) * T
Wakeup Patterns
Shifted Even and Odd Pattern
Shift the wakeup period of nodes in even levels by T/2
Delay = 0.5 * (#hops) * T
Wakeup Patterns
Ladder Pattern (DMAC: staggered wakeup)
Skew the wakeup period of nodes in the communication path
Forward and backward delays are asymmetric
Wakeup Patterns
Two-Ladders Pattern
To improve the delay in both directions
Combine the forward ladder with a backward ladderNodes in the middle levels wake up twice in every period T
Wakeup Patterns
Crossed-Ladders Pattern
Cross the two ladders at one point so that the same wakeup can be used for both directions
Wakeup Patterns
Multi-Parent Method
Embed multiple trees in the networkEach node has multiple paths and multiple parents to the sink
Depending on the packet arrival time, a node can choose the fastest path to get to the destination
SPEEDMAC: Speedy and Energy EfficientData Delivery MAC Protocol for
Real-Time Sensor Network Applications
ICC 2010
Motivation
Sleep delay is the dominant factor of WSN packet latencyA packet can traverse at most a single hop each cycle
Minimum packet latency = cycle time *hops
Most of WSN applications have real-time characteristicsDisaster monitoring, real-time target tracking, intrusion detection, health, etc.
However, it is practically impossible to obtain both low latency and low energy communi-cation at the same time
Sleep delay exists for both synchronous & asynchronous MACSynchronous scheduling (S-MAC, A-MAC)
A packet can traverse at most a single hop (or 2 with ‘adaptive listening’) each cycle since nodes beyond one-hop from the receiver cannot overhear the data.
Asynchronous scheduling (B-MAC, Wise-MAC, XMAC)A packet can traverse at most a single hop each cycle since a sender needs to send the pre-amble before starting the next-hop communication
Motivation
Synchronous skewed wakeup (DMAC) may be a solution!Schedule the wakeup time of each node in a pipelined fashion in the direc-tion of packet movement so that
No sleep delay during the packet movement
Issues with synchronous skewed wakeupMay fail to deliver the message when multiple sensors compete for the message delivery
A single event is likely to be detected by nearby multiple sensors
Multiple events may occur simultaneously, which leads to collisions and con-tentions
More idle listening
Since a node must wake up during the entire DATA transmission period in-stead of RTS period as in SMAC
May not be practically possible to use such wakeup scheduling techniques for real applications unless these issues are completely resolved.
Synchronous Skewed Wakeup
S
1
2 4
3
Sink
Sink
Node 1
Node 2
Node 3
ACK
DATA
DATA
ACK
DATA
ACK
DATA
Tx state
Rx state
Synchronous Skewed Wakeup
S
1
2 4
3
Sink
Sink
Node 1
Node 2
Node 3
ACK
DATA
DATA
DATA
Node 4DATA
DATA
Tx state
Rx state
SPEED MAC Ideas
Goal: Can we achieve both low-energy and low-latency at the same time?
1. A collision signal to detect multi-source events &for fast event deliveryA special control packet called SIGNAL packet is used. It has different electrical characteristics from background noise
2. Separate event report period from data delivery periodFaster event report using a short control signal
Lower energy consumption for idle period
To further reduce both the latency and the energy consumption
3. Adaptive wakeup for multi-source eventsFast pipelined data delivery for a single-source event
Full wakeup and CSMA-based data delivery for a multi-source event
Full duty-cycle operation for high-bandwidth transmission
Use RTS/CTS for busy periods
Synchronous Skewed Wakeup
Issues with Synchronous Skewed Wakeup
AssumptionsStationary sensor nodes and stationary sinksMany to one communication pattern from multiple sources to the sinks
IssuesContention
Only a single source can transmit the data and other sources may have to wait
CollisionWhen multiple nodes transmit at the same time, the packets will even-tually collide in an upper layer and no packet can be transmitted
Transmission errorWhen a transmission error occurs, the sender needs to wait for the next cycle
For single-source eventNo contention, no collision, only need to consider error
For multiple-source eventsNeed to consider contention, collision, and error
SPEED-MAC
Event announcement period: Fast Event AnnouncementIn this period, nodes announce the presence of an event by sending a small control packet called a SIGNAL packet.
SIGNAL packet: consists of receiver address and collision bit
There is NO ACK packet for the signal packet.
Collision detection for multi-source eventsThe collision bit tells that the event is a multi-source event.Need to distinguish transmission errors from collision
All the senders overhear the signal transmission from its parentTo distinguish a single source event from a multi-source event
Data transmission period: Adaptive WakeupIn this period, nodes transfer messages by sending DATA packets
For a single-source event, the period consists of DATA and ACKFixed scheduled data transmission for single-source events (not a CSMA)
For a multi-source event, the period consists of RTS/CTS/DATA/ACKContention-based data transmission for multi-source events (CSMA/CA)
SPEED-MAC: Single Source Event
No trafficNodes wakeup only during a signal rx slot.
Single source traffic: single-packet dataNodes wake up during signal rx/tx/rx slots and data slot
SPEED-MAC: Multi-Packet & Multi-Source Event
Single source traffic: multi-packet dataNodes wake up during signal rx/tx/rx slots and multiple data slots
Multi-source traffic Nodes wake up during signal rx/tx/rx slots and several RTS/CTS/DATA/ACK slots
SPEED-MAC with Multiple SinksWe can handle sink-to-sensor, sensor-to-sensor, and many sensors-to-many sinks scenarios
Collision/Error Differentiation
Transmission error can occur due to two reasonsNoise (Error)
Unwanted electrical signals interfering with the desired signal
The strength of the signal is irregular and variable
Collision
Multiple simultaneous transmission collide at the receiver
The strength of the signal is regular and stronger
Can be differentiated at the physical layer by tracking RSSI
In case of collision, the SIGNAL control packet is already destroyed.COLLISION SIGNAL does not contain the receiver address anymore.
COLLISION SIGNAL packet is broadcast to the nodes in the upper layers
False-positive delivery: Nodes in the upper layers after the collision may unnecessarily wakeup
Collision/Error Differentiation
NS-2 Simulation Parameters
# of nodes: 400 grid nodes + 1 sink nodePower
Tx : 30mW, Rx : 15mW, Idle : 15mW
Bandwidth: 20KbpsPacket size
Data packet: 100BSignal packet: 6BControl packet: 10B
Tx & Rx slot lengthData: 103ms, Signal: 22ms
Simulation time: 10 minTotal number of event: 20 events# of source nodes: 1, 2, 4, 8, 16 nodesBasic cycle time
SMAC: 1.44sSPEED-MAC, D-MAC: 2.88s
Single Source – LatencySMAC
SMAC suffers from the sleep delay and the additional buffering delay when the mes-sage generation interval is small.
SPEED-MAC vs. DMACDue to the signaling wakeup period, SPEED-MAC’s data latency is slightly higher than that of DMAC.
Signal delivery latency of SPEED-MAC is almost close to the minimum delay achiev-able and is much smaller than DMAC’s data delivery latency
Single Source - Energy
SMACAs the packet generation interval decreases SMAC spends more energy in re-peated wakeups and buffering.
SPEED-MAC vs. DMACSPEED-MAC can achieve an order of magnitude reduction in the energy consump-tion compared to DMAC
By reducing the idle listening overhead and
By removing unnecessary wakeups during idle periods
Multiple Sources - Latency
SMACLatency increases substantially as the number of source nodes increases.
This is due to the increased contention and buffering for multiple transactions.
SPEED-MAC vs. DMACConstant and faster signal delivery latency even in multi-source events
Noticeably higher data packet delay due to its adaptive wakeups and increased con-trol packet (RTS and CTS) overhead for multi-source events.
For DMAC we use their assumption that an interference range of a node is twice
larger than its transmission range to avoid collision for multi-source events.
SMACSMAC spends more energy due to its higher duty cycle operations
SPEED-MAC vs. DMACLike the single-source case, SPEED-MAC can substantially reduce the energy con-sumption by reducing the idle listening and removing unnecessary wakeups.
Multiple Sources - Energy
MICA-2 Mote ImplementationPacket size: control packet: 10B, data packet: 100B
Contention window: SYNC packet: 15 slots, Data packet: 31 slot
SINGLE SOURCE RESULTS MULTIPLE SOURCE RESULTS