Title: Medium Access Control in Sensor Networks
1Medium Access Control in Sensor Networks
- Huaming Li
- Electrical and Computer Engineering
- Michigan Technological University
2Outline
- Overview
- S-MAC an energy-efficient MAC protocol for
wireless sensor networks - Other MAC Techniques
- References
3Medium Access Control in Sensor Networks
- Sensor networks
- Consist of a set of sensor nodes
- Each node is equipped with one or more sensors
and is normally battery operated - Nodes communicate with each other via wireless
connection. - Medium Access Control (MAC)
- Fundamental task is to avoid collisions so that
two interfering nodes do not transmit at the same
time
4Characteristics of Sensor Network
- A special wireless ad hoc network
- Large number of nodes
- Battery powered
- Topology and density change
- Nodes for a common task
- In-network data processing
- Sensor-net applications
- Sensor-triggered bursty traffic
- Can often tolerate some delay
- Speed of a moving object places a bound on
network reaction time
Msg-level Latency
5MAC Protocols Classification
- Scheduling-Based MAC Protocols
- Contention-Based MAC
- Collision Free Real Time MAC
- Hybrid MAC
6Scheduling Based MAC
- Time is divided into slots
- Each node knows when to transmit
- Schedule is predetermined
- TDMA
- Synchronization problems
- Adaptability problems
7Contention Based MAC
- Carrier sensing collision avoidance
- In-band, out-band handshaking
- Busy-tone multiple access (BTMA)
- Multiple access with collision avoidance (MACA)
- High priority packets
8Common MAC Protocol Requirements
- Quality of service (QoS)
- Tolerate message loss
- Support real time guarantees
- Decentralized
- Global information may not be available
- Flexibility
- Diversity of applications
9MAC Requirements in Sensor Networks
- Important requirements of MAC protocols
- Collision avoidance
- Energy efficiency
- Scalability Adaptivity
- Latency
- Fairness
- Throughput
- Bandwidth utilization
10Energy Efficiency in MAC Design
- Energy is primary concern
- What causes energy waste on radio?
- Long idle time
- Control packet overhead
- Overhearing unnecessary traffic
- Collisions
- bursty traffic in sensor-net apps
- Idle listening consumes 50100 of the power for
receiving (Stemm97, Kasten)
11S-MAC Design Overview
- Major components in S-MAC
- Periodic listen and sleep
- Collision avoidance
- Overhearing avoidance
- Massage passing
12Periodic Listen and Sleep
- Reduce long idle time
- Reduce duty cycle to 10 (120ms on/1.2s off)
- Prefer neighboring nodes have same schedule
- easy broadcast low control overhead
13Periodic Listen Sleep
- Nodes are in idle for a long time if no sensing
event happens - Put nodes into periodic sleep mode
- i.e. in each second, sleep for half second and
listen for other half second
14Coordinated Sleeping
- Nodes coordinate on sleep schedules
- Nodes periodically broadcast schedules
- New node tries to follow an existing schedule
Schedule 1
Schedule 2
1
2
- Nodes on border of two schedules follow both
- Periodic neighbor discovery
- Keep awake in a full sync interval over long time
15Choose Maintain Schedule
- Each node maintains a schedule table that stores
schedules of all its neighbors - Nodes exchange schedules by broadcasting them to
its neighbors - Try to synchronize neighboring nodes together
16Choose Schedule
- If not hear a schedule from others, the node
randomly chooses a schedule and broadcast the
schedule - If receive a schedule, the node follows that
schedule, wait for a random delay then
rebroadcast this schedule - If receive a different schedule, the node adopt
both, broadcast its own schedule
17Maintain Synchronization
- Listen/sleep scheme requires synchronization
among neighboring nodes - Looser synchronization (compared to TDMA)
- Listen period is significantly longer than clock
error or drift - Use relative time rather than absolute
- Update schedule by sending SYNC packets
18Maintain Sync (contd.)
- Divide listen time into two parts
- For receiving SYNC packets
- For receiving data packets
- Each part is further divided into many time slots
for senders to perform carrier sense
19Maintain Sync (contd.)
CS carrier sense
20Collision Avoidance
- Adopt IEEE 802.11 collision avoidance
- Virtual carrier sense
- During field
- Network allocation vector (NAV)
- Physical carrier sense
- RTS/CTS exchange (for hidden terminal problem)
- Broadcast packets (SYNC) are sent without RTS/CTS
- Unicast packets (DATA) are sent with RTS/CTS
21Overhearing Avoidance
- Problem Receive packets destined to others
- Solution Sleep when neighbors talk
- Basic idea from PAMAS (Singh, Raghavendra 1998)
- But we only use in-channel signaling
- Who should sleep?
- All immediate neighbors of sender and receiver
- How long to sleep?
- The duration field in each packet informs other
nodes the sleep interval
22Example
- Who should sleep when node A is transmitting to
B? - All immediate neighbors of both sender receiver
should go to sleep
23Message Passing
- How to efficiently transmit a long message?
- Single packet vs. fragmentations
- Single packet high cost of retransmission if
only a few bits have been corrupted - Fragmentations large control overhead (RTS CTS
for each fragment), longer delay - Problem Sensor network in-network processing
requires entire message
24Message Passing
- Solution Dont interleave different messages
- Long message is fragmented sent in burst
- RTS/CTS reserve medium for entire message
- Fragment-level error recovery ACK
- extend Tx time and re-transmit immediately
- Other nodes sleep for whole message time
25Implementation on Testbed Nodes
- Configurable S-MAC options
- Low duty cycle with adaptive listen
- Low duty cycle without adaptive listen
- Fully active mode (no periodic sleeping)
26Implementation on Testbed Nodes
- Layered model on Motes
- MAC layer S-MAC
- Physical layer
- Radio state control, Carrier sense
- CRC checking, Channel coding, Byte buffering
- Nested headers
- Avoid memory
- copy across
- layers
27Test Bed
- Three test MAC modules
- Simplified IEEE 802.11 DCF
- Message passing with overhearing avoidance
- Complete S-MAC
- Topology in experiments
28Experiment Result
- Average source nodes energy consumption
- S-MAC consumes much less energy than 802.11-like
protocol w/o sleeping - At heavy load, overhearing avoidance is the major
factor in energy savings - At light load, periodic sleeping plays the key
role
29Experiment Result (contd.)
- Percentage of time source nodes in sleep
30Experiment Result (contd.)
- Energy consumption in the intermediate node
31S-MAC Conclusions
- Advantages
- Periodically sleep reduces energy consumption in
idle listening - Sleep during transmissions of other nodes
- Message passing reduces contention latency and
control packet overhead - Disadvantages
- Reduction in both per-node fairness latency
32Other MAC Techniques
- Timeout-MAC (T-MAC)
- S-MAC has fixed duty cycle and not optimal
- Reduce idle listening by transmitting data in
bursts - Sleep in between bursts to save power
- End the active time in an intuitive way
- Timeout on hearing nothing
33T-MAC
- Every node periodically wakes up and communicates
with its neighbors - A node will keep listening and potentially
transmitting, as long as it is in active period - An active period ends when no activation event
has occurred for time TA
34Activation event
- The firing of periodic timer
- The reception of any data on radio
- The sensing of communication on the radio
- The end of transmission of a nodes own data
packet - The knowledge through prior RTS and CTS packets
35T-MAC
- A node will sleep if it is not in an active
period - TA determines the minimum amount of idle
listening per frame - All communication occurs as a burst in the
beginning of the frame - Buffer capacity determines the upper bound on the
maximum frame time
36Evolution
Adaptive Rate Control
ARC
IEEE 802.11
IEEE 802.3
CSMA/CD
CSMA/CA
SMAC
TMAC
Carrier Sense Multiple Access with Collision
Detection
Carrier Sense Multiple Access with Collision
Avoidance
Fixed duty cycle
Adaptive duty cycle
DMAC/ MMAC
Directional Antennas
37Performance Analysis of 802.15.4 in WPAN
- One promising kind of sensor network Wireless
Personal Area Network (WPAN) - Medical sensing and control
- Wearable computing
- Location awareness and identification
- Implanted medical sensors (Focus)
- Coronary care
- Diabetes
- Optical aids
- Drug delivery
38Critical Metric Battery Life
- Implanted medical sensors (Main concern)
- Objective
- Make Batteries work 10-15 years
- Method
- Ensure that all sensors are powered down or in
sleep mode when not in active use - Tradeoff
- Battery life VS. latency
39Protocol Options
Possible options
Market Name Standard GPRS/GSM 1xRTT/CDMA Wi-Fi 802.11b Bluetooth 802.15.1 ZigBee 802.15.4
Application Focus Wide Area Voice Data Web, Email, Video Cable Replacement Monitoring Control
System Resources 16MB 1MB 250KB 4KB - 32KB
Battery Life (days) 1-7 .5 - 5 1 - 7 100 - 1,000
Network Size 1 32 7 255 / 65,000
Bandwidth (KB/s) 64 - 128 11,000 720 20 - 250
Transmission Range (meters) 1,000 1 - 100 1 - 10 1 - 100
Success Metrics Reach, Quality Speed, Flexibility Cost, Convenience Reliability, Power, Cost
40802.15.4 (LR-WPAN)
Upper Layers
Other LLC
IEEE 802.2 LLC
IEEE 802.15.4 MAC
IEEE 802.15.4
IEEE 802.15.4
2400 MHz
868/915 MHz
PHY
PHY
Physical Medium
41802.15.4 (LR-WPAN)
MAC Layer (prefer star topology)
Why star topology here?
42802.15.4 (LR-WPAN)
Why star topology here?
- Coordinator is external to the body
- PDA, mobile phone or bedside monitor station
- Easy to replace of charge batteries
- Easy to communicate with other networks
- Coordinator defines the start and end of a
superframe and is charge of the association and
disassociation of the other nodes
43IEEE 802.15.4 superframe structure
44Two Communication methods
- Beacon mode
- Pros Coordinator can communicate at will
- Cons Listeners have to keep awake
- Non-beacon mode
- Pros Nodes can sleep more
- Cons Communication latency
45Network scenarios and power analysis
Sensor power consumption with beacon reception
- Problem The sensor devices within a beacon
network have to wake up to receive the beacon
from the coordinator (Power consuming)
Timebase Tolerances
Warm-up time
46Network scenarios and power analysis
Data Transfer Mechanisms (Beacon)
- Data transfer to a coordinator (upload)
Is the upload period
47Network scenarios and power analysis
Data Transfer Mechanisms (Beacon)
- Data transfer from a coordinator (download)
Is the download period
48Results
Average Back-off
With a small number of sensors that are
effectively off most of the time, the probability
of a channel being free is greater than 99 .
Therefore, for the relatively small number of
sensors used in the WBAN networks explored here,
it would be more economical to keep the CSMA/CA
switched off. This is to ensure that the
automatic initial back-off is avoided.
49Node Lifetime in Beacon Networks
50Node Lifetime in Beacon Networks
51Node Lifetime in Beacon Networks
- 15-year lifetime may only be obtained for very
low upload rates. - It is under very limited data rate conditions and
a tight tolerance crystal, which typically must
be better than 25 ppm.
52GTS Option
The main drawback of using GTS is that the
receiver in the sensor remains on for the
duration of the timeslot regardless of the size
of the data packet.
53Non-Beacon Networks
54Non-Beacon Networks
55Conclusion
- As a solution to the challenge of the personal
area network, the IEEE 802.15.4 standard would
provide a limited answer in its non-beacon form. - Sensors that do not have large amounts of data to
transfer could be used, i.e., small packets of
data several times per hour.
56Questions and Comments
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