EECE 5155 Wireless Sensor Networks
(and The Internet of Things)
Prof. Francesco Restuccia Email: f.restuccia@northeastern.edu Office: 318 Dana
February 17, 2021
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Contention-Based Mac Protocols: S-MAC
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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S-MAC: Sleep MAC
W. Ye, J. Heidemann, D. Estrin, , “Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks,’‘ IEEE/ACM Trans. on Networking, June 2004.
➢ Problem: “Idle Listening” consumes significant energy
➢ Solution: Periodic listen and sleep
sleep sleep
time
➢ During sleeping, radio is turned off
➢ Reduce duty cycle to ~ 10% (Listen for 200ms and sleep
listen
listen
for 2s)
Latency
Energy
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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S-MAC
➢ Each node goes into periodic sleep mode during which it switches the radio off and sets a timer to awake later
➢ When the timer expires it wakes up and listens to the channel, to see if any other node wants to talk to it
➢ The duration of the sleep and listen cycles are application dependent and they are set the same for all nodes
➢ Requires a periodic synchronization among nodes to take care of any type of clock drift
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Periodic Sleep and Listen
➢ All nodes are free to choose their own listen/sleep schedules
➢ To reduce control overhead, only neighboring nodes are synchronized together
➢ Preferably, they listen at the same time and go to sleep at the same time
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Synchronization
➢ SYNC packets are exchanged periodically to maintain schedule synchronization
SYNC PACKET
Sender Node ID
Next Sleep Time
➢ SYNCHRONIZATION PERIOD: Period for a node to send a SYNC packet
➢ Receivers will adjust their timer counters immediately after they receive the SYNC packet
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Periodic Listen and Sleep
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Maintaining Synchronization
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Choosing and Maintaining Schedules
➢ Each node maintains a schedule table that stores schedules of all its known neighbors
➢ For initial schedule, do:
1. A node first listens to the medium for a certain amount of time
(at least the synchronization period)
2. If it does not hear a schedule from another node, it randomly chooses a schedule and broadcasts its schedule with a SYNC packet immediately
3. This node is called a Synchronizer
4. If a node receives a schedule from a neighbor before choosing its own schedule, it just follows this neighbor’s schedule, i.e. becomes a Follower, waits for a random delay and broadcasts its schedule
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Coordinated Sleeping
➢
➢ ➢
In a large network, we cannot guarantee that all nodes follow the same schedule
The node on the border will follow both schedules
When it broadcasts a packet, it needs to do it twice, first
for nodes on schedule 1 and then for those on schedule 2 Schedule
Schedule 1
2
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Collision Avoidance
➢ S-MAC is based on contention, i.e., if multiple neighbors want to talk to a node at the same time, they will try to send when the node starts listening
➢ Similar to IEEE 802.11, i.e. use RTS/CTS mechanism to address the hidden terminal problem
➢ Perform carrier sense before initiating a transmission
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Collision Avoidance
➢ If a node fails to get the medium, it goes to sleep and wakes up when the receiver is free and listening again
➢ Broadcast packets are sent without using RTS/CTS
➢ Unicast data packets follow the sequence of RTS/CTS/DATA/ACK
between the sender and receiver
➢ Duration field in each transmitted packet indicates how long the remaining transmission will be so if a node receives a packet destined to another node, it knows how long it has to keep silent
➢ The node records this value in network allocation vector (NAV) and sets a timer for it
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Collision Avoidance
➢ When a node has data to send, it first looks at NAV
➢ If this value is not zero, then medium is busy (virtual
carrier sense)
➢ The medium is determined as free if both virtual and physical carrier sense indicate the medium is free
➢ All immediate neighbors of both the sender and receiver should sleep after they hear RTS or CTS packet until the current transmission is over
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Message Passing Feature
➢ Long messages are broken down into smaller packets and sent continuously once the channel is acquired by RTS/CTS handshake
➢ Increases the sleep time, but leads to fairness problems
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Msg Passing
➢ S-MAC message passing
…
…
RTS
21
Data
19
Data
17
Data
CTS 20
ACK 18
ACK 16
ACK
0
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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S-MAC – Performance Evaluation
➢ Topology: Two-hop network with two sources and two sinks
➢ Sources periodically generate a sensing message which is divided
into fragments
➢ Traffic load is changed by varying the inter-arrival period of the messages: (for inter-arrival period of 5s, message is generated every 5s by each source. Here it varies between 1-10s)
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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S-MAC – Example
➢ In each test, there are 10 messages generated on each source node
➢ Each message has 10 fragments, and each fragment has 40 bytes (200 data packets to be passed from sources to sinks)
➢ The total energy consumption of each node is measured for sending this fixed amount of data
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Experiments
Average energy consumption in the source nodes A&B
1800
1600
1400
1200
1000
800
600
400
200
S-MAC without
periodic sleep
S-MAC with Periodic Sleep
0 2 4 6 8 10
Message inter-arrival period (traffic load) (second) (small value → heavy traffic load)
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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802.11-like protocol without sleep
Energy consumption (mJ)
Experiments
➢ S-MAC consumes much less energy than 802.11-like protocol without sleeping
➢ At heavy load, idle listening rarely happens, energy savings from sleeping is very limited.
➢ At light load, periodic sleeping plays a key role
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Let’s think…
What are the pros and cons of S-MAC? How would you improve it?
Think-Pair-Share!
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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S-MAC – Conclusions
➢ A mainly static network is assumed
➢ Trades off latency for reduced energy
consumption
➢ Redundant data is still sent with increased latency
➢ Increased collision rate due to sleep schedules
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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EECE 5155 Wireless Sensor Networks
(and The Internet of Things)
Prof. Francesco Restuccia Email: f.restuccia@northeastern.edu Office: 318 Dana
February 18, 2021
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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Contention-Based Mac Protocols: B-MAC
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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B-MAC
J. Polastre, J. Hill, D. Culler, “Versatile Low Power Media Access for WSNs”, Proc. of ACM SenSys, Nov. 2004.
➢ What is B-MAC?
– A configurable MAC protocol for WSNs
– Small core and factoring out higher-level functionality
– An adaptive bidirectional interface for WSN applications
➢ It’s a traditional MAC protocol for WSNs
– Work reasonably well for a large set of traffic workloads
(applications)
– Create a flexible set of functionalities able to provide solutions to a set of goals
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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B-MAC Goals
➢ Goals
– Low Power Operation
– Effective Collision Avoidance
– Simple Implementation, Small Code and RAM Size
– Efficient Channel Utilization
– Reconfigurable by Network Protocols
– Tolerant to Changing RF/Networking Conditions
– Scalable to Large Numbers of Nodes
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Classical vs. Minimalistic
➢ S-MAC
– A classical approach
– User pre-configures duty cycle
– Applications rely on S-MAC to adjust its operation as the environment evolves
➢ B-MAC
– A minimalistic approach
– Small core of medium access functionalities
– RTS/CTS, ACKs, etc. are considered higher layer functionalities (services)
– Uses Clear Channel Assessment (CCA) and packet backoffs for channel arbitration, link layer acknowledgments for reliability
– Uses Low Power Listening (LPL)
– More flexible and more tunable
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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B-MAC: Principles
➢ Reconfigurable parameters:
– Backoff/Timeouts
– Duty Cycle
– OptionalACKs
➢ Flexible control
➢ Feedback to higher protocols
– Model of operation
– Upward costs (e.g., link quality)
➢ Minimal implementation
➢ Minimal state
Link/Network Protocols
Data Control
B-MAC
PHY
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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B-MAC Tiny OS Interface
➢ Interfaces for flexible control of B-MAC by higher layer services
➢ Allow services to toggle CCA and ACKs, set backoffs on a per
message basis, and change the LPL mode for transmit and receive
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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B-MAC Important Design Aspects
➢ Clear Channel Assessment (CCA)
➢ Low Power Listening (LPL)
➢ Packet backoffs
➢ Link layer acknowledgments
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Clear Channel Assessment (CCA)
➢ Effective collision avoidance
➢ Find out whether the channel is idle
– If too pessimistic: waste bandwidth
– If too optimistic: more collisions
➢ Key points:
– Ambient noise may change significantly depending on the environment
– Packet reception has fairly constant channel energy
– What is noise? What is signal?
➢ Automatic gain control in software to estimate the noise floor
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Noise Floor Estimation
➢ Take a number of received signal strength indicator (RSSI) samples when the channel is assumed to be free/idle
➢ RSSI samples are entered into a FIFO queue
➢ Median of the queue is added to an exponentially weighted moving
average with decay a
➢ Median is used as a simple low pass filter to add robustness to the
noise floor estimate
At = a * St + (1 – a) * St-1
where a is assumed to be 0.06 and FIFO queue size of 10
➢ Once a good estimate of the noise floor is established, a request to transmit a packet starts the process of monitoring the received signal from the radio
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Single-Sample Thresholding vs Outlier Detection
➢ Common approach: take single sample, compare to noise floor
– Large number of false negatives → lower effective channel BW
➢ B-MAC: search for outliers
– If a sample has significantly lower energy than the noise floor during the sampling period, then the channel is clear
If 5 samples are taken and no outlier is found, the channel is busy
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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CCA vs. Threshold Techniques
Idle
Signal Strength Indicator (RSSI) from transceiver
● A packet arrives between 22 and 54ms. The middle graph shows the output of a thresholding CCA algorithm (1: channel clear, 0: channel busy)
● Bottom shows the output of an outlier detection algorithm
● Threshold: waste channel utilization
● CCA: Fully utilize the channel since a valid packet could have no outlier
significantly below the noise floor
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Clear Channel Assessment – Recap
➢ ➢
➢
Before transmission – take samples of the channel
If five samples are taken, and no outlier found => channel busy, take a random backoff
Noise floor updated when the channel is known to be clear, i.e., just after packet transmission
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Low Power Listening (LPL)
➢ Periodic Channel Sampling
– Energy Cost = RX + TX + Listen
– Goal: minimize idle listening
➢ Periodically
– Wake up, sample channel, sleep
➢ Properties
– Wake-up time fixed
– “Check Time” between wakeups variable
– Preamble length matches wakeup interval
➢ Overhear all data packets
Mica 2 motes
– Duty cycle depends on number of neighbors and traffic
Node 1 Node 2
sleep
sleep
sleep
time
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sleep
sleep
sleep time
TX [preamble]
RX
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
wakeup
wakeup
packet packet wakeup
wakeup
wakeup
wakeup
wakeup
Low Power Listening (LPL) – 2
➢ Goal: Minimize “Listen Cost”
➢ Principles
Low Power Listening
– Node periodically wakes up, turns radio on and checks activity on the channel
– If energy/activity on the channel is detected, node powers up and stays awake for the time required to receive the incoming packet
➢ Node goes back to sleep
– If the complete packet is received
– After a timeout (if no packet received (a false positive))
➢ Preamble length matches channel checking period
– No explicit synchronization required
➢ Noise floor estimation used to detect channel activity during LPL
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Check Interval for Channel Activity
➢ To reliably receive data, the preamble length is matched to the interval that the channel is checked for activity
➢ If the channel is checked for every 100 ms, the preamble must be at least 100 ms long for a node to wake up, detect activity on the channel, receive the preamble and then receive the message
➢ Interval between LPL samples is maximized so that the time spent sampling the channel is minimized
– Transmit mode ~~ Preamble length
– Listening mode ~~ Check interval
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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LPL Check Interval
➢ Sampling rate (traffic pattern) defines optimal check interval
➢ Check interval
– Too small: energy wasted on idle listening
– Too large: energy wasted on transmissions (long preambles)
➢ In general, it is better to have larger preambles than to check more often!
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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S-MAC and B-MAC: Comparison
➢ Experimental Setup:
– n nodes send as quickly as possible to saturate the channel
– Shown: throughput as a function of the number of nodes
➢ B-MAC has about 4.5 time higher throughput than S-MAC-unicast
– Not as much when ACK or RTS/CTS is used
– Differences less pronounced as # of nodes increases
– B-MAC has CCA, thus it backs off less frequently
topology
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Throughput vs Power Consumption
ALWAYS ON
S-MAC B-MAC
Throughput (bits/s)
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Power Consumed (mJ/sec)
Tradeoffs: Latency vs Reliability
➢ Reliability
– 98.5% of all packets delivered
– Some nodes 100% delivery
➢ …but communication links are volatile
– Retransmissions required
– After 5 retries, give up and pick a new parent
➢ Actual latency
– Retransmission delay
– Contention delay (infrequent)
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Tradeoffs: Latency for Energy
S-MAC Default Configuration
➢ 10-hop network
➢ Source sends a 100-byte
packet every 10 seconds
➢ SMAC: 10% Duty Cycle BMAC: choose optimal check interval
➢ Poor SMAC again performs worse…
– Reason: sync packets, probability of multiple schedules—less time to sleep
B-MAC Default Configuration
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Latency
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Let’s think…
Can you compare S-MAC vs B-MAC?
Think-Pair-Share!
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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ACK
Comparison of S-MAC and B-MAC
Collision avoidance
Message passing
Listen period
Listen interval
Schedule synchronization
Packet transmission
Code size
S-MAC
CSMA/CA
Yes
Yes
Pre-defined
Long
Required
Short preamble
6.3KB
B-MAC
CSMA
Optional
No
Pre-defined
Very short
Not required
Long preamble
4.4KB (LPL & ACK)
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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EECE 5155 Wireless Sensor Networks
(and The Internet of Things)
Prof. Francesco Restuccia Email: f.restuccia@northeastern.edu Office: 318 Dana
February 22, 2021
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Dr. Francesco Restuccia, EECE 5155 Wireless Sensor Networks and the Internet of Things
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IEEE 802.15.4 MAC LEVEL
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05 2004
Content
➢ Overview
➢ Topologies
➢ Superframe structure ➢ Frame formatting
➢ Data service
➢ Management service ➢ Interframe spacing ➢ CSMA procedure
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802.15.4 Architecture
Upper Layers
IEEE 802.15.4 SSCS (Service Specific Convergence Sublayer)
IEEE
802.2
LLC, Type
I
IEEE 802.15.4 MAC
IEEE 802.15.4 PHY
IEEE 802.15.4 PHY
868/915 MHz
2400 MHz
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What IEEE 802.15.4 Aims at
➢ Extremely low cost
➢ Ease of installation
➢ Reliable data transfer ➢ Short range operation ➢ Reasonable battery life
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MAC Overview
➢ Star and peer-to-peer topologies
➢ Association
➢ CSMA-CA channel access mechanism ➢ Packet validation and message rejection ➢ Optional guaranteed time slots (GTS)
➢ Guaranteed packet delivery
➢ Facilitates low-power operation
➢ Security
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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IEEE 802.15.4 Device Classes
➢ Full function device (FFD) – Any topology
– Capable to be a PAN coordinator – Talks to any other device
– Implements complete protocol set
➢ Reduced function device (RFD)
– Limited to star topology or end-device in a
peer-to-peer network.
– Cannot become a PAN coordinator
– Very simple implementation
– Reduced protocol set
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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IEEE 802.15.4 Definitions
➢ Network Device: An RFD or FFD implementation containing an IEEE 802.15.4 medium access control and physical interface to the wireless medium.
➢ Coordinator: An FFD with network device functionality that provides coordination and other services to the network.
➢ PAN Coordinator: A coordinator that is the principal controller of the PAN. A network has exactly one PAN coordinator.
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Low-Power Operation
➢ Duty-cycle control using superframe structure
– Beacon order and superframe order
– Coordinator battery life extension
➢ Indirect data transmission
➢ Devices may sleep for extended period over multiple beacons
➢ Allows control of receiver state by higher layers
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Star Topology
PAN coordinator
Master/slave
FFD
RFD
Communications flow
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05 2004
Peer-Peer Topology
PAN coordinators
Point to point FFD
RFD
Cluster tree Communications flow
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Combined Topology
Clustered stars – for example, cluster nodes exist between rooms of a hotel and each room has a star network for control.
FFD
RFD
Communications flow
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Optional Frame Structure
Battery life extension
Con
tention Access Period
Contentio
n Fr
ee Period
Slot
15ms * 2n
Network beacon
Beacon extension period
Contention period
Guaranteed Time Slot
where 0 ≥ n ≥ 14 Max 24.576s!
Transmitted by PAN coordinator. Contains network information, frame structure and notification of pending node messages.
Space reserved for beacon growth due to pending node messages
Access by any node using CSMA-CA
Reserved for nodes requiring guaranteed bandwidth [n = 0].
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Dr. Francesco Restuccia, EECE 5698 Wireless Sensor Networks
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Optional Frame Structure
15ms * 2SO
Inactive Period
where 0 ≥ SO ≥ 14
15ms * 2BO
where SO ≥ BO ≥ 14
➢ Superframe may have optional inactive period (duty cycle)
SO = Superframe order BO = Beacon order
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General MAC Frame Format
Frame control field
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Beacon Frame Format
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MAC Command Frame
➢ Command Frame Types – Association request
– Association response
– Disassociation notification – Data request
– PAN ID conflict notification
● Orphan Notification
● Beacon request
● Coordinator realignment ● GTS request
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Data Frame Format
Acknowledgement Frame Format
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Data Service
➢ Data transfer to neighboring devices
– Acknowledged or unacknowledged
– Direct or indirect
– Using GTS service
➢ Maximum data length (MSDU) aMaxMACFrameSize (102 bytes)
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MAC Data Primitives
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Data Transfer
Message Sequence Diagram
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Indirect Data Transfer Message Sequence Diagram
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Management Service
➢ Association / disassociation ➢ GTS allocation
➢ Message pending
➢ Node notification
➢ Network scanning/start
➢ Network synchronization/search
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MAC Management Primitives
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Association Message Sequence Diagram
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Disassociation Message Sequence Diagram
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Data Polling Message Sequence Chart
No data pending at the coordinator
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Data Polling Message Sequence Chart
Data pending at the coordinator
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Passive Scan
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Active Scan
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Orphaning Message Sequence Diagram
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Slotted CSMA Procedure
Used in beacon enabled networks.
1
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Un-slotted CSMA Procedure
TO BE EVALUATED IN HOMEWORK 1 !!!!
Used in non-beacon networks.
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