Week 8 Lectures
MAC Layer in WSNs
Medium Access Control in WSNs
1
Outline
•
•
•
•
•
•
Introduction to MAC
MAC attributes and trade-offs
Scheduled MAC protocols
Contention-based MAC protocols
Case studies
Summary
Medium Access Control in WSNs
2
Introduction to MAC
• The role of medium access control (MAC)
– Controls when and how each node can transmit in
the wireless channel
• Why do we need MAC?
– Wireless channel is a shared medium
– Radios transmitting in the same frequency band
interfere with each other – collisions
– Other shared medium examples: Ethernet
Medium Access Control in WSNs
3
Where Is the MAC?
• Network model from Internet
Application layer
Transport layer
Network layer
Link/MAC layer
Physical layer
End-to-end reliability, congestion control
Routing
Per-hop reliability, flow control, multiple access
Packet transmission and reception
• A sublayer of the Link layer
– Directly controls the radio
– The MAC on each node only cares about its
neighborhood
Medium Access Control in WSNs
4
Media access in wireless
• In wired link,
– Carrier Sense Multiple Access with Collision
Detection
– send as soon as the medium is free, listen into the
medium if a collision occurs (original method in IEEE
802.3)
• In wireless
– Signal strength decreases in proportional to at least
square of the distance
– Collision detection only at receiver
– Half-duplex mode
– Furthermore, CS is not possible after propagation
range
Medium Access Control in WSNs
5
What’s New in Sensor Networks?
• A special wireless ad hoc network
–
–
–
–
–
–
Large number of nodes
Limited computation ability and RAM
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
Medium Access Control in WSNs
6
Characteristics of Sensor Network
• A special wireless ad hoc network
–
–
–
–
–
Scalability & Self-configuration
Large number of nodes
Battery powered
Energy efficiency
Topology and density change
Adaptivity
Nodes for a common task
Fairness not important
In-network data processing
Message-level Latency
• Sensor-net applications
– Sensor-triggered bursty traffic
– Can often tolerate some delay
Adaptivity
Trade for energy
• Speed of a moving object places a bound on network
reaction time
Medium Access Control in WSNs
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Primary Concerns of MAC Attributes
• Collision avoidance
– Basic task of a MAC protocol
– Determine when and how to access the medium
• Energy efficiency
– One of the most important attributes for sensor
networks, since most nodes are battery powered
– Affect the overall node lifetime
Medium Access Control in WSNs
8
Primary Concerns of MAC Attributes
• Scalability and adaptivity
– Network size, node density and topology change
• Deployed ad-hoc and operate in uncertain
environments
• Nodes die
• Nodes join later
• Nodes move
– Good MAC accommodates changes gracefully
Medium Access Control in WSNs
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Other Concerns of MAC Attributes
• Channel utilization
– How well is the channel used?
– Also called bandwidth utilization or channel
capacity
• Latency
– Delay from sender to receiver
– Its importance depends on application
– single hop or multi-hop
Medium Access Control in WSNs
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Other Concerns of MAC Attributes
• Throughput
– The amount of data transferred from sender to
receiver in unit time
– Affected by efficiency of collision avoidance, channel
utilization, latency, control overhead…
– Goodput?
• Fairness
– Can nodes share the channel equally?
– All nodes cooperate for a single common task
• Less important in sensor networks
Medium Access Control in WSNs
11
Energy Efficiency in MAC Design
• Energy is primary concern in sensor networks
• What causes energy waste?
– Collisions
• Retransmission
– Long idle time
Dominant factor
• bursty traffic in sensor-net apps
• Idle listening consumes 50—100% of the power for
receiving (Stemm97, Kasten)
Medium Access Control in WSNs
12
Energy
• What causes energy waste?
– Overhearing unnecessary traffic
• Can be a dominant factor of energy waste when
– Heavy traffic load
– High node density
– Control packet overhead
• Reduce effective goodput
– Computation complexity
– With Motes, radio and CPU are two major energy
consumers
Medium Access Control in WSNs
13
Classification of MAC Protocols
• Schedule-based protocols
– Schedule nodes onto different Time slots or subchannels
– Examples: TDMA, FDMA, CDMA
• Contention-based protocols
– Nodes compete in probabilistic coordination
– Examples: ALOHA (pure & slotted), CSMA, S-MAC
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Schedule-based protocols
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16
Scheduled Protocols: TDMA
• Time division multiple access
– Divide time into subchannels
– Advantages
• No collisions
• Energy efficient — easily support low duty cycles
– Disadvantages
• Difficult to accommodate node changes
• Requires strict time synchronization
• Could limit available throughput
Medium Access Control in WSNs
Frequency Division Multiple Access (FDMA)
• Available frequency subdivided into a number of
subchannels
– FDMA is used in nearly all first generation mobile
communication systems, like AMPS (30 KHz channels)
• Require frequency synchronization, narrowband
filters, tunable receiver
– Transceiver more complex
Bandwidth
subChannel 1
subChannel 2
subChannel 3
subChannel 4
Time
Medium Access Control in WSNs
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Code Division Multiple Access (CDMA)
• Use different codes to separate the
transmissions
• Users encoded by different codes (keys)
coexist in time and frequency domains
– All parallel transmissions using other codes
appears as noise
– English vs. French
• Code management is complex and critical
Medium Access Control in WSNs
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Scheduled Protocols: Polling
• Master-slave configuration
– The master node decides which slave can send by
polling the corresponding slave
– Only direct communication between the master
and a slave
– A special TDMA without pre-assigned slots
– Examples
• IEEE 802.11 infrastructure mode (CPF)
• Bluetooth piconets
Medium Access Control in WSNs
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Scheduled Protocols: Bluetooth
• Wireless personal area network (WPAN)
– Short range, moderate bandwidth, low latency
– IEEE 802.15.1 (MAC + PHY) is based on Bluetooth
• Nodes are clustered into piconet
– Each piconet has a master and up to 7 active slaves
– scalability problem
– The master polls each slave for transmission
– CDMA among piconets
– Multiple connected piconets form a scatternet
• Difficult to handle inter-cluster communications
Medium Access Control in WSNs
20
Scheduled Protocols: Bluetooth
• Bluetooth (Cont.)
– How about Bluetooth radio with sensor networks?
– Scalability is a big problem
– Lack of multi-hop support
• No commercial Bluetooth radio supports scatternet so far
• Use two radios – expensive and energy inefficient
• A node temporarily leave one piconet and joins another –
high overhead and long delay
– Connection maintenance is expensive even with a
low-duty-cycle mode ([Leopold + 2003])
Medium Access Control in WSNs
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Scheduled Protocols: Self-Organization
• By Sohrabi and Pottie [Sohrabi+ 2000]
– Have a pool of independent channels
• Frequency band or spreading code
• Potential interfering links select different channels
– Talk to neighbors in different time slots
– Sleep in unscheduled time slots
– Looks like TDMA, but actual multiple access is
accomplished by FDMA or CDMA
• Any pair of two nodes can talk at the same time
– Low bandwidth utilization
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Scheduled Protocols: LEACH
• Low-Energy Adaptive Clustering Hierarchy —
by Heinzelman, et al. [Heinzelman+ 2000]
– Similar to Bluetooth
– CDMA between clusters
– TDMA within each cluster
•
•
•
•
Static TDMA frame
Cluster head rotation
Node only talks to cluster head
Only cluster head talks to base station (long dist.)
– The same scalability problem
Medium Access Control in WSNs
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Contention-based protocols
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Contention Protocols: Classics
• ALOHA
– Pure ALOHA: send when there is data
– Slotted ALOHA: send on next available slot
– Both rely on retransmission when there’s collision
• CSMA — Carrier Sense Multiple Access
– Listening (carrier sense) before transmitting
– Send immediately if channel is idle
– Backoff if channel is busy
• non-persistent, 1-persistent and p-persistent
Medium Access Control in WSNs
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ALOHA, Slotted-ALOHA
• Mechanism
– random, distributed (no central arbiter), time-multiplex
– Slotted Aloha additionally uses time-slots, sending must always start at
slot boundaries
• Aloha
collision
sender A
sender B
sender C
t
• Slotted Aloha
collision
sender A
sender B
sender C
Medium Access Control in WSNs
t
26
Contention Protocols: CSMA/CA
• Hidden terminal problem
a
b
c
Node a is hidden from c’s carrier sense
– CSMA is not enough for multi-hop networks
(collision at receiver)
• CSMA/CA (CSMA with Collision Avoidance)
– RTS/CTS handshake before send data
– Node c will backoff when it hears b’s CTS
Medium Access Control in WSNs
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Hidden terminal problem
• Hidden terminals
–
–
–
–
A sends to B, C cannot receive A
C wants to send to B, C senses a “free” medium (CS fails)
collision at B, A cannot receive the collision (CD fails)
A is “hidden” for C
A
B
C
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Exposed terminal problem
• Exposed terminals
– B sends to A, C wants to send to D
– C has to wait, CS signals a medium in use
– but A is outside the radio range of C, thus waiting
is not necessary
– C is “exposed” to B
A
B
C
D
Medium Access Control in WSNs
29
Contention Protocols: MACA and MACAW
• MACA — Multiple Access w/ Collision
Avoidance [Karn 1990]
– Based on CSMA/CA
– Add duration field in RTS/CTS informing other
node about their backoff time
• MACAW [Bharghavan+ 1994]
– Improved over MACA
– RTS/CTS/DATA/ACK
– Fast error recovery at link layer
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30
Contention Protocols: IEEE 802.11
• IEEE 802.11 ad hoc mode (DCF)
– Virtual and physical carrier sense (CS)
• Network allocation vector (NAV), duration field
– Binary exponential backoff
– RTS/CTS/DATA/ACK for unicast packets
– Broadcast packets are directly sent after CS
– Fragmentation support
• RTS/CTS reserve time for first (fragment + ACK)
• First (fragment + ACK) reserve time for second…
• Give up transmission when error happens
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Contention Protocols: IEEE 802.11 (cont.)
• Power save (PS) mode in IEEE 802.11 DCF
– Assumption: all nodes are synchronized and can
hear each other (single hop)
– Nodes in PS mode periodically listen for beacons &
ATIMs (ad hoc traffic indication messages)
– Beacon: timing and physical layer parameters
• All nodes participate in periodic beacon generation
– ATIM: tell nodes in PS mode to stay awake for Rx
• ATIM follows a beacon sent/received
• Unicast ATIM needs acknowledgement
• Broadcast ATIM wakes up all nodes — no ACK
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Contention Protocols: IEEE 802.11 (cont.)
• Unicast example of PS mode in 802.11 DCF
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Contention Protocols: Tx Rate Control
• By Woo and Culler [woo+ 2003]
– Based on a special network setup
• A base station tries to collect data equally from all
sensors in the network
– CSMA + adaptive rate control
– Promote fair bandwidth allocation to all sensors
• Nodes close to the base station forward more traffic,
and have less chances to send their own data
– Helps in congestion avoidance
Medium Access Control in WSNs
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Self-Organizing Medium Access Control for Sensor
network (SMACS) [Sohrabi+ 2000]
• Note: this is SMACS, not S-MAC (which will be discussed later)
• Trades bandwidth for increased energy efficiency
• Superframe, Tframe
Channel: a pair of time intervals
• Four types of message:
– TYPE1: a short invitation containing a node’s ID and number of attached
neighbors.
– TYPE2: a response to TYPE1, containing a node’s address and attached
state.
– TYPE3: response to TYPE2, including the sender’s decision about
communication peer, timing information, schedule of sender’s existing
link.
– TYPE4: response to TYPE3. It identifies the time slots available to both
sender and receiver, determines the channel for the new communication
link.
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SMACS Operation
Medium Access Control in WSNs
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Eavesdrop-And-Register (EAR) protocol
• EAR extends SMACS for use with mobile devices.
• Stationary nodes periodically broadcast a Broadcast Invitation
(BI) message to invite other nodes to join.
• A mobile node selects a BI from many BIs that it got, then
reply with a Mobile Invite (MI) message
• If the stationary node accepts MI request, it selects slots for
communication and replies with a Mobile Response (MR)
message.
• As the received SNR along the channel improves or degrades,
mobile nodes request a connection or disconnection (with an
MD) based on predetermined threshold
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38
Scheduled vs. Contention Protocols
Scheduled
Protocols
Contention
Protocols
No
Yes
Good
Need
improvement
Scalability and
adaptivity
Bad
Good
Multi-hop
communication
Difficult
Easy
Time
synchronization
Strict
Loose or not
required
Collisions
Energy efficiency
Medium Access Control in WSNs
Energy Efficiency in Contention
Protocols
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Energy Efficiency in Contention Protocols
• Contention-based protocols need to work
hard in all directions for energy savings
– Reduce idle listening – support low duty cycle
– Better collision avoidance
– Reduce control overhead
– Avoid unnecessary overhearing
Medium Access Control in WSNs
40
Energy-Efficient MAC Design
• Piconet scheme — [Bennett+ 1997]
– This scheme is not the same piconet in Bluetooth
– Low duty-cycle operation — energy efficient
• Sleep for 30s, beacon, and listen for a while
• Sending node needs to listen for receiver’s beacon first,
then
• CSMA before sending data
– May wait for long time before sending
Medium Access Control in WSNs
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Energy-Efficient MAC Design
• PAMAS: Power Aware Multi-Access with
Signalling — [Singh+ 1998]
– Improve energy efficiency from MACA
– Avoid overhearing by putting node into sleep
– Use separate control and data channels
• RTS, CTS, busy tone to avoid collision
• Probe packets to find neighbors transmission time
– Increased hardware complexity
• Two channels need to work simultaneously, meaning
two radio systems.
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Power Aware Multi-Access protocol with Signaling
(PAMAS)
• Using a separate signaling channel.
• Avoids the overhearing among neighboring nodes
• Nodes shut themselves off when overhear transmissions.
– If a node has nothing to transmit, and one of its
neighbors begins transmitting
– If at least one neighbor of a node is transmitting
or receiving
• A is sending data packets to B, C and D power off
E
C
A
B
D
Medium Access Control in WSNs
F
43
PAMAS
• Every node makes the decision to power off independently
• Node sends RTS on the signal channel before transmitting
data
• If no other transmission going on, target node replies a CTS
• If any neighboring node is receiving a transmission, it
responds with a busy tone; if a CTS is sent, it collides with the
busy tone. Then the sender will backoff and retry later.
• A node only powers off its data channel. The signaling
interface stays on all the time
• Powering off radios does not have any effect on the message
latency
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Energy-Efficient MAC Design
• Asynchronous sleeping – by Tseng, et al.
– Extend 802.11 PS mode to Multi-hops
– Nodes do not synchronize with each other
– Designed 3 sleep patterns — ensure nodes listen
intervals overlap, example:
• Periodically fully-awake interval: similar to S-MAC
• Problem on broadcast — wake up each neighbor
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46
Energy-Efficient MAC Design
• ZigBee
– Industry standard through
application profiles
running over IEEE
802.15.4 radios
– Target applications are
sensors networks,
interactive toys, smart
badges, remote controls,
and home automation
Medium Access Control in WSNs
Contention Protocols: ZigBee
• Based on IEEE 802.15.4 MAC and PHY
– Three types devices
• Network Coordinator
• Full Function Device (FFD)
– Can talk to any device, more computing power
• Reduced Function Device (RFD)
– Can only talk to a FFD, simple for energy conservation
– CSMA/CA with optional ACKs on data packets
– Optional beacons with superframes
– Optional guaranteed time slots (GTS), which supports
contention-free access
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Contention Protocols: ZigBee (cont.)
• Low power, low rate (250kbps) radio
• MAC layer supports low duty cycle operation
– Target node life time > 1 year
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Sensor Mac: Case Studies
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Case Study 1: S-MAC
• By Ye, Heidemann and Estrin
• Tradeoffs
Latency
Fairness
Energy
• Major components in S-MAC
– Periodic listen and sleep
– Collision avoidance
– Overhearing avoidance
– Message passing
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Coordinated Sleeping
• Problem: Idle listening consumes significant
energy
• Solution: Periodic listen and sleep
listen
sleep
listen
sleep
• Turn off radio when sleeping
• Reduce duty cycle to ~ 10% (120ms on/1.2s off)
Latency
Energy
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Coordinated Sleeping
• Schedules can differ
Node 1
Node 2
listen
sleep
listen
listen
sleep
sleep
listen
sleep
• Prefer neighboring nodes have same schedule
— easy broadcast & low control overhead
Schedule 1
Schedule 2
Medium Access Control in WSNs
Border nodes:
two schedules or
broadcast twice
52
Coordinated Sleeping
• Schedule Synchronization
– New node tries to follow an existing schedule
– Remember neighbors’ schedules
— to know when to send to them
– Each node broadcasts its schedule every few periods
of sleeping and listening
– Re-sync when receiving a schedule update
• Periodic neighbor discovery
– Keep awake in a full sync interval over long periods
Medium Access Control in WSNs
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Coordinated Sleeping
• Adaptive listening
– Reduce multi-hop latency due to periodic sleep
– Wake up for a short period of time at end of each
transmission
2
1
3
4
RTS
CTS
listen
CTS
listen
t1
listen
t2
 Reduce latency by at least half
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Periodic Listen and Sleep
• Choosing schedules
– The node randomly choose time to go to sleep.
– The node receives and follows its neighbor’s schedule by setting its
schedule to be the same.
– If the nodes receives a different schedule after it selects its own
schedule, it adopts its own schedule.
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Periodic Listen and Sleep
• Maintaining synchronization
– Update schedule by sending a SYNC packet
periodically.
• SYNC packet contains address of the sender and the
time of its next sleep.
– The new node follows the same procedure to
choose schedule.
• The initial listen period should be long enough.
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56
S-MAC: Coordinated Sleeping
Frame Schedule Maintenance
1. Choosing a schedule
• Listen to the medium for at least SP
• Nothing heard, choose a schedule
• Broadcast a SYNC packet (should contend for medium)
2. Following a schedule
• Receives a schedule before choosing/announcing
• Follows the schedule
• Broadcast a SYNC packet
3. Adopting multiple schedules
• Receives a schedule after choosing/announcing
• Can discard the new schedule; or
• Follow both the schedules – suffer more energy loss
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S-MAC: Coordinated Sleeping
Neighbor Discovery
• chance of failing to discover an existing neighbor
– corrupted SYNC packet, collisions, interference
– sensor – border of two schedules; discovers only the first
schedule, if schedules do not overlap
• Periodically, listen for the complete SP
– frequency?
•  - if a sensor has no neighbors
• S-MAC experimental values:
– SP = 10 seconds
– Neighbor discovery period = 2 minutes, if at least 1 nbr
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S-MAC: Coordinated Sleeping
Maintaining Synchronization
• Clock drifts – not a major concern (listen time = 0.5s – 105
times longer than typical drift rates)
• Need to mitigate long term drifts – schedule updating using
SYNC packet (sender ID, its next scheduled sleep time –
relative);
• Listen is split into 2 parts – for SYNC and RTS/CTS
Listen
Receiver
for SYNC
for RTS
for CTS
Sleep
• Once RTS/CTS is established, data sent in sleep interval
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S-MAC: Coordinated Sleeping
Example Scenarios
Listen
Receiver
for SYNC
for RTS
for CTS
Sleep
Tx SYNC
Sender 1
CS
Tx RTS
Got CTS
Send data
CS
Sender 2
Tx SYNC
CS
Tx RTS
Got CTS
CS
Send data
Sender 3
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S-MAC: Coordinated Sleeping
Adaptive Listening – Low-duty cycle to active mode
* Overhearing nodes – wakeup at the end of the current
transmission (duration field in RTS/CTS)
ListenR
Sender
Receiver
ListenON
RTS
DATA
CTS
ACK
Sleep (based on RTS)
Overhearing
nodes (ON)
Sleep (based on CTS)
Medium Access Control in WSNs
Wakes up even though it
is not the correct listeninterval
Not all receiver’s nexthop nodes can hear the
transmission, if
adaptive
61
S-MAC: Collision Avoidance
• S-MAC is based on contention
• Similar to IEEE 802.11 ad hoc mode (DCF)
– Physical and virtual carrier sense
– Randomized backoff time
– RTS/CTS for hidden terminal problem
– RTS/CTS/DATA/ACK sequence
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S-MAC: Collision Avoidance
• Collision avoidance
– The same procedure as 802.11.
– To adopt RTS/CTS exchange and physical/virtual
carrier sense.
– Randomized carrier sense time.
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S-MAC: Collision-Avoidance
Collision-Avoidance Strategy ~= 802.11
• RTS/CTS
• Physical carrier sense
• Virtual carrier sense: network allocation
vector (NAV)
Sender
Receiver
RTS
DATA
CTS
ACK
NAV (based on RTS)
Other
Sensors
Contend for
medium access
defer access
NAV (based on CTS)
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Overhearing 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
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Overhearing Avoidance
• To avoid overhearing by letting interfering nodes go to sleep
after they hear a RTS or CTS packet.
• Who must go to sleep?
E
C
A
B
D
F
– All immediate neighbors of both the sender and receiver should sleep.
• Sleep until NAV becomes zero.
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Message Passing
• Problem: Sensor net in-network processing
requires entire message
• Solution: Don’t 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
Fairness
Energy
Msg-level latency
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Message Passing
• Only one RTS and CTS packet are used to send the fragmented
long packet.
– To avoid control overhead.
• To do overhearing avoidance..
– Each RTS/CTS/DATA/ACK packet has its duration field.
– The duration field include expected transmission time of all fragment.
– Sleep until NAV becomes zero.
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S-MAC: Efficient Message Passing
• Sending a long message?
– As a single packet:  cost of re-transmission for
message corruption
Sender
Receiver
RTS
FRAG1
CTS
FRAG-N
ACK
ACK
NAV (based on RTS)
Other
Sensors
defer access
NAV (based on CTS)
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S-MAC: Efficient Message Passing
• RTS/CTS/ACK – has duration fields in it
• If ACK is not received, increase the
transmission time, retransmit. ACK will be also
be updated.
• Difference between 802.11 & S-MAC
– Medium is reserved upfront for the whole
transmission in S-MAC
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Msg Passing vs. 802.11 fragmentation
• S-MAC message passing
Data 19
RTS 21
ACK 18
CTS 20
...
Data 17
ACK 16
Data 1
...
ACK 0
 Fragmentation in IEEE 802.11
• No indication of entire time — other nodes keep listening
• If ACK is not received, give up Tx — fairness
Data 3
RTS 3
CTS 2
...
Data 3
ACK 2
ACK 2
Medium Access Control in WSNs
Data 1
...
ACK 0
71
Implementation and Experiments
• Platform: Mica Motes
• Topology: 10-hop linear
network
Energy consumption at different traffic load
Energy consumption (J)
30
25
No sleep cycles
20
15
10
10% duty cycle without
adaptive listen
5
• S-MAC saved a lot of
energy compared with a
MAC without sleep
10% duty cycle with adaptive listen
0
0
2
4
6
8
10
Message inter-arrival period (S)
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72
S-MAC: An Energy-Efficient MAC Protocol
• S- MAC protocol designed specifically for sensor networks to
reduce energy consumption while achieving good scalability
and collision avoidance by utilizing a combined scheduling
and contention scheme
• The major sources of energy waste are:
–
–
–
–
collision
overhearing
control packet overhead
idle listening
• S-MAC reduce the waste of energy from all the sources
mentioned in exchange of some reduction in both per-hop
fairness and latency
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Case Study 2: B-MAC [Polastre+ 2004]
• Another low-power MAC for sensor networks
• B-MAC design considerations
–
–
–
–
Simplicity: based on simple CSMA
Configurable options
Minimize idle listening
Based on model of periodic sensor data transfer
• B-MAC components
– CSMA without RTS/CTS
– Optional Low-power listening (LPL)
– Optional ACK
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Low-Power Listening
• Determine channel status by quick sampling
– Very low overhead on wake-up
Joe Polastre, et al., SenSys’04
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Low Duty Cycle with LPL
• Nodes periodically sleep and perform LPL
• Nodes do not synchronized on listen time
• Sender uses a long preamble before each packet
to wake up the receiver
• Shift most burden to the sender
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Comparison of S-MAC and B-MAC
S-MAC
B-MAC
CSMA/CA
CSMA
ACK
Yes
Optional
Message passing
Yes
No
Overhearing avoidance
Yes
No
Pre-defined + adaptive
listen
Pre-defined
Long
Very short
Required
Not required
Short preamble
Long preamble
6.3KB
4.4KB (LPL & ACK)
Collision avoidance
Listen period
Listen interval
Schedule
synchronization
Packet transmission
Code size
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Drawbacks of S-MAC
• Active (Listen) interval – long enough to
handle to highest expected load
– If message rate is less – energy is still wasted in
idle-listening
• S-MAC fixed duty cycle – is NOT OPTIMAL
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Case Study 3: T-MAC [Van Dam+ 2003]
• Fixed duty cycle like S-MAC, is not optimal.
– The nodes must be deployed with an active time that
can handle the highest expected load.
– Whenever the load is lower than that, the active time is
not optimally used and energy will be wasted on idle
listening
• An active period ends when no activation event
has occurred for a time TA
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T-MAC: Preliminaries
• Adaptive duty cycle:
Active
Active
Active
Sleep
TA
Sleep
TA
TA
• A node is in active mode until no activation event occurs for
time TA
– Periodic frame timer event, receive, carrier sense, send-done,
knowledge of other transmissions being ended
• Communication ~= S-MAC/802.11
• Frame schedule maintenance ~= S-MAC
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T-MAC: RTS Operation
Contention Interval
• waiting/listening for a random time within a
fixed contention interval (unlike exponential
back-off in 802.11)
– Tuned for max. load
• assumptions:
– load is always high, does not vary
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T-MAC: RTS Operation
RTS Retries
• No CTS reply for RTS?
– collision
– receiver should not reply due to another transmission
in progress (overhearing RTS/CTS of others)
– receiver is sleeping
• Solutions:
– wait for TA, go to sleep – receiver might be awake,and
start transmission!
– retransmit RTS if no answer, max of 2 retries
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T-MAC: Choosing TA
• Requirement: a node should not sleep while its
neighbors are communicating, potential next
receiver
• TA > C+R+T
– C – contention interval length;
– R – RTS packet length;
– T – turn-around time, time bet. end of RTS and start of
CTS;
• TA = 1.5 * (C+R+T);
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T-MAC: Overhearing Avoidance
• ~= S-MAC
• But implemented as an option in T-MAC
• Node – goes to sleep after overhearing RTS/CTS
of other nodes communication
– miss other RTS/CTS transmissions
– disturb the medium while waking up
– throughput decreases
– Overhearing avoidance should not used when
maximum throughput is required
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T-MAC: Asymmetric Communication
Early-Sleeping Problem – in convergecast (A to
D)
• C – may lose medium to B (RTS) or A (B’s CTS)
• C loses to B; D will hear CTS from C;
• C loses to A; D will hear nothing, since C is silent;
contend
RTS
CTS
DATA
ACK
A
B
contend
C
active
sleep
D
RTS?
TA
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T-MAC: Asymmetric Communication
Future RTS (FRTS)
• Let others know that it cannot access the medium; C – sends FRTS –
has duration field; receiver of FRTS – schedule timer;
• FRTS might affect data; so, DATA postponed until FRTS is over;
Prevent others from taking medium, send dummy DS packet;
contend
RTS CTS DS
DATA
ACK
A
B
contend
C
active
active
D
TA
FRTS
TA = C+R+T+CTS_length
Medium Access
Control in WSNs
RTS
86
T-MAC: Asymmetric Communication
Full-Buffer Priority – suitable for unidirectional
flows
• Buffer – almost full – prefer sending than receiving
• Receive RTS, send its own RTS back instead of CTS
• Higher chance of transmitting its own message, lesser probability
of early-sleeping, limited form of flow control
contend
A
contend
B
contend
C
active
RTS
D
TA
RTS CTS
DATA
ACK
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Homogeneous Local Unicast
Send one packet
to a random nbr.
T-MAC: OA, no
FRTS or priority
over full-buffers.
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Nodes to Sink Communication ~=
Convergecast
Send message to
corner node;
Shortest path routing;
No data aggregation;
T-MAC: OA, FRTS &
Full-buffer priority
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Early-Sleeping Problem & Solutions
Performance
Send message to
corner node;
Shortest path routing;
No data aggregation;
T-MAC:
FRTS
Vs. Priority
Vs. FRTS + Priority
Vs. No measures
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Event-Based Local Unicast
Event frequency 10s;
Event duration 5s;
Affect 9 nodes;
Send local unicast to
their neighbors.
Nbrs. reply with 20%
probability;
T-MAC: OA, no FRTS &
Full-buffer priority
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Event-Based Local Unicast,
Convergecast
No event, xchg local msgs.
(10bytes) every 20s; report
sink every 100s;
Event frequency 10s;
Event duration 5s;
Local unicast (30bytes) 4ps;
To sink (50bytes) 1ps;
Shortest path routing;
Data aggregation;
T-MAC: OA, no FRTS &
Full-buffer priority
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T-MAC: An Adaptive Energy-Efficient MAC
Protocol
–
T-MAC is a contention based Medium Access Control Protocol
–
Energy consumption is reduced by introducing an active/sleep duty cycle
–
Handles the load variations in time and location by introducing an
adaptive duty cycle

It reduces the amount of energy wasted on idle listening by dynamically
ending the active part of it
–
In T-MAC, nodes communicate using RTS, CTS, Data and ACK pkts
which provides collision avoidance and reliable transmission
–
When a node senses the medium idle for TA amount of time it
immediately switches to sleep
–
TA determines the minimal amount of idle listening time per frame
–
The incoming messages between two active states are buffered
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93
T-MAC: An Adaptive Energy-Efficient MAC
Protocol
–
The buffer capacity determines an upper bound on the maximum frame
time
–
Frame synchronization in T-MAC follows the scheme of virtual clustering
as in S-MAC
–
The RTS transmission in T-MAC starts by waiting and listening for a
random time within a fixed contention interval at the beginning of the each
active state
–
The TA time is obtained using TA > C + R + T
–
T-MAC suffers from early sleeping problem
–
Its overcome by sending Future request to send or taking priority on full
buffers
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94
T-MAC: An Adaptive Energy-Efficient MAC
Protocol
Advantages:
–
The T-MAC protocol is designed particularly for wireless sensor
networks and hence energy consumption constraints are taken into
account
–
The T-MAC protocol tries to reduce idle listening by transmitting all
messages in bursts of variable lengths and sleeping between burst
–
T-MAC facilitates collision avoidance and overhearing -- nodes transmit
their data in a single burst and thus do not require additional RTS/CTS
control packets.
–
By stressing on RTS retries, T-MAC gives the receiving nodes enough
chance to listen and reply before it actually goes to sleep -- this
increases the throughput in the long run
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T-MAC: An Adaptive Energy-Efficient MAC
Protocol
Disadvantages:
–
The authors do not outline how a sender node would sense a FRTS packet
and enable it to send a DS packet
–
Also sending a DS packet increases the overhead.
–
The network topology in the simulation considers that the locations of the
nodes are known
–
T-MAC has been observed to have a high message loss phenomenon
–
T-MAC suffers from early sleeping problem for event based local unicast
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T-MAC: An Adaptive Energy-Efficient MAC
Protocol
Suggestions/Improvements/Future Work:
–
If a buffer is full there would be a lot of dropped packets decreasing the
throughput. A method to overcome this drawback is that we could have
the node with its buffer 75% full broadcast a special packet Buffer Full
Packet
–
MAC Virtual Clustering technique needs to be further investigated
–
An adaptive election algorithm can be incorporated where the schedule
and neighborhood information is used to select the transmitter and
receivers for the current time slot, hence avoiding collision and
increasing energy conservation
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MAC Design for Sensor Networks
• MAC protocols can be classified as scheduled and
contention-based
• Major considerations
– Energy efficiency
– Scalability and adaptivity to number of nodes
• Major ways to conserve energy
–
–
–
–
Low duty cycle to reduce idle listening
Effective collision avoidance
Overhearing avoidance
Reducing control overhead
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References
[Ye+ 2002] W. Yei, et al., Energy-Efficient MAC Protocol for Wireless Sensor Networks, Proceedings of the
Twenty First International Annual Joint Conference of the IEEE Computer and Communications Societies
(INFOCOM 2002), New York, NY, USA, June 23-27 2002.
[Woo+ 2003] A. Woo et al., A Transmission Control Scheme for Media Access in Sensor Networks,
Proceedings of the ACM/IEEE International Conference on Mobile Computing and Networking, Rome, Italy,
July 2001, pp. 221-235.
[Van Dam+ 2003] T. V. Dam et al., An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks,
ACM SenSys, Los Angeles, CA, November, 2003.
[Polastre+ 2004] J. Polastre et al., Versatile Low Power Media Access for Wireless Sensor Networks, In
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3-5, 2004
[Leopold + 2003] M Leopold, M. B. Dydensborg, and P. Bonnet, Bluetooth and Sensor Networks: A Reality
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[Heinzelman+ 2000]W. R. Heinzelman, A. Chandrakasan, and H. Balakrishnan, Energy-efficient
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[Bharghavan+ 1994] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, MACAW: A
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[Sohrabi+ 2000] K. Sohrabi et al., Protocols for Self-Organization of a Wireless Sensor Network,” IEEE Pers.
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[Bennett+ 1997] F. Bennett et al., Piconet: Embedded mobile networking,” IEEE Personal Communications
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[Tseng+ 2002] Yu-Chee Tseng et al., Power-saving protocols for IEEE 802.11-based multi-hop ad hoc
networks,” in Proc. of the IEEE Infocom, New York, NY, June 2002, pp. 200–209.
[Singh+ 1998] S. Singh et al., PAMAS: Power aware multi-access protocol with signalling for ad hoc networks,”
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