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UNIT III
18CSE451T -Wireless Sensor
Networks
Prepared by
Dr.N.Noor Alleema
AP , SRM IST
• Distributed MAC Protocols
– Distributed Foundation Wirelesss MAC
(DFWMAC)
– Eliminate Yield – Non-Preemptive Priority
Multiple Access (EY-NPMA)
• Centralized MAC Protocols
– Random Access
• Idle Sense Multiple Acces (ISMA)
• Randomly Addressed Polling (RAP)
• Resource Auction Multiple Access (RAMA)
– Guaranteed Access
• Zhang’s and Acampora’s Proposals
• Disposable Token MAC Protocol (DTMP)
– Hybrid Access
• Random Reservation Protocols (RRA)
• Packet Reservation Multiple Access (PRMA)
• Random Reservation Access – Independent Stations
Algorithm (RRA-ISA)
• Distributed Queuing Request Updated Multiple Access
(DQRUMA)
• Moble Access Scheme based on Contention and
Reservation for ATM (MASCARA)
Distributed Wireless Network
• ad hoc network
• No central administration
• Multi-hop wireless networks
• Wireless Sensor Nets
Centralized Wireless Network
• Last Hop Network
• Very common
– Corporate, Academic, and Cellular uses.
• Has a controlling Base Station, with variable
intelligence
– Wireless Access Point
– Cellular Tower
Centralized Wireless Network
• Last Hop Network
• Very common
– Corporate, Academic, and Cellular uses.
• Has a controlling Base Station, with variable
intelligence
– Wireless Access Point
– Cellular Tower
Characteristics of MAC Protocols in
Sensor Networks
• Energy Efficiency
• Scalability
• Adaptability
• Low Latency and Predictability
• Reliability
MAC Protocols in Sensor Networks
• ALOHA
• CSMA
– Persistent
• L persistent
• P persistent
– Non Persistent
MAC Protocols in Sensor Networks
• CSMA
– CSMA/CD
• Truncated binary exponential back-off algorithm
– CSMA/CA
• Hidden-node problems
• Exposed-node problems
Hidden-node scenario in wireless
sensor networks
Exposed node scenario in wireless
sensor networks
Collision avoidance using RTS/CTS
handshake
Collision avoidance failure using
RTS/CTS handshake
Collision avoidance failure using
RTS/CTS handshake
Contention-Free MAC Protocols
• Traffic-Adaptive Medium Access
• Y-MAC
• Low energy Adaptive Clustering
Traffic-Adaptive Medium Access
• TRAMA assumes a time-slotted channel,
where time is divided into
– Periodic random access intervals (signaling slots)
• Neighbor Protocol (NP)
– Scheduled-access intervals (transmission slots)
• Schedule Exchange Protocol (SEP)
Overview
•Routing in WSNs is challenging due to distinguish
from other wireless networks like mobile ad hoc
networks or cellular networks.
•First, it is not possible to build a global
addressing scheme for a large number of sensor
nodes. Thus, traditional IP-based protocols may
not be applied to WSNs. In WSNs, sometimes
getting the data is more important than knowing
the IDs of which nodes sent the data.
•Second, in contrast to typical communication
networks, almost all applications of sensor
networks require the flow of sensed data from
1
8
Overview
(cont.)
• Third, sensor nodes are tightly constrained in
terms of energy, processing, and storage
capacities. Thus, they require carefully resource
management.
• Fourth, in most application scenarios, nodes in WSNs
are generally stationary after deployment except for,
may be, a few mobile nodes.
• Fifth, sensor networks are application specific, i.e.,
design
requirements of a sensor network change with
application.
• Sixth, position awareness of sensor nodes is
important since data collection is normally based
on the location.
• Finally, data collected by many sensors in WSNs is
typically based on common phenomena, hence there
is a high probability that this data has some
redundancy.
1
9
Overview
(cont.)
• The task of finding and maintaining routes in
WSNs is nontrivial since energy restrictions and
sudden
changes in node status (e.g., failure) cause
frequent and unpredictable topological
changes.
• To minimize energy consumption, routing
techniques proposed for WSNs employ some
well-known routing strategies, e.g., data
aggregation and in-network processing,
clustering, different node role assignment, and
data-centric methods were employed.
Overview
• Data-centric communication
• Data is named by attribute-
value pairs
• Different form IP-
style
communication
• End-to-end delivery service
• e.g.
• How many pedestrians do
you observe in the
geographical region X?
Event
Sources
2
0
Sink Node
Directed
Diffusion
A sensorfield
2
1
Overview(cont.)
• Data-centric communication (cont.)
• Human operator’s query (task) is diffused
• Sensors begin collecting information about query
• Information returns along the reverse path
• Intermediate nodes aggregate the data
• Combing reports from sensors
• Directed Diffusion is an important milestone in
the data centric routing research of sensor
networks
2
2
HierarchicalRouting
2
3
Overview
• In a hierarchical architecture, higher energy nodes
can be used to process and send the information
while low energy nodes can be used to perform
the sensing of the target.
• Hierarchical routing is mainly two-layer routing
where one layer is used to select cluster heads
and the other layer is used for routing.
• Hierarchical routing (or cluster-based routing),
e.g., LEACH, PEGASIS, TTDD, is an efficient
way to lower energy consumption within a
cluster and by performing data aggregation and
fusion in order to decrease the number of
transmitted messages to the base stations.
Y-MAC
• Y-MAC divides time into
– Frames
– Slots
• Each frame contains a broadcast period and a
unicast period
• Every node must wakeup at the beginning of a
broadcast period
Y-MAC
2
6
LEACH
Low-Energy Adaptive Clustering Hierarchy
Low energy Adaptive Clustering
Hierarchy (LEACH)
• Setup Phase
• Steady-State Phase
Low energy Adaptive Clustering
Hierarchy (LEACH)
Low energy Adaptive Clustering
Hierarchy (LEACH)
3
0
LEACH
• LEACH (Low-Energy Adaptive Clustering
Hierarchy), a clustering-based protocol that
minimizes energy dissipation in sensor
networks.
• LEACH outperforms classical clustering
algorithms by using adaptive clusters and
rotating cluster-heads, allowing the energy
requirements of the system to be distributed
among all the sensors.
• LEACH is able to perform local computation in
each cluster to reduce the amount of data that
must be transmitted to the base station.
• LEACH uses a CDMA/TDMA MAC to reduce
inter- cluster and intra-cluster collisions.
3
1
LEACH(cont.)
• Sensors elect themselves to be local cluster-headsat
any given time with a certainprobability.
• Each sensor node joins a cluster-head thatrequires
the minimum communicationenergy.
• Once all the nodes are organized into clusters, each
cluster-head creates a transmission schedule forthe
nodes in itscluster
.
• In order to balance the energy consumption, the
cluster-head nodes are not fixed; rather
,thisposition
is self-elected at different timeintervals.
LEACH
1 0 0 m
叢 集 區
觀 測 區 域
B a s e S t a t i o n
S e n s o r ( N o n C l u s t e r H e a d )
S e n s o r ( C l u s t e r H e a d )
I n i t i a l D a t a
A g g r e g a t e d D a t a
~ 1 0 0 m
1
2
LEACH:AdaptiveClustering
• Periodic independent self-election
• Probabilistic
• CSMA MAC used to advertise
• Nodes select advertisement with strongest signal
strength
• Dynamic TDM
A
cycles
All nodes marked with a given symbol belong to the same cluster, and
the cluster head nodes are marked with a ● .
3
3
3
4
Algorithm
• Periodic process
• Two phases per round:
• Setup phase
• Advertisement: Execute election algorithm
• Members join to cluster
• Cluster-head broadcasts schedule
• Steady-State phase
• Data transmission to cluster-head using
TDMA
• Cluster-head transfers data to BS
(Base Station)
Algorithm(cont.)
15
Advertisement
phase
Cluster setup phase Broadcast schedule
Time slot
1
Time slot
2
Time slot
3
Setup phase Steady-state phase
Self-election of cluster
heads
Cluster heads compete
with CSMA
Members
compete with
CSMA
Cluster head Broadcast
CDMA code to members
Fixed-length cycle
1
5
Algorithm Summary(cont.)
• Set-up phase
• Cluster heads assign a TDMA schedule for their
members where each node is assigned a time slot
when it can transmit.
• Each cluster communications using different CDMA
codes to reduce interference from nodes belonging
to other clusters.
• TDMA intra-cluster
• CDMA inter-cluster
• Spreading codes determined randomly
• Broadcast during advertisement phase
1
6
1
7
Algorithm Summary
(cont.)
• Steady-state phase
• All source nodes send their data to their cluster
heads
• Cluster heads perform data aggregation/fusion
through local transmission
• Cluster heads send aggregated data back to the BS
using a single direct transmission
An Example of a
LEACHNetwork
• While neither of these diagrams is the optimum scenario, the
second is better because the cluster-heads are spaced out and
the network is more properly sectioned
1
8
Node
Cluster-Head Node
Node that has been cluster-head in the last 1/P
rounds
Cluster
Border
X
Good case scenario Bad case scenario
3
9
Conclusions
• Advantages
• Increases the lifetime of the network
• Even drain of energy
• Distributed, no global knowledge required
• Energy saving due to aggregation by CHs
• Disadvantages
• LEACH assumes all nodes can transmit with enough
power to reach BS if necessary (e.g., elected as
CHs)
• Each node should support both TDMA & CDMA
• Need to do time synchronization
• Nodes use single-hop communication
Contention-Based MAC Protocols
• Power Aware Multi-Access with signaling
• Sensor MAC
• Timeout MAC
• Data gathering MAC
Power Aware Multi-Access with
Signaling
Sensor MAC
Timeout MAC
Timeout MAC
Data gathering MAC
DATA GATHERING TREE
Data gathering MAC
CONVERGE CAST COMMUNICATION
Contents
• Basic Concepts
• S-MAC
• T-MAC
• B-MAC
• P-MAC
• Z-MAC
Basic Concepts
• Problem
• TDMA
• CSMA
• RTS / CTS
Hidden Nodes
A B C
MAC Challenges
• Traditionally
– Fairness
– Latency
– Throughput
• For Sensor Networks
– Power efficiency
– Scalability
S-MAC - Sensor MAC
• Nodes periodically sleep
• Trades energy efficiency for lower throughput
and higher latency
• Sleep during other nodes transmissions
Listen Sleep t
Listen Sleep
S-MAC
• Listen significantly longer than clock drift
• Neighboring nodes exchange SYNC msgs
• Exchanged timestamps are relative rather than
absolute
• RTS/CTS avoids hidden terminal
• Message passing provided
• Packets contain expected duration of message
• Every packet must be acknowledged
• Adaptive listening can be used so that potential
next hop nodes wake up in time for possible
transmissions
S-MAC Results
Latency and throughput are problems, but
adaptive listening improves it significantly
S-MAC Results
• Energy savings significant compared to
“non-sleeping” protocols
T-MAC - Timeout MAC
• Transmit all messages in bursts of variable
length and sleep between bursts
• RTS / CTS / ACK Scheme
• Synchronization similar to S-MAC
T-MAC Operation
T-MAC Results
• T-MAC saves energy compared to S-MAC
• The “early sleeping problem” limits the maximum
throughput
• Further testing on real sensors needed
B-MAC - Berkeley MAC
• B-MAC’s Goals:
– Low power operation
– Effective collision avoidance
– Simple implementation (small code)
– Efficient at both low and high data rates
– Reconfigurable by upper layers
– Tolerant to changes on the network
– Scalable to large number of nodes
B-MAC’s Features
• Clear Channel Assessment (CCA)
• Low Power Listening (LPL) using preamble
sampling
• Hidden terminal and multi-packet mechanisms
not provided, should be implemented, if needed,
by higher layers
Sleep
t
Receive
Receiver
Sleep
t
Preamble
Sender Message
Sleep
B-MAC Interface
• CCA on/off
• Acknowledgements on/off
• Initial and congestion backoff in a per packet
basis
• Configurable check interval and preamble
length
B-MAC Lifetime Model

E  Erx  Etx  Elisten  Ed  Esleep
Erx  trxcrxbV
Etx  ttxctxbV
Elisten  Esample
1
ti
Ed  tdcdataV
Esleep  tsleepcsleepV
• E can be calculated if hardware constants, sample rate, number
of neighboring nodes and check time/preamble are known
• Better: E can be minimized by varying check time/preamble if
constants, sample rate and neighboring nodes are known
B-MAC Results
• Performs better than the other studied
protocols in most cases
• System model can be complicated for
application and routing protocol developers
• Protocol widely used because has good results
even with default parameters
P-MAC - Pattern MAC
• Patterns are 0*1 strings with size 1-N
• Every node starts with 1 as pattern
• Number of 0’s grow exponentially up to a threshold  and then linearly up to N-1
• TR = CW + RTS + CTS + DATA + ACK
• N = tradeoff between latency and energy
Patterns vs Schedules
Local
Pattern Bit
Packet to
Send
Receiver
Pattern Bit
Local
Schedule
1 1 1 1
1 1 0 1-
1 0 * 1-
0 1 1 1
0 1 0 0
0 0 * 0
P-MAC Evaluation
• Simulated results are better than SMAC
• Good for relatively stable traffic conditions
• Adaptation to changes on traffic might be
slow
• Loose time synchronization required
• Needs more testing and comparison with
other protocols besides S-MAC
Z-MAC - Zebra MAC
• Runs on top of B-MAC
• Combines TDMA and CSMA features
CSMA
 Pros
 Simple
 Scalable
 Cons
 Collisions due to
hidden terminals
 RTS/CTS is
overhead
TDMA
 Pros
 Naturally avoids
collisions
 Cons
 Complexity of
scheduling
 Synchronization
needed
Z-MAC Initialization
• Neighborhood discovery through ping messages
containing known neighbors
• Two-hop neighborhood used as input for a
scheduling algorithm (DRAND)
• Running time and message complexity of DRAND is
O(), where  is the two-hop neighborhood size
• The idea is to compensate the initialization energy
consumption during the protocol normal operation
Z-MAC Time Slot Assignment
2a1
 Fi  2a
1
l2a
 si ( for: l  0,1,2...)
Z-MAC Transmission Control
The Transmission Rule:
• If owner of slot
– Take a random backoff within To
– Run CCA and, if channel is clear, transmit
• Else
– Wait for To
– Take a random backoff within [To,Tno]
– Run CCA and, if channel is clear, transmit
Z-MAC HCL Mode
• Nodes can be in “High Contention Level” (HCL)
• A node is in HCL only if it recently received an
“Explicit Contention Notification” (ECN) from a two-
hop neighbor
• Nodes in HCL are not allowed to contend for the
channel on their two-hop neighbors’ time slots
• A node decides to send an ECN if it is losing too many
messages (application ACK’s) or based on noise
measured through CCA
Z-MAC Receiving Schedule
• B-MAC based
• Time slots should be large enough for
contention, CCA and one B-MAC packet
transmission
• Slot size choice, like in B-MAC, left to
application
Z-MAC Results
• Z-MAC performs better than B-MAC when load is high
• As expected, fairness increases with Z-MAC
• Complexity of the protocol can be a problem
Conclusions
• Between the protocols studied, B-MAC still
seems to be the best one for applications in
general
• Application developers seem not to use B-
MAC’s control interface
• Middleware service could make such
optimizations according to network status

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Wsn protocols

  • 1. UNIT III 18CSE451T -Wireless Sensor Networks Prepared by Dr.N.Noor Alleema AP , SRM IST
  • 2. • Distributed MAC Protocols – Distributed Foundation Wirelesss MAC (DFWMAC) – Eliminate Yield – Non-Preemptive Priority Multiple Access (EY-NPMA) • Centralized MAC Protocols – Random Access • Idle Sense Multiple Acces (ISMA) • Randomly Addressed Polling (RAP) • Resource Auction Multiple Access (RAMA) – Guaranteed Access • Zhang’s and Acampora’s Proposals • Disposable Token MAC Protocol (DTMP)
  • 3. – Hybrid Access • Random Reservation Protocols (RRA) • Packet Reservation Multiple Access (PRMA) • Random Reservation Access – Independent Stations Algorithm (RRA-ISA) • Distributed Queuing Request Updated Multiple Access (DQRUMA) • Moble Access Scheme based on Contention and Reservation for ATM (MASCARA)
  • 4. Distributed Wireless Network • ad hoc network • No central administration • Multi-hop wireless networks • Wireless Sensor Nets
  • 5. Centralized Wireless Network • Last Hop Network • Very common – Corporate, Academic, and Cellular uses. • Has a controlling Base Station, with variable intelligence – Wireless Access Point – Cellular Tower
  • 6. Centralized Wireless Network • Last Hop Network • Very common – Corporate, Academic, and Cellular uses. • Has a controlling Base Station, with variable intelligence – Wireless Access Point – Cellular Tower
  • 7. Characteristics of MAC Protocols in Sensor Networks • Energy Efficiency • Scalability • Adaptability • Low Latency and Predictability • Reliability
  • 8. MAC Protocols in Sensor Networks • ALOHA • CSMA – Persistent • L persistent • P persistent – Non Persistent
  • 9. MAC Protocols in Sensor Networks • CSMA – CSMA/CD • Truncated binary exponential back-off algorithm – CSMA/CA • Hidden-node problems • Exposed-node problems
  • 10. Hidden-node scenario in wireless sensor networks
  • 11. Exposed node scenario in wireless sensor networks
  • 12. Collision avoidance using RTS/CTS handshake
  • 13. Collision avoidance failure using RTS/CTS handshake
  • 14. Collision avoidance failure using RTS/CTS handshake
  • 15. Contention-Free MAC Protocols • Traffic-Adaptive Medium Access • Y-MAC • Low energy Adaptive Clustering
  • 16. Traffic-Adaptive Medium Access • TRAMA assumes a time-slotted channel, where time is divided into – Periodic random access intervals (signaling slots) • Neighbor Protocol (NP) – Scheduled-access intervals (transmission slots) • Schedule Exchange Protocol (SEP)
  • 17. Overview •Routing in WSNs is challenging due to distinguish from other wireless networks like mobile ad hoc networks or cellular networks. •First, it is not possible to build a global addressing scheme for a large number of sensor nodes. Thus, traditional IP-based protocols may not be applied to WSNs. In WSNs, sometimes getting the data is more important than knowing the IDs of which nodes sent the data. •Second, in contrast to typical communication networks, almost all applications of sensor networks require the flow of sensed data from
  • 18. 1 8 Overview (cont.) • Third, sensor nodes are tightly constrained in terms of energy, processing, and storage capacities. Thus, they require carefully resource management. • Fourth, in most application scenarios, nodes in WSNs are generally stationary after deployment except for, may be, a few mobile nodes. • Fifth, sensor networks are application specific, i.e., design requirements of a sensor network change with application. • Sixth, position awareness of sensor nodes is important since data collection is normally based on the location. • Finally, data collected by many sensors in WSNs is typically based on common phenomena, hence there is a high probability that this data has some redundancy.
  • 19. 1 9 Overview (cont.) • The task of finding and maintaining routes in WSNs is nontrivial since energy restrictions and sudden changes in node status (e.g., failure) cause frequent and unpredictable topological changes. • To minimize energy consumption, routing techniques proposed for WSNs employ some well-known routing strategies, e.g., data aggregation and in-network processing, clustering, different node role assignment, and data-centric methods were employed.
  • 20. Overview • Data-centric communication • Data is named by attribute- value pairs • Different form IP- style communication • End-to-end delivery service • e.g. • How many pedestrians do you observe in the geographical region X? Event Sources 2 0 Sink Node Directed Diffusion A sensorfield
  • 21. 2 1 Overview(cont.) • Data-centric communication (cont.) • Human operator’s query (task) is diffused • Sensors begin collecting information about query • Information returns along the reverse path • Intermediate nodes aggregate the data • Combing reports from sensors • Directed Diffusion is an important milestone in the data centric routing research of sensor networks
  • 23. 2 3 Overview • In a hierarchical architecture, higher energy nodes can be used to process and send the information while low energy nodes can be used to perform the sensing of the target. • Hierarchical routing is mainly two-layer routing where one layer is used to select cluster heads and the other layer is used for routing. • Hierarchical routing (or cluster-based routing), e.g., LEACH, PEGASIS, TTDD, is an efficient way to lower energy consumption within a cluster and by performing data aggregation and fusion in order to decrease the number of transmitted messages to the base stations.
  • 24. Y-MAC • Y-MAC divides time into – Frames – Slots • Each frame contains a broadcast period and a unicast period • Every node must wakeup at the beginning of a broadcast period
  • 25. Y-MAC
  • 27. Low energy Adaptive Clustering Hierarchy (LEACH) • Setup Phase • Steady-State Phase
  • 28. Low energy Adaptive Clustering Hierarchy (LEACH)
  • 29. Low energy Adaptive Clustering Hierarchy (LEACH)
  • 30. 3 0 LEACH • LEACH (Low-Energy Adaptive Clustering Hierarchy), a clustering-based protocol that minimizes energy dissipation in sensor networks. • LEACH outperforms classical clustering algorithms by using adaptive clusters and rotating cluster-heads, allowing the energy requirements of the system to be distributed among all the sensors. • LEACH is able to perform local computation in each cluster to reduce the amount of data that must be transmitted to the base station. • LEACH uses a CDMA/TDMA MAC to reduce inter- cluster and intra-cluster collisions.
  • 31. 3 1 LEACH(cont.) • Sensors elect themselves to be local cluster-headsat any given time with a certainprobability. • Each sensor node joins a cluster-head thatrequires the minimum communicationenergy. • Once all the nodes are organized into clusters, each cluster-head creates a transmission schedule forthe nodes in itscluster . • In order to balance the energy consumption, the cluster-head nodes are not fixed; rather ,thisposition is self-elected at different timeintervals.
  • 32. LEACH 1 0 0 m 叢 集 區 觀 測 區 域 B a s e S t a t i o n S e n s o r ( N o n C l u s t e r H e a d ) S e n s o r ( C l u s t e r H e a d ) I n i t i a l D a t a A g g r e g a t e d D a t a ~ 1 0 0 m 1 2
  • 33. LEACH:AdaptiveClustering • Periodic independent self-election • Probabilistic • CSMA MAC used to advertise • Nodes select advertisement with strongest signal strength • Dynamic TDM A cycles All nodes marked with a given symbol belong to the same cluster, and the cluster head nodes are marked with a ● . 3 3
  • 34. 3 4 Algorithm • Periodic process • Two phases per round: • Setup phase • Advertisement: Execute election algorithm • Members join to cluster • Cluster-head broadcasts schedule • Steady-State phase • Data transmission to cluster-head using TDMA • Cluster-head transfers data to BS (Base Station)
  • 35. Algorithm(cont.) 15 Advertisement phase Cluster setup phase Broadcast schedule Time slot 1 Time slot 2 Time slot 3 Setup phase Steady-state phase Self-election of cluster heads Cluster heads compete with CSMA Members compete with CSMA Cluster head Broadcast CDMA code to members Fixed-length cycle 1 5
  • 36. Algorithm Summary(cont.) • Set-up phase • Cluster heads assign a TDMA schedule for their members where each node is assigned a time slot when it can transmit. • Each cluster communications using different CDMA codes to reduce interference from nodes belonging to other clusters. • TDMA intra-cluster • CDMA inter-cluster • Spreading codes determined randomly • Broadcast during advertisement phase 1 6
  • 37. 1 7 Algorithm Summary (cont.) • Steady-state phase • All source nodes send their data to their cluster heads • Cluster heads perform data aggregation/fusion through local transmission • Cluster heads send aggregated data back to the BS using a single direct transmission
  • 38. An Example of a LEACHNetwork • While neither of these diagrams is the optimum scenario, the second is better because the cluster-heads are spaced out and the network is more properly sectioned 1 8 Node Cluster-Head Node Node that has been cluster-head in the last 1/P rounds Cluster Border X Good case scenario Bad case scenario
  • 39. 3 9 Conclusions • Advantages • Increases the lifetime of the network • Even drain of energy • Distributed, no global knowledge required • Energy saving due to aggregation by CHs • Disadvantages • LEACH assumes all nodes can transmit with enough power to reach BS if necessary (e.g., elected as CHs) • Each node should support both TDMA & CDMA • Need to do time synchronization • Nodes use single-hop communication
  • 40. Contention-Based MAC Protocols • Power Aware Multi-Access with signaling • Sensor MAC • Timeout MAC • Data gathering MAC
  • 41. Power Aware Multi-Access with Signaling
  • 45. Data gathering MAC DATA GATHERING TREE
  • 46. Data gathering MAC CONVERGE CAST COMMUNICATION
  • 47. Contents • Basic Concepts • S-MAC • T-MAC • B-MAC • P-MAC • Z-MAC
  • 48. Basic Concepts • Problem • TDMA • CSMA • RTS / CTS
  • 50. MAC Challenges • Traditionally – Fairness – Latency – Throughput • For Sensor Networks – Power efficiency – Scalability
  • 51. S-MAC - Sensor MAC • Nodes periodically sleep • Trades energy efficiency for lower throughput and higher latency • Sleep during other nodes transmissions Listen Sleep t Listen Sleep
  • 52. S-MAC • Listen significantly longer than clock drift • Neighboring nodes exchange SYNC msgs • Exchanged timestamps are relative rather than absolute • RTS/CTS avoids hidden terminal • Message passing provided • Packets contain expected duration of message • Every packet must be acknowledged • Adaptive listening can be used so that potential next hop nodes wake up in time for possible transmissions
  • 53. S-MAC Results Latency and throughput are problems, but adaptive listening improves it significantly
  • 54. S-MAC Results • Energy savings significant compared to “non-sleeping” protocols
  • 55. T-MAC - Timeout MAC • Transmit all messages in bursts of variable length and sleep between bursts • RTS / CTS / ACK Scheme • Synchronization similar to S-MAC
  • 57. T-MAC Results • T-MAC saves energy compared to S-MAC • The “early sleeping problem” limits the maximum throughput • Further testing on real sensors needed
  • 58. B-MAC - Berkeley MAC • B-MAC’s Goals: – Low power operation – Effective collision avoidance – Simple implementation (small code) – Efficient at both low and high data rates – Reconfigurable by upper layers – Tolerant to changes on the network – Scalable to large number of nodes
  • 59. B-MAC’s Features • Clear Channel Assessment (CCA) • Low Power Listening (LPL) using preamble sampling • Hidden terminal and multi-packet mechanisms not provided, should be implemented, if needed, by higher layers Sleep t Receive Receiver Sleep t Preamble Sender Message Sleep
  • 60. B-MAC Interface • CCA on/off • Acknowledgements on/off • Initial and congestion backoff in a per packet basis • Configurable check interval and preamble length
  • 61. B-MAC Lifetime Model  E  Erx  Etx  Elisten  Ed  Esleep Erx  trxcrxbV Etx  ttxctxbV Elisten  Esample 1 ti Ed  tdcdataV Esleep  tsleepcsleepV • E can be calculated if hardware constants, sample rate, number of neighboring nodes and check time/preamble are known • Better: E can be minimized by varying check time/preamble if constants, sample rate and neighboring nodes are known
  • 62. B-MAC Results • Performs better than the other studied protocols in most cases • System model can be complicated for application and routing protocol developers • Protocol widely used because has good results even with default parameters
  • 63. P-MAC - Pattern MAC • Patterns are 0*1 strings with size 1-N • Every node starts with 1 as pattern • Number of 0’s grow exponentially up to a threshold  and then linearly up to N-1 • TR = CW + RTS + CTS + DATA + ACK • N = tradeoff between latency and energy
  • 64. Patterns vs Schedules Local Pattern Bit Packet to Send Receiver Pattern Bit Local Schedule 1 1 1 1 1 1 0 1- 1 0 * 1- 0 1 1 1 0 1 0 0 0 0 * 0
  • 65. P-MAC Evaluation • Simulated results are better than SMAC • Good for relatively stable traffic conditions • Adaptation to changes on traffic might be slow • Loose time synchronization required • Needs more testing and comparison with other protocols besides S-MAC
  • 66. Z-MAC - Zebra MAC • Runs on top of B-MAC • Combines TDMA and CSMA features CSMA  Pros  Simple  Scalable  Cons  Collisions due to hidden terminals  RTS/CTS is overhead TDMA  Pros  Naturally avoids collisions  Cons  Complexity of scheduling  Synchronization needed
  • 67. Z-MAC Initialization • Neighborhood discovery through ping messages containing known neighbors • Two-hop neighborhood used as input for a scheduling algorithm (DRAND) • Running time and message complexity of DRAND is O(), where  is the two-hop neighborhood size • The idea is to compensate the initialization energy consumption during the protocol normal operation
  • 68. Z-MAC Time Slot Assignment 2a1  Fi  2a 1 l2a  si ( for: l  0,1,2...)
  • 69. Z-MAC Transmission Control The Transmission Rule: • If owner of slot – Take a random backoff within To – Run CCA and, if channel is clear, transmit • Else – Wait for To – Take a random backoff within [To,Tno] – Run CCA and, if channel is clear, transmit
  • 70. Z-MAC HCL Mode • Nodes can be in “High Contention Level” (HCL) • A node is in HCL only if it recently received an “Explicit Contention Notification” (ECN) from a two- hop neighbor • Nodes in HCL are not allowed to contend for the channel on their two-hop neighbors’ time slots • A node decides to send an ECN if it is losing too many messages (application ACK’s) or based on noise measured through CCA
  • 71. Z-MAC Receiving Schedule • B-MAC based • Time slots should be large enough for contention, CCA and one B-MAC packet transmission • Slot size choice, like in B-MAC, left to application
  • 72. Z-MAC Results • Z-MAC performs better than B-MAC when load is high • As expected, fairness increases with Z-MAC • Complexity of the protocol can be a problem
  • 73. Conclusions • Between the protocols studied, B-MAC still seems to be the best one for applications in general • Application developers seem not to use B- MAC’s control interface • Middleware service could make such optimizations according to network status