Media Access Control in Wireless Sensor Networks - II

1
Media Access Control in Wireless Sensor Networks
- II
2
What We Have Learned Last Time

• B-MAC ?
• CSMA LPL Noise Floor Estimation Explicit
  ACK
• X-MAC ?
• B-MAC Early ACK Encoded preamble

3
Outline

• Overview
• TDMA/CSMA
• Advantages and disadvantages
• S-MAC
• Z-MAC
• Design concepts, performance evaluation and issues

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Classification of Multiple Access Protocols
Multiple Access Protocols
Random Access
Controlled Access
ALOHA
Static channel allocation
CSMA
B-MAC
TDMA (FDMA, CDMA)
X-MAC
X-MAC
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CSMA

• CSMA listen before transmit
• If channel sensed idle transmit entire pkt
• If channel sensed busy, defer transmission
• Persistent CSMA retry immediately with
  probability p when channel becomes idle (may
  cause instability)
• Non-persistent CSMA retry after random interval
• human analogy dont interrupt others!

Me too
I want to talk now
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TDMA

• TDMA Time Division Multiple Access
• Access to channel in "rounds"
• Each station gets a fixed length slot (length
  pkt Tx time) in each round
• Unused slots go idle
• Example 6-station LAN, 1,3,4 have pkts, slots
  2,5,6 idle

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Medium Access Paradigms

• Contention Based (CSMA)
• Random-back off and carrier-sensing
• Simple, no time synch, and robust to network
  changes
• High idle listening and overhearing overheads
• Solve this by duty cycling
• TDMA Based (or Schedule based)
• Nodes within interference range transmit during
  different times, so collision free
• Requires time synch and not robust to changes.
• Low throughput and high latency even during low
  contention.
• Low idle listening and overhearing overheads
• Wake up and listen only during its neighbor
  transmission

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TDMA vs. CSMA for Sensor Networks
Parameter TDMA CSMA
Energy for Synchronization Bad Good
Throughput Good for multiple sources Good for single source
complexity Bad Good
Fairness Good Bad
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TDMA vs. CSMA for Sensor Networks cont
Parameter TDMA CSMA
Scalability Bad Good
Latency Bad/Good Good/Bad
Dealing with node failures, new node arrivals Bad Good
Energy for collision Avoidance Good Bad
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How about combine TDMA and CSMA?

• S-MAC for the benefit of Energy Efficiency
• Z-MAC for the benefit of throughput

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Z-MAC a Hybrid MAC for Wireless Sensor
Networks-Injong Rhee, Ajit Warrier, Mahesh Aia
and Jeongki Min
Medium Access Control with Coordinated Adaptive
Sleeping for Wireless Sensor NetworksWei Ye,
John Heidemann and Deborah Estrin
TON 2004
Sensys 2005
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S-MAC Introduction

• S-MAC stands for Sensor-MAC
• Key Idea in SMAC
• Combine key advantages of scheduled (TDMA) and
  unscheduled (CSMA) protocols

S-MAC lite-802.11 Scheduling
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S-MAC Energy savings

• S-MAC tries to reduce wastage of energy from at
  least 3 sources of energy inefficiency
• Nodes periodically sleep to reduce energy
  consumption in listening to an idle channel
• Resolve contention by using RTS and CTS
• Avoid overhearing S-MAC sets the radio to sleep
  during transmissions of other nodes

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Periodic sleeping
listen
listen
sleep
sleep

• Problem Idle listening consumes significant
  energy
• Solution Periodic listen and sleep
• Turn off radio when sleeping
• Reduce duty cycle to 10 (120ms on/1.2s off)

Difference between S-MAC toggling and B-MAC
toggling?
15

• Deal with global synchronization

Schedules can differ, prefer neighbouring nodes
to have same schedule
Border nodes may have to maintain more than one
schedule.
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Overhearing Avoidance

• Problem
• Receive packets destined to others
• Solution Sleep when neighbors talk
• 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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SMAC Pros and Cons

• Pros
• Well-designed, complete protocol that addresses
  deficiencies of 802.11 if applied to a sensor
  network.
• Schedules sleep and transmit times to enable
  low-power data transfer with reasonable-latency.
• Cons
• SMAC incurs some drawbacks of TDMA schemes
• Topology maintenance, need for synchronization,
  additional complexity at border nodes between two
  schedules
• Monolithic system architecture similar to 802.11
• Combines carrier sense, link-layer reliability,
  RTS/CTS and sleep scheduling into MAC layer.

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Z-MAC Motivation Throughput
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Z-MAC Hybrid Contention Resolution

• Z-MAC (Zebra MAC) a Hybrid MAC protocol
  combines the strengths of both CSMA and TDMA at
  the same time discounting their weaknesses
• Z-MAC uses a base TDMA schedule as a hint to
  schedule the transmissions of the nodes, and it
  differs from TDMA by allowing non-owners of slots
  to 'steal' the slot from owners if they are not
  transmitting

Channel Utilization
MAC
Low Contention
High Contention
CSMA
High
Low
TDMA
Low
High
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Z-MAC Features

• Adaptability to the level of contention in the
  network
• Under low contention behaves like CSMA
• Under high contention behaves like TDMA

CSMA
Channel Utilization
TDMA
of Contenders
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Z-MAC Design

• Z-MAC has the setup phase in which the following
  operations are run in sequence
• Neighbor discovery
• Time slot assignment (DRAND)
• Local frame exchange
• Time synchronization

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Neighbor discovery

• When a node starts up, it runs a neighbor
  discovery protocol
• Periodically broadcasts a ping to its one-hop
  neighbors
• Ping message contains the current list of its
  one-hop neighbors
• Through this message, each node gathers neighbor
  information

Q How many hops neighbor information is need to
avoid interference? 1 Hop, 2 Hop, More than 2
Hop? Whats the reality?
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Timeslot Assignment

• The two-hop neighbor list is used as an input to
  a time-slot assignment algorithm
• Current implementation of Z-MAC uses DRAND a
  distributed implementation of RAND to assign time
  slots to every node in the network
• DRAND ensures no two nodes within a two-hop
  communication neighborhood are assigned to the
  same slot.
• This assignment guarantees that no transmission
  by a node to any of its one-hop neighbors
  interferes with any transmission by its two-hop
  neighbors.

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DRAND slot assignment
Radio Interference Map
1
0
3
2
DRAND slot assignment
0
1
Input Graph
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Z-MAC Transmission Control

• A node can be in one of two modes
• Low Contention Level (LCL) or
• High Contention Level (HCL)
• Node is in HCL only
• when it receives an explicit contention
  notification (ECN) message from a two-hop
  neighbor within the last tECN period.
• Otherwise, the node is in LCL.
• A node sends an ECN when it experiences high
  contention

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Z-MAC Transmission Control cont

• In LCL, any node can compete to transmit in any
  slot
• But in HCL, only the owners of the current slot
  and their one-hop neighbors are allowed to
  compete for the channel
• In both modes, owners have higher priority over
  non-owners.
• If a slot does not contain an owner or its owner
  does not have data to send, non-owners can steal
  the slot.
• This feature achieves high channel utilization
  even under low contention as a node can transmit
  as soon as the channel is available.
• Z-MAC implements LCL and HCL using the back off,
  CCA and LPL interfaces of B-MAC

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Transmission rule (for the owner)

• Owner takes a random back off within a fixed
  time period To
• When the back off timer expires, it runs CCA and
  if the channel is clear, transmits the data.
• If the channel is not clear, then it waits until
  the channel is not busy and repeats the above
  process.

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Transmission rule (non owner - LCL)

• Waits for To and then performs a random back off
  within a contention window To, Tno
• When the back off timer expires, it runs CCA and
  if the channel is clear, then it starts
  transmission.
• If the channel is not clear, it waits until the
  channel is clear, and repeats the above process.

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Z-MAC Transmission Control (Continued)
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Z-MAC Performance Evaluation

• Setup
• Single-hop, Two-hop and Multi-hop topology
  experiments on Mica2 motes.
• Comparisons with B-MAC (default MAC of Mica2),
  with different back-off window sizes
• Metrics Throughput energy efficiency

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Experimental Setup Single Hop

• Single-Hop Experiments
• Mica2 motes equidistant from one node in the
  middle.
• All nodes within one-hop transmission range.
• Tests repeated 10 times and average/standard
  deviation errors reported.

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Z-MAC Two-Hop Experiments
Sink
Sources
Sources

• Setup Two-Hop
• Dumbbell shaped topology
• Transmission power varied between low (50) and
  high (150) to get two-hop situations.
• Aim See how Z-MAC works when Hidden Terminal
  Problem manifests itself

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Experimental Setup - Test bed

• 42 Mica2 sensor motes in a Lab.
• Wall-powered and connected to the Internet via
  Ethernet ports.
• Programs uploaded via the Internet, all mote
  interaction via wireless.
• Links vary in quality, some have loss rates up to
  30-40.

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Multi Hop Results Throughput

• Why B-MAC is better than Z-MAC in low traffic ?
• Why B-MAC is worse than Z-MAC in high traffic ?

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Multi Hop Results Energy Efficiency
B-MAC
Z-MAC HCL
MULTI-HOP
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Summary of Z-MAC

• Hybrid MAC Combines strengths of TDMA and CSMA
• Uses the TDMA schedule created by DRAND as a
  'hint' to schedule transmissions
• The owner of a time-slot always has priority
  over the non-owners while accessing the medium.
• Unlike TDMA, non-owners can 'steal' the
  time-slot when the owners do not have data to
  send.
• This enables Z-MAC to switch between CSMA and
  TDMA depending on the level of contention.
• Hence, under low contention, Z-MAC acts like CSMA
  (i.e. high channel utilization and low latency),
  while under high contention, Z-MAC acts like TDMA
  (i.e. high channel utilization, fairness and low
  contention overhead).

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Discussion

• Limitation of Z-MAC

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Take Away Messages

• Asynchronous MAC
• B-MAC CSMA LPL Noise Floor Estimation
  Explicit ACK
• X-MAC B-MAC Early ACK Encoded preamble

• Hybrid Synchronous MAC
• S-MAC lite-802.11 Scheduling
• Z-MAC CSMA within TDMA slots

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MAC Summary

• Commercial MAC (802.X, Bluetooth) are suitable
  for wireless LAN with much more powerful devices.
  Energy is secondary concern compared with
  throughput.
• Asynchronous MAC (B/X-MAC) is flexible and works
  well in low traffic scenario (why is so widely
  used!)
• Hybrid synchronous MAC (S/Z-MAC) can achieve
  better performance in high traffic scenario.