Radio%20and%20Medium%20Access%20Control

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Radio and Medium Access Control
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Learning Objectives

• Understand important concepts about radio signals
• Understand radio properties of WSNs
• Understand schedule-based medium access protocols
  in WSNs
• Understand contention-based medium access
  protocols in WSNs
• Understand S-MAC, B-MAC, and X-MAC

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Prerequisites

• Module 2
• Basic concepts of wireless communications
• Basic concepts of computer networks

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Radio Properties
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Some Basic Concepts

• RSSI
• dbm
• Noise floor see wikipedia
• CCA thresholding algorithms
• Duty cycle
• LPL

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Signal Transmission
Ref Fig. 2.9 of Wireless Communications and
Networks by William Stallings
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Packet Reception and Transmission

• Ref Hardware_1 Figure 5

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Signal

• An electromagnetic signal
• A function of time
• Also a function of frequency
• The signal consists of components of different
  frequencies

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802.15.4 Physical Layer
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dB

• dB (Decibel)
• Express relative differences in signal strength
• dB 10 log10 (p1/p2)
• dB 0 no attenuation. p1 p2
• 1 dB attenuation 0.79 of the input power
  survives 10 log10(1/0.79)
• 3 dB attenuation 0.5 of the input power
  survives 10 log10(1/0.5)
• 10 dB attenuation 0.1 of the input power
  survives 10 log10(1/0.1)

• http//en.wikipedia.org/wiki/Decibel
• http//www.sss-mag.com/db.html

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dBm

• The referenced quantity is one milliwatt(mW)
• dBm 10 log10 (p1/1mW)
• 0 dBm p1 is 1 mW
• -80 dBm p1 is 10-11W 10pW

• http//en.wikipedia.org/wiki/DBm

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Received Signal Strength Indicator (RSSI)

• The strength of a received RF signal
• Many current platforms provide hardware indicator
• CC2420, the radio chip of MicaZ and TelosB,
  provides RSSI indicator and LQI (Link Quality
  Indicator)

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LQI (Link Quality Indicator)

• A measure of chip error rate
• Error rate
• The rate at which errors occur
• Error
• 0 is transmitted while 1 is received
• 1 is transmitted while 0 is received

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Noise Floor

• The measure of the signal created from the sum of
  all the noise sources and unwanted signals

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Signal Noise Ratio (SNR)

• The ratio of the power in a signal to the power
  contained in the noise that is present
• Typically measured at the receiver
• Take CC2420 as the example
• Noise Floor the RSSI register from the CC2420
  chip when not receiving a packet
• For example -98dBm
• The strength field from the received packet RSSI
  of the received packet

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Radio Spectrum Frequency Allocation

• http//www.ntia.doc.gov/osmhome/allochrt.pdf

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Radio Irregularity

• Spherical radio range is not valid
• When an electromagnetic signal propagate, the
  signal may be
• Diffracted
• Reflected
• Scattered
• Radio irregularity and variations in packet loss
  in different directions

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Radio Signal Property

• Anisotropic Signal Strength Different path
  losses in different directions

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Radio Signal Property

• Anisotropic Packet Loss Ratio Packet Reception
  Ratio (PRR) varies in different directions

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Radio Signal Property

• Anisotropic Radio Range The communication range
  of a mote is not uniform

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Medium Access Control (MAC)
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Introduction

• A radio channel cannot be accessed simultaneously
  by two or more nodes that are in a radio
  interference range
• Nodes may transmit at the same time on the same
  channel
• Medium Access Control
• On top of Physical layer
• Control access to the radio channel

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MAC Protocol Requirements

• Energy Efficiency
• Sources of energy waste
• Collision, Idle Listening, Overhearing, and
  Control Packet Overhead
• Effective collision avoidance
• When and how the node can access the medium and
  send its data
• Efficient channel utilization at low and high
  data rates
• Reflects how well the entire bandwidth of the
  channel is utilized in communications
• Tolerant to changing RF/Networking conditions
• Scalable to large number of nodes

Ref MAC_2 Section I, II
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Two Basic Classes of MAC Protocol Slotted and
Sampling

• Slotted Protocols (Synchronous Protocols)
• Nodes divides time into slots
• Radio can be in receive mode, transmit mode, or
  powered off mode
• Communication is synchronized
• Data transfers occur in slots
• TDMA, IEEE 802.15.4, S-MAC, T-MAC, etc.
• Also Ref J. Polastre Dissertation Section 2.4
  http//www.polastre.com/papers/polastre-thesis-fin
  al.pdf

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Two Basic Classes of MAC Protocol Slotted and
Sampling

• Sampling Protocols
• Nodes periodically wake up, and only start
  receiving data if they detect channel activity
• Communication is unsynchronized
• Data transfer wakes up receiver
• Must send long, expensive messages to wake up
  neighbors
• B-MAC, Preamble sampling, LPL, etc.

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Slotted Protocol Example 802.15.4

• Each node beacons on its own schedule
• Other nodes synchronize with the received Beacons

CSMA Contention Period
Beacon
Ack
Beacon
Data
Data
sleep
Superframe Duration
Beacon Frame Duration
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IEEE 802.15.4 Superframe
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Using 802.15.4
send done
SP
wake forbeacon period
beaconTX
packet received
send
Coordinator
15.4
stop radio superframe complete
start radio send beacon
Beacon
Data
Data
Ack
RF Channel
If yes,wake up
beacon RX
TX first packet
Ack received
15.4
Neighbors
are messagespending?
send donereliability set
Update schedule
TXdone
Stopradio
packet RX
SP
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Main MAC Protocols
Wireless medium access
Centralized
Distributed
Contention-based
Schedule-based
Contention-based
Schedule-based
Fixedassignment
Demandassignment
Fixedassignment
Demandassignment
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Scheduled Protocols

• TDMA divides the channel into N time slots

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Contention-based Protocols

• A common channel is shared by all nodes and it is
  allocated on-demand
• A contention mechanism is employed
• Advantages over scheduled protocols
• Scale more easily
• More flexible as topologies change
• No requirement to form communication clusters
• Do not require fine-grained time synchronization
• Disadvantage
• Inefficient usage of energy
• Node listen at all times
• Collisions and contention for the media

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CSMA

• Listening before transmitting
• Listening (Carrier Sense)
• To detect if the medium is busy
• Hidden Terminal Problem

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Hidden Terminal Problem

• Node A and C cannot hear each other
• Transmission by node A and C can collide at node
  B
• On collision, both transmissions are lost
• Node A and C are hidden from each other

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CSMA-CA

• CA
• Collision Avoidance to address the hidden
  terminal problem
• Basic mechanism
• Establish a brief handshake between a sender and
  a receiver before transmission
• The transmission between a sender and a receiver
  follows RTS-CTS-DATA-ACK

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Centralized Medium Access

• Idea Have a central station control when a node
  may access the medium
• Example Polling, centralized computation of TDMA
  schedules
• Advantage Simple, quite efficient (e.g., no
  collisions), burdens the central station
• Not directly feasible for non-trivial wireless
  network sizes
• But Can be quite useful when network is somehow
  divided into smaller groups
• Clusters, in each cluster medium access can be
  controlled centrally compare Bluetooth
  piconets, for example
• ! Usually, distributed medium access is
  considered

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Schedule- vs. Contention-based MACs

• Schedule-based MAC
• A schedule exists, regulating which participant
  may use which resource at which time (TDMA
  component)
• Typical resource frequency band in a given
  physical space (with a given code, CDMA)
• Schedule can be fixed or computed on demand
• Usually mixed difference fixed/on demand is
  one of time scales
• Usually, collisions, overhearing, idle listening
  no issues
• Needed time synchronization!

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Schedule- vs. Contention-based MACs

• Contention-based protocols
• Risk of colliding packets is deliberately taken
• Hope coordination overhead can be saved,
  resulting in overall improved efficiency
• Mechanisms to handle/reduce probability/impact of
  collisions required
• Usually, randomization used somehow

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Possible Solutions

• CSMA (Carrier Sense Multiple Access)
• Advantage
• No clock synchronization required
• No global topology information required
• Disadvantage
• Hidden terminal problem serious throughput
  degradation
• RTS/CTS can alleviate hidden terminal problem,
  but incur high overhead

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Possible Solutions

• TDMA (Time-division multiple access)
• Advantage
• Solve the hidden terminal problem without extra
  message overhead
• Disadvantage
• It is challenging to find an efficient time
  schedule
• Need clock synchronization
• High energy overhead
• Handling dynamic topology change is expensive
• Given low contention, TDMA gives much lower
  channel utilization and higher delay

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Effective Throughput CSMA vs. TDMA
CSMA
Sensitive to Time synch. errors, Topology
changes, Slot assignment errors.
Channel Utilization (The fraction of time that
the channel is transmitting data)
TDMA
Do not use any topology or time synch.
Info. Thus, more robust to time synch. errors and
changes.
of Contenders
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MAC Energy Usage

• Four important sources of wasted energy in WSN
• Idle Listening (required for all CSMA protocols)
• Overhearing (since RF is a broadcast medium)
• Collisions (Hidden Terminal Problem)
• Control Overhead (e.g. RTS/CTS or DATA/ACK)

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S-MAC

• During sleep, the node turns off its radio, and
  sets a timer to awake itself

S-MAC Figure 2
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S-MAC

• Requires periodic synchronization among
  neighboring node
• Negotiate a schedule
• Prefer that neighboring nodes listen at the same
  time and go to sleep at the same time
• Use SYNC message

S-MAC
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S-MAC

• All senders perform CS (Carrier Sense) before
  initiating a transmission
• Broadcast packets are sent without using RTS/CTS
• Unicast packet follow RTS/CTS/DATA/ACK
• To avoid collision

S-MAC
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S-MAC Overhearing Avoidance

• To avoid overhearing let interfering nodes go to
  sleep after they hear an RTS or CTS packet

S-MAC
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S-MAC Adaptive Listening

• To improve latency caused by the periodic sleep
  of each node in the multi-hop network
• Let each node who overhears its neighbors
  transmissions (RTS and CTS) wake up a short
  period of time at the end of transmission

S-MAC
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S-MAC with Adaptive Listening
Ref Figure 1 of DW-MAC
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B-MAC

• A set of primitives that other protocols may use
  as building block
• Provide basic CSMA access
• Optional link level ACK, no link level RTS/CTS
• CSMA backoffs configurable by higher layers
• Carrier Sensing using Clear Channel Assess (CCA)
• Sleep/Wake scheduling using Low Power Listening
  (LPL)

• Ref Section 1, 3 of ref. MAC_1
• LPL See Section 2.1 of ref. Energy_1

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B-MAC

• Does not solve hidden terminal problem
• Duty cycles the radio through periodic channel
  sampling Low Power Listening (LPL)

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B-MAC Clear Channel Assessment

• Before transmission take a sample of the
  channel
• If the sample is below the current noise floor,
  channel is clear, send immediately.
• If five samples are taken, and no outlier found
  gt channel busy, take a random backoff
• Noise floor updated when channel is known to be
  clear e.g. just after packet transmission

• Ref Section 1, 3 of ref. MAC_1

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A Trace of Power Consumption
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B-MAC Low Power Listening

• Similar to ALOHA preamble sampling
• Wake up every Check-Interval
• Sample Channel using CCA
• If no activity, go back to sleep for
  Check-Interval
• Else start receiving packet
• Preamble gt Check-Interval

Goal minimize idle listening
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Low Power Listening

• Purpose
• Energy cost RX TX Listen
• Save energy
• How
• Duty cycle the radio while ensuring reliable
  message delivery
• Periodically wake up, sample channel, and sleep
• The duty cycling receiver node performs short and
  periodic receive checks
• If the channel is checked every 100ms
• 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.

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X-MAC

• Asynchronous duty-cycled MAC protocol
• Provide the following advantages over B-MAC
• Avoid overhearing problem embedding the Target
  ID in the Preamble
• Reduce excessive preamble strobed preamble

• Ref Section 1, 3 of ref. MAC_4

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X-MAC and B-MAC

• Ref Section 1, 3 of ref. MAC_4

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802.15.4 Frame Format

• Page 36 of CC2420 Data Sheet

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TinyOS Implementation of CSMA o CC2420 - CCA

• Hardware
• CC2420 has CCA as a pin that can be sampled to
  determine if another node is transmitting
• See CC2420 Data Sheet Figure 1 CC2420 Pinout
• Software
• CC2420Transmit has the option to send the message
  with or without CCA
• See CC2420TransmitP.send()

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TinyOS Implementation of CSMA of CC2420 - Ack

• Hardware Ack
• If MDMCTRL0.AUTOACK of CC2420 is enabled
• Software Ack
• SACK strobe in CC2420ReceiveP can be used to set
  software ack

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Lab

• Please add the Low Power Listening feature to the
  PingPong application. That is, the packet from A
  to B and from B to A should be received using
  LPL.
• Please follow TEP 126 - CC2420AckLplP /
  CC2420NoAckLplP

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Assignment

• 1. In many research papers about wireless sensor
  networks, spherical radio range is assumed. Is
  this true or not? Please brief explain.
• 2. What is the main idea of Low Power Listening
  (LPL)? Why do we need LPL?
• 3. What is the purpose of Clear Channel
  Assessment?
• 4. What are the main advantages of X-MAC over
  B-MAC?

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Project

• This is a group project. Each group can have up
  to 3 students
• Project Description
• In this project, you will develop a multi-hop
  data collection tree protocol based on TinyOS
  2.x.
• 1. Development of a multi-hop data collection
  tree protocol
• 1.1 The protocol to form a multi-hop data
  collection tree
• The base station locally broadcast a tree
  construction message, which includes its own ID
  and its depth to be 0
• a. When a node, say A, receives a tree
  construction message from node B at its first
  time (i.e., node A has not joined the data
  collection tree yet), node A assigns its depth
  to be the depth of node B plus one, and its
  parent to be node B. After this, node A
  rebroadcasts the tree construction message.

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Project - continue

• b. When a node, say A, already joins the data
  collection tree and receives a tree construction
  message from node B, node A just simply
  disregards the tree construction message.
• c. When a node, say A, already joins the data
  collection tree (suppose that node As parent is
  node B and node As depth is n) and receives a
  tree construction message from node C
• 1. suppose that if node A selects node C as its
  parent, node As depth is m
• 2. suppose m lt n
• node A will change its parent to node C.

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Project - continue
one example multi-hop data collection tree
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Project - continue

• Also see attached slide tree.ppt for a dynamic
  view about how to construct a multi-hop data
  collection tree.
• 1.2 After the multi-hop data collection tree is
  formed, each node senses and transmits its light
  intensity to the base station every one second.
  For each received message, the base station
  displays the following information
• Node ID which originates the message
• Tree depth of the Node
• Sensed light value

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Project - continue

• Basic Steps
• Please follow the steps listed below
• 1. Setting up TinyOS environment - XubunTOS
• XubunTOS can be downloaded here
• http//toilers.mines.edu/Public/XubunTOS
• 2. Go through the TinyOS tutorials at
  http//docs.tinyos.net/index.php/TinyOS_Tutorials