# Radio and Medium Access Control

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```					Radio and Medium Access
Control

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Learning Objectives
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

3

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Some Basic Concepts
•   dbm
•   Noise floor: see wikipedia
•   CCA thresholding algorithms
•   Duty cycle
•   LPL

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Signal Transmission

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Ref: Fig. 2.9 of “Wireless Communications and Networks” by William Stallings
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
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• http://www.sss-mag.com/db.html
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              11
• The strength of a received RF signal
• Many current platforms provide hardware
indicator
– CC2420, the radio chip of MicaZ and TelosB,
Indicator)

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• 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

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

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• 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
• Anisotropic Signal Strength: Different path
losses in different directions

Figure 1: Signal Strength over Time in Four Directions

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

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

Medium Access Control (MAC)

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Introduction
• A radio channel cannot be accessed
simultaneously by two or more nodes that are
– Nodes may transmit at the same time on the same
channel
• Medium Access Control
– On top of Physical layer

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MAC Protocol Requirements
• Energy Efficiency
– Sources of energy waste
• Collision, Idle Listening, Overhearing, and Control Packet
• 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
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Ref: [MAC_2] Section I, II
Two Basic Classes of MAC Protocol –
Slotted and Sampling
• Slotted Protocols (Synchronous Protocols)
– Nodes divides time into slots
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-
final.pdf
[MAC_3]: Section 4                     24
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

Beacon
Data

Data
Ack
sleep
Superframe Duration

Beacon Frame Duration

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IEEE 802.15.4 Superframe

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Using 802.15.4

Coordinator
wake for beacon                       send                     packet                    SP
send
15.4
send beacon                                                          superframe complete
Beacon

Data

Data

Ack
RF Channel

If yes,            beacon          TX first                 Ack

Neighbors
wake up              RX              packet                   received                  15.4

are messages                Update                      TX          send done          Stop
pending?               schedule                  done          reliability set   radio

packet                                                 SP
RX                                                                 28
Main MAC Protocols
Wireless medium access

Centralized
Distributed

Schedule-           Contention-
based                based             Schedule-           Contention-
based                based
Fixed      Demand
assignment   assignment                    Fixed      Demand
assignment   assignment

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Scheduled Protocols
• TDMA divides the channel into N time slots

[MAC_2]: Figure 1           30
Contention-based Protocols
• A common channel is shared by all nodes and it is
allocated on-demand
• A contention mechanism is employed
–   Scale more easily
–   More flexible as topologies change
–   No requirement to form communication clusters
–   Do not require fine-grained time synchronization
– Inefficient usage of energy
• Node listen at all times
• Collisions and contention for the media
[MAC_2]: Section IV         31
CSMA
• Listening before transmitting
• Listening (Carrier Sense)
– To detect if the medium is busy
• Hidden Terminal Problem

[MAC_2]: Section IV   32
Hidden Terminal Problem
• Node A and C cannot
hear each other
• Transmission by node A
and C can collide at
node B                                  A   B   C

• On collision, both
transmissions are lost
• Node A and C are
hidden from each other

[MAC_2]: Section IV               33
CSMA-CA
• CA
– Collision Avoidance: to address the hidden
terminal problem
• Basic mechanism
– Establish a brief handshake between a sender and a
– The transmission between a sender and a receiver
follows RTS-CTS-DATA-ACK

[MAC_2]: Section IV            34
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

37
Possible Solutions
• CSMA (Carrier Sense Multiple Access)
• No clock synchronization required
• No global topology information required
• Hidden terminal problem: serious throughput
• RTS/CTS can alleviate hidden terminal problem, but

38
Possible Solutions
• TDMA (Time-division multiple access)
• Solve the hidden terminal problem without extra
• It is challenging to find an efficient time schedule
• Need clock synchronization
• 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

IDEAL

CSMA
Sensitive to
Channel          Do not use               TDMA   Time synch.
Utilization      any topology
or time
errors,

(The fraction    synch. Info.
Topology
changes,
of time that     Thus, more
Slot
the channel is
robust to
assignment
time synch.
errors.
transmitting     errors and
changes.
data)

# of Contenders                  40
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)

[MAC_2]: Section II.B          41
S-MAC

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

[S-MAC]: Figure 2
42
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]
44
S-MAC – Overhearing Avoidance
• To avoid overhearing: let interfering
nodes go to sleep after they hear an RTS
or CTS packet

[S-MAC]
45
• To improve latency caused by the periodic
sleep of each node in the multi-hop network
• Let each node who overhears its neighbor’s
transmissions (RTS and CTS) wake up a short
period of time at the end of transmission

[S-MAC]
46

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
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]   48
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 => channel
busy, take a random backoff
- Noise floor updated when
A packet arrives between 22 and        channel is known to be clear
54ms.                                  e.g. just after packet
The middle graph shows the output
of a thresholding CCA algorithm.
transmission
( 1: channel clear, 0: channel busy)
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• Ref: Section 1, 3 of ref. [MAC_1]
A Trace of Power Consumption

[MAC_1]: Figure 3    51
B-MAC Low Power Listening
Check      Carrier sense
Interval

Sender                         Long Preamble       Data Tx

 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 > Check-Interval
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Goal: minimize idle listening
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
• 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]                54
X-MAC and B-MAC

• Ref:
Section 1,
3 of ref.
[MAC_4]

55
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();

http://www.mail-
archive.com/tinyos-
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

https://www.millennium.berkeley.e
du/pipermail/tinyos-help/2008-
Lab
the PingPong application. That is, the packet
from A to B and from B to A should be

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 briefly 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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