Lecture 2 - MAC
TDDI07 Distributed Embedded Software and Networks
Mikael Asplund
Department of Computer and Information Science
Linköping University
Wireless signal strength
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Brighter color = stronger signal
© Jochen Schiller, FU Berlin
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2011-02-17
Baspresentation LiU
Today's lecture
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Recap of MAC protocol basics
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Energy-efficient MAC protocols
2011-02-17
Baspresentation LiU
What is Medium Access Control (MAC)?
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Performance metrics
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Performance metrics
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Throughput
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Energy efficiency
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Latency
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Fairness
Channel allocation
• TDMA – Time Division Multiple Access
• FDMA – Frequency Division Multiple Access
– OFDMA Orthogonal frequencies
• CDMA – Code Division Multiple Access
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MAC Taxonomy
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Basic wireless protocols
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ALOHA (Pure and slotted)
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Busy tone
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CSMA
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CSMA/CA, (CSMA/CD)
CSMA (1-persistent)
Create frame
No
Idle
channel?
Wait random time
Yes
Transmit
Got ack?
Yes
End
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No
CSMA/CA (nonpersistent)
Create frame
Idle
channel?
No
Yes
Transmit
Got ack?
Yes
End
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No
Wait random time
CSMA/CA (p-persistent)
Create frame
Idle
channel?
No
Yes
No
rand() < p?
Yes
Transmit
Got ack?
Yes
End
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No
Wait for next slot
Throughput, CSMA and ALOHA
[Tanenbaum]
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Basic wireless protocols
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ALOHA (Pure and slotted)
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Busy tone
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CSMA
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CSMA/CA, (CSMA/CD)
Problems
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Hidden terminal
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Exposed terminal
RTS/CTS
A
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B
C
D
RTS/CTS
A
B
RTS
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C
D
RTS/CTS
A
B
RTS
NAV
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C
D
RTS/CTS
A
B
C
RTS
CTS
NAV
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D
RTS/CTS
A
B
C
D
RTS
CTS
NAV
NAV
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RTS/CTS
A
B
C
D
RTS
CTS
NAV
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Data
NAV
RTS/CTS
A
B
C
D
RTS
CTS
NAV
Data
NAV
ACK
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RTS/CTS Problems
A
B
C
D
Data
B
C
D
RTS
RTS
CTS
A
RTS
RTS
CTS
CTS
Data
RTS
CTS
Data
Ack
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802.11
[Anastasi, Borgia, Conti, Gregori, 2005]
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802.11 Measured throughput
[Anastasi, Borgia, Conti, Gregori, 2005]
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Energy-Efficient MAC Protocols
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Transmissions are costly
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Receiving about as expensive as transmitting
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Idling can be cheaper but is still expensive
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Sleeping is cheapest, but does not allow communication
Energy Problems for MAC Protocols
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Energy Problems for MAC Protocols
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Collisions – wasted effort when two packets collide
Overhearing – waste effort in receiving a packet destined for
another node
Idle listening – sitting idly and trying to receive when nobody is
sending
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Protocol overhead - control packets, additional encoding bits,…
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Overemitting – Sending when listener is not ready
WiFi Energy consumption
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Duty cycling
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Wake up only a small percentage of the time
Two variants
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Synchronous
Centralised
– Decentralised
Asynchronous
–
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–
–
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Sender-initiated
Receiver-initiated
Example: TRAMA
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Traffic-adaptive medium access (TRAMA)
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Assumes synchronized nodes
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Time divided into cycles, divided into
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●
●
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Random access periods
Scheduled access periods
Three components:
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Neighbour Protocol (NP)
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Schedule Exchange Protocol (SEP)
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Adaptive Election Algorithm (AEA)
TRAMA - NP and SEP
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Neighbour Protocol (NP)
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●
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Learning about their two-hop neighbourhood
During random access period, send small, incremental neighbourhood
update information in randomly selected time slots
Schedule Exchange Protocol (SEP)
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Traffic-based transmission schedule for each node
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Give up slots if not needed
TRAMA – Adaptive Election
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Use node identifier x and globally known hash function h
For time slot t, compute priority p = h (x  t)
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Compute p for next k time slots for all two-hop neighbours
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Node uses those time slots for which it has the highest priority
Example:
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t = 0 t =1
t = 2 t=3
t = 4 t =5
A
14
23
9
56
3
26
B
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64
8
12
44
6
C
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18
6
33
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2
Example: Sensor-MAC (S-MAC)
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Idea: Switch nodes off, ensure that neighboring nodes turn on
simultaneously to allow packet exchange (rendez-vous)
Active period
Wakeup period
Sleep period
SYNCH
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RTS/CTS/DATA/ACK
S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
Time
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S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
A
A
A
A
Time
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S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
A
A
A
A
B
Time
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S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
A
A
A
A
B
B
B
B
Time
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S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
A
A
A
A
B
B
B
B
C
Time
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S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
C
A
A
A
A
B
B
B
B
C
C
C
Time
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S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
C
A
A
A
A
B
B
B
B
C
D
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C
C
Time
S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
C
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A
A
A
A
B
B
B
B
C
C
C
D
D
D
Time
S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
A
A
A
A
B
B
B
B
E
C
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C
C
C
D
D
D
Time
S-MAC Synchronized Islands
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Follow neighbour schedule (if available)
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If multiple schedules, follow all
A
A
A
A
A
B
B
B
B
E
C
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E
E
E
E
C
C
C
D
D
D
E
Time
Asynchronous sender-initiated
duty cycle
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Synchronizing is complex
Alternative: long preambles
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Examples: WiseMAC, B-MAC, LPL
Start transmission:
Check
channel
Long preamble Actual packet
Check
channel
Stay awake!
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Check
channel
Check
channel
Receiver-initiated duty-cycle
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●
Receivers send when ready to receive
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Hidden terminal problem reduced
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Allows extremely long duty cycles (for the receiver)
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Supports broadcast
IEEE 802.15.4
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IEEE standard for low-rate WPAN applications
Goals:
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low-to-medium bit rates,
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moderate delays without too stringent guarantee
requirements,
low energy consumption
802.15.4 Architecture
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IEEE 802.15.4
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Physical layer
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20 kbps over 1 channel @ 868-868.6 MHz
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40 kbps over 10 channels @ 905 – 928 MHz
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250 kbps over 16 channels @ 2.4 GHz
MAC protocol
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Single channel at any one time
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Combines contention-based and schedule-based schemes
Asymmetric: nodes can assume different roles: device,
coordinator, PAN coordinator (personal area network
coordinator)
IEEE 820.15.4 Topologies
Coordinator
Star
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Coordinator
Peer-to-peer
802.15.4 Superframe
Active period
Beacon
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Contention
access
period
Inactive period
Guaranteed time
slots (GTS)
802.15.4 Data Transfer
Coordinator
Device
Beacon
Data
request
Acknowledgement
Data
Acknowledgement
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Readings for lecture 2
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Carrano, R.; Passos, D.; Magalhaes, L.; Albuquerque, C., "Survey
and Taxonomy of Duty Cycling Mechanisms in Wireless Sensor
Networks," Communications Surveys & Tutorials, IEEE doi:
10.1109/SURV.2013.052213.00116
Section 6.1 - Zigbee (802.15.4), overall architecture is more relevant
than the particular detailed frame structure
www.liu.se
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