02 2 networking aspects
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[SelfOrg] 2-2.1
Self-Organization in AutonomousSensor/Actuator Networks
[SelfOrg]
Dr.-Ing. Falko Dressler
Computer Networks and Communication Systems
Department of Computer Sciences
University of Erlangen-Nürnberg
http://www7.informatik.uni-erlangen.de/~dressler/
dressler@informatik.uni-erlangen.de
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[SelfOrg] 2-2.2
Overview
Self-Organization
Introduction; system management and control; principles andcharacteristics; natural self-organization; methods and techniques
Networking Aspects: Ad Hoc and Sensor Networks Ad hoc and sensor networks; self-organization in sensor networks;evaluation criteria; medium access control; ad hoc routing; data-centricnetworking; clustering
Coordination and Control: Sensor and Actor NetworksSensor and actor networks; coordination and synchronization; in-network operation and control; task and resource allocation
Bio-inspired Networking
Swarm intelligence; artificial immune system; cellular signalingpathways
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[SelfOrg] 2-2.3
M AC Protocols for Ad Hoc and Sensor Networks
Principles and Classification
M AC A / M AC AW
S-M AC
Power Control M AC
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[SelfOrg] 2-2.4
Principal Options and Difficulties
Medium access in wireless networks is difficult mainly because of
Impossible (or very difficult) to send and to receive at the same time
Interference situation at receiver is what counts for transmission success,
but can be very different to what sender can observe
High error rates (for signaling packets) compound the issues
Requirements
As usual: high throughput, low overhead, low error rates, «
Additionally: energy-efficient, handle switched off devices!
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[SelfOrg] 2-2.5
Requirements for Energy-efficient M AC Protocols
Recall
Transmissions are costly
Receiving about as expensive as transmitting
Idling can be cheaper but is still expensive
Energy problems
Collisions ± wasted effort when two packets collide
Overhear ing ± waste effort in receiving a packet destined for another
node
Id l e list ening ± sitting idly and trying to receive when nobody is sending
P r otocol overhead
Always nice: Low complexity solution
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[SelfOrg] 2-2.6
Design Issues
Distributed nature/lack of central coordination
Nodes must be scheduled in a distributed fashion
Exchange of control information
control packets must not consume too much of network bandwidth
Mobility of nodes
Very important factor affecting the performance (throughput) of the
protocol
Bandwidth reservations or control information exchanged may end up
being of no use if the node mobility is very high
Protocol design must take this mobility factor into consideration
system performance should not significantly affected due to nodemobility
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[SelfOrg] 2-2.7
Classification of M AC Protocols
M AC Protocols for Ad
Hoc Wireless Networks
Contention-Based Protocols
Contention-Based
Protocols with Reservation
Mechanisms
Contention-Based
Protocols with
Scheduling Mechanisms
Other M AC Protocols
Sender-Initiated
Protocols
Receiver-Initiated
Protocols
Synchronous
Protocols
Asynchronous
Protocols
Single-Channel
Protocols
Multichannel
Protocols
M AC AW
F AM A
BTM A
DBTM A
RI-BTM A
M AC A-BI
HRM A
FPRP
M AC A/PR
RTM AC
DPS DLPS
MM AC MCSM A
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[SelfOrg] 2-2.8
Classification of M AC Protocols
Cont ention-based pr otocols
No a priori resource reservation
Whenever a packet should be transmitted, the node contends with its
neighbors for access to the shared channel
Cannot provide QoS guarantees
Sender-i nitiat ed protocols±
packet transmissions are initiated by thesender node
n Single-channel sender-initiated protocols ± the total bandwidth is used
as it is, without being divided
n Multi-channel sender-initiated protocols ± available bandwidth is
divided into multiple channels; this enabled several nodes to
simultaneously transmit data
R ec eiv er-i nitiat ed protocols ± the receiver node initiates the contention
resolution protocol
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[SelfOrg] 2-2.9
Classification of M AC Protocols
Cont ention-based pr otocols w it h reservation mec hanisms
Support for real-time traffic using QoS guarantees Using mechanisms for reserving bandwidth a priori
Synchronous protocols ± require time synchronization among all nodes inthe network global time synchronization is generally difficult to achieve
Asynchronous protocols ± do not require any global time synchronization,usually rely on relative time information for effecting reservations
Cont ention-based pr otocols w it h sc hedu ling mec hanisms
Focus on packet scheduling at nodes and also scheduling nodes for
access to the channel requirement for fair treatment and no starvation
Used to enforce priorities among flows
Sometimes battery characteristics, such as remaining battery power, areconsidered while scheduling nodes for access to the channel
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[SelfOrg] 2-2.10
Contention-based Protocols: Main Problems
Hidden and exposed terminals - unique problem in wireless networks
H i dd en t erminal pr obl em±
collision of packets due to the simultaneous
transmission of those nodes that are not within the direct transmission
range of the sender but are within the transmission range of the receiver
Ex posed t erminal pr obl em ± inability of a node, which is blocked due to
transmission by a nearby transmitting node, to transmit to another node
S1 S2
R
R1 R2
S1 S2
Hidden terminal Exposed terminal
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[SelfOrg] 2-2.11
Main Options to Shut Up Senders
Receiver informs potential interferers whil e a reception is on-going
By sending out a signal indicating just that
Problem: Cannot use same channel on which actual reception takes
place
Use separate channel for signaling
Bu sy tone protocol
Receiver informs potential interferers bef ore a reception is on-going
Can use same channel
Receiver itself needs to be informed, by sender, about impending
transmission
Potential interferers need to be aware of such information MAC A protocol
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[SelfOrg] 2-2.12
BTM A ± Busy Tone Multiple Access
The transmission channel is split into
data and control channel
General behavior
When a node wants to transmit a packet,
it senses the channel to check whether
the busy tone is active
If not, it turns on the busy tone signal and
starts transmission
Problem: very poor bandwidth utilization
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[SelfOrg] 2-2.13
M AC A ± Multiple Access Collision Avoidance
Use of additional signaling packets
Sender asks receiver whether it is able to receive a transmission - R eq u est to Send ( RT S) Receiver agrees, sends out a Cl ear to Send (C T S )
Sender sends, receiver acks
Potential interferers overhear RTS/CTS
RTS/CTS packets carry the expected duration of the data transmission
Store this information in a N et w ork Alloc ation Vector ( N AV)
Node 1
Sender
Receiver
Node 4
RTS
CTS ACK
D AT A
N AV
N AV
time
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[SelfOrg] 2-2.14
M AC A ± Problems
RTS/CTS ameliorate, but do not solve hidden/exposed terminal
problems
Node 1
Node 2
Node 3
Node 4
RTS
CTS
D AT A
CTS
RTS
time
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[SelfOrg] 2-2.15
M AC A ± continued
Collision handling
If a packet is lost (collision), the node uses the binary exponential back-off (BEB) algorithm toback off for a random time interval before retrying
Each time a collision is detected, the node doubles its maximum back-off window
Idle listening: need to sense carrier for RTS or CTS packets
In some form shared by many CSM A variants; but e.g. not by busy tones
Simple sleeping will break the protocol
MAC A pr otocol (used e.g. in I EEE 802.11)
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[SelfOrg] 2-2.16
M AC AW Protocol
The binary back-off mechanism can lead to starvation of flows
Example
S1 and S2 are generating a high volume of traffic
If one node (S1) starts sending, the packets transmitted by S2 get collided
S2 backs off and increases its back-off window
the probability of node S2 acquiring the channel keeps decreasing
Solution
Each packet carries the current back-off window of the sender
A node receiving this packet copies this value into its back-off counter
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[SelfOrg] 2-2.17
M AC AW Protocol
Large variations in the back-off values
the back-off window increases very rapidly and is reset after eachsuccessful transmission
Solution
multiplicative increase and linear decrease (MILD) back-off mechanism(increase by factor 1.5)
Fairness
M AC A: per node fairness
M AC AW: per flow fairness (one back-off value per flow)
Error detection
Originally moved to the transport layer Slow and introducing much overhead
Solution
New control packet type: data-sending (DS)
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[SelfOrg] 2-2.18
M AC AW Protocol
Exposed terminal problem
RTS/CTS mechanism does not
solves the exposed terminal
problem
Solution
New control packet type: data-sending (DS), a small packet
(30 Byte) containing information
such as the duration of the
forthcoming data transmission
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[SelfOrg] 2-2.19
Contention-Based Protocols with Reservation
M AC A/PR ± M AC A with Piggy-Backed Reservation
Multi-hop routing protocol based on M AC AW
Main components
M AC protocol
Reservation protocol
QoS routing protocol
Differentiation of real-time and best-effort packets General behavior
S lott ed mec hanisms
Maintenance of a reservation table (RT) at each node that records all thereserved transmit and receive slots / windows of all nodes within itstransmission range
Network allocation vectors (N AV) for cycles Destination sequenced distance vector (DSDV) used for routing
TDM-like system for real-time traffic
Best-effort traffic using M AC AW in free slots
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[SelfOrg] 2-2.20
M AC A/PR Protocol
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[SelfOrg] 2-2.21
M AC Protocol Using Directed Antennas
Properties
One receiver per node, which can transmit and receive only one packet atany given time
Each transceiver is equipped with M
directional antennas
Each antenna has a conical radiation
pattern spanning an angle of 2/M radians Basic RTS/CTS scheme (as used in M AC A)
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[SelfOrg] 2-2.22
M AC Protocol Using Directed Antennas
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[SelfOrg] 2-2.23
Power-Control M AC Protocol (PCM)
Properties
RTS/CTS are transmitted with maximum power pmax
RTS-CTS handshake to determine the required transmission power pdesired
RTS is received at the receiver with a signal level pr
Calculation of pdesired
R xthresh is the minimum necessary received signal strength
c « constant
c x p
p
p threshr
ax
desired *
!
measured
known in advance
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[SelfOrg] 2-2.24
Power-Control M AC Protocol
RTS/CTSrange
1 2 3 6 7 8Data
transmission
D AT A/ ACK
range
4
carrier sensingrange
5
pmax pdesired
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[SelfOrg] 2-2.25
Power-Control M AC Protocol
Properties
Adaptation to changing conditions, e.g. caused by mobility Instantaneous check and re-calculation of the necessary transmission power pdesired
Collision avoidance
Periodic bursts (after each EIFS) using pmax to notify neighbors about
ongoing transmissions
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[SelfOrg] 2-2.26
Sensor-M AC (S-M AC)
Primary goal
To retain f l e x ibility of contention-based protocols while i mpr ov ing ener gy eff ici ency in multi-hop networks
(M AC A¶s idle listening is particularly unsuitable if average data rate is low - most of
the time, nothing happens)
Idea: Switch nodes off, ensure that neighboring nodes turn on simultaneously
to allow packet exchange (rendez-vous) Only in these acti ve per iod s, packet exchanges happen
Need to also exchange wakeup schedule between neighbors
When awake, essentially perform RTS/CTS
Coarse-grained sleep/wakeup cycle with duty cycle D = / T
time
Listen Sleep Listen Sleep
X
T
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[SelfOrg] 2-2.27
S-M AC ± Scheduling
Use SYNC, RTS, CTS phases
Scheduling
Low-duty-cycle operation (1-10%)
All nodes choose their own listen/sleep schedules
These schedules are shared with their neighbors to make communication
possible between all nodes
Each node periodically broadcasts its schedule in a SYNC packet, which
provides simple time synchronization
To reduce overhead, S-M AC encourages neighboring nodes to adopt
identical schedules
time
Sync Data/Sleep
X
T
RTS/CTS Sync RTS/CTS
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[SelfOrg] 2-2.28
S-M AC ± Synchronization
Nodes try to pick up schedule synchronization from neighboring nodes
If no neighbor found, nodes pick some schedule to start with
If additional nodes join, some node might learn about two different
schedules from different nodes
³Synchronized islands´
To bridge this gap, it has to follow both schemes
Complete algorithm
1. Listen for ³waiting time´ (at least one complete busy/sleep cycle) for
SYNC messages ± if nothing happens, the node chooses its own
schedule
2. If a node receives a SYNC bef ore setting up its own schedule, it takesover the received schedule
3. If a node receives a SYNC af t er setting up its own schedule, its adopts
both schedules to bridge two islands
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[SelfOrg] 2-2.29
S-M AC ± Synchronization
S1 S1Start: Node 1
Waiting time
R1 S1Start: Node 2
S4 S4Start: Node 4
Waiting time
R1 S4Start: Node 3
Abbreviatedwaiting time
R4
Abbreviated
waiting time
Adapted sync
Adapted sync
Adapted sync
S1
S1
S1
time
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[SelfOrg] 2-2.30
S-M AC ± Performance Aspects
Standard S-M AC
Energy saving through periodic sleep
Depending on the duty cycle, the end-to-end performance is increasing as
n Per busy period, exactly one packet can be transmitted within a
common radio range
n If rather short packets need to be transmitted either long sleep
intervals must be prevented (energy wastage) or the per-hop delay isfurther increased
Improved S-M AC
Ad apti ve list ening allows additional energy savings (nodes wake up
immediately after the exchange completes for immediate contention for
the channel)
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[SelfOrg] 2-2.31
S-M AC ± Performance Aspects
Standard S-M AC w/o adaptive listening
S R/C
Data
Sleep
S R/C
Data
S R/C
Data
Sleep
C
TimeListen/Sleep
R
C A
Sleep
Sleep
Sleep
Slot n Slot n+1 Slot n+2
S Sync R/C RTS/CTS R RTS C CTS A ACK
Listen/Sleep
R
C A
Sleep
Sleep
Sleep
Sleep
Listen/Sleep
R
C A
Sleep
Sleep
Sleep
A
B
D
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[SelfOrg] 2-2.32
S-M AC ± Performance Aspects
Improved S-M AC w/ adaptive listening
A
B
C
S R/CTime
R
C
Data
A Sleep
Slot n Slot n+1 Slot n+2
S Sync R/C RTS/CTS R RTS C CTS A ACK
S R/C
R
C
Data
A Sleep
Sleep
S R/C
R
C
Data
A
Sleep
Sleep
Sleep
Sleep
ALP ALP
Adaptive Listening ALP
D
Sleep
Sleep
Sleep
Sleep
Sleep
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[SelfOrg] 2-2.33
S-M AC ± Performance Evaluation
Experimental setup
Ten nodes in a line
Analyzed S-M AC modes
Mode1: no periodic sleep (= M AC A)
Mode2: 10% duty cycle, w/o adaptive listening (= standard S-M AC)
Mode3: 10% duty cycle, w/ adaptive listening (= improved S-M A
C)
1 2 3 8 9 10«
source sink
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[SelfOrg] 2-2.34
S-M AC ± Performance Evaluation
M ean energy consumption per byt e ± the total energy consumed by all
nodes divided by the total number of bytes received by the sink
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[SelfOrg] 2-2.35
S-M AC ± Performance Evaluation
E nd -to-end good put ± the total number of bytes received by the sink
divided by the time from the first packet generated at the source untilthe last packet was received by the sink
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[SelfOrg] 2-2.36
S-M AC ± Performance Evaluation
M ean end -to-end delay ± the sum of all end-to-end delays divided by
the total number of packets
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[SelfOrg] 2-2.37
Summary (what do I need to know)
Well-established M AC protocols in the ad hoc domain
M AC A / M AC AW / 802.11 Similar solutions for hidden/exposed terminal problem
Applicability for wireless sensor networks
S c al ability ± M AC A/802.11 needs a global sync; adaptive solutions aredemanded
E ner gy eff ici ency - limited sleeping time in M AC A/802.11; low dutycycles and/or adjustments of the transmission power are needed
Specific developments
P C M ± well-controlled transmission power, can be combined with any
RTS/CTS based M A
C protocol S - MAC ± supports multiple schedules and long sleep cycles with adaptive
listening
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[SelfOrg] 2-2.38
References
V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, "M AC AW: A Media Access
Protocol for W
ireless L A
N's," Proceedings of A
CM SIGCOMM'94, London, UK,September 1994, pp. 212-225.
P. Karn, "M AC A: a new channel access method for packet radio," Proceedings of
ARRL/CRRL Amateur Radio 9th Computer Networking Conference, London, Ontario,
Canada, 1990, pp. 134-140.
IEEE, "Wireless L AN Medium Access Control (M AC) and Physical Layer (PHY)Specification," IEEE Std. 802.11-1999 edition, 1999.
E.-S. Jung and N. Vaidya, " A Power Control M AC Protocol for Ad Hoc Networks,"Proceedings of ACM/IEEE MobiCom, September 2002.
W. Ye, J. Heidemann, and D. Estrin, " An Energy-Efficient M AC Protocol for WirelessSensor Networks," Proceedings of 21st International Annual Joint Conference of theIEEE Computer and Communications Societies (INFOCOM), vol. 3, New York, NY,US A, June 2002, pp. 1567-1576.
W. Ye, J. Heidemann, and D. Estrin, "Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks," IEEE/ACM Transactions on
N etworki
ng (TON), vol. 12 (3), pp. 493-506, June 2004.
F. Chen, F. Dressler, and A. Heindl, "End-to-End Performance Characteristics inEnergy- Aware Wireless Sensor Networks," Proceedings of Third ACM InternationalWorkshop on Performance Evaluation of Wireless Ad Hoc, Sensor, and UbiquitousNetworks ( ACM PE-WASUN'06), Torremolinos, Malaga, Spain, October 2006, pp. 41-47.
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