DECENTRALIZED CONGESTION CONTROL FOR ITS‐G5ETSI STF469
Dieter Smely 6th ITS Workshop, Feb. 2014, Berlin, Germany
© ETSI 2013. All rights reserved
Overview
Introduction
Basics
Solutions
Conclusions
2 © ETSI 2011. All rights reserved Dieter Smely , STF469
Introduction
What is Decentralized Congestion Control (DCC)?What is Decentralized Congestion Control (DCC)?DCC shapes the ITS‐G5 ad hoc network traffic.
Why to shape the network traffic?To ensure proper operation of safety applications.To ensure proper operation of safety applications.Since high channel loads cause ...• ... long channel access delays andg y• ... packet collisions, which cause radio range degradation
Therefore, for safety relevant systems, y ycritical channel loads must be avoided!
3 © ETSI 2011. All rights reserved Dieter Smely , STF469
Basics
How to shape the network traffic?How to shape the network traffic?
Decentralized Congestion Control (DCC)g ( )• IEEE 802.11 (MAC and PHY Layer)
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)• Enhanced Distributed Coordination Function (EDCA)• Quality of Service (QoS) and Access Categories (AC)
• ETSI TC ITS • Cross layer extension to IEEE 802.11
4 © ETSI 2011. All rights reserved Dieter Smely , STF469
Basics
IEEE 802.11 CSMA/CA and QoS for broadcast packetsIEEE 802.11 CSMA/CA and QoS for broadcast packets
lot t
ime
3 µsarbitration interframe space
Message prioritization by PHY timing parameters.
AIFS[i] sl 13
timebusy medium transmission
CW random number of slots
EDCA mechanismcontention window
AC CW i CW AIFSAC CWmin CWmax AIFSAC_VO 3 7 58 µsAC_VI 7 15 71 µsAC_BE 15 1 023 110 µs
4 priorities (access categories) historically called:VO ... voiceVI ... videoBE best effort
5 © ETSI 2011. All rights reserved Dieter Smely , STF469
AC_BK 15 1 023 149 µsBE ... best effortBK ... background
Basics
EDCA limitations for broadcast packetsEDCA avoids most packet collisions up to moderate channel loads.For high channel loads the probability increases that the CWs end simultaneously for several nodes.both nodes can transmit at the same time.
ETSI TC ITS DCC• DCC avoids this channel overload by ...• ... message rate control, and in future• ... by additional transmit power control, and later on
6 © ETSI 2011. All rights reserved Dieter Smely , STF469
y p ,• ... by offloading data traffic to other channels.
Basics
Control input to the DCC algorithmControl input to the DCC algorithm
• Local measurement simple to implement:• Local measurement simple to implement: • Local Channel Busy Ratio (CBR)
• Global knowledge improves fairness and safety: • CBR disseminationCBR dissemination• Message rate dissemination• Transmit power level dissemination
7 © ETSI 2011. All rights reserved Dieter Smely , STF469
Basics
CBR determination principle*) and accuracy
RSSI threshold
Busy
timeRSSI ... Received Signal Strength Indicator
8 © ETSI 2011. All rights reserved Dieter Smely , STF469
*) IEEE 802.11 specifies a more complex CBR determination.
Specification
How to specify DCC?f b d bl b h l
0 81,0
• Specification based on a testable system behaviour limit.• Proposal: Upper CBR limit vs. node number
0,00,20,40,60,8
CBR
0 100 200 300 400 500 600 700
Number of ITS‐S
• For a message length of 1 ms themessage rate limit is:
5101520
sage Rate
[Hz]
• For a message length of 1 ms the message rate limit is:
9 © ETSI 2011. All rights reserved Dieter Smely , STF469
05
0 100 200 300 400 500 600 700
Mess [
Number of ITS‐S
Implementation examples ...
1,0
State Machine (Toff proportional to CBR)
0,4
0,6
0,8
CBR
0,0
0,2
0 100 200 300 400 500 600 700
Number of ITS‐S
Limit Limit Cycle+ Limit Cycle‐
Number of ITS‐S
Continuous PI controller with upper and lower rate limits
0,81,0
0,00,20,40,6,
CBR
CBR Limit Continuous PI controller
10 © ETSI 2011. All rights reserved Dieter Smely , STF469
Any other algorithm that matches the limit!PI ... proportional integral
0 100 200 300 400 500 600 700Number of ITS‐S
Single channel with local DCC information
Facilities LayerCAM DENM Service Appl.
DCC Facility
Message generation parameters
y
aram
.
Network and Transport Layer
Traffic class prioritizationDCC parm. evaluational
DC
C p
a
. .DCC queues
Power control
Local CBR
evaluation
CEN DSRC coexistence
inte
rna
Flow controlcoexistence
DCC Access
Access Layer11 © ETSI 2011. All rights reserved Dieter Smely , STF469
PHY / MAC / DRIVERS / RADIO (Chipset)
Multi channel with global DCC information
Facilities LayerCAM DENM Service Appl.
DCC Facility
Message generation parameters
DCC Facility
Global DCC
Neighbour TableChannel DCC param.
y
aram
.
Global DCC TX param.
y
Network and Transport Layer
Traffic class prioritization
Global CCRX param.
DCC parm. evaluational
DC
C p
a
DCC parm. evaluation
Traffic class prioritization...
.
.
.
.
.
.
.
.
.DCC queues
Power control
Local CBR
evaluation
CEN DSRC coexistence
inte
rna evaluation
Local CBR
.
.
.
.
.
.
.
.
.
.
.
.DCC queues
Power controlCEN DSRC coexistence
Flow controlcoexistence
DCC AccessDCC Access
Flow controlcoexistence
Access Layer12 © ETSI 2011. All rights reserved Dieter Smely , STF469
PHY / MAC / DRIVERS / RADIO (Chipset)
Conclusions
IEEE 802.11 CSMA/CA plus EDCA works fine for moderate network loads.For safety related applications the network load has to b ll d b ddi i l l i hbe controlled by an additional DCC algorithm.The first implementation step will be done by a l l CBR t d t t llocal CBR measurement, and a message rate control.To improve fairness and safety, information sharing between the ITS Ss will be necessary later onbetween the ITS‐Ss will be necessary later on. A coordinated power control will be necessary when high penetration rates are reached
13 © ETSI 2011. All rights reserved Dieter Smely , STF469
high penetration rates are reached.In addition, multi‐channel DCC can be used.
Decentralized Congestion Control for ITS‐G5
Dieter Smely
Kapsch TrafficCom AGAm Europlatz 2 | A-1120 Vienna | Austria
Tel.: +43 50 811 2217Fax: +43 50 811 2209
E-mail: [email protected] kapschtraffic comwww.kapschtraffic.com
www.kapsch.net
Thank you!
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