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Controlling Unfairness due to Physical Layer Capture and Channel Bonding in 802.11n+s Wireless Mesh Networks Sandip Chakraborty, Sukumar Nandi Department of Computer Science and Engineering Indian Institute of Technology Guwahati, Guwahati 781039 INDIA 06 January, 2015 Networking Track ICDCN 2015 06 January, 2015 1 / 21

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Page 1: Controlling Unfairness due to Physical Layer Capture and ...cse.iitkgp.ac.in/~sandipc/presentations/ICDCN_2015.pdf · Controlling Unfairness due to Physical Layer Capture and Channel

Controlling Unfairness due to Physical Layer Capture andChannel Bonding in 802.11n+s Wireless Mesh Networks

Sandip Chakraborty, Sukumar Nandi

Department of Computer Science and EngineeringIndian Institute of Technology Guwahati,

Guwahati 781039 INDIA

06 January, 2015

Networking Track ICDCN 2015 06 January, 2015 1 / 21

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Preface: IEEE 802.11n

High speed extension for wireless networks: 600 Mbps

Extensions over IEEE 802.11b/g

- MIMO - Spatial diversity and spatial multiplexing

- Channel Bonding: Combine two 20 MHz channels to one 40 MHzchannel (also knowm as 20/40 semantics)

- MAC Layer Enhancements: Frame aggregation and blockacknowledgements

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Channel Bonding in IEEE 802.11n

Channel bonding combines two consecutive 20 MHz channels to asingle 40 MHz channel.

Source:http://wifijedi.com/2009/01/25/how-stuff-works-channel-bonding/

Wider channel provides more bandwidth, hence more physical datarate.

However, wider channels are also susceptible to channel errors,channel errors are more in 40 MHz channel compared to a 20 MHzchannel.Networking Track ICDCN 2015 06 January, 2015 3 / 21

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Wireless Mesh Network: IEEE 802.11s

Wireless mesh network is a multi-hop communication network thatcan provide last mile broadband wireless connectivity.

Source:http://www.cnri.dit.ie/research.mesh.congestion.html

IEEE has standardized mesh network through IEEE 802.11s extensionin the well-deployed Wi-Fi standard.

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Physical Layer Capture

In a mesh network, multiple mesh routers (called mesh stations) triesto gain access to the common channel for data communication.

As a consequence, interference effect is significantly high in meshnetworks.

Physical layer capture (PLC) is used to recover packets in thepresence of interference.

According to the PLC, a receiver can recover data from the interferedsignal if the received signal to noise ratio (SNR) is more than athreshold, called the capture threshold.

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High Throughput Wireless Mesh Extension - IEEE802.11n+s

IEEE 802.11n can be used for supporting high-throughputcommunication over mesh backbone - through IEEE 802.11n at thephysical layer and IEEE 802.11s at the MAC layer.

This paper analyzes the compatibility of IEEE 802.11n in a meshenvironment - with the presence of both 20 MHz and 40 MHzcommunication supported nodes and Physical Layer Capture (PLC).

A practical high throughput mesh networking testbed has beendeployed at IIT Guwahati research labs. The performance of IEEE802.11n channel bonding in a mesh scenario has been evaluated overthe testbed.

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Testbed Scenario

STA-1 STA-2 STA-3

STA-1 STA-2STA-3 STA-4

Scenario 1: Direct Interference

Scenario 2: Indirect Interference

Link-11 Link-12

Link-21Link-22

In scenario-1, STA-2 is the common receiver from SCN-1 and SCN-3(symmetric and direct interference).

In scenario 2, the communication between STA-3 and STA-4 interfereswith the communication between STA-1 and STA-2. However, theSTA-1 to STA-2 communication does not interfere with STA-3 toSTA-4 communication (asymmetric and indirect interference).

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Testbed Setup Parameters

Ralink RT-3352 router-on-chip as the routers

The RT-3352 router-on-chip combines 802.11n draft compliant 2T2RMAC along with BBP/PA/RF MIMO

This chip can support up to 300 Mbps data rate with the maximumtransmission power of 16dB.

Software: Kernel 3.12.24 with open80211s reference model for IEEE802.11s mesh networks

Capture threshold for the RT-3352 chipset is approximately 1.6 dB.

Rate adaptation is supported for IEEE 802.11n

Data Traffic- Constant bit rate: 200 Mbps (network saturation)

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Modulation and Coding Schemes (MCS)

IEEE 802.11n supports different modulation and coding schemes(MCS) for different data rates.

Two reference MCS levels - MCS 7 and MCS 15 are used in theexperiment.

MCS 7 provides 72.20 Mbps at 20 MHz channel and 150 Mbps at 40MHz channel.

MCS 14 provides 144.40 Mbps at 20 MHz channel and 300 Mbps at40 MHz channel.

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Effect of PLC over Channel Bonding

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Capture Disabled, MCS 7

Link-11: STA-1 -> STA-2Link-12: STA-3 -> STA-2

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Link-11: STA-1 -> STA-2Link-12: STA-3 -> STA-2

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Capture Enabled, MCS 7

Link-11: STA-1 -> STA-2Link-12: STA-3 -> STA-2

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Capture Enabled, MCS 15

Link-11: STA-1 -> STA-2Link-12: STA-3 -> STA-2

Direct Interference Scenario

Observation: When PLC is enabled, significant unfairness is observedwhen one link uses 20 MHz and another one uses 40 MHz. The link with40 MHz attains very less throughput.

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Effect of PLC over Channel Bonding

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Capture Disabled, MCS 7

Link-21: STA-1 -> STA-2Link-22: STA-3 -> STA-4

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Capture Disabled, MCS 15

Link-21: STA-1 -> STA-2Link-22: STA-3 -> STA-4

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Capture Enabled, MCS 7

Link-21: STA-1 -> STA-2Link-22: STA-3 -> STA-4

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Capture Enabled, MCS 15

Link-21: STA-1 -> STA-2Link-22: STA-3 -> STA-4

Indirect Interference Scenario

Observation: Similar observation like direct interference scenario. WhenPLC is enabled, link-21 attains very less throughput when it uses 40 MHzand the other link uses 20 MHz.

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Observations from the Experiments

PLC significantly improves network throughput in the presence ofinterference, when all the interfering links use similar bondingopportunity (either 20 MHz or 40 MHz)

The network goes through severe unfairness in the presence ofinterference, when one of the interfering link uses 40 MHz and theinterferer links use 20 MHz.

Though the 40 MHz channel bonding significantly improves networkthroughput in the absence of interference or in the presence ofinterference from another 40 MHz link with PLC enabled, it performsworst when interfered by a 20 MHz link. The PLC always biases thethroughput towards the 20 MHz link.

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Should We Disable PLC?

The answer is NO. PLC significantly reduces data loss due tointerference, and helps 20 − 20 or 40 − 40 communications to sustain.The data loss due to interference is much more compared to theeffect of unfairness.

Solution: Intelligent channel bonding - avoid interference between 20MHz and 40 MHz channels.

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Intelligent Channel Bonding

A 40 MHz communication is allowed, only when there is no ongoing20 MHz communications in the vicinity.

A 20 MHz communication is allowed only when all the ongoingcommunication in the neighborhood uses 20 MHz only.

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Distributed Selection of Bonding Opportunity

A mesh STA (transmitter) selects 20 MHz only when there is no 40MHz communication (receiver) in the vicinity (otherwise it will affectthe 40 MHz communication).

A receiver accepts 40 MHz communication if there is no ongoing 20MHz transmitter in the vicinity. (otherwise it will be affected due tointerference from the 20 MHz)

The proposed protocol is termed as High Throughput Fair Mesh(HT-Fair Mesh)

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Protocol Architecture: HT-Fair Mesh

Rate/Bonding Opportunities

Selection Engine (RBOSen)

Rate/Bonding Opportunities

Selection Engine (RBOSen)

Frame MonitorChannel Quality

Estimator

MCCA Reservation

Engine

MCCA Reservation

Engine

MCCA Setup Request

MCCA Setup Reply

MCS Request

MCS Feedback

802.11n

802.11s

Transmitter Receiver

InEn

InEn

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Performance of HT-Fair Mesh: Scenario

STA-1

STA-2

STA-3

STA-4

STA-5

STA-6

STA-7

STA-10

STA-11

STA-8 STA-13/Gate

STA-9 STA-12

Internet

Testbed Scenario for Performance Evaluation

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Comparative Protocols

IEEE 802.11n+s with Mistrel: The default rate adaptation (MCSselection) protocol for Linux kernel. This protocol selects the optimaldata rate and bonding opportunity based on previous history ofpacket losses. With a few consecutive packet losses, the protocolreduces the MCS level and vice versa.

Dynamic MCS Selection: This protocol selects bonding opportunityon the fly, based on whether 20 MHz or 40 MHz performs better inthe previous run. It does not consider fairness.

The proposed protocol differs from the dynamic MCS selection in thesense that it selects the bonding opportunity and MCS selectionbefore the communication, while considering fairness.

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Comparison: Network Performance

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1 10 100 1000

Avera

ge N

etw

ork

Th

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Traffic Generation Rate (Mbps), log scale

(a) Average Network Throughput

IEEE 802.11n+s + MinstrelDynamic MCS Selection

HT-Fair Mesh

0

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1

1 10 100 1000

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irn

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Traffic Generation Rate (Mbps), log scale

(b) Jain Fairness Index

IEEE 802.11n+s + MinstrelDynamic MCS Selection

HT-Fair Mesh

HT-Fair Mesh significantly improves network performance in terms ofnetwork throughput and fairness.

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Comparison: End-to-End Performance

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nd

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ug

hp

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(Mb

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Traffic Generation Rate (Mbps), log scale

(a) Average End-to-end Throughput

IEEE 802.11n+s + MinstrelDynamic MCS Selection

HT-Fair Mesh

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Avera

ge

En

d-t

o-e

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lay (

ms

)Traffic Generation Rate (Mbps), log scale

(b) Average End-to-end Delay (ms)

IEEE 802.11n+s + MinstrelDynamic MCS Selection

HT-Fair Mesh

HT-Fair Mesh significantly improves network performance in terms ofend-to-end throughput and end-to-end delay.

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Conclusion

This paper explores the severity of unfairness in IEEE 802.11n+s highthroughput mesh networks in the presence of PLC and channelbonding.

The 40 MHz channel gets affected severely through interference froma 20 MHz channel in the presence of PLC.

This paper proposes a dynamic, localized and distributed protocol forthe selection of bonding opportunities that ensures fairness.

Future directions: How MCS selection interfere with the selection ofchannel bonding opportunities?

For any query, please contact the author

Dr. Sandip Chakraborty([email protected])

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Conclusion

This paper explores the severity of unfairness in IEEE 802.11n+s highthroughput mesh networks in the presence of PLC and channelbonding.

The 40 MHz channel gets affected severely through interference froma 20 MHz channel in the presence of PLC.

This paper proposes a dynamic, localized and distributed protocol forthe selection of bonding opportunities that ensures fairness.

Future directions: How MCS selection interfere with the selection ofchannel bonding opportunities?

For any query, please contact the author

Dr. Sandip Chakraborty([email protected])

Networking Track ICDCN 2015 06 January, 2015 21 / 21