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Alleviating MAC Layer Self-Contention in Ad-hoc Networks. Zhenqiang Ye, Dan Berger, Prasun Sinha † , Srikanth Krishnamurthy, Michalis Faloutsos, Satish K. Tripathi Dept. of CSE, UC Riverside † Dept of CIS, Ohio State University. Motivation. Self-contention

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alleviating mac layer self contention in ad hoc networks

Alleviating MAC Layer Self-Contention in Ad-hoc Networks

Zhenqiang Ye, Dan Berger, Prasun Sinha† ,

Srikanth Krishnamurthy, Michalis Faloutsos, Satish K. Tripathi

Dept. of CSE, UC Riverside

† Dept of CIS, Ohio State University

motivation
Motivation

Self-contention

Contention between packets of same transport connection

Intra-stream contention

Contention caused by packets of the

same stream at different nodes

Inter-stream contention

Contention between DATA packet

stream and ACK packet stream

DATA stream (TCP or UDP)

TCP DATA stream

destination

source

source

destination

ACK stream

Contention for shared media

Contention for shared media

prior MAC solutions: [Fu et. al., Infocom ’03]

prior MAC solutions: (none)

  • Self-contention is best resolved at the MAC layer because…
  • Self-contention arises in the MAC layer
  • Requires no changes to widely deployed transport protocols
  • IEEE 802.11 is an evolving standard and is amenable to changes
the main contributions
The Main Contributions
  • Propose two mechanisms:
    • Fast Forward alleviates intra-stream contention by withholding transmission until previous packet has reached beyond interference range.
    • Quick Exchange alleviates inter-stream contention by exchanging TCP data and TCP ACK packets in the same RTS-CTS-ACK dialogue.
  • Observe significant performance improvement:
    • Up to 250% goodput improvement
    • Up to 19% backoff time reduction
    • 22% MAC layer overhead reduction
fast forward ff key idea don t send next packet till previous packet is out of interference range
Fast-Forward (FF)Key Idea: don’t send next packet till previous packet is out of interference range

Receiver

Sender

Next hop Receiver

RTS

CTS

Lower avg back-off time per packet

No backoff precedes FFPKT tx

Fewer False Link Failures

No explicit contention for FFPKT

Reduced control packet overhead

No RTS for FFPKT

DATA

Time

ACK (with Implicit RTS)

ACK( with implicit RTS)

CTS

DATA

(fast forwarded packet)

FFPKT: Fast Forwarded Packet

ACK( with implicit RTS)

2

6

4

6

6

2

Bytes:

Modified ACK

(with RTS for next-hop)

Frame

Control

Duration

Destination

Address

FCS

RTS dest

Address

Source

Address

Needed by the next hop to respond with CTS

Identifies the intended RTS recepient (next hop)

quick exchange qe key idea subsume contention caused by reverse stream
Quick-Exchange (QE)Key Idea: subsume contention caused by reverse stream

Receiver

Sender’s Neighbor

Sender

Receiver’s Neighbor

RTS

Lower avg back-off time per packet

No backoff precedes DATA2 tx

Fewer False Link Failures

No explicit contention for DATA2

Reduced control packet overhead

No RTS/CTS for DATA2

Piggybacked ACK1

NAV

(RTS)

CTS

Time

DATA1

NAV

(CTS)

ACK1

DATA2

NAV

(DATA1)

NAV

(ACK1)

ACK2

2

2

6

4

2

Bytes:

HCS : Header Check Sequence

FCS : Frame Check Sequence

BSSID : Basic Service Set ID

(unique network ID)

Frame

Control

Duration

Extra

Duration()

Destination

Address

FCS

CTS

2

6

4

6

2

6

2

0 - 2308

4

Bytes:

DATA2

(with ACK1)

Frame

Control

Duration

Destination

Address

HCS

Source

Address

BSSID

Sequence

Control

Body

FCS

ACK Header

MAC Header

performance goodput in string topology
Performance: Goodput in String Topology

Single UDP flow in a string topology

Single TCP flow in a string topology

Goodput increase up to 250%

Goodput increase up to 45%

performance normalized goodput in random topology
Performance: Normalized Goodput in Random Topology
  • 100 nodes in 2500m  1000m
  • Average of 50 scenarios
  • 180 sec sim time per scenario

TCP flows in a random topology

Normalized Goodput increases by up to 30%

goodput improvement factors scenario tcp flows in random topology
Goodput Improvement Factors(Scenario: TCP flows in random topology)

Normalized Backoff Time

(backoff time per MAC packet tx)

Normalized Control Packet Overhead

(#Control packets per unicast packet)

Reduction by up to 19%

Reduction from 3.2 to 2.5 (approx.)

Number of Link Failures

Reduction by up to 66%

conclusions
Conclusions
  • Quick-Exchange alleviates inter-stream self-contention
  • Fast-Forward alleviates intra-stream self-contention
  • UDP goodput improves by 250% in string topology
  • TCP goodput improves by 45% in string topology

Ongoing Work

  • Goodput studies for scenarios with mobility
  • Analytical model of goodput gains for
  • fast-forward and quick-exchange