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Collisions & Virtual collisions in IEEE 802.11 networks Libin Jiang EE228a Communication Networks

Collisions & Virtual collisions in IEEE 802.11 networks

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Collisions & Virtual collisions in IEEE 802.11 networks. Libin Jiang EE228a Communication Networks. Outline. Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion. T2. R1. R2. T1. PCS. 1. 2. Range. - PowerPoint PPT Presentation

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Page 1: Collisions & Virtual collisions in IEEE 802.11 networks

Collisions & Virtual collisions in IEEE 802.11 networks

Libin JiangEE228a Communication Networks

Page 2: Collisions & Virtual collisions in IEEE 802.11 networks

Outline

Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion

Page 3: Collisions & Virtual collisions in IEEE 802.11 networks

Problem of Carrier-Sensing in 802.11 Hidden-terminal problem (causing collisions)

T1R1

T2

Basic Mode |T2-R1|<IR;

|T1-T2|<PCSRange

PCSRange

R2

1 2

Link contention Graph(Link: a Transmitter-Receiver pair)

T1 R1T2

VCSRange VCS

Range

R2RTS/CTS mode

|T2-R1|<IR;VCSRange>IR

IR: Interference RangePCSRange: Physical Carrier Sensing RangeVCSRange: Virtual Carrier Sensing Range

RTS/CTS

Page 4: Collisions & Virtual collisions in IEEE 802.11 networks

Problem of Carrier-Sensing in 802.11 Collisions & unfairness still exist

if a receiver can sense “busy” channel but the transmitter can’t

Transmitter does not know when to transmit

Collisions Virtual collisions

T2 can send RTS to R2, but R2 does not reply with a CTS

(May not be a real collision) Information asymmetry

T1 knows Link 3 T3 does not know Link 1,

resulting in collisions Link 3 gets a much lower

throughput than Link 1 Cannot be solved by just using

a large CS Range

Collision Virtual Collision

T1 R1R3

RTS/CTS Mode

VCSRange>IR

VCSRange VCS

Range

T3

T2

R2

Page 5: Collisions & Virtual collisions in IEEE 802.11 networks

Problem of Carrier-Sensing in 802.11 Similar situation exists in "Basic Mode“, if the receiver

cannot “restart” to receive a stronger packet

T1R1

T2

PCSRange

R2Preamble, length MAC Data

Preamble, length MAC Data (T2->R2)

If |T1-R2|<PCSRange, R2 can miss thepacket T2->R2Basic Mode

Packets on Link 2 are often lost, for any PCSRange.

Packet T1->R1

Packet T2->R2

Packets arriving at R2

Page 6: Collisions & Virtual collisions in IEEE 802.11 networks

Outline

Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion

Page 7: Collisions & Virtual collisions in IEEE 802.11 networks

Symptoms

Frequent packet collisions cause many problems [1] Throughput Unfairness Routing Instability

[1] Xu, S.; Saadawi, T., “Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks?”, Communications Magazine, IEEE, Volume: 39, Issue: 6, June 2001, Pages:130 - 137

Page 8: Collisions & Virtual collisions in IEEE 802.11 networks

Symptoms Throughput Unfairness

0 10 20 30 40 500

0.5

1

1.5

2

2.5

Time (s)

Thr

ou

ghp

ut (

Mb

ps) TCP 1

TCP 2

TCP 1: node 1 node 3, starts earlier at 3.0 sec

TCP 2: node 6 node 4. starts at 10.0 sec.

1 2 3 4 5 6

Tool: Network Simulator 2 Nodes are spaced by 140m No RTS/CTS, PCSRange =

550m. 3 hops<PCSRange<4 hops

Data rate: 11Mbps Packet size: 1460 Bytes Routing protocol: AODV (Ad-

hoc On-demand Distance Vector Routing)

Page 9: Collisions & Virtual collisions in IEEE 802.11 networks

Symptoms Routing Instability

A UDP flow: node 1 node 12 (11 hops).

Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability

1 2 3 4 5 6 11 12...

0 10 20 30 40 500

0.2

0.4

0.6

0.8

1

1.2

Time (s)

Thr

ough

put

(Mbp

s)

Page 10: Collisions & Virtual collisions in IEEE 802.11 networks

Outline

Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion

Page 11: Collisions & Virtual collisions in IEEE 802.11 networks

The New Design--for IEEE 802.11 Basic Mode

Range Requirement: Transmitter must sense the interfering link(s)

Receiver Requirement: Receiver assumes no role in Carrier-Sensing “Restart”: If a stronger packet arrives later, the receiver switches to

receiving the packet In any case, return ACK if receiving a DATA packet

max max

Each link has an "Interference Range": ( ),

where (.) is an increasing function. ( )

links , are "interfering" if | | , where

{ , }, { , }.

i i

i j i j

i i i j j j

i IR f d

f IR f d

i j X X IR or IR

X T R X T R

max max

max{| |}, interfering links ,

max{| |} 2

i j

i j

PCSRange T T i j

T T d IR

T1 R1 R2

IRmaxdmax dmax

T2

Definitions

Page 12: Collisions & Virtual collisions in IEEE 802.11 networks

The New Design--for IEEE 802.11 RTS/CTS Mode Range Requirement: transmitter must receive the RTS or CTS from

interfering link(s)

Receiver Requirement: Receiver assumes no role in Carrier-Sensing Same as before, except… In any case, return CTS/ACK if receiving a RTS/DATA packet

max max

max{min(| |,| |)}, interfering links ,i j i jVCSRange T T T R i j

VCSRange d IR

T1 R1

R2

VCSRange VCS

Range

T2

RTS/CTS modeIf VCSRange>dmax+IRmax,

then |R1-R3|>IRmax

IRmax

dmax

dmax

T3

R3

Page 13: Collisions & Virtual collisions in IEEE 802.11 networks

Outline

Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion

Page 14: Collisions & Virtual collisions in IEEE 802.11 networks

Performance Evaluation TCP unfairness

0 10 20 30 40 500

0.5

1

1.5

2

2.5

Time (s)

Thr

ou

ghp

ut (

Mb

ps) TCP 1

TCP 2

TCP 1: node 1 node 3, starts earlier at 3.0 sec

TCP 2: node 6 node 4. starts at 10.0 sec.

1 2 3 4 5 6

0 10 20 30 40 500

0.5

1

1.5

2

2.5

Time (s)

Thr

ou

ghp

ut (

Mb

ps)

TCP 1TCP 2

Page 15: Collisions & Virtual collisions in IEEE 802.11 networks

Performance Evaluation Routing Instability

A UDP flow: node 1 node 12 (11 hops).

Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability

1 2 3 4 5 6 11 12...

0 10 20 30 40 500

0.5

1

1.5

Time (s)

Thr

ough

t (M

bps)

Before

After

Page 16: Collisions & Virtual collisions in IEEE 802.11 networks

Outline

Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion

Page 17: Collisions & Virtual collisions in IEEE 802.11 networks

Throughput-collision tradeoff In the design, CSRange/dmax seems

to be large:

A smaller PCSRange can not remove hidden-terminals, but may give a higher throughput

To study the tradeoff, consider a random network M=4 16 APs, 64 randomly located

clients D/M=175m dmax=175/root(2) PCSRange>468m satisfies

Range Requirement

AP1 AP2

AP3 AP4

D

D/MD

max max

max max

2PCSRange d IR

VCSRange d IR

Page 18: Collisions & Virtual collisions in IEEE 802.11 networks

Throughput-collision tradeoff

Collision Probability vs. PCS Range Total throughput vs. PCS Range

Total Throughput

0

10

20

30

40

50

60

70

80

90

0 100 200 300 400 500

PCS Range (m)

To

tal T

hro

ug

hp

ut

(Mb

ps)

After meeting "Receiver Requirement" IEEE 802.11

Collision Probability

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 100 200 300 400 500PCS Range (m)

Co

llisi

on

Pro

bab

ility

After meeting "Receiver Requirement" IEEE 802.11

Range Requirement met

Page 19: Collisions & Virtual collisions in IEEE 802.11 networks

Throughput-collision tradeoff

The tradeoff always exists

The tradeoff is improved by meeting the Receiver Requirement

Throughput-collision tradeoff

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100

Total Throughput (Mbps)

Co

llis

ion

Pro

bab

ilit

y

After meeting "Receiver Requirement" IEEE 802.11

Page 20: Collisions & Virtual collisions in IEEE 802.11 networks

Conclusion

802.11 does not avoid hidden-terminal-induced collisions & virtual collisions

It is the root of many problems (symptoms) 2 requirements (Range Requirement and Receiver

Requirement) is sufficient to solve the problem Tradeoff between throughput & collisions

Page 21: Collisions & Virtual collisions in IEEE 802.11 networks

Thank you!