doc.: ieee 802.11-05-0367-00-0000s submission may 2005 jan kruys,ciscoslide 1 mesh networking...

May 2005

Jan Kruys,CiscoSlide 1

doc.: IEEE 802.11-05-0367-00-0000s

Submission

Mesh Networking

Performance Considerations

Jan [email protected]

+31 20357 2447

mailto:[email protected]

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11.

Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <[email protected]> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at <[email protected]>.

AuthorsName Address Compan

yPhone Email

Jan Kruys Haalerbergweg 10, CH1101Netherlands

Cisco Systems Int’l

+ 31 348 453719

[email protected]

Mesh Networking – Performance Considerations



May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Purpose• Show some of the main factors

that determine mesh performance

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

A Simple Example

main mesh links

back-up connections

Mesh link interference area for 12 Mb/s (Pl exp = 3.3)

AP coverage

Feed point

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Observations• Traffic aggregation near the fibre pop:

– requires high throughput – see next slide• The interference area shown is for 12Mb/s links and

omni antennas– all mesh links share the capacity of ONE RF

channel is this area• because the defer threshold must be set low enough to assure

sufficient SIR• Possible starvation of further away nodes

– their packets have to content for channel access many times• every contention stage decreases throughput exponentially• does not play a role at low load levels (reduced contention)

• This is closely related to the fairness issue brought up by Violeta in the March meeting

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Traffic aggregation (topology effect)• The tree type

mesh is a good model for a group of nodes supported by a wired root node – multiple trees

build a 3600 coverage.

• Link level loads increase rapidly towards the wired root node– even a small fan-

out at each hop causes exponential traffic growth at the root link

Required Mesh Link capacity

0.0

50.0

100.0

150.0

200.0

250.0

1 2 3 4 5 6

Hops from the edge

Rela

tive

load

Serial (fan-out=1)

fan-out=2

fan-out=2.2

Required Mesh link Capacity

0.0

10.0

20.0

30.0

40.0

50.0

1 2 3 4

Hops from the edge

Rela

tive

Load

Series1

Series2

Series3

fan-out=1fan-out=1.7fan-out=2.2

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Starvation of the edge (single RF channel)

• This graph is for a 5 hop deep mesh with a fan-out of 1.6– for a total of 26 APs– a packet from the edge has to contend 4 x with 25 others to get

through to the root = 2.10^-6• this effect becomes noticeable at medium to high traffic loads

Relative Medium access probability as function of nr of hops

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1 2 3 4 5 6

nr of hops

dB o

f pro

babi

lity

low fan out

high fan out

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Down/up ratio limitations (topology effect)

• For the same data rate, delivering packets to the next hop down requires progressively less time – Higher fan-out allows higher down/up ratio– Lower traffic loads also allow higher down/up ratio

1.5

2.5 4

1.00

0.400%

20%40%60%80%

100%

link load

fan-out ratio

Maximum Down traffic % as function of fan-out and link load

1.00

0.80

0.60

0.50

0.40

0.30

0.20

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Taking stock• “Deep” and/or “wide” meshes have a problem with

traffic aggregation near the root– the effect is exponential: ~1/fan-out-1)*(fan-

out^hops)• “Deep” meshes have a problem with down/up traffic

ratio• RF channel access contention causes starvation near

the edge – even if the mesh is narrow– the effect is exponential: (1/nodes)^hops for a single

RF channel• At low offered loads, none of this may be very clearly

noticeable

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Benefits of Sectorization

Reduces the aggregation effect – in proportion to # of

sectorsReduces the starvation effect

– by reducing the contending population

Reduces the re-use exclusion area (for the same EIRP)– improves spectral

efficiency

Feedpoint

600 sector

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Multiple RF channels per node

Requires two backhaul radios per nodeFrequency planning reduces/eliminates re-use and starvation issuesAllows optimization of data rate wrt SIRRetains sharing behaviour of CSMA/CAAggregation and down/up limitation still applyFeed

point

600 sector

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

A second look at starvation• Starvation is caused by repeated

contention for resources– keeping the contending population low reduces

starvation• one way: spread the contenders over multiple

RF channels: e.g. (1/nodes)^hops becomes (1/fan-out)^hops

• second way: operate the link at the lowest possible SIR = lowest possible data rate– throughput requirements drop with distance from the root– the effect is less in a narrow mesh but it still has benefits

r.e. sharing with adjacent networks– keeping the number of hops down is highly

effective• conversely, sharing the RF channel, omni

fashion, is asking for trouble

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

What about small meshes?• Small: fan-out < 3, hops <2, <9 devices, one

root• Performance with one RF channel:

– aggregation: < 2^3 or a factor 8 max.• means “no two HDTV streams on the second hop from the

DSL feed”– down/up ratio is limited to 60% or so at full

load• same impact as aggregation

– starvation for a 6 node network: 1/6^2 = 1/36• hurts most on the second hop nodes: no HDTV viewing and

slow uploads.– probably ok for the home and SMB

• Larger meshes have major performance issues– notably when operated at a single RF channel

May 2005


doc.: IEEE 802.11-05-0367-00-0000s

Submission

Summary• Mesh network performance depends strongly on three

factors:– mesh depth and width (nr of hops/nr of links per node)

• drives aggregation and starvation effects– traffic pattern (down/up ratio)

• limits the mesh depth– RF channel

• limits data rates and increases starvation• can be improved with fire brigade or multi-radio designs

– these factors further complicate the QoS problem• The optimum configuration depends on specific

requirements: – underloaded networks can support a “large” mesh depth or

width– high performance networks require a low fan-out and

shallow mesh• Bottom line: there is no “best” solution - TGs standard

should be flexible enough to support a wide variety of mesh solutions

doc.: ieee 802.11-05-0367-00-0000s submission may 2005 jan kruys,ciscoslide 1 mesh networking...

Documents