fractel: building (rural) mesh networks with predictable performance on the feasibility of the link...

FRACTEL: Building (Rural) Mesh Networks with Predictable Performance

On the Feasibility of the Link Abstraction in (Rural) Mesh Networks

Dattatraya Gokhale (Indian Navy),Sayandeep Sen (U. Wisconsin-Madison),

Kameswari Chebrolu (IIT Bombay),Bhaskaran Raman (IIT Bombay)

Work done at IIT KanpurPresentation at Infocom 2008, Phoenix, Apr 2008

FRACTEL DeploymentwiFi-based Rural data ACcess & TELephony

Cherukumilli

Juvvalapalem

Point-to-Point 802.11 Links withDirectional Antennas

Landline: wired gateway to the

Internet

Point-to-Multi-Point802.11 link-sets using Sector Antennas

19 Km

19.5 Km

Local-Gateway: gateway to

LDN

LACN: Local-ACcess

Network at one of the villages (desired, not

deployed)

LDN: Long-Distance Network (deployed, in the Ashwini project)

Jalli Kakinada

Ardhavaram

Kasipadu

Alampuram

Tetali

Pippara

Kesavaram

Korukollu Polamuru

Jinnuru

Lankala KoderuBhimavaram

Tadinada

IBhimavaram

Need to support real-time apps.

Link Abstraction: Background

• Understanding link behaviour has implications on– Network Planning, Protocol Design, Application Design

Pac

ket

Err

or

Rat

e (%

)

Average RSSI (dBm)

Link ExistsNegligible error rates

• Link abstraction:– Either link exists

or does not

– That is, 0% packet reception, or ~100%

– Abstraction holds in wired networks

Link doesn’t exist.

Steep change in Error Rate

DGP, Roofnet, FRACTEL LACN

Typical

link distances

Network architecture

Environment

Multipath effects

SNR or RSSI

External interference

Link abstraction

Long-distance

mesh networks

(e.g. DGP)

Up to few tens of

kms

High gain directional &

sector antennas on

tall towers or masts

Rural setting

studied in depth

Effect not apparent

Has strong correlation with link quality

Affects links performance

Valid

Rooftop mesh

networks (e.g.

Roofnet)

Mostly < 500 m

Mostly omni-directional antennas on

rooftops

Dense urban setting

studied in-depth

Reported as a

significant component

Not useful in

predicting link quality

Reported as not

significant

Not valid

FRACTEL LACNs

Mostly < 500 m

Would like to avoid tall

towers

Rural, campus,

residential

To be determined

To be determined

To be determined

To be determ

ined

Experimental Methodology

• Two kinds of environment– Five locations on campus, One village location

• One transmitter, Multiple receiver positions• Broadcast 6K 1400 byte pkts with 20ms gap.

• Hardware same as in DGP study, Roofnet study

Results: Err Rate vs RSSI

• For Interference free positions– If RSSI > Threshold

Error rates are low and stable

• For Interference prone positions– Intermediate error rates

Data Rate: 1Mbps

MIT Roofnet: Conclusions• No correlation between SNR and Error Rate

– Loss rate high at high SNR

• No correlation between lost packets and foreign packets observed

• Introducing delay spread causes high loss rates

Multipath induced delay spread and not interference is the culprit

MIT Roofnet data: Fresh Analysis

Data Rate: 1Mbps, Average RSSI > -80 dBm, 80% >Error Rate > 20%



Noise band as high as 16 dB, Ours ~ 2 dB



Roofnet Max Noise = -75 dBm, Ours = -94 dBm



What is the cause of increased noise level? Multipath does not cause high noise level

Is it Interference ?

Understanding Interference

1. Does interference on affect noise levels ?

2. Can pkts. loss be related to no. of foreign pkts seen?

3. Can reported noise level be used to gauge the level of interference?

4. Can we estimate the link performance based on the average measured noise floor?



2. Does pkts. loss correlate with foreign pkts. ?

3. Use reported noise level to gauge level of interf.?

4. Estimate the link perf. based on the avg. measured noise floor?

Controlled Interference Expt

• Experimental Setup

• A and B Hidden Nodes to one another• B’s power fixed at -75 dBm• A’s power varied: -90, -85, -80, -75 dBm

Does Interference affect noise levels?

Noise extends right up to

-65 dBm

RoofnetControlled Experiment

Avg Received RSSI from A is

-85 dBm and B is -75 dBm

P1: Interference causes noise level to be high and variable

Can packet loss be related to foreign packets seen?

As far as B’s packets are concerned:B’s loss = 18.3%; A’s loss = 99.2% 4 foreign pkts /sec

P2: Packet loss high even though number of observed foreign packets low

Hidden node, receiver outside of interferer’s reception range

P3: Packet loss can be low even though number of observed foreign packets is high

IN RANGE

NODE ‘R’

NODE ‘B’NODE ‘A’

P2 P3Support Roofnet’s observation but disprove conclusion

Non-hidden-node case

Can the reported noise level be used to gauge the level of interference?


– Instantaneous noise levels show variability

Large Noise Band


– Noise levels reported differ from known level

P4: On this H/W, gauging level of interference is error prone

Actual Interference Perceived Interference

Can link performance be estimated based on average noise floor?

P5: It is not possible to estimate the link quality based on reported noise floor

From Roofnet Data

Operating near the RSSI threshold

Village Location

Variable Loss Rates


Variable Loss Rates

Village LocationNo Interference


Variable Loss Rates

Village LocationNo Interference

What is the reason behind Intermediate error rates ?


Village Location

Pac

ket

Err

or

Rat

e (%

)

Average RSSI (dBm)

RSSI Below Thresh.


Village Location

Pac

ket

Err

or

Rat

e (%

)

Average RSSI (dBm)

RSSI Above Thresh.


Village Location

Cannot distinguish between links with loss rates between 0-100%Cannot use routing metrics based on ETX or WCETT (going to be unstable)

Small variation in RSSI large variation in error rates

Design Implications• Link abstraction:

– Absence of external interference can plan links

• Routing:– Metrics to distinguish between links with

intermediate loss rates (e.g. ETX and WCETT) are likely unstable

– Gauging interference can be error prone in interference-aware routing

• MAC:– CSMA/CA will lead to unpredictable loss rates,

due to self-interference and hidden node cases

Conclusion

• Multipath and delay spread:– Not a likely factor in rural areas– May or may not be the main culprit in urban areas– Roofnet: extrapolates 900 MHz multipath measmts.

• Lesson: double-check measurements, analysis

• Open questions:– 802.11g and 802.11a?– Future of unplanned deployments?

• For further information on FRACTEL:– http://www.cse.iitb.ac.in/~br/

http://www.cse.iitb.ac.in/~br/

Backup slides


Village Location

• Cannot distinguish between links with loss rates between 0-100%

• Cannot use routing metrics based on ETX or WCETT

• In the absence of external interference

• Intermediate error rates due to operation in steep region

• Small variation in RSSI large variation in error rates

Interference free locations: RSSI Stability

• Short-term stability (2-min expt): Mostly within 3-4 dB

Hardware Quirk

Person standing near antenna

Band: Difference between the 95%-ile and 5%-ile values of RSSI

Long-term Stability

• Band variation depends on environment but within 4dB in most cases

Design Implication: Link Abstraction

• Absence of external interference can plan links with predictable performance.

• Simplifies higher layer protocol design considerably

RSSI threshold-79dBm

RSSI variation3-4dB

Modified RSSI Threshold Value

-75dBm

Design Implication: MAC

• CSMA/CA unsuitable in multihop mesh networks

• External interference can worsen CSMA/CA performance

• Roofnet data indicates considerable sources at interference range but not reception range– RTS/CTS will not help in this setting

FRACTEL: Ongoing Work

• Scope for TDMA-based mesh network– Lots of theory research, little in systems

• Systems issues in TDMA-based networks:– Interference mapping– Synchronization– Schedule dissemination, dynamic scheduling– Scaling

• For further information:– http://www.cse.iitb.ac.in/~br/

http://www.cse.iitb.ac.in/~br/

Long Distance Networks (LDNs)

"Long-Distance 802.11b Links: Performance Measurements and Experience'', Kameswari Chebrolu, Bhaskaran Raman, and Sayandeep Sen, MOBICOM 2006

Link abstraction holds

Measurements on the DGP network, Kanpur, India

Local Access Networks (LACNs)

• Prior Studies: MIT Roofnet study– Outdoor WiFi mesh, Boston/Cambridge area– Most links have intermediate loss rates, between

0% and 100%– Multi-path (not external interference) is a major

cause of losses– No Link Abstraction!– Work around: design of appropriate routing

metrics

Experimental Methodology: Environment

• Two kinds of environment– Five locations on campus– One village location

• Link lengths: 150-400m • One transmitter position• Up to 6 receiver positions

– Good – Avg. RSSI ≈ -70 dBm– Medium – Avg. RSSI ≈ -75 dBm– Bad – Avg. RSSI ≈ -80 dBm

FRACTEL Measmt. Study: IITK

Source: Google Maps

Dense buildings, academic area, student dormitories, campus housing, several trees



2. Can pkts. loss be related to no. of foreign pkts seen?

3. Can reported noise level be used to gauge the level of interference?

4. Can we estimate the link performance based on the average measured noise floor?

FRACTEL Measmt. Study: Amaur

Source: Google Maps

Dense buildings, 2-3 storey tall

Experimental Methodology• Hardware

– Laptops with Senao 2511CD Plus 802.11b PCMCIA cards

– Antennas• Sector Antenna – 17 dBi

• Omni Directional Antenna – 8 dBi

• Software– Linux – kernel 2.6.11– Modified HostAP driver – ver 0.4.9– Each experiment broadcasts 6000 1400 byte pkts

with 20ms gap at 4 different data rates

Hardware same as in DGP study, Roofnet study

Controlled Interference Expt

• Experimental Setup

• A: 1400-byte packets, 2ms interval, 11Mbps• B: 1300-byte packets, 2ms interval, 11Mbps• B’s power fixed at -75 dBm• A’s power varied: -90, -85, -80, -75 dBm


– Instantaneous noise levels show variability– Noise levels reported differ from known level

P4: On this H/W, gauging level of interference is error prone

fractel: building (rural) mesh networks with predictable performance on the feasibility of the link...

Documents

directional sector antennas

tall towersrural

mastsrural setting

tens of kmshigh

rooftopsdense urban

fractel deploymentwifi

apparenthas strong correlation

iit kanpurpresentation