Transcript Presentation - CSE, IIT Bombay

FRACTEL: Building (Rural)
Mesh Networks with
Predictable Performance
On the Feasibility of the Link
Abstraction in (Rural) Mesh Networks
Bhaskaran Raman, Kameswari Chebrolu
IIT Bombay
Dattatraya Gokhale, Sayandeep Sen
Work done at IIT Kanpur
Presentation at AirTight Networks, Pune, 22 Dec 2007
LocalGateway:
gateway to
LDN
FRACTEL Deployment
wiFi-based Rural
data ACcess &
TELephony
Alampuram
Tetali
Point-to-Multi-Point
802.11 link-sets using
Sector Antennas
Kasipadu
LACN: LocalPoint-to-Point
ACcess Network
802.11 Links with
at one of the
Directional
villages (desired,
Antennas
not deployed)
Pippara
Ardhavaram
Kesavaram
Jalli Kakinada
19 Km
Korukollu
Polamuru
IBhimavaram
Tadinada
Juvvalapalem
19.5 Km
Cherukumilli
Bhimavaram
Landline: wired
gateway to the
Internet
Jinnuru
Lankala Koderu
Network has
a fractal
structure!
LDN: LongDistance Network
(deployed, in the
Ashwini project)
FRACTEL Goals
• Support a variety of applications:
–
–
–
–
HTTP/FTP
Voice over IP
Video-conferencing based
Real-time applications particularly important in rural
setting
• Quality of Service is necessary
• Scalable operation:
– Deployment for a few hundred nodes in a district
Link Abstraction: Background
– Either link exists
or does not
– That is, 0%
packet reception,
or ~100%
– Abstraction holds
in wired
networks
Packet Error Rate (%)
• Link abstraction:
Link doesn’t exist.
Steep change in Error Rate
Link Exists
Negligible error rates
Average RSSI (dBm)
• Understanding link behaviour has implications on
– Network Planning
– Protocol Design
– Application Design
Long Distance Networks (LDNs)
Link abstraction holds
"Long-Distance 802.11b Links: Performance Measurements and Experience'',
Kameswari Chebrolu, Bhaskaran Raman, and Sayandeep Sen, MOBICOM 2006
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
Link Abstr.: DGP, Roofnet, FRACTEL
Typical
link
distances
Network
architecture
Environme
nt
Up to few
tens of
kms
High gain
directional &
sector
antennas on
tall towers or
masts
Rooftop
mesh
networks
(e.g.
Roofnet)
LACNs
Longdistance
mesh
networks
(e.g. DGP)
Multipath
effects
SNR or
RSSI
External
interference
Link
abstra
ction
Rural
setting
studied in
depth
Effect not
apparent
Has strong
correlation
with link
quality
Affects links
performance
Valid
Mostly <
500 m
Mostly omnidirectional
antennas on
rooftops
Dense
urban
setting
studied indepth
Reported as
a significant
component
Not useful
in
predicting
link quality
Reported as
not
significant
Not
valid
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:
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
FRACTEL Measmt. Study: Amaur
Source: Google Maps
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
Results: Err Rate vs RSSI
• For Interference prone positions
– Intermediate error rates
• For Interference free positions
– If RSSI > Threshold
Error rates are low and stable
Data Rate: 1Mbps
MIT Roofnet Data: 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 %)
Roofnet: Noise band as high as 16dB; Max noise = -75dBm
Our expts: Noise band ~ 2 dB, Max Noise = -94 dBm
What is the cause of increased noise level?
Multipath does not cause high noise level
Is it Interference ?
Understanding Interference
• Does interference increase the noise level
reported by the card?
• Can packet loss be related to the number
of foreign packets seen?
• Can the reported noise level be used to
gauge the level of interference?
• Can we estimate the link performance
based on the average measured noise
floor?
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
Does Interference affect noise levels?
Controlled Experiment
Roofnet
Avg Received RSSI from A is
-85 dBm and B is -75 dBm
Noise extends right up to
-65 dBm
P1:
Interference causes noise level to be high and
variable
Can packet loss be related to foreign
packets seen?
Col-1
Col-2
Col -3
Col-4
Col-5
Col-6
Col-7
Col-8
Col-9
Expt
No
Src
Mean
RSSI
Loss
%
Mean
Noise
5
%-ile
95
%-ile
Noise
Band
Max
Noise
(dBm)
(dBm)
(dBm)
(dB)
(dBm)
-85
-85
9
9
-85
-84
(dBm)
2
2
A
B
-85.23 99.2 -92.53 -94
-74.68 18.3 -89.34 -94
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
IN RANGE
NODE ‘A’
NODE ‘B’
NODE ‘R’
Node
A
P3:
P2
• Senses
Medium,
Clear
• Starts
TX
Node
B
• Senses
Medium
• Hears A
• Backs off
Node
R
No of
packets
seen low
Packet loss can be low even though number
of observed foreign packets is high
P3
Support Roofnet’s
observation but disprove
conclusion
Can the reported noise level be used
to gauge the level of interference?
Col-1
Col-2
Col -3
Col-4
Col-5
Col-6
Col-7
Col-8
Col-9
Expt
No
Src
Mean
RSSI
Loss
%
Mean
Noise
5
%-ile
95
%-ile
Noise
Band
Max
Noise
(dBm)
(dBm)
(dBm)
(dB)
(dBm)
-85
-85
9
9
-85
-84
(dBm)
2
2
A
B
-85.23 99.2 -92.53 -94
-74.68 18.3 -89.34 -94
– Instantaneous noise levels show variability
– Noise levels reported differ from known level
P4:
On this H/W, gauging level of interference is
error prone
Can link performance be estimated
based on average noise floor?
From Roofnet
Data
P5:
It is not possible to estimate the link quality
based on reported noise floor
Conclusion so far
• External interference can cause high loss
rates (from our expts)
• Intermediate loss rates in Roofnet possibly
due to interference and not multi-path
• Estimating level of interference from noise
floor (for use in routing etc) is difficult
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
Location
Rx
Posn
LoS ?
Dur.
(hrs)
RSSI
95%-ile
(dBm)
RSSI
5%-ile
(dBm)
RSSI
Band
(dB)
(Y/N)
Apt
1
Y
48
-66
-69
3
Apt
2
N
(foliage)
48
-69
-77
8
Apt
3
N
(foliage)
48
-76
-82
6
Apt2Dorm
1
Y
24
-75
-77
2
Apt2Dorm
2
Y
24
-70
-71
1
Apt2Dorm
3
N
(foliage)
24
-79
-81
2
Operating near the threshold
Village Location
• Cannot distinguish between links with loss rates
between 0-100%
• Cannot use routing metrics based on ETX or WCETT
Design Implications: Link
Abstraction
• Allows us to plan links with predictable
performance.
• Simplifies higher layer protocol design
considerably
RSSI Threhold
-79dBm
Modified RSSI
Threshold Value
-75dBm
RSSI variation
3-4dB
Design Implication: Routing
• Routing metrics (ETX and WCETT) that
tend to distinguish between links with
intermediate loss rates are unstable
• Opportunistic routing (EXOR) cannot give
performance guarantees
• Gauging interference can be error prone in
interference-aware routing
Design Implications: 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
Discussion
• Multipath and delay spread:
– Not a likely factor in rural areas
– May or may not be the culprit in urban areas
– Roofnet extrapolated multipath measurements
done in 900 MHz cellular band!
• Lesson: wireless measurements  pay
attention to detail, double-check
• Open questions:
– 802.11g and 802.11a?
– Future of unplanned deployments?
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/