mswim, october 2010 1 throughput characteristics of free-space-optical mobile ad-hoc networks mehmet...
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MSWiM, October 2010
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Throughput Characteristics of Free-Space-Optical Mobile Ad-hoc Networks
Mehmet Bilgi and Murat Yuksel{mbilgi,yuksem}@cse.unr.edu
Computer Science and EngineeringUniversity of Nevada – Reno
Project Website: http://www.cse.unr.edu/~yuksem/fso-manet.htm
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Collaborators
Faculty: Murat Yuksel ([email protected]), Univ. of
Nevada, Reno Mona Hella ([email protected]), Rensselaer
Polytechnic Institute
Students: Abdullah Sevincer ([email protected]) (M.S.),
UNR Mehmet Bilgi ([email protected]) (Ph.D.), UNR Michelle Ramirez ([email protected])
(B.S.), UNR
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Outline
Motivation & Vision FSO Simulation Modules
FSO Propagation LOS Alignment Protocol
Validation Simulations Throughput Simulations Summary and Conclusions
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4Wireless: Spectrum Constraints
Source: Chris Ramming/DARPA: CBMANETS overview
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Dense Deployment: No Help Beyond a Point
As we add more RF nodes, per-node throughput diminishes
Dense deployment of many omni-directional antennas increase interference
sqrt(N) as N increases (Gupta, Kumar, Tran. on Inf. Theo. 2000)
Can become linear with hierarchical cooperative MIMO imposing constraints on topology and mobility pattern (Ozgur et al., Tran. on Inf. Theo. 2006)
None is able to totally eliminate the scaling problemThe RF spectrum is getting saturated.. We need alternative communication spectrum resources.
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6Free-Space-Optical (FSO): open spectrum
Open spectrum: 2.4GHz, 5.8GHz, 60GHz, > 300 GHz
Lots of open spectrum up in the optical regime!
FSO usage: point-to-point links interconnects indoor infrared
communications
DoD use of FSO: Satellite communications DARPA ORCL project: air-to-
ground, air-to-air, air-to-satellite
802.11a/g, 802.16e,Cellular (2G/3G)
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7Optical Wireless: Commodity components
Many FSO components are very low cost and available for mass production.
Lasers…
LEDs…
VCSELs…
IrDAs…
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FSO-MANETs VisionFree-Space-Optical
(FSO) Communications
Mobile Ad-Hoc Networking
• High bandwidth• Low power• Dense spatial reuse• License-free band of operation
• Mobile communication• Auto-configuration
Free-Space-OpticalAd Hoc Networks
• Spatial reuse and angular diversity in nodes• Low power and secure• Electronic auto-alignment• Optical auto-configuration (switching, routing)
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Optical Wireless: Why? Positive points:
More Secure: Highly directional + small size & weight => low probability of interception (LPI)
High-brightness LEDs (HBLEDs) are very low cost and highly reliable components
35-65 cents a piece, and $2-$5 per transceiver package + upto 10 years lifetime
Very low power consumption (100 microwatts for 10-100 Mbps!)
Even lower power for 1-10 Mbps 4-5 orders of magnitude improvement in
energy/bit compared to RF Huge spatial reuse => multiple parallel
channels for huge bandwidth increases due to spectral efficiency
Issues: Need line-of-sight (LOS); and alignment of LOS
Can we leverage these benefits while solving the issues?
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FSO Issues/Disadvantages Limited range (no waveguide, unlike fiber optics) Need line-of-sight (LOS)
Any obstruction or poor weather (fog, heavy rain/snow) can increase BER in a bursty manner
Bigger issue: Need tight LOS alignment: LOS alignment must be changed/maintained with
mobility or sway! Effects of relative distance and mobility Received power
Spatial profile: ~ Gaussian drop off
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FSO Modules: Alignment Protocol
Goal: Provide an FSO link with “seamless” alignment
Steer the data transmission among the transceivers as the nodes move with respect to each other
Need a 3-way handshake among the transceivers to assure a bidirectional alignment
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FSO Modules: Alignment Protocol
Send “search” frames periodically
need an “alignment timer”
Receive data frames only after alignment is established
might still get wrong or erroneous frames – leave them to the higher layers
State diagram of LOS alignment protocol
SendingSYN_ACK
Target Node = i
Recv(ACK, i)
Recv(SYN | SYN_ACK | DATA)
Recv(ACK, j)
Discard
Not AlignedSending SYN
Recv(SYN_ACK, i)
Recv(SYN, i)Start
Recv(ACK | DATA)
Discard
Aligned
Target Node = iRecv(SYN_ACK | ACK)
Recv(DATA, j)
Recv(DATA, i)
Discard
SendingACK
Target Node = i Recv(DATA, i)
Recv(SYN | SYN_ACK | ACK)
Recv(DATA, j)
Discard
ProcessData
Recv(SYN, i)
Alignment TimerTimeout
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FSO Modules: Alignment Protocol
Maintain “alignment lists” to keep track of which transceiver is aligned with which neighbor
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FSO Modules: Propagation & Interference
FSO Propagation Geometric Attenuation
divergence angle receiver’s surface
Atmospheric Attenuation
visibility
FSO Interference Must consider the FSO
signals coming from other nodes too
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FSO Simulations in NS-2 How good/bad the
transport performance will be if we have FSO nodes with
mobility multiple transceivers?
Needed to add several things to NS-2
multi-transceiver nodes
LOS alignment protocol
FSO propagation obstacles
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TCP Throughput over FSO-MANETs
Performed several simulations..
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FSO Simulations in NS-2
Propagation validation
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FSO Simulations in NS-2 Propagation
validation
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FSO Simulations in NS-2 Propagation
validation
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FSO Simulations in NS-2Mobility is a major problem for throughput scaling!
Nodes with wider divergence angle transceivers perform better due to resemblance to RF.
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FSO Simulations in NS-2
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FSO Simulations in NS-2
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Summary & Future Work
Contributed multi-transceiver simulation modules for free-space-optical communication.
Accurate simulation of multi-transceiver FSO structures reveals differences with RF in TCP behavior.
Intermittent connectivity pattern requires re-consideration of network layers to enable cross-layer buffering.
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Thank you!
THE END
AcknowledgmentsThis work was supported by the U.S. National Science Foundation under awards 0721452 and 0721612 and
DARPA under contract W31P4Q-08-C-0080