Capacity Scaling in Free-SpaceOptical Mobile Ad-Hoc Networks Mehmet

Capacity Scaling in Free-SpaceOptical Mobile Ad-Hoc Networks Mehmet

Capacity Scaling in Free-SpaceOptical Mobile Ad-Hoc Networks Mehmet Bilgi University of Nevada, Reno Mehmet Bilgi Department of Computer Science and Engineering Agenda RF and FSO Basics FSO Propagation Model FSO in Literature Mobility Model and Alignment Simulation Results Conclusions Future Work Mehmet Bilgi 2

Department of Computer Science and Engineering RF and FSO Illustration Receiver Transmitter Receiver Directional FSO antenna Transmitter Omni-directional RF antenna Different natures of two technologies: omni-directional and directional Mehmet Bilgi 3 Department of Computer Science and Engineering RF Saturation A well-known fact: RF suffers from frequency saturation and RFMANETs do not scale well Omni-directional nature of the frequency propagation causes: n as n is increased [1] Linear scalability can be achieved with hierarchical cooperative MIMO [2] imposing constraints on topology and mobility pattern Channel is a broadcast medium, overhearing

Security problems Increased power consumption to reach a given range End-to-end per-node throughput vanishes: approaches to zero as more nodes are added 1 Gupta, P. Kumar, P.R. , The capacity of wireless networks, IEEE Transactions on Information Theory, 00 2 Ozgur et al., Hierarchical Cooperation Achieves Optimal Capacity Scaling in Ad Hoc Networks, IEEE Transactions on Information Theory, 06 Mehmet Bilgi 4 Department of Computer Science and Engineering Fiber Optical Solutions As of 2003; Laying fiber to every house and business is costly and takes a long time Considered as sunk cost: no way to recover Only ~5% of buildings have fiber connections ~75% of these buildings are within 1 mile range of fiber Purchase land to lay fiber Digging ground Maintenance of fiber cable is hard

Modulation hardware is sensitive and expensive ISPs are uneager to deploy aggressively because of initial costs They are deploying gradually Attempts existed in near past: California, Denver, Florida (before 2000) 1 Source: 02-146 ExParte FCC WTB Filing by Cisco Systems, May 16, 2003 Mehmet Bilgi 5 Department of Computer Science and Engineering FSO Advantages Materials: cheap LEDs or VCSELs with Photo-Detectors, commercially available, <$1 for a transceiver pair Small (~1mm ), low weight (<1gm) Amenable to dense integration (1000+ transceivers possible in 1 sq ft) Reliable (10 years lifetime) Consume low power (100 microwatts for 10-100 Mbp) Can be modulated at high speeds (1 GHz for LEDs/VCSELs and higher for lasers)

Offer highly directional beams for spatial reuse/security Propagation medium is free-space instead of fiber, no dedicated medium No license costs for bandwidth, operate at near-infrared wavelengths 2 Mehmet Bilgi 6 Department of Computer Science and Engineering FSO Disadvantages FSO requires clear line-of-sight (LOS) Maintaining LOS is hard even with slight mobility Node often looses its connectivity: intermittent connectivity Loss of connectivity is different than RFs channel fading Investigated the effects of intermittent connectivity on higher layers: Especially TCP Mehmet Bilgi

7 Department of Computer Science and Engineering FSO Propagation Model Atmospheric attenuation, geometric spread and obstacles contribute to BER Atmospheric attenuation: Absorption and scattering of the laser light photons by the different aerosols and gaseous molecules in the atmosphere Mainly driven by fog, size of the water vapor particles are close to near-infrared wavelength Braggs Law [1]: AL 10 log e R is the attenuation coefficient, defined by Mie scattering: 3.91 V 550

q V is the atmospheric visibility, q is the size distribution of the scattering particles whose value is dependent on the visibility 1.6, V 50km q 1.3, 6km V 50km Pubs, 1 H. Willebrand and B. S. Ghuman. Free Space Optics. Sams 0 V 1/ 31, Edition. V 6km .5852001. st Mehmet Bilgi 8 Department of Computer Science and Engineering FSO Propagation Model Geometric spread is a function of transmitter radius , the radius of the receiver , divergence angle of the transmitter , the distance between the transmitting node and receiving node R [1]:

AG 10 log 200 R 2 Error in the approximate model (receiver radius) Rmax FSO Receiver (e.g. PD) R Maximum range (our approximate model: triangle + half-circle) Geometrical Spread of the Beam FSO Transmitter (e.g. LED) Uncovered Area Coverage Area Maximum range (Lambertian model) 1 H. Willebrand and B. S. Ghuman. Free Space Optics. Sams Pubs, 2001. 1st Edition.

Mehmet Bilgi 9 Department of Computer Science and Engineering FSO Literature High Speed Terrestrial last-mile applications Roof-top deployments Metropolitan / downtown areas Point-to-point high speed links Use high-powered laser light sources Traditional roof-top FSO deployment Use additional beams to handle swaying of buildings Gimbals for tracking the beam Limited spatial reuse

Some indoor applications with diffuse optics (more on this later) Mehmet Bilgi 10 Department of Computer Science and Engineering FSO Literature High Speed Free-Space-Optical Interconnects Hybrid FSO/RF applications Inside the large computers to eliminate latency Short distances(1-10s cm) Remedy vibrations in the environment Use backup beams, misalignment detectors Expensive, highly-sensitive tracking instruments Consider FSO as a back-bone technology No one expects pure-FSO MANETs Single optical beam No effort to increase the coverage of FSO via spatial reuse Interconnect with misalignment detector [1] Deep space communications 1 M. Naruse et al., Real-Time Active Alignment Demonstration for Free-Space Optical Interconnections, IEEE Photonics Tech. Letters,

Nov. 2001 Mehmet Bilgi 11 Department of Computer Science and Engineering FSO Literature Mobile FSO Communications Indoor, single room using diffuse optics Suitable for small distances Outdoor (roof-top and space) studies focus on swaying and vibration Scanning, tracking via beam steering using gimbals, mechanical autotracking Instruments are slow and expensive We propose electronical steering methods Effects of directional communication on higher layers Choudhury et al. worked on RF directionality, directional MAC

Traditional flooding based routing algorithms are effected badly Directionality must be used for localization also (future work) Mehmet Bilgi 12 Department of Computer Science and Engineering Mobility Model Design an antenna with FSO transceivers to Exploit directionality and spatial reuse Target mobility Multi-element antenna using commercially available components Disconnections will still occur 1 Multi-element optical antenna design: Honeycombed arrays of directional transceivers 3 But with a reduced amount

Recoverable with special techniques (auto-alignment circuit) 10 2 11 15 12 16 4 8 14 13 Our work: FSO in MANET context with mobility Mehmet Bilgi 13 9 7 5 6 Department of Computer Science and Engineering Mobility Model in NS-2

No network simulator has FSO simulation capabilities Each transceiver keeps track of its alignments A table based implementation Alignment timers Example scenario: B-5 (Pos-1) A 2 A 1 A 8 2 1 8 Node-B in Pos-1 3

4 5 2 nodes with 8 interfaces each Node-B has relative mobility w.r.t. Node-A Observe the changes in alignment tables of 2 different transceivers in two nodes 7 6 Alignment tables in interface 5 of node B and interface 1 of node A B-4 (Pos-2) Node-B in Pos-2 A-7 A-8 A-1 BB 342 BB 453 BB 564 2 3 1 5

1 A 8 A 7 Alignment tables in interface 4 of node B and interface 8 of node A B-3 (Pos-3) 8 Node-A 4 A 7 Node-B in Pos-3 6 A 8 A 7 A 6 Mehmet Bilgi

14 Alignment tables in interface 3 of node B and interface 7 of node A Department of Computer Science and Engineering Misaligned Aligned Train looses and re-gains its alignment in a short amount of time: intermittent connectivity Measured light intensity shows the connection profile Complete disruption of the underlying physical link: different than RF fading Auto-alignment circuitry: Received Light Intensity from the moving train 70 60 Monitors the light intensity in all interfaces Aligned 50 Handles auto hand-off among different transceivers

Misaligned 40 30 20 128 121 112 105 9 7 .5 8 8 .5 79 72 65 5 1 .5 4 0 .5 33 23 17 0 11 10 0

Detector Threshold Initiates the search phase Angular Position of the Train (degree) Denser packing will allow fewer interruptions (and smaller buffering), but more handoffs. Search Phase: When misaligned, an interfaces sends out a search signal (pre-determined bit sequence), freq of search signal Waits for reception L O S o p -a m p & filte r 2 3 M U X L O S o p -a m p & filte r L E D P D P D L E D L E D P D P D

L E D L E D P D P D L E D L E D P D o p -a m p & filte r L O S o p -a m p & filte r L O S o p -a m p & filte r M U X L O S M U X o p -a m p & filte r 1 3 0 1

Interfaces restore the data transmission phase L O S M U X o p -a m p & filte r When senses a search signal, responds it 0 2 M U X XUMED M U X D a ta S in k P D L E D re do c nE 1 yt iro i rP 2 -oT -4 1 4-To-2 Priority Encoder 0 0 MUX L ig h t In t e n s it y ( lu x )

Mobility Experiment D a ta S o u rc e L O S M U X o p -a m p & filte r L O S M U X We want to observe TCP behaviour over FSOMANETs Mehmet Bilgi 15 Department of Computer Science and Engineering 210 meters 30 meters 49 nodes in a 7 x 7 grid Every node establishes an FTP session to every other node: 49x48 flows 4 interfaces per node, each with its own MAC 3000 sec simulation time 210 meters Simulations 30 meters Divergence angle 200 mrad Per-flow throughputs are depicted

Random waypoint algorithm, conservative mobility IEEE 802.11 MAC limitation (20 Mbps) Mehmet Bilgi 16 Department of Computer Science and Engineering Stationary RF and FSO Comparison RF and FSO comparison in stationary case, no mobility Mehmet Bilgi 17 Department of Computer Science and Engineering Throughput (KB/s) Stationary RF and FSO Comparison 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 Number of Interfaces RF and FSO comparison with different number of interfaces Mehmet Bilgi 18 Department of

Computer Science and Engineering Mobile FSO: TCP is adversely affected Mobility Effect in FSO. TCP is adversely effected. Mehmet Bilgi 19 Department of Computer Science and Engineering Mobile RF and FSO Comparison RF/FSO comparison w.r.t. Speed Mehmet Bilgi 20 Department of Computer Science and Engineering Node Density Effect Fixed power: 49 nodes Increase the separation b/w nodes and the area Keep the source transmit power same Adjusted power:

49 nodes Increase the separation b/w nodes and the area Adjust the source transmit power so that they can reach increased distance Mehmet Bilgi 21 Department of Computer Science and Engineering Node Density with Fixed Power Both performs poorly in a larger area when power is not adjusted accordingly Mehmet Bilgi 22 Department of Computer Science and Engineering Node Density with Adjusted Power RF performs better when power is adjusted, Uncovered regions causes FSOs loss RFs power consumption is way bigger than FSOs Mehmet Bilgi 23 Department of Computer Science and Engineering Mobile UDP Results 1.4 Throughput (KB/s) 1.2 1 0.8

TCP 0.6 UDP 0.4 0.2 0 4 9 Number of Nodes UDP and TCP mobile throughput comparison Mehmet Bilgi 24 Department of Computer Science and Engineering Conclusions FSO MANETs are possible and provides significant benefit via spatial reuse Mobility affects TCP performance severely RF and FSO are complementary to each other; coverage + throughput Mehmet Bilgi 25 Department of Computer Science and Engineering Future Work Introduce buffers at LL and/or Network Layer

Group concept Directional MAC Effect of search signal sending frequency Mehmet Bilgi 26 Department of Computer Science and Engineering Questions Mehmet Bilgi 27 Department of Computer Science and Engineering

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