Mehmet Bilgi Department of Computer Science and Engineering 1 Capacity Scaling in Free-Space-Optical Mobile Ad-Hoc Networks Mehmet Bilgi University of Nevada, Reno

Mehmet Bilgi Department of Computer Science and Engineering 2 Agenda RF and FSO Basics FSO Propagation Model FSO in Literature Mobility Model and Alignment Simulation Results Conclusions Future Work

Mehmet Bilgi Department of Computer Science and Engineering 3 RF and FSO Illustration Different natures of two technologies: omni-directional and directional Omni-directional RF antenna Directional FSO antenna Transmitter Receiver Transmitter Receiver

Mehmet Bilgi Department of Computer Science and Engineering 4 A well-known fact: RF suffers from frequency saturation and RF- MANETs do not scale well  √n as n is increased [1]  Linear scalability can be achieved with hierarchical cooperative MIMO [2] imposing constraints on topology and mobility pattern Omni-directional nature of the frequency propagation causes:  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 RF Saturation

Mehmet Bilgi Department of Computer Science and Engineering 5 Fiber Optical Solutions As of 2003;  Only ~5% of buildings have fiber connections  ~75% of these buildings are within 1 mile range of fiber Laying fiber to every house and business is costly and takes a long time Considered as sunk cost: no way to recover  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: ExParte FCC WTB Filing by Cisco Systems, May 16, 2003

Mehmet Bilgi Department of Computer Science and Engineering 6 FSO Advantages Materials: cheap LEDs or VCSELs with Photo-Detectors, commercially available, <$1 for a transceiver pair Small ( ~1mm 2 ), low weight ( <1gm ) Amenable to dense integration ( transceivers possible in 1 sq ft ) Reliable ( 10 years lifetime ) Consume low power ( 100 microwatts for 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

Mehmet Bilgi Department of Computer Science and Engineering 7 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 RF’s channel fading Investigated the effects of intermittent connectivity on higher layers:  Especially TCP

Mehmet Bilgi Department of Computer Science and Engineering 8 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  Bragg’s Law [1] :  σ is the attenuation coefficient, defined by Mie scattering:  V is the atmospheric visibility, q is the size distribution of the scattering particles whose value is dependent on the visibility 1 H. Willebrand and B. S. Ghuman. Free Space Optics. Sams Pubs, st Edition.

Mehmet Bilgi Department of Computer Science and Engineering 9 FSO Propagation Model Geometric spread is a function of  transmitter radius γ,  the radius of the receiver ς,  divergence angle of the transmitter θ,  the distance between the transmitting node and receiving node R [1] : R max (receiver radius) Maximum range (our approximate model: “triangle + half-circle”) Maximum range (Lambertian model) Coverage Area Uncovered Area R  Error in the approximate model FSO Transmitter (e.g. LED) FSO Receiver (e.g. PD) Geometrical Spread of the Beam 1 H. Willebrand and B. S. Ghuman. Free Space Optics. Sams Pubs, st Edition.

Mehmet Bilgi Department of Computer Science and Engineering 10 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  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 ) Traditional roof-top FSO deployment

Mehmet Bilgi Department of Computer Science and Engineering 11 Free-Space-Optical Interconnects  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 Hybrid FSO/RF applications  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 Deep space communications 1 M. Naruse et al., Real-Time Active Alignment Demonstration for Free-Space Optical Interconnections, IEEE Photonics Tech. Letters, Nov FSO Literature – High Speed Interconnect with misalignment detector [1]

Mehmet Bilgi Department of Computer Science and Engineering 12 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 auto- tracking  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 ) FSO Literature

Mehmet Bilgi Department of Computer Science and Engineering 13 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 But with a reduced amount Recoverable with special techniques (auto-alignment circuit)  Our work: FSO in MANET context with mobility Multi-element optical antenna design: Honeycombed arrays of directional transceivers

Mehmet Bilgi Department of Computer Science and Engineering 14 Mobility Model in NS-2 B-5 (Pos-1) A2 A1 A8 A-1 B4 B5 B6 A-8 B3 B4 B5  No network simulator has FSO simulation capabilities  Each transceiver keeps track of its alignments A table based implementation Alignment timers Node-A Node-B in Pos-1 Node-B in Pos-2 Node-B in Pos-3 A-7 B2 B3 B4 B-3 (Pos-3) A8 A7 A6 B-4 (Pos-2) A1 A8 A7 Alignment tables in interface 5 of node B and interface 1 of node A Alignment tables in interface 4 of node B and interface 8 of node A Alignment tables in interface 3 of node B and interface 7 of node A  Example scenario: 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

Mehmet Bilgi Department of Computer Science and Engineering 15 Mobility Experiment Misaligned Aligned Denser packing will allow fewer interruptions (and smaller buffering), but more handoffs. Received Light Intensity from the moving train Detector Threshold 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: Monitors the light intensity in all interfaces Handles auto hand-off among different transceivers Initiates the search phase Search Phase: When misaligned, an interfaces sends out a search signal ( pre-determined bit sequence ), freq of search signal Waits for reception When senses a search signal, responds it Interfaces restore the data transmission phase We want to observe TCP behaviour over FSO- MANETs MisalignedAligned

Mehmet Bilgi Department of Computer Science and Engineering 16 Simulations 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 Divergence angle 200 mrad Per-flow throughputs are depicted Random waypoint algorithm, conservative mobility IEEE MAC limitation (20 Mbps) 210 meters 30 meters

Mehmet Bilgi Department of Computer Science and Engineering 17 Stationary RF and FSO Comparison RF and FSO comparison in stationary case, no mobility

Mehmet Bilgi Department of Computer Science and Engineering 18 Stationary RF and FSO Comparison RF and FSO comparison with different number of interfaces

Mehmet Bilgi Department of Computer Science and Engineering 19 Mobile FSO: TCP is adversely affected Mobility Effect in FSO. TCP is adversely effected.

Mehmet Bilgi Department of Computer Science and Engineering 20 Mobile RF and FSO Comparison RF/FSO comparison w.r.t. Speed

Mehmet Bilgi Department of Computer Science and Engineering 21 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 Department of Computer Science and Engineering 22 Node Density with Fixed Power Both performs poorly in a larger area when power is not adjusted accordingly

Mehmet Bilgi Department of Computer Science and Engineering 23 Node Density with Adjusted Power RF performs better when power is adjusted, Uncovered regions causes FSO’s loss RF’s power consumption is way bigger than FSO’s

Mehmet Bilgi Department of Computer Science and Engineering 24 Mobile UDP Results UDP and TCP mobile throughput comparison

Mehmet Bilgi Department of Computer Science and Engineering 25 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 Department of Computer Science and Engineering 26 Future Work Introduce buffers at LL and/or Network Layer Group concept Directional MAC Effect of search signal sending frequency

Mehmet Bilgi Department of Computer Science and Engineering 27 Questions