Building Blocks for Mobile FreeSpaceOptical Networks

1
Building Blocks for Mobile Free-Space-Optical
Networks

• Jayasri Akella, Chang Liu, David Partyka, Murat
  Yuksel,
• Shiv Kalyanaraman, and Partha Dutta
• Rensselaer Polytechnic Institute
• Emails sri_at_networks.ecse.rpi.edu,
  yuksem_at_ecse.rpi.edu

shiv rpi
2
Outline

• Context and Motivation
• Auto-configurable optical antenna design.
• Tessellated Spherical Optical Antenna
• Auto-alignment Circuit
• Mobility Experiment
• Simulating Mobile FSO Networks
• Results and summary

3
Bringing Optical Communications and Ad Hoc
Networking Together
This paper proposes initial building blocks for
this vision
4
Current Commercial FSO
Point-to-Point Links in dense metros, competing
with wires and leased lines Issues How to
achieve link reliability/availability despite
weather
5
Current RF-Based Ad Hoc Networks

• 802.1x with omni-directional RF antennas
• High-power typically the most power consuming
  part of laptops
• Low bandwidth typically the bottleneck link in
  the chain
• Error-prone, high losses

6
Contributions Ad-hoc FSO BBlocks

• New optical antenna design
• Spherical/Honeycomb structure with FSO
  trans-receiver modules
• Potential logical links function even when
  antennas are in relative motion.
• Auto-configuration circuit that enables physical
  FSO channel handoff
• Integrated with optical antenna design
• Simulation models in ns-2 to enable future
  studies of FSO MANETs
• Initial tests suggest need to revisit routing and
  TCP layer designs

7
FSO Basics

• High-brightness LEDs (HBLEDs) are very low cost
  and highly reliable components
• 35-65 cents a piece, 10 years lifetime
• Low power consumption (100 microwatts for 10-100
  Mbps!)
• 4-5 orders of magnitude improvement in energy/bit
  compared to RF
• Directional gt Huge spatial reuse
• ButFSO also requires
• availability of unobstructed line-of-sight (LOS)
  and,
• alignment of LOS between the transmitter and the
  receiver.

8
LOS Alignment Optical Antenna Concept

• Tessellated spheres with trans-receiver pairs
• Line-of-sight (LOS) auto-alignment electronics
• Rapid alignment handoff gt enables mobility or
  sway, while maintaining the logical link.

• Tessellated Sphere b) Showing a
  Line of Sight Sphere
• Tessellated with LEDPD transceivers.

9
Auto-Alignment Circuit Design

• Pilot signal sent
• If aligned, signal is fed-back
• Feedback signal detection gt alignment!
• Handoff logical link transmit data

10
Alignment Circuit (Contd)
11
Optical antenna Multiple Alignment Circuits

• Multiple channels gt
• Connected to a bank of auto-alignment circuits
• Eg 4-circuit block diagram
• shown below

12
Optical antenna Experimental Platform (Contd)

• LEDs high divergence angle
• PDs angular field of view
• gt the LED-PD pair forms a transceiver cone.
• The transceiver cone covers a significant volume
  of 3-dimensional space.
• Key appropriate packing density to cover entire
  360 steradian of surrounding space.

Tessellated Spherical antennas on stable optical
testing platforms
13
In Action 4-channel spherical optical antenna
Not Aligned Searching phase to locate an LOS
(all channels searching)
Aligned Data Transmission phase (only one
channel active)
14
Mobility Experiment

• UDP data transfer between the moving toy train on
  a circular track and a data-sink at the center of
  the circle.

15
Auto-alignment Search Phase
16
Auto-alignment Aligned! Data Transfer Phase
17
Intensity vs Mobility _at_ The optical antenna
Aligned
Not aligned
Detector Threshold
Denser packing will allow fewer interruptions
(and smaller buffering), but more handoffs
18
Simulation Model Mobile Ad-Hoc FSO nodes

• NS-2 Model
• FSO propagation model (weather effects)
• FSO antennas (sphere model)
• Additional parameters
• directional normals, transmission and receiving
  angles
• to assist the propagation model and LOS
  calculations.

19
Mobile FSO Simulation (Contd)

• Initial proof-of-concept
• 2-D scenario on the XY-plane
• Spatial reuse and angular diversity features
  illustrated.
• Single mobile FSO node
• Circles around four stationary FSO nodes
• Stationary nodes are connected via wired links to
  a single central node.

20
Simple Experiments

• Four experiments, varying
• Speed of mobile node and
• Distance of the mobile node from the central node

21
TCP sequence numbers in Experiment 1
22
Experiment 1 Data Transfer in Bursts After LOS
discovery
23
Experiment 2 Higher distance gt TCP interactions
lower throughput
24
Experiment 3 Lower distance, higher speeds
TCP affected by higher loss rates periodic
disconnections
25
Observations

• Need dense tessellation and packing.
• Need rapid auto-alignment
• TCP may be affected with increasing distance and
  speed.
• End-to-end connection is not the same as physical
  link alignment
• Key
• Need to provide either bit-level buffering and/or
• Link-layer hybrid ARQ/FEC to mask such losses
  from TCP
• Interactions with transport and network level
  protocols will need to be studied and optimized
• Ongoing work

26
Indoor Ad-Hoc FSO Music App
27
Summary

• Ad-hoc FSO communication
• Different from pt-pt FSO and ad-hoc RF
• Key building blocks
• Optical antenna tessellated sphere with dense
  packing of trans-receivers
• Auto-Alignment optoelectronic circuit (simple
  feedback design)
• Absence of mechanical parts such as motors or
  moving mirrors typically used for auto-alignment
  purpose.
• Significant savings in power consumption and
  improved alignment reliability.
• Simple demonstration optical data transmission
  between toy train and ground nodes
• NS-2 simulation components
• FSO propagation models
• Mobile FSO antennas.
• Initial simulation points to need for optimizing
  interactions w/ transport and network level
  protocols.

28
Thanks!
Students Jayasri Akella, sri_at_networks.ecse.rpi.e
du Dr. Murat Yuksel (post-doc)
yuksem_at_ecse.rpi.edu Chang Liu, c.liu_at_ee.unimelb.ed
u.au David Partyka, partyd_at_rpi.edu Sujatha
Sridharan
shiv rpi
Ps Online free videos of all my advanced
networking classes
29
Details
30
Simulation of mobile FSO nodes continued

• An FTP session is kept alive between the central
  node and the mobile node. For our experiments,
  all wired links are 100 Mbps with 2ms delays and
  Drop Tail queues, while the FSO nodes are
  configured to only transmit at 20 Mbps. (20Mbps
  is just our configuration limitation, and is not
  a physical limitation as modulation speeds can be
  in the order of GHz in optical bands)
• Initially, the experiment starts with the mobile
  node and one of the stationary nodes in LOS.
• Soon after the session is established, the node
  moves around the stationary nodes at a constant
  rate of speed. Routing is performed by ad hoc
  DSDV routing agents and MAC is facilitated by
  802.11 that is already present in NS-2.

31
Simulation of mobile FSO nodes continued

• We can see that using FSO propagation model in
  the simulation, it is possible to achieve
  connectivity through mobile FSO communication
  even with a very small number of transceivers on
  the spherical optical antenna.
• The experiments were configured in such a manner
  that LOS is not always present, thus showing that
  connectivity is reestablished when the nodes are
  back in LOS. This is demonstrated by the periods
  of inactivity in the utilization graphs and by
  the plateaus in the TCP sequence number graphs,
  which is shown in the figure.
• The TCP sequence numbers for the other
  experiments also showed similar behavior, where
  plateaus exist for connectivity periods.
• Furthermore, increase in the TCP sequence numbers
  imply that
• All simulation components from physical layer to
  transport layer are setup properly, thereby
  provides validity of our simulation building
  blocks.
• Transport level good-put can be achieved over a
  highly variant (i.e. frequent LOS changes) FSO
  environment.