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@networks.ecse.rpi.edu, yuksem@ecse.rpi.edu
: “shiv rpi”
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
1
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
2
Bringing Optical Communications and Ad Hoc
Networking Together…
Free-Space-Optical
Communications (FSO)
Ad Hoc
Networking
High bandwidth
Low power
Directional
Mobile communication
Auto-configuration
Free-Space-Optical
Ad Hoc Networks
Spatial reuse and angular diversity in nodes
Low power and secure
Electronic auto-alignment
Optical auto-configuration (switching, routing)
This paper proposes initial building blocks for this vision…
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
3
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
4
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
5
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
6
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 => Huge spatial reuse
But…FSO also requires:
 availability of unobstructed line-of-sight (LOS) and,
 alignment of LOS between the transmitter and the receiver.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
7
LOS Alignment: Optical Antenna Concept



Tessellated spheres with transreceiver pairs
Line-of-sight (LOS) auto-alignment
electronics
Rapid alignment & handoff =>
enables mobility or sway, while
maintaining the logical link.
LED
PD
(a) Tessellated Sphere
b) Showing a Line of Sight Sphere
Tessellated with LED+PD transceivers.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
8
Auto-Alignment Circuit Design




Pilot signal sent
If aligned, signal is
fed-back
Feedback signal
detection =>
alignment!
Handoff logical link
& transmit data
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
9
Alignment Circuit (Contd)
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
10
Optical antenna: Multiple Alignment Circuits
Universal Mounting
sockets of LED/PDs
Multiple channels =>
 Connected to a bank of
auto-alignment circuits
 Eg: 4-circuit block diagram
shown below

Universal Interface of the
wires of the LED/PDs
Interface to
the local
network
Board (Controllor)
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
11
Optical antenna: Experimental Platform (Contd)
LEDs: high divergence angle
 PDs: angular field of view
=> the LED-PD pair forms a
transceiver cone.


The transceiver cone covers a
significant volume of 3dimensional space.

Key: appropriate packing density
to cover entire 360 steradian of
surrounding space.
Tessellated Spherical antennas on stable
optical testing platforms
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
12
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)
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
13
Mobility Experiment

UDP data transfer between the moving toy train on a circular track and a
data-sink at the center of the circle.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
14
Auto-alignment: Search Phase
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
15
Auto-alignment: Aligned!
Data Transfer Phase
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
16
Intensity vs Mobility @ The optical antenna
Light Intensity (lux)
70
Aligned
60
50
Not aligned
40
30
Detector
Threshold
20
10
128
121
112
105
97.5
88.5
79
72
65
51.5
40.5
33
23
17
11
0
0
Angular Position of the Train (degree)
Denser packing will allow fewer interruptions (and smaller
buffering), but more handoffs
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
17
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
18
Mobile FSO Simulation (Contd)

Initial proof-of-concept:
 2-D scenario on the XYplane
 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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
19
Simple Experiments…

Four experiments, varying:
 Speed of mobile node and
 Distance of the mobile node from the central node
Experiment
Mobile Node’s Velocity
Mobile Node’s Path Radius
1
1.5 meters/second
25 meters
2
1.5 meters/second
35 meters
3
2.5 meters/second
25 meters
4
2.5 meters/second
35 meters
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
20
TCP sequence numbers in Experiment 1
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
21
Experiment 1: Data Transfer in Bursts After LOS discovery
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
22
Experiment 2: Higher distance => TCP interactions & lower
throughput
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
23
Experiment 3: Lower distance, higher speeds
TCP affected by higher loss rates & periodic disconnections
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
24
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…
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
25
Indoor Ad-Hoc FSO: Music App
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
26
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
27
Thanks!
Students:
Jayasri Akella, sri@networks.ecse.rpi.edu
Dr. Murat Yuksel (post-doc): yuksem@ecse.rpi.edu
Chang Liu, c.liu@ee.unimelb.edu.au
David Partyka, partyd@rpi.edu
Sujatha Sridharan
: “shiv rpi”
Ps: Online free videos of all my advanced networking classes
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
28
Details
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
29
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
30
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
31