R V COLLEGE ENGINEERING.

DEPARTMENT OF TELECOMMUNICAION ENGINEERING

BYSANTHOSHKUMAR (1RV14TE410 ).Prashnat m angadi(1RV14TE407) smitha (1RV14TE411)

Topic- FREE SPACE OPTICS COMMUNICATION(FSO).

 CONTENTS

• INTRODUCTION

• HISTORY OF FSO

• HOW FSO WORKS

• ARCHITECTURE

• FSO SECURITY

• APPLICATIONS

• MERIT

• DEMERIT

• CONCLUSION

• REFERENCES.

FREE SPACE OPTICS COMMUNICATIPON

INTRODUCTION

• Free Space Optics (FSO) communications, also called Free

Space Photonics (FSP) or Optical Wireless, refers to the

transmission of modulated visible or infrared (IR) beams

through the atmosphere to obtain optical communications.

Free-space optical communication- (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking.

HOW FREE SPACE OPTICS WORKS

• Free Space Optics (FSO) transmits invisible, eye-safe light

beams from one "telescope" to another using low power

infrared laser in the teraHertz spectrum.

• The beams of light in Free Space Optics (FSO) systems are

transmitted by laser light focused on highly sensitive photon

detector receivers.

• These receivers are telescopic lenses able to collect the photon

stream and transmit digital data containing a mix of Internet

messages, video images, radio signals or computer files.

*How FSO works:FSO systems use optical wireless link heads each having:

• a transceiver with a laser or LED transmitter, a lens or telescope (can have

more that one) .shaping overcomes building movement

• a receiver usually a semiconductor May also employ servo motors, voice

coils, mirrors, CCD arrays, and even liquid crystals and micro-

electromechanical systems (MEMS) for tracking and acquisition.

• FSO operates in the infrared (IR) range around 850 and 1550 nm (frequencies

around 200 THz).

• FSO can use Power Over Ethernet (PoE).

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TRANSMITTER RECEIVER

FSO Block-Diagram

1 Network traffic converted into pulses of invisible light representing 1’s and 0’s

2 Transmitter projects the carefully aimed light pulses into the air

5 Reverse direction data transported the same way.• Full duplex

3 A receiver at the other end of the link collects the light using lenses and/or mirrors

4 Received signal converted back into fiber or copper and connected to the network

1.Both parties can communicate with eachOther simultaneously.

Better Modulation Techniques Classic systems use a relatively simple modulation

• Called “Non-Return to Zero” (NRZ).

(Allow the medium to flow in only one direction.) • Each symbol encodes 1 bit worth of data.

But there are other more efficient modulations

• If we can’t signal faster, carry more data in each signal.

• Some modulation schemes currently being adopted are:

• Duo-binary

• DPSK (Differential Phase Shift Keying)

• DQPSK (Differential Quaternary Phase-Shift Keying)

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Modulation Method

Architecture

Free space Optical Amplifiers.

Free space Optical Amplifiers. FSO Optical amplifiers increase the intensity of a signal

• There are different types, for different spectrums of light.

• The most common is the Erbium Doped Fiber Amplifier(EDFA).

• Another method is Raman Amplification, typically for ultra

long-haul.

In an EDFA, a piece of fiber is “doped” with Erbium ions.

Additional laser power at 980nm and/or 1480nm is pumped in via

a coupler.

The interaction between the Erbium and the pump laser causes the

emission of light in the C-band spectrum, amplifying the signal.

Free space Optic Transmission Bands. There are several frequency “windows” available:

• 850nm – The First Window

• Highest attenuation, only used for short reach applications today.

1310nm – The Second Window (O-band)

• The point of zero dispersion on classic SMF, but high attenuation.

• Primarily used for medium-reach applications (up to 10km) today.

1550nm – Third Window (C-band)

• Stands for “conventional band”, covers 1525nm – 1565nm.

• Has the lowest rate of attenuation over SMF.

• Used for almost all long-reach and DWDM applications today.

• Also called the “Erbium Band”, the frequencies which support EDFAs.

Forth Window (L-band)

• Stands for “long band”, covers 1570nm – 1610nm.

Free space Optic Transmission Bands

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Noise in FSO Systems

Background Radiation (e.g. sun light)

Shot Noise (Poisson distributed)

Thermal Noise (Gaussian distributed)

Scintillation Noise

Building Motion – Thermal Expansion

Results from Seattle Deployment:

• 15% of buildings move more than 4mrad

• 5% of buildings move more than 6mrad

• 1% of buildings move more than 10mrad

SERVICE TYPES AND NETEORK TRANSMISSION OF FSO.

Two basic service types (switching technologies) Connection-oriented Connectionless

Connection-oriented Based on circuit switching (setup, connect, tear-down)Example: Public Switching Telephone Network (PSTN)Originally only supported voice Not good for bursty traffic

ConnectionlessBased on sending datagrams Examples: Packet, massage, burst switching Improves bandwidth and network utilization

* LAYERD PROTOCOL*1.Physical layer methods.

•Free space optical terminal –Field proven adaptive optics system reduces beam spread, increases collection efficiency.

•Optical Automatic Gain Control system – Field proven system substantially reduces receive power variations. 60 dB dynamic range, < 1 ms response time

•Optical modem and FEC with near theoretical sensitivity

2.Network layer methods.

•Packet retransmission systems (link or network) –Assures delivery of packets lost during deep fades.

•Deep queues at the nodes (link or network)

•Network re-routing or re-pointing (network only)

* Unguided Media OF FSO SYSTEM.

*Unguided media transport electromagnetic waves without using

a physical conductor. This type of communication is often

referred to as wireless communication. Signals are normally

broadcast through free space and thus are available to anyone

who has a device capable of receiving them.

*Microwaves

* Radio Waves

*Infrared

MAC Sublayer

• In Standard Ethernet, the MAC sublayer governs the operation

of the access method. It also frames data received from the

upper layer and passes them to the physical layer.

Frame Format

• The Ethernet frame contains seven fields: preamble, SFD, DA,

SA, length or type of protocol data unit (PDU), upper-layer data,

and the CRC

• Ethernet does not provide any mechanism for acknowledging

received frames, making it what is known as an unreliable

medium. Acknowledgments must be implemented at the higher

layers.

IEEE Project 802 has created a sublayer called media access control that defines the specific access method for each LAN. For example,it defines CSMA/CD as the media access method for Ethernet LANs and the tokenpassing-method for Token Ring and Token Bus LANs. A part of the framing function is also handled by the MAC layer.

Ethernet frame

Preamble - The first field of the 802.3 frame contains 7 bytes (56 bits) of alternating Os and 1s that alerts the receiving system to the coming frame and enables it to synchronize its input timing.The pattern provides only an alert and a timing pulse.

Start frame delimiter (SFD) -The second field (l byte: 10101011) signals the beginning of the frame. The SFD warns the station or stations that this is the last chance for synchronization. The last 2 bits is 11 and alerts the receiver that the next field is the destination address.

Destination address (DA)- The DA field is 6 bytes and contains the physical address of the destination station or stations to receive the packet.

Data- This field carries data encapsulated from the upper-layer protocols. It is a minimum of 46 and a maximum of 1500 bytes,

CRC- The last field contains error detection information.CRC is calculated over the address, types and data field.

If the receiver calculates the CRC and finds that it is not zero(corruption transmission),it discards the frame.

Applications

• Metro Area Networks (MAN)

• Last Mile Access

• Enterprise connectivity

• Fiber backup

• Backhaul

• Service acceleration

Merits

• Flexible network solution over conventional broadband

services.

• Straight forward deployment- no licenses required

• Low initial investment

• Ease of installation

• Re-deployability

• High bit rates and low error rates

Demerits

• Fog

• Physical obstructions

• Scintillation

• Solar interference

• Scattering

• Absorption

• Building sway / Seismic activity

[1] .V. W. S. Chan, “Coherent optical space communications system: Architecture and technology issues,” in SPIE Control Communication Technol. Laser Syst., vol. 295, 1981, pp. 10–17.

[2]. V. W. S. Chan, “Space coherent optical communication systems—An introduction,” IEEE J. Lightwave Technol., vol. LT-5, pp. 633–637, Apr. 1987.

[3]. Couch, L. Digital and Analog Communication Systems. Upper Saddle River, NJ:

Prentice Hall, 2000

[4]. Garcia, A. and Widjaja, I, Communication Networks. New York, NY: McGraw-Hill,

2003

[5]. Keshav, S. An Engineering Approach to Computer Networking. Reading,MA: Addison-

Wesley, 1997.

[6]. Kumar A., Manjunath, D., and Kuri, 1. Communication Networking. San Francisco,

CA: Morgan, Kaufmans, 2004.

[7].Data communication and networking.Behrouz A.Forouzan.

REFERENCES.

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