1. Fiber-Optic Communication Systems An Introduction

2. Why Optical Communications?
Optical Fiber is the backbone of the modern communication networks
The Optical Fiber Carries:
Almost all long distance phone calls
Most Internet traffic (Dial-up, DSL or Cable)
Most Television channels (Cable or DSL)
One fiber can carry up to 6.4 Tb/s (1012 b/s) or 100 million conversations simultaneously
Information revolution wouldn’t have happened without the Optical Fiber
‘Triple Play’

4. Why Optical Communications?
Lowest Attenuation: 0.2 dB/km at 1.55 µm band resulting in 100s of km fiber links without repeaters Highest Bandwidth of any communication channel: Single Mode Fiber (SMF) offers the lowest dispersion  highest bit rate  rich content (broadband) up to 100 Gb/s or more Enormous Capacity: Via WDM that also offer easy upgradability, The ‘Optical Layer’: Wavelength routing, switching and processing all optically, which adds another layer of flexibility

5. Elements of a Fiber Optic Link3

6. Elements of OPTICOM System
The Fiber – that carries the light
Single Mode Fiber (only one EM mode exists), offers the highest bit rate, most widely used
Multi Mode Fiber (multiple EM modes exist), hence higher dispersion (due to multiple modes) cheaper than SMF, used in local area networks
Step Index Fiber – two distinct refractive indices
Graded Index Fiber – gradual change in refractive index

7. Elements of OPTICOM System
Optical Transmitter converts the electrical information to optical format (E/O)
Light Emitting Diode (LED): cheap, robust and used with MMF in short range applications
Surface emitting and edge emitting LED
LASER Diode: high performance and more power, used with SMF in high speed links
Distributed Feedback (DFB) Laser – high performance single mode laser
Fabry-Perrot (FP) lasers – low performance multimode laser

8. Elements of OPTICOM System
Optical Receiver converts the optical signal into appropriate electrical format (E/O)
PIN Photo Diode: Low performance, no internal gain, low cost, widely used
Avalanche Photo Diode (APD): High performance with internal (avalanche) gain
Repeater: receives weak light signal, cleans-up, amplifies and retransmits (O/E/O)
Optical Amplifier: Amplifies light in fiber without O/E/O

9. Wavelength Division Multiplexing
Fiber has the capability to transmit hundreds of wavelengths
Coarse WDM (CWDM) has ~20 nm wavelength spacing
Dense WDM (DWDM) has up to 50 GHz spacing
Once the fiber is in place, additional wavelength can be launched by upgrading transceivers

10. Optical Amplifier & EDFA
Continuous Wave
(Constant)
An optical amplifier amplifies the light signal without converting to electrical
Very useful is WDM systems
Erbium Doped Fiber Amplifier (EDFA) works in 1550 nm band 2

11. Brief Intro on Telecom Networks

12. Basics of Communication Networks
Long Haul Network

13. Brief History of Networks
Copper Telecom Networks:
4 kHz analog voice local loop (between customers and central office – access end) still in Bell Telephone lines & 56k modems
Digital interoffice trunks using DS-1 (Digital Signal Type 1)
A voice signal digitized at a sampling rate of 8 kHz  8 bits/samples is DS-0 (64 kb/s)
Carried on a single twisted copper-wire pair
Required repeaters every 2 km to compensate for attenuation

14. Digital Transmission Hierarchy (DTH)
64-kb/s circuits are multiplexed into higher-bit-rate formats
Called Telephony or T-Networks
This is Copper network 1

15. First Generation Fiber Optic Systems
Purpose:
Eliminate repeaters in T-1 systems used in inter-office trunk lines
Technology:
0.8 µm GaAs semiconductor lasers
Multimode silica fibers
Limitations:
Fiber attenuation
Intermodal dispersion
Deployed since 1974

16. Second Generation Systems
Opportunity:
Development of low-attenuation fiber (removal of H2O and other impurities)
Eliminate repeaters in long-distance lines
Technology:
1.3 µm multi-mode semiconductor lasers
Single-mode, low-attenuation silica fibers
DS-3 signal: 28 multiplexed DS-1 signals carried at Mb/s
Limitation:
Fiber attenuation (repeater spacing ≈ 6 km)
Deployed since 1978

17. Third Generation Systems
Opportunity:
Deregulation of long-distance market
Technology:
1.55 µm single-mode semiconductor lasers
Single-mode, low-attenuation silica fibers
OC-48 signal: 810 multiplexed 64-kb/s voice channels carried at Gb/s
Limitations:
Fiber attenuation (repeater spacing ≈ 40 km)
Fiber dispersion
Deployed since 1982

18. Fourth Generation Systems
Opportunity:
Development of erbium-doped fiber amplifiers (EDFA)
Technology (deployment began in 1994):
1.55 µm single-mode, narrow-band semiconductor lasers
Single-mode, low-attenuation, dispersion-shifted silica fibers
Wavelength-division multiplexing of 2.5 Gb/s or 10 Gb/s signals
Nonlinear effects limit the following system parameters:
Signal launch power
Propagation distance without regeneration/re-clocking
WDM channel separation
Maximum number of WDM channels per fiber
Polarization-mode dispersion limits the following parameters:

19. Evolution of Optical Networks

20. History of Attenuation5

21. Fiber Network Topologies
Who Uses it?
Span (km)
Bit Rate
(bps)
Multi-plexing
Fiber
Laser
Receiver
Core/
LongHaul
Phone Company, Gov’t(s)
~103
~1011
(100’s of Gbps)
DWDM/
TDM
SMF/ DCF
EML/ DFB
APD
Metro/
Regional
Phone Company, Big Business
~102
~1010
(10’s of Gbps)
DWDM/CWDM/TDM
SMF/ LWPF
DFB
APD/ PIN
Access/
LocalLoop
Small Business, Consumer
~10
~109
(56kbps- 1Gbps)
TDM/ SCM/
SMF/ MMF
DFB/ FP
PIN
Core - Combination of switching centers and transmission systems connecting switching centers.
Access- that part of the network which connects subscribers to their immediate service providers
LWPF : Low-Water-Peak Fiber, DCF : Dispersion Compensating Fiber, EML : Externally modulated (DFB) laser

22. Synchronous Optical Networks
SONET is the TDM optical network standard for North America (called SDH in the rest of the world)
De-facto standard for fiber backhaul networks
OC-1 consists of 810 bytes over 125 us; OC-n consists of 810n bytes over 125 us
Linear multiplexing and de-multiplexing is possible with Add-Drop-Multiplexers

23. SONET/SDH Bandwidths SONET Optical Carrier Level SONET Frame Format
SDH level and Frame Format
Payload bandwidth (kbps)
Line Rate (kbps)
OC-1
STS-1
STM-0
50,112
51,840
OC-3
STS-3
STM-1
150,336
155,520
OC-12
STS-12
STM-4
601,344
622,080
OC-24
STS-24 –
1,202,688
1,244,160
OC-48
STS-48
STM-16
2,405,376
2,488,320
OC-192
STS-192
STM-64
9,621,504
9,953,280
OC-768
STS-768
STM-256
38,486,016
39,813,120
OC-3072
STS-3072
STM-1024
153,944,064
159,252,480

24. Last Mile Bottle Neck and Access Networks
Infinite Bandwidth Backbone
Optical Fiber Networks A few (Gb/s)
Virtually infinite demand end user
Few Mb/s
The Last Mile ? ?
Additionally, supporting different QoS

25. Fiber in the Access End
Passive Optical Networks (PON) – No active elements or O/E conversion
Fibre-Coaxial (analog) or DSL (digital) fibre-copper systems
Radio over fibre (Fibre-Wireless) Systems
Currently Drives the Market

26. PON Bit-Rates & Timeline

27. Digital Subscriber Loop (DSL)
Fiber Link
Digital fiber-copper (fiber-twisted pair) link
Multimedia (video, voice and data)
At least 3.7 Mb/s streaming is needed for quality video
Bit rate heavily depend on the length of the twisted pair link
New techniques like very high rate DSL (VDSL)
Many buildings in GTA have access to video over DSL

28. Radio over Fiber (ROF)
RF signals are transmitted over fiber to provide broadband wireless access
An emerging very hot area
Many advantages
Special areas
Underground
Olympics London
Niagara Tunnel

29. ROF for Fiber-Wireless NetworksY
Central
Base
Station
Radio over Fiber (ROF)
RAP
(Simple)
Up/Down links Y
RAP
802.11
voice Y
RAP
Single ROF link can support voice and
data simultaneously
Micro
Cell