Fiber-Optic

Communication Systems
An Introduction

Pablo A. Costanzo Caso

Brief Intro on Telecom
Networks
Basics of Communication Networks

Bariloche Neuquén
 Brief History of Networks

Copper Telecom Networks:

• 4 kHz analog voice local loop (between customers and central
office – access end)
• 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
• Digital interoffice trunks using DS-1 (Digital Signal Type 1)
• DSx refers to service type and Tx data rate at physical link
 Digital Transmission Hierarchy
(DTH)

64-kb/s circuits are multiplexed into

higher-bit-rate formats

Called Telephony or T-Networks

This is Copper network

What is the overhead for T1?

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
 SONET/SDH Bandwidths

SONET (synchronous SDH (synchronous digital

SONET Frame Payload bandwidth Line Rate (kbps) / Voice
optical network) Optical hierarchy) level and
Format (kbps) Channels
Carrier Level Frame Format

OC-1 STS-1 (synchronous STM-0 (synchronous 50,112 51,840 / 672

transport signal) transport module)

OC-3 STS-3 STM-1 150,336 155,520 / 2,016

OC-12 STS-12 STM-4 601,344 622,080 / 8,064

OC-24 STS-24 – 1,202,688 1,244,160

OC-48 STS-48 STM-16 2,405,376 2,488,320 / 32,256

OC-192 STS-192 STM-64 9,621,504 9,953,280 / 129,024

OC-768 STS-768 STM-256 38,486,016 39,813,120 / 516,096

OC-3072 STS-3072 STM-1024 153,944,064 159,252,480 / 2,064,384

 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
 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 44.736
Mbits/s
Limitation:
• Fiber attenuation (repeater spacing ≈ 6 km)
Deployed since 1978
 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 2.488 Gbits/s
Limitations:
• Fiber attenuation (repeater spacing ≈ 40 km)
• Fiber dispersion
Deployed since 1982
 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:
• Propagation distance without regeneration/re-clocking
Evolution of Optical Networks
History of
Attenuation
Three
Windows
based on
Wavelength
 Fiber Network Topologies
Who Uses Span Bit Rate Multi- Fiber Laser Receiver
it? (km) (bps) plexing
Core/ Phone ~103 ~1011 DWDM/ SMF/ DCF EML/ APD
LongHaul Company, (100’s of TDM DFB
Gov’t(s) Gbps)
Metro/ Phone ~102 ~1010 DWDM/C SMF/ DFB APD/ PIN
Regional Company, Big (10’s of WDM/TD LWPF
Business Gbps) M
Access/ Small ~10 ~109 TDM/ SMF/ DFB/ FP PIN
LocalLoop Business, (56kbps- SCM/ MMF
Consumer 1Gbps)

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
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)
– Most LAN, WAN and much more
• One fiber can carry > 8 Tb/s (1012 b/s) or 125 million
conversations simultaneously
 Multimedia over Fiber
• Fiber carries various media
– Voice (SONET/Telephony) - The largest traffic
– Video (TV) over
• Hybrid Fiber Coaxial (HFC) or
• Fiber-Twisted Pair/Digital Subscriber Loops (DSL)
– Data – Internet traffic
– These three are called the ‘Triple Play’

Information revolution wouldn’t have

happened without the Optical Fiber
 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
Elements of OPTICOM System
 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
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
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
 Wavelength Division Multiplexing

• Fiber has the capability to transmit hundreds of wavelengths

• Cost effective only in long haul links in the past
• With low cost Coarse WDM (CWDM) equipment this is possible even in
the access front
• Once the fiber is in place, additional wavelength can be launched at both
ends by replacing transceivers
 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
 Last Mile Bottle Neck and
Access Networks

“Infinite” Bandwidth Backbone

Optical Fiber Networks  > Gb/s

Few Mb/s The Last Mile ?

Copper, wireless,
?
fiber optic?
Virtually infinite demand end user
 Last Mile Bottle Neck and
Access Networks. Wired Solutions
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

PON Bit-Rates & Timeline
NG-PON2
[40Gb/s]

2013
 PON Flavours
• APON/BPON: ATM/Broadband PON
– Uses ATM as bearer protocol
– 155 or 622 Mbps downstream, 155 upstream.

• EPON: Ethernet PON

– Uses Ethernet frames for data transfer
– 10G-EPON aims at reaching high data rates of 10 Gb/s

• GPON: Gigabit capable PON - successor of BPON

– Enables the transmission of both ATM cells and Ethernet packets in
the same transmission frame structure.

• WPON: WDM-PON
– Support multiple wavelengths
 PON Comparison
Downstream Upstream Standard

APON 155 Mb/s 155 Mb/s ITU-T (FSAN)

622 Mb/s 155 MB/s
BPON 155 Mb/s 155 Mb/s IEEE 802.3ah
622 Mb/s 622 MB/s
EPON 10-1000 Mb/s 10-1000 Mb/s ITU-T G.983
(FSAN)
GPON 1.244 Gb/s 155 Mb/s ITU-T G.983
2.488 Gb/s 622 Mb/s (FSAN – Full
1.244 Gb/s Services Access
2.488 Gb/s Network)
10G EPON Vs 10G GPON
Hybrid/Fiber Coax (HFC) TV
Networks
Digital
Subscriber
Loop

• DSL consists of fiber-twisted pair

• This is a digital fiber-copper link
• Multimedia (video and data) supported over voice
• 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) are tried
 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
 ROF for Fiber-Wireless Networks
Y
Central Radio over Fiber (ROF)
Base RAP
Station (Simple)
Up/Down links

Y
RAP
802.11 voice
Y
RAP
(Radio Access Point)

Single ROF link can support voice and Micro

Cell
data simultaneously
End