CS 6250: Computer Networks

1
CS 6250 Computer Networks

• Optical Communication
• The Why, the How, and the Whats Next in Networks
• Michael Medina
• Brian Butts

2
Introduction

• Why Should We Use Light?
• Consumer Needs and Wants
• Speed Equates to Money
• Performance Issues
• Digital vs. Analog
• Copper vs. Light
• Speed vs. Price

3
Copper

• Copper Connections
• Flow of Electrons
• Fairly Quick Data Transfer
• Limitations Being Reached
• Problems with Copper Connections
• Electron Flow Causes Heat
• Heat Leads to Slower Speeds

4
Fiber Optics

• The Invention of the Laser
• 1960s First Laser
• LEDs Often Used Instead of Lasers
• LEDs Not as Pure
• LED Source More Dispersed
• LEDs Less Expensive
• First Studies of Fiber Optics
• Groundwork for All Optical Communications

5
Fiber Optics (cont.)

• Basic Composition
• Glass Core Region
• Silicate Material
• Produced from Common Sand
• High Ability to Transmit Light
• Cladding Region
• Oxide Materials
• Generally Opaque to Reflect Light to Core
• Acts Much Like Backing of Mirror
• Shields Against External Radiation

6
All-Glass vs. PCD Fibers

• All-Glass Fiber
• Both Inner Core and Cladding Region Made of Glass
• Different Indexes of Refraction
• Utilized for Long Distances
• Plastic Clad Fiber
• Inner Core and Cladding Region Made of Plastic
  Material
• Leads to More Flexibility

7
All-Glass vs. PCD Fibers (cont.)

• Why Not PCD Fiber Instead of All-Glass?
• PCD Used for Inner Office Uses or Other Uses
  Under 100 Meters
• PCD Tends to Lose Data Over Long Distances
• All-Glass Less Flexible

8
Decibel Loss

• Major Interest in Decibel Loss
• Early 70s, First Fibers had Loss of 20dB/km
• Tremendous Loss in Comparison to Todays Fibers
• Today All-Glass Fibers Loss Less Than 1 dB/km
• All-Glass Allows for Transmission Over
  Trans-Atlantic Distances

9
Wavelength

• First Generation
• 1978 Right Outside of Chicago
• .85 um
• Transmission of 45 Mbps and Repeater Distance of
  Over 10 km
• Englands Technology at Same Time at 144 Mbps and
  Repeater Distance of 9 km

10
Wavelength (cont.)

• Next Generation
• Implemented in Early 80s
• .85 um Limits Were Reached
• Next Generation Included 1.30 um
• Bit-rate Increased to 100 Mbps
• Repeater Distance of 80 km
• Now Optical Networks Began to Be Implemented Over
  Long Distances

11
Wavelength (cont.)

• Fifth Generation
• 1.55 um with Bit-rates of 4 Gbps
• Repeated Distance of 200 km
• Used in Combination with Previous Wavelengths for
  Multiplexing
• Small Variations in Wavelength for Future
  Generations

12
Transmission

• Modulation of Signal
• Electrical Signal Modulated to Light Signal
• Modulated by Either Return-To-Zero or
  Non-Return-To-Zero
• This Source is Modulated by Either Amplitude
  Shift Key, Frequency Shift Key, or Phase Shift
  Key Modulating
• The Combination is the Modulated Light Signal
  Needed for Transmission

13
Transmission (cont.)

• Modulated Light Drives Laser or LED
• Characteristics
• Must Operate at Room Temperature
• Different Modulating Frequencies
• Small Emitting Area to Prevent Loss When
  Transmitting
• Signal Sent into a Broadband Amplifier
• Amplifier Controlled by a Controller to Modify
  the Signal to Allow for a Output Power Range

14
Receiver

• Signal Detected by Photodetector
• Characteristics
• Efficiency
• Speed
• Noise
• Compatibility
• Photodetector Produces Current
• Current Drives a Pre-amplifier

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Receiver (cont.)

• Pre-Amplifier Converts Current to Voltage
• Post-Amplifier Boosts or Lowers Voltage for
  Compatibility
• Post-Amplifier Controlled by a Controller
• Controller Allows for Different Voltage Levels
  for Different Networks

16
Wave Division Multiplexing (WDM)

• Why Multiplexing?
• At 1.55 um Bit-rates Can Reach 30 Tbps
• No Single User Needs Such a Large Bandwidth
• Multiple Users on One Line (Telcos)
• Many LANs Multiplex on a Single Connection

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Wave Division Multiplexing (cont.)

• Combines Two Signals By Modulating the Wavelength
  of the Two Signals or Multiple Signals
• Spacing Between Signals Vary
• Demodulation Can Occur Tunable Receivers
• Tunable Lasers to Optimize Traffic Flow

18
Synchronous Optical Network (SONet)

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Overview

• Optical Transmission Interface
• Proposed by BELLCORE during early 80s
• Standardized by ANSI
• STS-1 Frame(51.84 Mbps) basic building block
• Compatible with ITU-T Standards
• Synchronous Digital Hierarchy
• OSI/ISO Model
• OSI Layer 1 (physical layer) standard

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Who uses SONET?

• Regional Bells
• USWest, Ameritech, Bell Atlantic, etc.
• Inter-Exchange Carriers
• ATT, Spring, MCI, etc.
• High Performance Users
• vBNS, other Gigabit networks

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Initial SONET Requirements

• New system for multiplexing over high capacity
  optical fiber networks
• Increased Operations, Administration,
  Maintenance, and Provisioning (OAMP)
• Provided standardized interconnection between
  different services

22
STS-1 Frame
90 Octets
87 Octets
9 Rows
Transport Overhead 3 octets
23
SONET Signal
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SONET Layer Hierarchy

• Four Layers
• Photonic SONET physical layer, sets network
  specifications
• Section Creates frames, converts electronics
  signals to optical
• Line Synchronizes, multiplexes and switches
  data to SONET frames
• Path End-to-end transport of user data at
  appropriate speed

25
ATM over SONET

• Cell Based Physical Layer
• SONET Transport Overhead maintained
• Synchronization achieved by HEC algorithm
• SONET OAM embedded in Section, Line and Path is
  useful for ATM links

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SONET Summary

• SONET Implementation
• Carriers committed to SONET technology
• SONET enabling broadband technology
• Voice, data, multi-media
• ATM
• SONET improvements
• Increasing data rates
• OC-96 (4.8 Gbps) being tested

27
Photonic Switching

• Implementation
• Interconnects nodes through single point
• Increases bandwidth efficiencies, reduces idle
  resources
• Decreases network complexity

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Photonic Switching (cont.)

• Increased additive bandwidth with Multiplexed
  lines over single fiber
• Avoids optoelectric conversion
• Preserves phases and frequency information

29
Star Network Setup
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Photonic Switching w/ WDM

• Used in tandem with Wavelength-Division
  Multiplexing (WDM)
• 100 total channels possible
• Ideal for large scale broadband networks
• Small users multiplexed to form single path