Optical Networks

1
Optical Networks

• Introduction

2
Why Optical?

• Bandwidth
• Low cost (0.30/yard)
• Extremely low error rate (10-12 vs. 10-6 for
  copper
• Low signal attenuation
• Low power requirement
• More secure

3
History

• 1st Generation Copper is transmission medium
• 2nd Generation Optical Fiber (late 80s)
• Higher data rates longer link lengths
• Dense Wavelength-Division Multiplexing (DWDM,
  1994)
• Fiber exhaust forces DWDM
• Erbium-doped fiber amplifiers (EDFAs) lower DWDM
  transmission cost
• 3rd Generation Intelligent optical networking
  (1999)
• Routing and signaling for optical paths

4
Medium Characteristics

• Attenuation
• Wavelength dependent
• 0.85, 1.3, 1.55 micron windows
• Attenuation caused by impurities as well as
  scattering
• Dispersion
• Inter-modal
• Chromatic

5
Wavelength Division Multiplexing(WDM)

• All the bandwidth could not be used due to the
  electronic bottleneck
• Two breakthroughs
• WDM
• Erbium-doped fiber amplifier (EDFA)
• WDM vs. FDM
• WDM is passive and hence reliable
• WDM carrier frequency orders of magnitude higher

6
Wavelength Division Multiplexing(WDM)
Frequency-registered transmitters
Receivers
OA
OA
WDM DeMux
WDM Mux
7
Regenerators

• 3R
• Reshaping
• Re-clocking
• Amplification
• 2R
• Reshaping
• Amplification
• 1R (Example EDFA)
• Amplification

8
DWDM Evolution

• Faster (higher speed per wave),
• 40 Gb/s on the horizon
• Thicker (more waves),
• 160 waves possible today
• Longer (link lengths before regeneration)
• A few thousand km possible today
• 160 waves at 10 Gb/s 1.6 Tb/s
• 25 million simultaneous phone calls
• 5 million books per minute

9
WADMs WXC

• WADM (Wave Add-Drop Mux)
• Evolution from p-t-p
• Can add and drop traffic at various locations
• WXC (Wave crossconnect)
• Similar to ADM except that multiple fibers on the
  input side with the capability to switch colors
  between fibers

10
Enabling Technologies

• Fiber and laser technology
• EDFA
• MEMS (Micro-Electro Mechanical Systems)
• Opaque vs. all-optical networks

11
Current Protocol Stack
IP
ATM
SONET
WDM
12
How Did We Get Here?

• SONET over WDM
• Conventional WDM deployment is using SONET as
  standard interface to higher layers
• IP over ATM
• IP packets need to be mapped into ATM cells
  before transporting over WDM using SONET frame
• OEO conversions at every node is easier to build
  than all optical switch

13
Problems with Multilayer

• Inefficient
• In IP over ATM over SONET over WDM network, 22
  bandwidth used for protocol overhead
• Layers often do not work in concert
• Every layer now runs at its own speed. So, low
  speed devices cannot fill the wavelength
  bandwidth.
• Under failure, different layers compete for
  protection

14
The Roadmap
15
WDM

• Network Architecture

16
Classes of WDM Networks

• Broadcast-and-select
• Wavelength routed
• Linear lightwave

17
Broadcast-and-Select
w0
Passive Coupler
w1
18
Wavelength Routed

• An OXC is placed at each node
• End users communicate with one another through
  lightpaths, which may contain several fiber links
  and wavelengths
• Two lightpaths are not allowed to have the same
  wavelength on the same link.

19
WRN (contd)

• Wavelength converter can be used to convert a
  wavelength to another at OXC
• Wavelength-convertible network.
• Wavelength converters configured in the network
• A lightpath can occupy different wavelengths
• Wavelength-continuous network
• A lightpath must occupy the same wavelength

20
A WR Network
21
Linear Lightwave Networks

• Granularity of switching in wave bands
• Complexity reduction in switches
• Inseparability
• Channels belonging to the same waveband when
  combined on a single fiber cannot be separated
  within the network

22
Routing and Wavelength Assignment (RWA)

• To establish a lightpath, need to determine
• A route
• Corresponding wavelengths on the route
• RWA problem can be divided into two sub-problems
• Routing
• Wavelength assignment
• Static vs. dynamic lightpath establishment

23
Static Lightpath Establishment (SLE)

• Suitable for static traffic
• Traffic matrix and network topology are known in
  advance
• Objective is to minimize the network capacity
  needed for the traffic when setting up the
  network
• Compute a route and assign wavelengths for each
  connection in an off-line manner

24
Dynamic Lightpath Establishment (DLE)

• Suitable for dynamic traffic
• Traffic matrix is not known in advance while
  network topology is known
• Objective is to maximize the network capacity at
  any time when a connection request arrives at the
  network

25
Routing

• Fixed routing predefine a route for each
  lightpath connection
• Alternative routing predefine several routes for
  each lightpath connection and choose one of them
• Exhaust routing use all the possible paths

26
Wavelength Assignment

• For the network with wavelength conversion
  capability, wavelength assignment is trivial
• For the network with wavelength continuity
  constraint, use heuristics

27
Wavelength Assignment under Wavelength Continuity
Constraint

• First-Fit (FF)
• Least-Used (LU)
• Most-Used (MU)
• Max_Sum (MS)
• Relative Capacity Loss (RCL)

28
First-Fit

• All the wavelength are indexed with consecutive
  integer numbers
• The available wavelength with the lowest index is
  assigned

29
Least-Used and Most-Used

• Least-Used
• Record the usage of each wavelength
• Pick up a wavelength, which is least used before,
  from the available wavelength pool

• Most-Used
• Record the usage of each wavelength
• Pick up a wavelength, which is most used before,
  from the available wavelength pool

30
Max-Sum and RCL

• Fixed routing
• MAX_SUM Chooses the wavelength, such that the
  decision will minimize the capacity loss or
  maximize the possibility of future connections.
• RCL will choose the wavelength which minimize the
  relative capacity loss.