optical networks - bu

Optical Networks

Poompat Saengudomlert

Session 1

Introduction & Outline

P. Saengudomlert (2017) Optical Networks Session 1 1 / 16

Basic Course Information

Lecture hours: Mon 13.00−14.30 & Wed 9.00−10.30

Instructor: Poompat Saengudomlert ([email protected])

Class website: http://bucroccs.bu.ac.th/courses/

Grading policy:Assignments and quizzes (20%)Class project (10%)Mid-semester exam (30%)Final exam (40%)

Exam policy: Closed-book exams with sheets of notes allowed


Distribution of Topics

Aspects of optical networks

Basic optical components (≈10%)

Optical transmissions (≈20%)

Optical networking (≈70%)

Largest emphasis on optical networking

Wavelength-routed optical networks (for WANs) (≈50%)

Traffic grooming (for MANs) (≈25%)

Optical access networks (for LANs) (≈25%)


Well Known Reference Textbooks

In the alphabetical order based on first authors’ last names:

G. Keiser, Optical Fiber Communications. McGraw-Hill, 2010.

G. Kramer, Ethernet Passive Optical Networks. McGraw-Hill, 2005.

B. Mukherjee, Optical WDM Networks. Springer, 2006.

R. Ramaswamy, K. Sivarajan, and G. Sasaki,Optical Networks: APractical Perspective, Fourth Edition. Morgan Kaufmann, 2010.

J.M. Simmons, Optical Network Design and Planning. Springer, 2008.

T.E. Stern, G. Ellinas, and K. Bala, Multiwavelength OpticalNetworks: Architectures, Design and Control, Second Edition.Cambridge, 2008.


1.1 Background and Motivation

Optical transmission is common, especially in core networks.

For transmission rates up to 40 Gbps, a fiber can be used as a singletransmission channel.

Commonly used standards

Synchronous Optical Network (SONET)Synchronous Digital Hierarchy (SDH)

With more traffic, wavelength division multiplexing (WDM) is morecost effective for transmissions than using multiple fibers.


Electronic Switching Node

In current networks, traffic switching is mostly done in electronics.

electronicswitch

end user equipments

fiber

DMUX MUX

switching node

wavelengthchannels

The above architecture is called electronic switching nodearchitecture.


Electronic Bottleneck

Potential problem with electronic switching: switching speed cannotkeep up with transmission speed

⇒ referred to as electronic bottleneck

Example:

Electronic witching speed = 1 Tbps, 32 wavelengths per fiber, 10 Gbpsper wavelength

⇒ 320 Gbps per fiber

⇒ maximum input fibers at switching node = 3 (not enough!)

Currently, we don’t have “much” traffic, but future Gbps applicationsmay cause the electronic bottleneck.


Opaque Optical Switching Node

In the opaque optical switching node architecture, optical switches are used

to overcome the electronic bottleneck.

opticalswitch

fiber

DMUX MUX

switching node

wavelengthchannels

electronicswitch

end user equipments


Optical Bypass

With optical switching, some traffic can pass through without goingthrough electronic switching.

⇒ referred to as optical bypass

With O-E-O conversion at optical switch ports, any input can beconnected to any output.

NOTE: Signals are converted to the same wavelength and thenswitched optically.

Opaque optical switching vs. electronic switching

Lower switching cost thanks to optical bypassHigher transmission cost due to less multiplexing

⇒ trade-off between switching cost and transmission cost


Example (Trade-off between switching cost and transmissioncost):

The electronic switching architecture uses 1wavelength channel on each link. Theelectronic switch at node B processes thetraffic amount of 15 Gbps.

A B C

D

With optical bypass at node B, the opaqueoptical switching architecture uses 2 additionalwavelength channels. The electronic switch at node B processes the traffic amount of 5 Gbps.

A B C

D

traffic Gbps, 10 Gbps per wavelength channel

Electronically switched traffic down by 10 Gbps

Two additional wavelength channels used


Transparent Optical Switching Node

Can reduce switching cost further (O-E-O conversion still costly) by using

the transparent optical switching node architecture.

fiber

DMUX MUX

switching node

wavelengthchannels

electronicswitch

end user equipments

opticalswitch

opticalswitch


Transparent vs. Opaque Optical Switching

Lower switching cost due to no O-E-O conversionLess flexible due to the wavelength continuity constraint

Example:

Traffic of 1 wavelength from 1 to 4, from 3 to 6, and from 5 to 2

6

4

2

35

1

6

4

2

35

1

opaque optical switching transparent optical switching

wavelength

Opaque optical switching needs 2 wavelengths.

Transparent optical switching needs 3 wavelengths.


Properties of Transparent Networks

With wavelength continuity constraint,

routing ⇒ routing and wavelength assignment (RWA)

An “all-optical” connection is called a lightpath.

Maximum distance limit of a lightpath is called the optical reach.

Decreases with transmission bit rateBypass-enabled network instead of all-optical network

One key advantage of a lightpath is transparency.

A lightpath can support traffic at any rate and in any format.Equipment upgrades needed only at network edgesFuture-proof fiber infrastructure


First/Last-Mile Bottleneck

Current traffic in several core networks is not large enough to justifyoptical switching.

Optical industry moves towards end users to overcome thefirst/last-mile bottleneck.

cable modems and DSLs:typically in order of 1 Mbps

Standard bit ratesof 2.5-10 Gbps perwavelength channel

MAN or WAN

Typically 1-GHz clock rate 32-bit bus = potentialinternal transfer rate of

32 Gbps of which 1-3 Gbpsis achievable with hard drives

Internals of personal computer

Bottleneck between two Gbps worlds [Gre04]

⇒ optical access networks, e.g. fiber-to-the-home (FTTH)


Passive Optical Networks

Passive optical networks (PONs) are well accepted to serve as opticalaccess networks.

A PON contains passive devices internally.Low deployment cost per user (important for user adoption)

Typically tree topology

Internet

centraloffice

fiber

splitter/combiner

endusers

fiber

Key aspects of PONs

Single fiber for both upstream and downstream transmissionsDownstream traffic is broadcast (one-to-many).Upstream traffic (many-to-one) requires multiple access control (MAC).


1.2 Course Structure

Core networks (WANs)

RWA for static and dynamic traffic

Recovery from node/link failure

Dynamic lightpath setup/release

Distribution networks (MANs)

Efficient traffic multiplexing called traffic grooming

Allocation of electronic multiplexers at nodes

Optical access networks (LANs)

MAC protocol for downstream and upstream traffic

Dynamic transmission resource allocation

NOTE: Energy efficiency is a recent concern in all networks.


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