Advances in Optical Networks: SONET By Sean Goggin April 19, 2005

Advances in Optical Networks:SONET

By

Sean Goggin

April 19, 2005

Tuesday, April 19, 2005 Advances in Optical Networks: SONET

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Overview

• Fundamentals of Optical Networks

• SONET

• SDH

• Future of SONET


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Fundamentals of Optical Networks

• Fiber Optic Medium

• Variants of Fiber and Optical Networks

• Multiplexing Methods

• Optical Network Equipment

• Topologies of Optical Networks


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Fiber Optic Medium

• Core– Medium Where Light

Travels

• Cladding– Reflects Light Back

into the Core

• Buffer Coating– Protective Coating


5

Variants of Fiber Optic

• Single-Mode– Small Core

Approximately 9 Microns

– Uses IR Laser Light Transmitter

– Greater then 10 Miles*

– Most Expensive

• Multi-Mode– Large Core

Approximately 62.5 Microns

– Uses Light Emitting Diode Transmitter

– Less then 10 Miles*– Least Expensive

*Without Regeneration


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Types of Multiplexing

• Time Division Multiplexing (TDM)– Simplest Implementation– Uses Single Wavelength

• Wavelength Division Multiplexing (WDM)– Complex Implementation– Multiple Wavelengths on

a Single Fiber to Increase Bandwidth


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Types of Optical Networks

• Opaque– Weak Signals are

Boosted with a Repeater

– Optical-Electronic-Optical (OEO) Repeater

– Incurs Pricey Conversion Delay

• All-Optical (Pure)– Weak Signals are

Boosted with a Amplifier

– Erbium-Doped Fiber Amplifier (EDFA)

– Complete Photonic Boost


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OEO Repeater

OpticalTransceiver

OpticalTransceiver

Processor

StrongOpticalSignal

WeakOpticalSignal

ElectronicCircuit

Pathway


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Erbium Doped Fiber Amplifier

CouplerErbium-Doped Fiber

StrongOpticalSignal

WeakOpticalSignal

Pump Laser

Filter


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Optical Network Equipment

• Repeaters (OEO) & Amplifiers (EDFA)• Optical Crossconnects (OXC)

– Photonic Switch with N Full-Duplex Ports

• Optical Add-Drop Multiplexer (OADM)– Wavelengths Can Be Added and Removed

from the Photonic Flow– Ex: Remove Traffic for Inbound T1 and Traffic

for Outbound T1– Needed for WDM


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Optical Add-Drop Multiplexer


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Topologies

• Ring Topology– Data Moves in One Direction around 1st Ring– If Failure Occurs, Traffic is Rerouted in Opposite

Direction on 2nd Ring– Each Ring is ½ Total Capacity– Self-Maintaining

• Mesh Topology– Locations are Linked to 2 or More Other Locations– If a Link Fails, Traffic is Rerouted around the Failure– Requires Routes to be Established Before Failure


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Ring Topology

Internet


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Mesh Topology


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Sample of Optical Network


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Telecom Terminology

• Synchronous Optical Network (SONET)

• Asynchronous Transfer Mode (ATM)

• Digital Signal (DS)

• Synchronous Transport Signal (STS)

• Optical Carrier (OC)


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Telecom Circuits

• Digital Signal Levels– DS-0: 64 Kb Transmission Channel– DS-1(T1): 1.5 Mb; Formed of 24 DS-0– DS-3(T3): 44.7 Mb; Formed of 672 DS-0

• Synchronous Transport Signals Channels– STS-1: 52 Mb; Formed of 28 DS-0 or a Single DS-3– STS-3: 155 Mb; Formed of 84 DS-0 or 3 DS-1– Electric Signal is Converted to an Optical Signal it

Becomes OC


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SONET Connections

Megabits OC STS SONET Channels

Per Second Level Level DS-1 DS-3

52 OC-1 STS-1 28 1

155 OC-3 STS-3 84 3

622 OC-12 STS-12 336 12

2,488 OC-48 STS-48 1,344 48

9,953 OC-192 STS-192 5,376 192

39,812 OC-768 STS-768 21,504 768


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Background of SONET

• Conceived by MCI During the Mid-1980’s• Designed from the Ground-Up to Hasten

the Adoption of Optical Technology• Capacity and Distance Increased Rapidly

Due to Technological Developments– Increased Purity of Fiber Optic Cable

• Longer Distance without Regeneration• Iron, Nickel, and Hydroxyl Ions Cause Impurities• 1970’s 20dB/km Loss, Today .2 dB/km Loss


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– Development of Laser Technology• Lasers Yield Higher Energy then LEDs Allowing for

Longer Distance Before Regeneration

– Development of Pure-Optical Technology• Eliminating Optical-Electronic-Optical Conversion

for Regeneration & Routing Increase Speed• Possibility to Breach 10 Gb Barrier

– Wave Division Multiplexing & Dense Wave Division Multiplexing

• Using Multiple Wavelengths Capacity Can Be Increased Upwards of 92 Times the Capacity of a Single Wavelength


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• ANSI Transmission Standard– United States– Canada– Korea– Taiwan– Hong Kong

• SDH used in Rest of the World– Interoperable with SONET


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Description

• Physical-Layer Standard

• Four-Layer Protocol Stack

• TDM Creates Synchronous Channels

• Multiplex Many Types of Traffic into Uniform Streams onto Fiber Optic Cabling

• Used Primarily as Backbone for ATM


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• Not Well Suited for Data Because of Native 64 kilobit “chunks”

• Utilizes Ring Topology for Reliability

• Low Maintenance do to Automatic Protection Switching (APS)

• Operations, Provisioning, Monitoring and Maintenance Functions are Done Uniformly and Efficiently


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• Typical SONET Ring is Single Wavelength Opaque Network (Circa 2000)

• Entire Ring Must Operate at the Same Speed

• Adding Capacity to Rings Takes a Long Time and Typically Constitute a New Ring Due to Convenience

• Recent use of IP Over SONET


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Four-Layers of SONET

• Photonic: STS Electrical Data is Converted into OC Light Pulses and Vice Versa

• Section: Operates between Optical repeaters, Helping to Transmit STS Frames

• Line: Synchronizes and Multiplexes Multiple Streams into One Stream, Invokes APS When Required

• Path: Used for End-to-end Communications and Control


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STS-1 Frame

• Section, Line, and Path Stack Layers are Overheard in the basic STS-1 Frame

• Frame is comprised of 9 Rows by 90 Columns = 810 bytes

• 1st 3 Columns of Each Row Addresses Section and Line Overhead (27-Bytes)

• 4th Column of Each Row Addresses Path Overhead (9-Bytes)

• 86 Columns are Payload (774-Bytes)


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Section Overhead

• 9-Bytes• Supports

– Performance Monitoring (STS-N Signal)

– Local Orderwire– Data Communication

Channels– Framing


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Line Overhead

• 18-Bytes• Supports

– Locating the Payload in the Frame

– Multiplexing or Concatenating Signals

– Performance Monitoring

– Automatic Protection Switching (APS)

– Line Maintenance


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Path Overhead

• 9 Evenly Distributed Path Overhead Bytes per 125 Microseconds

• Supports– Performance

Monitoring of Payload– Signal Label– Path Status– Path Trace


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SONET Virtual Tributaries

• SONET is Capable of Accommodating Large and Small Capacities

• STS-1 Frame Payload Can be Sub-Divided to Create Virtual Tributaries (VT)

• Services Below DS3 are Transported via VTs in SONET

• VTs are Multiplexed to Reach Capacity of STS Payload


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SONET Multiplexing Hierarchy


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ATM Over SONET

• Data-Link Layer Standard• Voice Packets are Synchronous and

Continuous, Data Packets are Asynchronous and Burst

• ATM Dynamically Allocates “Cells” to Voice and Data on Synchronous and Continuous Connection

• Provides Routing, Quality of Service (QoS), and Flexible Traffic Engineering


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ATM Cell

• ATM Cell is 53-Bytes = 48-Bytes User Data + 5-Byte Header

• Fixed-Size Cell is More Manageable and Easy to Hardware Route

• Cell Header Contains Information Pertaining to the Cell’s Path, Priority, and Other Useful Information


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ATM Cell Header

• General Flow Control (GFC, 4-bit)– Used for Local Functions, i.e. Identifying Multiple

Stations that Share an ATM Interface. Typically not Used, Set to a Default Value

• Virtual Path Identifier (VPI, 8-bit)– Used with the VCI, to Identify Next Destination of a

Cell as it Passes through a Series of Routers on the Way to the Destination

• Virtual Channel Identifier (VCI, 16-bit)– Used with the VPI, to Identify Next Destination of a

Cell as it Passes through a Series of Routers on the Way to the Destination


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• Payload Type (PT, 3-bit)– First Bit Indicates Whether the Cell Contains User or

Control Data. If Cell Contains User Data, the Second Bit Indicates Congestion, and the Third Bit Indicates Whether the Cell is the Last in a Series of Cells

• Congestion Loss Priority (CLP, 1-bit)– Indicates Whether the Cell Should be Discarded if it

Encounters Extreme Congestion as it Moves through the Network

• Header Error Control (HEC, 8-bit)– Checksum Calculated Only on the Header Itself


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ATM Header


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Problems with ATM

– “Cell Tax” 53-Byte ATM Cells are too Small for Most Data Traffic

– Ex: Requires Two 53-Byte ATM Cells to Transfer the Smallest IP Data Packet (64-Bytes)

– 5-Byte Tax for Every 48-Bytes of Data for ATM vs. 1,500-Bytes with Minimal Overhead in Ethernet (Best Case)

– IP over ATM losses 20% of SONET Rate


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IP Over SONET

• Transports IP Utilizing Point-to-Point Protocol (PPP) and High-level Data Link Control (HDLC)– PPP Provides Multi-Protocol Encapsulation,

Error Control, and Link Initialization Control– HDLC Frames the PPP-Encapsulated IP

Datagrams into the STS-1 Frame’s Payload

• Requires STS-3c (3 Multiplexed STS-1)


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Promising Future

• WDM Allows ATM and IP to Coexist on SONET

• Pure-IP Networks Adopting Rapidly– ISPs (AOL)– Carriers (Sprint, GTE, Level 3, Qwest)

• Telephony Traffic Remains Static, IP Traffic Increasing 7% to 20% Per Month

• Cheaper then ATM


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Issues to be Addressed

• Generating Traffic for STS-3 (155 Mbps)

• PPP Establishes Direct Link– No Addressing Capabilities– No Routing Capabilities

• PPP has No Flow Control– Additional Router Buffer Maybe Necessary

• Multiple Links Need to Be Provisioned in Event of Link Failure


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• Without ATM’s Layer-2 QoS, QoS Must be Added at Layer-3– Multi-Protocol Label Switching (MPLS)

Utilized for QoS, Processor Intensive?

• HDLC Poor Scaling Hampers Connections Above OC-48– Lucent Proposes Simplified Data Link (SDL)


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SDH

• Synchronous Digital Hierarchy (SDH) Published in 1989 by CCITT

• Addressing Synchronization of ANSI and CCITT Standards, Establishing a World Standard

• 32 64-kb Channels (E0) are Multiplexed into a 2 Mbps E1 Signal

• 21 E1 are Multiplexed into a STM-0 (52 Mbps)


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SONET vs. SDH

• 1.5 Mbps DS1 vs. 2 Mbps E1

• 52 Mbps STS-1 vs. 155 Mbps STM-1

• Multiplexing Smaller Connections into Larger is Similar to SONET

• SDH can Accommodate SONET By Changing SONET Signal from Bit-Interleaving to Byte-Interleaving.


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SDH Connections

Megabits SDH STM SDH Channels

Per Second Level Level E1 E4

52 STM-0 21

155 SDH-1 STM-1 63 1

622 SDH-4 STM-4 252 4

2,488 SDH-16 STM-16 1,008 16

9,953 SDH-64 STM-64 4,032 64

39,812 SDH-256 STM-256 16,128 256


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Connection ComparisonMbps OC Level STS Level SDH Level STM Level

52 OC-1 STS-1 STM-0

155 OC-3 STS-3 SDH-1 STM-1

622 OC-12 STS-12 SDH-4 STM-4

1244 OC-24 STS-24 SDH-8 STM-8

2488 OC-48 STS-48 SDH-16 STM-16

4976 OC-96 STS-96 SDH-32 STM-32

9953 OC-192 STS-192 SDH-64 STM-64

19906 OC-384 STS-384 SDH-128 STM-128

39812 OC-768 STS-768 SDH-256 STM-256


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Future of SONET

• 10 Gb Barrier– OC-768– Tunable Lasers

• SONET and Metro Ethernet– Which is Best for MAN?

• IP Over SONET vs. IP Over Fiber– Fiber Infrastructure without SONET


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Documents

Advances in Optical Networks: SONET By Sean Goggin April 19, 2005