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Advances in Optical Networks:
SONET
By
Sean Goggin
April 19, 2005
Overview
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Fundamentals of Optical Networks
SONET
SDH
Future of SONET
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SONET
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Fundamentals of Optical
Networks
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•
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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
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Variants of Fiber Optic
• Single-Mode
– Small Core
Approximately 9
Microns
– Uses IR Laser
Light Transmitter
– Greater then 10
Miles*
– Most Expensive
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• 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-ElectronicOptical (OEO)
Repeater
– Incurs Pricey
Conversion Delay
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• All-Optical (Pure)
– Weak Signals are
Boosted with a
Amplifier
– Erbium-Doped
Fiber Amplifier
(EDFA)
– Complete Photonic
Boost
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OEO Repeater
Processor
Strong
Optical
Signal
Optical
Transceiver
Optical
Transceiver
Weak
Optical
Signal
Electronic
Circuit
Pathway
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Erbium Doped Fiber Amplifier
Pump Laser
Strong
Optical
Signal
Filter
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ErbiumDoped
Fiber
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SONET
Coupler
Weak
Optical
Signal
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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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SONET
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Optical Add-Drop Multiplexer
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SONET
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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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SONET
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Sample of Optical Network
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SONET
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Telecom Terminology
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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
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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 SubDivided 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 (64Bytes)
– 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 BitInterleaving to Byte-Interleaving.
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SDH Connections
Megabits
SDH
STM
Per Second
Level
Level
52
SDH Channels
E1
E4
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 Comparison
Mbps
OC Level STS Level SDH Level
52 OC-1
STS-1
155 OC-3
STS-3
SDH-1
622
1244
2488
4976
OC-12
OC-24
OC-48
OC-96
9953 OC-192
19906 OC-384
39812 OC-768
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STM Level
STM-0
STM-1
STS-12
STS-24
STS-48
STS-96
SDH-4
SDH-8
SDH-16
SDH-32
STM-4
STM-8
STM-16
STM-32
STS-192
STS-384
STS-768
SDH-64
SDH-128
SDH-256
STM-64
STM-128
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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