NETE0510
Optical Networking
Supakorn Kungpisdan
supakorn@mut.ac.th
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Outline
 History of Optical Networking
 SONET/SDH Standards
 DWDM
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History of Optical Networking
 Optical telegraph was invented in the 1790s
 In 1880, Alexander Graham Bell patented the optical
telephone system
 Modern optical communications started in 1950s with the
development of pulsing-laser technology (light) across
fiber (glass or plastic) with low loss rates to achieve highspeed data and voice communications transfer
 SONET/SDH define basic transmission rates and
characteristics, frame formats and testing, and an optical
interface-multiplexing scheme
 Had been main WAN transport technology through 1990s
 Now DWDM allows 160 wavelengths per fiber, each 2.510 Gbps
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Outline
 History of Optical Networking
 SONET/SDH Standards
 DWDM
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SONET/SDH
 SONET and SDH are U.S. and International
standards, respectively, for optical
telecommunication transport
 Provide a technology that enables the major
service providers to internationally standardize
and control broadband network transport media
through a common fiber interface called a
“midspan meet”.
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SONET/SDH Advantages
 Reduction of equipment needed and increase
network reliability and availability
 Centralized fault isolation and management of
payload (traffic carried)
 Synchronous multiplexing formats for DS1 and
E1 allowing easy access for switching and
multiplexing
 International vendor interoperability
 Flexible architecture able to accommodate
future requirements
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Synchronous, Plesiochronous,
Asynchronous
 Synchronous
All clocks are traceable to one Stratum 1 primary
reference clock (PRC)
All digital transitions in the signals occur at exactly the
same rate
 Plesiochronous
Clocks are extremely accurate and almost exact, but
small difference between them
 Asynchronous
The clocks do not have to match or be equal
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Layers in SONET
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Layers in SONET (cont’d)
 Physical Layer
 Define physical fiber type, path, and characteristics
 Include many electrical interfaces which become virtual channels
within the Synchronous Transport Signal-1 (STS-1) frame – the
base level building block of SONET
 Section Layer
 Build SONET frames from either lower SONET interfaces or
electrical interfaces
 Line Layer
 Provide synchronization, channel multiplexing, and protection
switching
 Path Layer
 Manage actual data transport across the SONET network
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SONET Network Structure
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SONET Network Structure (cont’d)
 Path: information carried end-to-end
 Line: information carried for STS-n signals between
multiplexers
 Section: information carried for communication between
adjacent network equipment, e.g. regenerator
 Path-terminating equipment (PTE): user interface at
the CPE
 Line-terminating equipment (LTE): a terminal, switch,
add/drop multiplexer, or cross-connect
 Section-terminating equipment (STE): a regenerator
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SONET Structure
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SONET Structure (cont’d)
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Frame Format
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SONET Frame Format
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STS-1 Overhead
 Section Overhead (9 bytes  3 columns x 3 rows)
 Performance monitoring
 Framing
 Messaging communication between STEs for control,
monitoring, administration, and other communication needs\
 Voice communication between STE
 Line Overhead (18 bytes  3 columns x 6 rows)
 Locating the SPE in the frame
 Multiplexing or concatenating signals
 Performance monitoring
 Automatic protection switching
 Line maintenance
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STS-1 Overhead(cont’d)
 Path Overhead (9 bytes  1 column x 9 rows)
 Performance monitoring of the SPE
 Path signal label, which indicates the content of the SPE
 Path status, which conveys status and performance back to
the originating terminal
 Path trace, which allows verification of continues connection
with the originating terminal
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STS-1 Synchronous Payload Envelope
(SPE)
 SPW is defined as 783 bytes  87 columns x 9 rows
 The first column is the path overhead
 Columns 30 and 59 are not used for payload, but
designated to fixed stuff columns
 So it remains 84 columns x 9 rows  756 bytes of
payload
 To support service that requires a payload larger than
STS-1, SONET allows concatenating STS-1s together to
support
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Pointers
Located in the line overhead of each frame
Used for frame synchronization
Identify subchannels down to the DS0
level within a SONET transmission
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Pointers
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Virtual Tributary (VT)
 VTs are the building blocks of the SPE
 VTxx designates VTs of xx Mbps
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Virtual Tributary (VT)
7 VT groups
• 4 VT1.5s
• 3 VT2s
• 2 VT3s
•1 VT6
•2 bit-stuffed unused columns
•1 path overhead column
Locked mode: fix the VT
structure within an STS-1
and is designed for
channelized operation
Floating mode: allow
these values to be
changed by crossconnects and switches
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Multiplexing
 SONET provides direct multiplexing of both SONET speeds and
current asynchronous and synchronous services into the STS-n
payload
 Payload types range from DS1 and DS3 to OC-3c and OC-12c
payloads
 STS-1 supports direct multiplexing of DS1 and DS3 into single or
multiple STS-1 envelopes, which are called VTs
 Multiple STS-1 envelopes are multiplexed into an STS-n signal
 Each individual signal down to DS1 can be accessed without the
need to demultiplex and remultiplex the entire OC-n level signal
  use a SONET digital cross-connect (DXC) or multiplexer
 SONET multiplexing requires an extremely stable clocking source
with a stable reference point
 Frequency of every clock within the network must be the same as or
synchronous with the others
 The central clocking source is typically a Stratum 1 source
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Multiplexing
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SONET Hardware
 Most common equipment term used is the
SONET terminal equipments:
SONET Terminating Multiplexer
SONET Add/Drop Multiplexer (SADM)
SONET Digital-Loop Carrier Systems (DLCs)
SONET Digital Cross-Connects (SDXCs)
SONET Regenerators and Optical Amplifiers
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SONET Terminating Multiplexer
 Provide user or customer premises equipment (CPE)
access to the SONET network
 Aka terminal adapter, edge multiplexer, or terminal
 Similar to M13 multiplexer and allow low-speed access
to SONET backbone
 Turn electrical interfaces into optical signals by
multiplexing multiple DS1, DS3, or E1 VTs into the STSn signals required for OC-n transport
 Arranged in point-to-point configuration
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SONET Add/Drop Multiplexer (SADM)
 Add/Drop Multiplexer
 Add one or more lower-bandwidth signal on to a highbandwidth stream
 Drop or extract other signals, removing them from the stream
and redirecting them to some other network paths
 Traditional ADM is asynchronous at DS3 and lower
speed.
 Require multiple equipments e.g. M13 MUX
 SADM enables provider to drop and add not only the
lower SONET rates, but also electrical interface rates
sown to the DS1 level
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SONET Add/Drop Multiplexer (SADM)
 Standard features:
 drop-and-insert: a process that diverts (drops) a portion of the
multiplexed aggregate signal at an intermediate point, and
introduces (inserts) a different signal for subsequent
transmission in the same position, e.g., time slot or frequency
band, previously occupied by the diverted signal.
 drop-and-continue: drop some signals while allowing others to
pass
 Used for distributed point-to-point network connectivity
 A CO device forming the building blocks of the SONET network
 Enable easy expansion and are often used in SONET ring
architectures
 Operate at the higher transmission speeds of OC-3 through OC192
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SONET Add/Drop Multiplexer (SADM)
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SONET Digital-Loop Carrier Systems
(DLCs)
 Concentrate multiple DS0 traffic from remote
terminals into a single OC-3 signal
 Situated at local service providers and handle
both voice and data traffic providing a SONET
network interface for non-SONET equipment
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SONET Digital Cross-Connects (SDXCs)
Act as a gateway to
SONET network
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SONET Regenerators and Optical
Amplifiers
 Both perform optical signal regeneration over
fiber optics
 Optical amplifiers just amplify the signal and
noise
 Regenerators reshape, retime, and retransmit
signals that have incurred dispersion or
attenuation over long transmission distances
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SONET Network Configurations
 Point-to-point
Two PTE pieces of equipment are connected directly
together with a STE/regenerator in line
 Point-to-multipoint
The PTE equipment is connected to a LTE/SADM that
enables circuits to be added or dropped along the way
 Ring
Are deployed in most large-scale service provider
networks
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SONET Ring Architecture
Normal operation
Fiber cut of both pairs
Working pair outage
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SONET Advantages
 Reduced network complexity and cost through SADM and
SDXC capabilities
 Ability to transport all forms of traffic: voice, data (ATM, IP),
and video
 Capability to build optical interconnects between carriers
 Efficient management of bandwidth at the physical layer
 Aggregation of low-speed data channels into common highspeed backbone trunk transport
 Standard optical interface and format specification providing
vendor interoperability
 Increased reliability and restoration over electrical systems
 Increased bandwidth management through logical path
grooming
 Smart OAM&P features with uniformity
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SONET Disadvantages and Challenges
 Strict synchronization schemes required
 Complex and costly SONET equipment contrast
to chapter optical Ethernet and other alternate
MAN technologies
 High percentage of SONET protocol overhead
 Fiber laying unutilized in a ring architecture,
waiting on a failure
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Outline
 History of Optical Networking
 SONET/SDH Standards
 DWDM
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Dense Wavelength Division Multiplexing
(DWDM)
 Three methods to relieve capacity shortage:
Increase the bit rate of existing systems, such as
moving OC-48 systems to OC-192 systems
Install new fiber
Optimize the use of existing fiber using methods like
increasing the number of wavelengths (and thus
bandwidth available) per fiber
 DWDM
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WDM
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DWDM VS WDM
 Similar  enable more than one wavelength to
be added to a single-mode fiber
Increased capacity depends on the number of
wavelengths added
Current systems support 160 wavelengths per fiber
 DWDM spaces the wavelengths closer than
WDM and therefore has a greater overall
capacity than WDM
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Advantages of DWDM
Every wavelength is independent of the
others
Can transport SONET, gigabit Ethernet, or
native ATM on the same optical fiber cable
Do not require a large amount of overhead as
that of SONET
Optical amplifier can apply to all wavelengths
 cost savings
Having 160 wavelengths on a DWDM fiber will
save amplifier 160:1
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DWDM Hardware
 DWDM multiplexer/demultiplexer
 Combine multiple optical signal into a single optical fiber
 Separate optical wavelengths into a single wavelength fiber
 Optical add/drop multiplexer (OADM)
 Like SADM, but in the optical domain
 Allow wavelengths to be split or added to a DWDM fiber
 OXC
 A cross-connect between n-input ports and m-output ports
 Perform management of wavelengths at the optical layer
 Optical amplifier
 Amplify signal strength to travel over long distances
 Regenerator
 Same functionality as amplifier with resharing and retiming
capabilities
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Optical Amplifiers/Regenerators
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Interfaces
DWDM supports many different types of
interfaces:
SONET
Ethernet (1Gbps, 10Gbps, and fast)
Fiber channel
ATM
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DWDM Network Configuration
 Point-to-point
 The network must have the characteristics of ultra high-speed
channels (10-40Gbps), high signal integrity and reliability, and
fast path restoration
 Distance btw transmitters and receivers can be several hundred
kms with less than 10 amplifiers
 Ring
 SONET rings can be built with the combination with DWDM
 Mesh (partial or full)
 Mesh architectures connect all-optical nodes together with two
routes, and implement intelligence in the notes to reroute
wavelengths on faults
 Extremely expensive to implement andmanage
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Advantages of DWDM
 Support 160 wavelengths  over 1 Tbps of
traffic to be carried
 Each wavelength can be a different traffic type
e.g. SONET, gigabit Ethernet, IP over PPP, and
can operate at different speeds
 Optical amplifiers provide cost saving
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Disadvantages of DWDM
 Some fiber plant are not suitable for DWDM and
do not support DWDM
 Difficult to troubleshoot, manage, and provision.
Need to manage DWDM-specific equipment
 Vendor interoperability issues
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Questions?
Next Lecture
Physical Layer Protocols and
Access Technologies
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