Optical Transport Technologies and Trends Dion Leung, Ph.D Director, Solutions and Sales Engineering (BTI)

1. Optical Transport Technologies and Trends
August 20, 2015
Dion Leung, Director of Solutions and Sales Engineering
dleung@btisystem.com

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§ Logical connectivity is presented between the routers/switches
§ The underlying “physical” network is an abstract layer
§ One often requires to know if the routers have 10G, 40G, 100G
interfaces and how many of these interfaces are available
In Data/Packet Networking World…
P P
PE
PE
CE
CE

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§ To engineer an optical transmission network, one would need to know
EXACTLY the underlying fiber topology, the fiber details and
characteristics, so that the optical layer can be designed accordingly.
§ Economics of regional network <> metro network <> ULH network
In Optical Transport Networking World…
DWDM
40km
DWDM
30km
CE
DWDM
DWDM
DWDM
CE
14km
35km
10km
15km
35km

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§ Technology Enabler: Wavelength Division Multiplexing
– A transmission technology that multiplexes multiple optical carrier
signals on a single fiber by using different wavelengths (colors) of
laser light to carry different signals of frequencies.
– Frequency (in THz) and wavelength (in nm) are often used to label a
wavelength and the frequency of a signal is inversely proportional to
wavelength. e.g. 193 x 1012 THz or 1551.9 nm
Wavelength Division Multiplexing (WDM) –
Similar to Sharing Spectrum over Air, Except Medium here is Fiber
M
U
X
Individually
Colored
Wavelengths
D
E
M
U
X
Single Transmission Fiber
Individually
Colored
Wavelengths
Equally spaced channels (aka standard ITU grid)

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§ Optical light transmitted through fiber will lose power
§ Attenuation caused by Scattering, Absorption and Stress
§ Other related parameter: fiber length, fiber type, transmission bands,
and external loss components such as connectors & splices
§ Typical fiber loss: 0.20 dB/km – 0.35 dB/km, although in some
regions fiber loss can be as high as ~0.5 dB/km
§ Basic Link Budget Engineering:
– Fiber loss + spice loss + connector loss + safety margin ≤ Power
Budget (i.e. Transmitted – Received Power)
Fiber Characteristic #1:
Fiber Attenuation or Loss (measured in dB or dB/km)

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Transmission Windows:
Attenuation in Optical Fiber (measured in dB or dB/km)
800 900 1000 1100 1200 1300 1400 1500 1600
Wavelength in nanometers (nm)
0.2 dB/km
0.5 dB/km
2.0 dB/km
C-­band (1530 –1565 nm)
L-­band (1565 –1625 nm)
Note: Frequency = 3 x 108
/ wavelength
850 nm Range
1310 nm Range
Also known as the three “Transmission” Windows

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§ Different wavelengths travel at different speeds through a given fiber
causing optical pulses to broaden or to “spread”
– e.g. Wavelength Channel #1 travels faster than Channels #2, #3, etc..
§ Excessive spread can cause pulses to overlap, and therefore receivers
would have a hard time to distinguish overlapped pulses
§ The longer the distance (or the higher the bitrate) is, the worst the
spread would be.
Fiber Characteristic #2:
Chromatic Dispersion (measured in ps / km-­nm)

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Lego Blocks of a Simple Point to Point DWDM System
Transponder
Muxponder
Transceiver
Mux
Demux

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§ Multiplex / Demultiplexer (aka. Mux/Demux) Comes with Various Sizes…
– Use light’s reflection and refraction properties to separate and combine wavelengths from a
fiber strand (e.g. logically think of a prism)
– Common technologies: thin film filters, fiber bragg gratings and arrayed waveguides (AWG)
– Passive device which requires no power
– Higher the channel counts means higher the insertion loss
Lego Block to Create the Highway Lanes
Common Mux/Demux Selections from most vendors…
DWDM Mux-­Demux (8λ Add-­Drop)
CWDM Mux-­Demux (4λ Add-­Drop)
OADMs (1,2, and 4λ Add-­Drop)
DWDM Mux-­Demux (40λ Add-­Drop)
DWDM Mux-­Demux (96λ Add-­Drop)

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Transponder and Muxponders:
Converting “Grey” to “Color”
Transponder
Muxponder
Client Signal (Grey optics)
(e.g. from a switch, router)
Line Signal (Color optics)
(to DWDM mux / outside plant)
10GE LAN PHY
(STM64, 10G FC)
a 10G Wavelength
(e.g. Channel 3)
a 10G Wavelength
(e.g. Channel 4)
GbE
STM16
2G FC
STM4
GbE
1 in – 1 out
Many in – 1 out

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Transceivers
§ Typical Line Rates
– 2.5G
– 10G
– 100G
§ Transceivers
– SFP
– XFP/SFP+
– QSFP+
– CFP
§ Selection of which type mainly depends on Speed, Reach
– 850nm, 1310nm, 1550nm, CWDM, DWDM Fixed Channel, DWDM Tunable
§ Each type of transceiver’s has its transmit power and receive power sensitivity
(e.g. TX = [-­3,1] dBm, RX = [-­25, -­5] dBm) à Max Budget = 1-­(-­25) = 26dB
Optical Transceivers – The Pluggable Optics

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Additional Lego Blocks of Multi-­Node Linear DWDM System
Transponder
Muxponder
Transceiver
Mux
Demux
Optical
Amplifier
Dispersion
Compensation

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Overcome Fiber Attenuation…
Optical Amplifiers (EDFA and Raman)
Optical Amplifiers are Needed in Order to be Sure Optical
Signals Can Be Accurately Detected by Receivers
Two Common Types of Optical Amplifiers
Erbium Doped Fiber Amplifier RAMAN Amplifier
Most common used and
simple to deploy
Fixed gain or Variable gain
For high span loss and long
distance transmission
Used in Conjunction With
EDFAs

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End End
0
Max
Min
Receiver
Tolerance
CD
Dispersion Compensation Over Multi-­span Route
(Note: for 100G coherent transmission, CD is less of an issue…)
DCM DCM DCM
Span-­by-­Span CD Compensation for 10G/40G transmission
Simply match fiber distance to DCM type (e.g. Use 60km DCM for ~60km link)

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As Network Grows and Evolves to Ring / Mesh Topologies…
Advanced Technology Enabler Makes Operation & Planning Simpler
From www.datacentermap.com

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§ For initial Point-­to-­Point network, Fixed OADM (FOADM) network
architecture worked fine.
§ A problem arises when we have intermediate location that requires
“partial” adding/dropping of traffic à manual patch work is needed
Unexpected Network Expansion or Node Insertion
Wavelength power management can become tricky to engineer
A C
40km
10dB
B
20km
5dB
§ 10 x 10GbE circuits are now between Site A and Site B
§ 10 x 10GbE circuits are now between Site A and Site C (via Site B)
10 x 10GbE
10 x 10GbE

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§ Since not all wavelengths need to be dropped, manual padding, patching works
are required to connect wavelength across intermediate site(s)
§ Patching through makes sense for small λ counts, but with 40/96 DWDM
channels, this can be prone to human errors and difficult to manage –
a better solution is warranted.
A Closer Look: Channel Patching Work is Required
Intermediate Site (at Site B)
DEMUX
MUX
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
1. The insertion loss
affects the overall link budget
2. Each wavelength added
needs to be re-­balanced
λ 3. Regeneration is often
needed due to deficit power budget
From Site A At Site B To Site C

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How a 4-­Degree ROADM Node Works…
A/D
Ex2
Ex4
Ex3
R
O
A
D
M
Fiber
Line
A/D
Ex3
Ex2
Ex4
R
O
A
D
M
Fiber
Line
A/D
Ex2
Ex4
Ex3
R
O
A
D
M
Fiber
Line
λ
Mux / Demux
λ
λ A/D
R
O
A
D
M
Fiber
Line
Ex2
Ex3
Ex4
Mux / Demux
λ
λA Single Express
Cable
Automatic
Power
Equalized
Wavelengths
In-­Service Network
Expansion by Simply
Adding ROADM module

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Network-­wide Benefit of ROADM:
Reconfigurability, Flexibility and Ease of Expansion
• Individual wavelengths can be easily steered from
any node to any node
• Site visit only at the add/drop locations
• Simplify operations and network planning
O
O
O
OO
O

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Optical Layer
Lego Block
Uses & Benefits Additional Design Notes
Fiber Pair For DWDM transmission • G.652 / SMF is preferred
Mux / Demux (M/D) For dividing fiber into virtual
highway lanes or wavelength
channels
• Passive element (no power
required)
• Various M/D have different
insertion loss
Dispersion
Compensation Fiber
or Module
(DCF/DCM)
For compensation CD for
80km or longer span or multi-­
span network
• DCF is usually classified by
distance
• DCF has insertion loss and
add latency
Amplifier For overcoming fiber span
loss and minimizing
regeneration cost for multi-­
span network
• Choice of EDFA (commonly
used metro) and Raman (for
regional/long-­haul)
• Amplifier has various gain
levels, noise figure
Reconfigurable
Optical Add/Drop
Multiplexer
(ROADM)
For flexible wavelength
add/drop/bypass and simpler
operation
• Single module combines
amplifier and WSS
• Per-­channel auto power
balancing
Summary of Essential Lego Blocks for Building a Flexible
Optical Network

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DWDM
40km
DWDM
30km
CE
DWDM
DWDM
DWDM
CE
14km
35km
10km
15km
35km
§ One school of thought: Router vendors have integrated DWDM or color optical
interfaces onto their routing platform
§ Another school of thought: Optical vendors have integrated packet functionalities
(L2/L3) onto their optical platform
§ Which option is better? Which option is more cost effective? It depends.
Remember this View?
P P
PE
PE
CE

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School of Thought #1… Simple Point to Point
Converged Packet Optical Networking
Edge
Router
Core
Router
Core
Router
§ Optics on routers simplify the need for a separate optical platform
§ Limited to point-­to-­point or simple fiber topology. Remember the
fundamental optical rules and lego blocks don’t disappear.
§ Optical reach depends on the integrated optical transceiver specifications
§ 100G coherent technology helps overcome dispersions
Integrated Optics on Routers
Edge
Router

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School of Thought #2… Beyond Point to Point
Converged Packet Optical Networking
NMS
Edge
Router
Core
Router
Core
Router
§ Pre-­integrated solution using 10G/100G colored interfaces from existing
feature-­rich routing platform, or grey optics handoff
§ ROADM layer can provide additional layer of flexibility at physical layer
§ OSS/NMS integration can simplify operations and troubleshooting
Dynamic
ROADM Core
Edge
Router

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School of Thought #3… DC+C Provider Centric
Converged Packet Optical Networking
• If many Layer 3 features go unused in routers, you paid for them anyway
• If most of Layer 3 traffic is actually MPLS LSR switching…
• MPLS LER features are higher cost than simple LSR
• Do you use all the RFC’s and functionalities in your routers today?
• MPLS LER and LSR functions do not need to be on same equipment
LER
WDM
LER/LSR
$$$$$
LER
WDM
LSR
$$
LER
$$ $$
$
LER
WDM+LSR

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