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Core router History
1
At that time, several breakthroughs in fiber
optic telecommunications (notably DWDM
and EDFA) technologies combined to
lower bandwidth costs that in turn drove a
sudden dramatic increase in core link
speeds: by 2000, a core link operated at
2.5 Gbit/s and core Internet companies
were planning for 10 Gbit/s speeds.
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Passive optical network - Passive optical components
1
These components are used in interoffice,
loop feeder, Fiber In The Loop (FITL),
Hybrid Fiber-Coaxial Cable (HFC),
Synchronous Optical Network (SONET),
and Synchronous Digital Hierarchy (SDH)
systems; and other telecommunications
networks employing optical
communications systems that utilize
Optical Fiber Amplifiers (OFAs) and Dense
Wavelength Division Multiplexer (DWDM)
systems
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Passive optical network - Passive optical components
Dense Wavelength Division
Multiplexers (DWDMs) are optical
components that split power over at
least four wavelengths
1
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OSI model - Examples
1
1 Physical X.25 (X.21bis, EIA/TIA-232,
EIA/TIA-449, EIA-530, G.703)
MTP,
Q.710 RS-232, RS-422, STP, PhoneNet
Twinax UMTS Physical layer or L1 RS232, Full duplex, RJ45, V.35, V.34, I.430,
I.431, T1, E1, 10BASE-T, 100BASE-TX,
1000BASE-T, POTS, SONET, SDH, DSL,
802.11a/b/g/n PHY, ITU-T G.hn PHY,
Controller Area Network, Data Over
Cable Service Interface Specification
(DOCSIS), DWDM
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Moore's law - Other formulations and similar laws
Optical networking and dense
wavelength-division multiplexing
(DWDM) is rapidly bringing down the
cost of networking, and further
progress seems assured
1
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Metro Ethernet
1
Ethernet on the MAN can be used as
pure Ethernet, Ethernet over SDH,
Ethernet over MPLS, or Ethernet over
DWDM
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Next-generation network - Implementations
The core optical network uses dense
wavelength-division multiplexing (DWDM)
technology to provide scalability to many
hundreds of gigabits per second of
bandwidth, in line with growth demand
1
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Single-mode optical fiber - Characteristics
By using optical amplifiers and
dispersion-compensating devices,
state-of-the-art DWDM optical systems
can span thousands of kilometers at 10
Gbit/s, and several hundred kilometers
at 40 Gbit/s.
1
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Soliton - Solitons in fiber optics
The GH jitter requires sophisticated,
expensive compensatory solutions that
ultimately makes dense wavelengthdivision multiplexing (DWDM) soliton
transmission in the field unattractive,
compared to the conventional non-returnto-zero/return-to-zero paradigm
1
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Google platform - Network topology
These custom switch-routers are
connected to Wavelength-division
multiplexing|DWDM devices to
interconnect data centers and point of
presences (PoP) via dark fibre.
1
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Diode laser - Distributed feedback lasers
1
Distributed feedback lasers (DFB) are the most
common transmitter type in DWDM-systems
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Synchronous Digital Hierarchy - Path virtual envelope
DWDM circuits are the basis for all
modern submarine communications cable
systems and other long-haul circuits.
1
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Synchronous Digital Hierarchy - SONET/SDH and relationship to 10 Gigabit Ethernet
However, 10 Gigabit Ethernet does not
explicitly provide any interoperability at the
bitstream level with other SDH/SONET
systems. This differs from WDM system
transponders, including both coarse and
dense wavelength-division multiplexing
systems (CWDM and DWDM) that
currently support OC-192 SONET signals,
which can normally support thin-SONET–
framed 10 Gigabit Ethernet.
1
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Distributed feedback laser
1
Integrated DFB lasers are often used in
optical communication applications,
such as DWDM where a tunable laser
signal is desired as well as in sensing
where extreme narrow line width is
required, or in gas sensing
applications, where the signal of the
absorbing gas is detected while
wavelength tuning the DFB laser
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Dark fibre - Market
1
Dark fibre capacity is typically used by
network operators to build SONET and
dense wavelength division multiplexing
(DWDM) networks, usually involving
meshes of self-healing rings
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Dark fibre - Variations
DWDM systems generally require
central management because their
closely spaced wavelengths are
subject to disruption by signals on
adjacent wavelengths that are not
within tightly controlled parameters,
especially if amplification is required
for signal transmission over 100km.
1
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Dark fibre - Rate of expansion
The availability of dense wavelengthdivision multiplexing DWDM and coarse
wavelength division multiplexing CWDM is
rapidly bringing down the cost of
networking, and further progress seems
assured.
1
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Telecommunications - Telephone
Lastly, improvements in multiplexing
have led to an exponential growth in the
data capacity of a single fibre.[
http://www.cisco.com/univercd/cc/td/d
oc/product/mels/cm1500/dwdm/dwdm
_ovr.pdf Fundamentals of DWDM
Technology], CISCO Systems, 2006.[
http://www.lightreading.com/document
.asp?doc_id=31358 Report: DWDM No
Match for Sonet], Mary Jander, Light
Reading, 2006.
1
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Dense WDM - WDM systems
Dense wavelength division
multiplexing (DWDM) uses the same
transmission window but with denser
channel spacing
1
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Dense WDM - WDM systems
Coarse wavelength division
multiplexing (CWDM) in contrast to
conventional WDM and DWDM uses
increased channel spacing to allow
less sophisticated and thus cheaper
transceiver designs
1
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Dense WDM - WDM systems
1
WDM, DWDM and CWDM are based on
the same concept of using multiple
wavelengths of light on a single fiber,
but differ in the spacing of the
wavelengths, number of channels, and
the ability to amplify the multiplexed
signals in the optical space
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Dense WDM
1
Dense wavelength division multiplexing
(DWDM) refers originally to optical
signals multiplexed within the 1550nm
band so as to leverage the capabilities
(and cost) of Optical amplifier#Erbiumdoped fibre amplifiers|erbium doped
fiber amplifiers (EDFAs), which are
effective for wavelengths between
approximately 1525–1565nm (C band),
or 1570–1610nm (L band)
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Dense WDM - DWDM systems
(Early DWDM systems contained 4 or 8
'wavelength converting transponders' in the
mid 1990s
1
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Dense WDM - DWDM systems
1
# A DWDM 'terminal
demultiplexer'
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Dense WDM - DWDM systems
Today's DWDM systems use 50GHz or
even 25GHz channel spacing for up to 160
channel
operation.http://www.infinera.com/products
/ils2.html
1
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Dense WDM - DWDM systems
These factors of smaller volume and
higher performance result in DWDM
systems typically being more expensive
than CWDM.
1
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Dense WDM - DWDM systems
1
Recent innovations in DWDM transport
systems include pluggable and softwaretunable transceiver modules capable of
operating on 40 or 80 channels. This
dramatically reduces the need for discrete
spare pluggable modules, when a handful
of pluggable devices can handle the full
range of wavelengths.
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Dense WDM - Wavelength converting transponders
At this stage, some details concerning
Wavelength Converting Transponders
should be discussed, as this will clarify the
role played by current DWDM technology
as an additional optical transport layer. It
will also serve to outline the evolution of
such systems over the last 10 or so years.
1
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Dense WDM - Wavelength converting transponders
1
As stated above, wavelength converting
transponders served originally to translate
the transmit wavelength of a client-layer
signal into one of the DWDM system's
internal wavelengths in the 1550nm band
(note that even external wavelengths in
the 1550nm will most likely need to be
translated, as they will almost certainly not
have the required frequency stability
tolerances nor will it have the optical
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Dense WDM - Wavelength converting transponders
; 1R: Retransmission. Basically, early
transponders were garbage in garbage
out in that their output was nearly an
analogue 'copy' of the received optical
signal, with little signal cleanup
occurring. This limited the reach of early
DWDM systems because the signal had
to be handed off to a client-layer receiver
(likely from a different vendor) before
the signal deteriorated too far. Signal
monitoring was basically confined to
optical domain parameters such as
received power.
1
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Dense WDM - Reconfigurable optical add-drop multiplexer (ROADM)
1
This is costly, and in some systems
requires that all active traffic be
removed from the DWDM system,
because inserting or removing the
wavelength-specific cards interrupts
the multi-wavelength optical signal.
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Dense WDM - Enhanced WDM
Cisco's Enhanced WDM system
combines 1 GB Coarse Wave Division
Multiplexing (CWDM) connections
using SFPs and GBICs with 10 GB
Dense Wave Division Multiplexing
(DWDM) connections using XENPAK,
X2, or XFP DWDM modules. These
DWDM connections can either be
passive or boosted to allow a longer
range for the connection
1
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Dense WDM - Transceivers versus transponders
1
** Dense WDM (DWDM) Transceivers:Channel 17
to Channel 61 according to ITU-T.
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Dense WDM - Implementations
1
There are several simulation tools that can
be used to design WDM systems. Popular
commercial tools have been developed by
[http://optiwave.com/applications/dwdm/
Optiwave Systems Inc.] and VPI Systems.
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Core network
Technologies used in the core and
backbone facilities are data link layer and
network layer technologies such as
SONET, DWDM, ATM, IP, etc
1
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Next-generation network - Implementations
The core optical network uses dense
wavelength-division multiplexing (DWDM)
technology to provide scalability to many
hundreds of gigabits per second of
bandwidth, in line with growth demand
1
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Magyar Telekom - Optical systems
1
In high-speed (10, 40 Gbit/s) DWDM
systems, especially in pure optical
networks, the optimum location of the
optical amplifiers and the dispersion
compensating elements to a great
extent influences the performance
(throughput) capabilities of the
network
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Small form-factor pluggable transceiver - Types
1
** CWDM and DWDM transceivers at various
wavelengths achieving various maximum
distances
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Belgacom - BICS's coverage
1
BICS owns and operates a high capacity
MPLS enabled global network based on a
wholly owned and operated 100Gbit/s
capable DWDM network in the
Netherlands, France, Germany, Italy,
Switzerland, Luxemburg, UK and Belgium
augmented with leased capacity and
submarine cable assets to serve the global
footprint.
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EDPnet
1
'Edpnet' is a Belgian telecom provider
that offers services worldwide. The
company focuses on three markets:
retail (in Belgium and The
Netherlands), business to business (in
Europe, Russia and North America)
and wholesale (in Europe, Russia and
North America). Edpnet has its own
fully redundant transcontinental fiber
network based on the latest
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Self phase modulation - Mitigation strategies in DWDM systems
1
In long-haul single-channel and DWDM
systems SPM is one of the most
important reach limiting nonlinear
effects. It can be reduced by:
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Ultra high definition television - 2003–2011
1
On November 2, 2006, NHK demonstrated
a live relay of a UHDTV program over a
260kilometer (km) distance by a Fiberoptic communication|fiber-optic network.
Using Wavelength-division
multiplexing#Dense WDM|dense
wavelength division multiplex (DWDM),
24 Data rate units#Gigabit per second
|Gbit/s speed was achieved with a total of
16 different wavelength signals.
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Computational power - Other formulations and similar laws
Optical networking and Dense
WDM|dense wavelength-division
multiplexing (DWDM) is rapidly bringing
down the cost of networking, and further
progress seems assured
1
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Optical Internetworking Forum - 100G Implementation Agreements
* Multisource Agreement for 100G Long-Haul
DWDM Transmission Module - Electromechanical
1
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Transponder - Optical communications
Fujitsu
[http://www.fujitsu.com/downloads/
TEL/fnc/pdfservices/DWDM_Tutorial
.pdf Fujitsu DWDM Primer, 2004]
1
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Optical performance monitoring
1
'Optical performance monitoring'
(OPM) is used for managing high
capacity dense wavelength division
multiplexing (DWDM) optical
communications|optical
transmission and switching systems
in Next Generation Networks (NGN)
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Optical performance monitoring
The optical component used for this
purpose in DWDM networks is known
as optical performance monitor (OPM)
or optical channel monitor (OCM),
which measures channel power,
wavelength, and optical signal-to-noise
ratio (OSNR) for each channel.
1
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Optical mesh network
Technological advancements in
optical transport equipment in the
first decade of the 21st century, along
with continuous deployment of DWDM
systems, have led
telecommunications service
providers to replace their SONET ring
architectures by mesh-based
architectures
1
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Optical mesh network - History of transport networks
1
in the first decade of the 21st century,
along with continuous deployment of
dense wavelength-division
multiplexing (DWDM) systems, have
led telecommunications service
providers to replace their SONET ring
architectures by mesh-based
architectures for new traffic
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Ciena - Sprint and IPO
In late 1994, during the public-to-private
transition of the Internet, Ciena began working
with Sprint – an earlier carrier of exploding
internet traffic – to develop “high-capacity
fiber optic transmission systems called Dense
Wave Division Multiplexing (DWDM).”
https://aresu.dsi.cnrs.fr/IMG/pdf/dwdm_ciena.p
df The outcome of their effort was announced
in June 1996 when Ciena said it had increased
“Sprint's nationwide, all-digital fiber-optic
network by a stunning 1,600 percent.” Sprint
soon became one of the world’s largest
carriers of Internet traffic
1
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GAIL - GAILTEL
1
GAILTEL services include long term lease of
Dark Fibre and Duct under IRU, Tower space
collocation facilities and point-to-point leased
line Bandwidth services. With SDH DWDM
as the core layer, GAILTEL network is built
largely along the highly secured GAIL’s cross
country pipeline corridor and ensure highly
reliable and error free service to its internal
external customers. The network is managed
centrally on 24X7 basis from a state-of-art
Network Management Centre at Noida.
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BT 21CN - Core Node
1
The core nodes are MPLS switches, with
the MPLS traffic carried over optical
(DWDM) transport. They are completely
unaware of customer IP traffic, and only
switch based on MPLS tags (all customer
IP traffic is encapsulated with an MPLS
header by the Metro node PEs). Native IP
is only used by the Core nodes for
protocols such as MP-BGP, an IGP, LDP,
and RSVP to exchange routing and label
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Telecommunications industry in China - Historical overview
These resources consisted of a
2,200,000km long nation-wide optical
network, based on Asynchronous Transfer
Mode (ATM), Synchronous Digital
Hierarchy (SDH) and Wavelength-division
multiplexing|Dense Wavelength Division
Multiplexing (DWDM) technologies and
several submarine cables, in particular
with the USA, Japan, Germany and
Russia.
1
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Coriant - History
1
The enterprise is selling hardware and
software for optical transmission in the
Backbone network of voice, data and
mobile networks. These are mainly the
products hiT 7300 for optical
multiplexing DWDM over fiber optic
cables, hiT 7100 for electrical cross
connection of optical paths and hiT 70xx
for multi service provisioning. Software
products for management and planning
are TNMS (network management) and
Transnet/Transconnect (network
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Coriant - History and Technology
1
In the late 1990s and early 2000s DWDM
emerged to allow for even higher
transmission capacity (in the
Terabit_per_second#Terabit_per_secon
d|Terabit per second region)
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Single-mode fiber - Characteristics
By using optical amplifiers and
dispersion-compensating devices,
state-of-the-art DWDM optical
systems can span thousands of
kilometers at 10 Gbit/s, and several
hundred kilometers at 40 Gbit/s.
1
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Go Daddy - Infrastructure
1
Go Daddy has a 65,000 square foot data
center in The United States. This data
center is connected to an optical fiber
DWDM Ethernet backbone with a speed of
20 Gigabits per second. In 2013, 'Go
Daddy' reached more than 55 million
domain names under management.In a
May 2012 publication (used here under
name=vance2012), a figure of 53 million
was provided, but the specific timing of
this figure was not provided.
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Cisco CRS-1 - CRS-3
1
CRS-3 also supports IPoDWDM
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Electronic communication - Telephone
1
Lastly, improvements in multiplexing have
led to an exponential growth in the data
capacity of a single fibre.[
http://www.cisco.com/univercd/cc/td/doc/pr
oduct/mels/cm1500/dwdm/dwdm_ovr.pdf
Fundamentals of DWDM Technology],
CISCO Systems, 2006.[
http://www.lightreading.com/document.asp
?doc_id=31358 Report: DWDM No Match
for Sonet], Mary Jander, Light Reading,
2006.
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Communication systems - Telephone
1
Lastly, improvements in multiplexing
have led to an exponential growth in
the data capacity of a single
fibre.[http://www.cisco.com/univerc
d/cc/td/doc/product/mels/cm1500/d
wdm/dwdm_ovr.pdf Fundamentals of
DWDM Technology], CISCO Systems,
2006.[http://www.lightreading.com/d
ocument.asp?doc_id=31358 Report:
DWDM No Match for Sonet], Mary
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Cubao - Tandang Sora
1
Culiat, a Muslim compound, is also to
be found here, as are the central
offices of the Iglesia ni Cristo, New
Era University and the studios and
transmitter of DZEC-TV|Net 25 DZCETV|INC TV 49, DZEC-AM|Radyo
Agila, DWDM-FM|Pinas FM and
DZEM|INC Radio of the Eagle
Broadcasting Corporation and
Christian Era Broadcasting Service
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Optical buffer - Optical networks
1
DWDM technologies enable
bitrates well beyond 1 Tbit/s
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Frequency grid
1
The most common frequency grid used for
fiber-optic communication is that used for
channel spacing in wavelength-division
multiplexing|Dense Wavelength Division
Multiplexing (DWDM) at wavelengths around
1550Nanometre|nm and defined by ITU-T
G.694.1.ITU-T G.694.1, Spectral grids for
WDM applications: DWDM frequency grid
[http://www.itu.int/rec/T-REC-G.694.1/en ITUT website] The grid is defined relative to
193.1Terahertz radiation|THz and extends
from 191.7THz to 196.1THz with 100GHz
spacing
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Internet in Russia - Backbone
1
Plenty of local commercial ISPs function in
large cities, but most of the existing
country-wide cable lines are held by small
number of large operators such as former
monopolist, the state-controlled
Rostelecom and the railways-affiliated
Transtelecom, which operates country
biggest DWDM fiber backbone
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Internet in Russia - Backbone
1
Until 2011 the backbone network in Russia
was based on DWDM technology with a
capacity of 10 and 40 Gbit/s
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CWDM - WDM systems
Dense wavelength division
multiplexing (DWDM) uses the same
transmission window but with denser
channel spacing
1
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CWDM - DWDM systems
In the mid-1990s DWDM systems
contained 4 or 8 wavelength-converting
transponders; by 2000 or so, commercial
systems capable of carrying 128 signals
were available
1
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CWDM - DWDM systems
1
# A DWDM 'terminal
demultiplexer'
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CWDM - DWDM systems
1
These factors of smaller volume and
higher performance result in DWDM
systems typically being more
expensive than CWDM.
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CWDM - DWDM systems
1
Recent innovations in DWDM transport
systems include pluggable and softwaretunable transceiver modules capable of
operating on 40 or 80 channels. This
dramatically reduces the need for discrete
spare pluggable modules, when a handful
of pluggable devices can handle the full
range of wavelengths.
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CWDM - Wavelength-converting transponders
1
At this stage, some details concerning
wavelength-converting transponders
should be discussed, as this will clarify
the role played by current DWDM
technology as an additional optical
transport layer. It will also serve to
outline the evolution of such systems
over the last 10 or so years.
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CWDM - Wavelength-converting transponders
As stated above, wavelengthconverting transponders served
originally to translate the transmit
wavelength of a client-layer signal
into one of the DWDM system's
internal wavelengths in the 1,550nm
band (note that even external
wavelengths in the 1,550nm will most
likely need to be translated, as they
will almost certainly not have the
1
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CWDM - Wavelength-converting transponders
; 1R: Retransmission. Basically, early
transponders were garbage in garbage
out in that their output was nearly an
analogue copy of the received optical
signal, with little signal cleanup
occurring. This limited the reach of early
DWDM systems because the signal had
to be handed off to a client-layer receiver
(likely from a different vendor) before
the signal deteriorated too far. Signal
monitoring was basically confined to
optical domain parameters such as
received power.
1
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CWDM - Enhanced WDM
1
Cisco Systems|Cisco's Enhanced WDM
system combines 1 GB Coarse Wave
Division Multiplexing (CWDM) connections
using SFPs and GBICs with 10 GB Dense
Wave Division Multiplexing (DWDM)
connections using XENPAK, 10-gigabit
Ethernet|X2 or XFP DWDM modules.
These DWDM connections can either be
passive or boosted to allow a longer range
for the connection.
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CWDM - Transceivers versus transponders
** Dense WDM (DWDM) Transceivers:
Channel 17 to Channel 61 according to ITU-T.
1
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Internet in Greece - ADSL
1
HOL has the largest (amongst alternative
carriers) optical backhaul networks based
on DWDM technology
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SONET/SDH - SONET/SDH and relationship to 10-gigabit Ethernet
However, 10-gigabit Ethernet does not
explicitly provide any interoperability at the
bitstream level with other SDH/SONET
systems. This differs from WDM system
transponders, including both coarse and
dense wavelength-division multiplexing
systems (CWDM and DWDM) that
currently support OC-192 SONET signals,
which can normally support thin-SONET–
framed 10-gigabit Ethernet.
1
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64QAM
1
QAM is being used in optical fiber systems
as bit rates increase; QAM16 and QAM64
can be optically emulated with a 3-path
interferometer.[http://kylia.com/QAM.html
Kylia products], dwdm mux demux, 90
degree optical hybrid, d(q) psk
demodulatorssingle polarization
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Add-drop multiplexer
1
ADMs can be used both in long-haul core
networks and in shorter-distance
Metropolitan area|metro networks,
although the former are much more
expensive due to the difficulty of scaling
the technology to the high data rates and
wavelength division multiplexing|dense
wavelength division multiplexing
(DWDM) used for long-haul
communications. The main optical
filtering technology used in add-drop
multiplexers is the Fabry–Pérot etalon.
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Abilene Network - 2006/2007 Upgrade
1
Unlike the previous architecture, Level 3
manages and operated an Infinera
Networks based DWDM system devoted
to Internet2
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G.709
1
'ITU Telecommunication Standardization
Sector|ITU-T Recommendation G.709'
Interfaces for the Optical Transport
Network (OTN) describes a means of
communicating data over an optical
network. It is a standardized method for
transparent transport of services over
optical wavelengths in DWDM systems.
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Next Generation Networking - Implementations
The core optical network uses dense
wavelength-division multiplexing (DWDM)
technology to provide scalability to many
hundreds of gigabits per second of
bandwidth, in line with growth demand
1
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Liu Gang - In the United States
He also proposed a special class of
Optical devices, SPIDER (2001, ISSN
:1089-7089) : optical routers, dense
wavelength division multiplexing (DWDM)
systems, and cross connects of
unprecedented capacities are on the verge
of large-scale commercial deployment
1
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Liu Gang - Works
*Gang Liu, E. Bouillet, I. Saniee (2000).
Algorithms for DWDM Mesh Network
Design: Routing and Wavelength
Assignment for Dedicated Protection, Ring
Auto-Recovery and Optical Cross-Connect
Restoration in Core Optical
Networks.http://cm.belllabs.com/who/gang/pub.html Bell
Laboratories Technical Memorandum,
Murray Hill, New Jersey. 1999. 10009626-
1
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Network Protocol Stack E6 - Motivation
1
Ethernet over DWDM solutions.
It ejects xDSL in access
networks with
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London Internet Exchange - IXManchester
The links between sites will be
provisioned on a dark fibre provided
by NTL and lit by LINX with the use of
SFP+ DWDM transceivers and passive
DWDM MUX units. Initial ISL capacity
will be configured at 20Gbit/s but
easily upgradeable to 80Gbit/s if
required.
1
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Railtel Corporation of India
1
RailTel has created state of the art
multimedia telecom network using
SDH/DWDM-based transmission
systems and high end routers for
MPLS-IP network
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Railtel Corporation of India
The long haul network of RailTel is on
STM-16 which has already been upgraded
to 100 G DWDM which can be further
augmented up to 400 Gpbs.
1
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Railtel Corporation of India
1
The network is also being upgraded to
Giga capacity overlaid on new DWDM
network, so as to provide gigabit VPN
ports for IP based voice, video and data
applications.
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Semiconductor laser - Distributed feedback lasers
DFBs are the most
common transmitter
type in DWDM-systems
1
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Laser acronyms - D
*DWDM – dense
wavelength division
multiplexing
1
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Tunable laser - Narrowband tuning
To get wideband tuning using this
technique, some such as Santur
Corporation or Nippon Telegraph and
Telephone (NTT Corporation) Full Tunable
DFB Laser Array Copackaged with InP
Mach-Zehnder Modulator for DWDM
Optical Communication Systems, K
1
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Lambda rail network - Description
Links in the network use dense
wavelength-division multiplexing (DWDM),
which allows up to 64 individual optical
wavelengths to be used (depending on
hardware configuration at each end)
separated by 100GHz spacing. At present,
individual wavelengths are used to carry
traditional Optical Carrier|OC-X (OC3,
OC12, OC48 or OC192) time-division
multiplexing circuits or Ethernet signals for
Gigabit Ethernet or 10 Gigabit Ethernet.
1
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Communications network - Optical Transport Network (OTN)
1
A 40G pipe can carry 40 different channels
as a result of Dense Wave Division
Multiplexing (DWDM) transmission
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