Download Optical switching networks

Tabriz university
school of new science and technology
Optical switching networks
Designed by : N.pasyar
Optical switching network
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optical switching networks refers to all the various types of
flexible and reconfigurable optical networks that use any of the
multiplexing , tuning , and switching techniques.

optical switching networks are single-channel or multichannel
(WDM) networks whose configuration can be changed
dynamically in response to varying traffic loads and network
failures.
End-to-end optical networks
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Optical switching networks have been used in backbone
networks in order to cope with the ever-increasing amount of
traffic and bandwidth-hungry applications.
They are also used in metro(politan), access, and local area
networks.
Long-haul backbone Networks
Wide Area
Core Ring
Metro Area
Edge Ring
Edge Ring
Access/Local Area
Fig 2.1 Hierarchy of optical switching networks.
Last/first mile
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phone companies deploy digital subscriber line (DSL) while cable
providers deploy cable modems .so, copper is used on the final
network segment to connect subscribers.
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copper-based network segment forms a bandwidth bottleneck.
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The bandwidth bottleneck is referred to as the so-called last mile
bottleneck, or also as the first mile bottleneck in order to
emphasize its importance to the end users.
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To remove the first/last mile bottleneck, fiber is brought close
or even all the way to the subscribers, who might be either
business or residential users.

Depending on the demarcation point of fiber, these optical
access networks are called fiber to the X (FTTX) networks,
where X denotes the various possible demarcation points of
fiber.
PON
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To build cost-effective optical access networks FTTX networks
are based on passive optical components without using amplifiers
or any other powered devices. these FTTX networks are called
passive optical networks (PONs).
They simplify their operation, administration, and maintenance
(OAM) as well as their management due to the completely
passive nature of their underlying components.
The transport of data traffic over PONs may be based on
asynchronous transfer mode (ATM), resulting in the so-called
ATM PON (APON).
Ethernet frames may be transported in their native format in the
so-called Ethernet PON (EPON),
whether APON or EPONs are suitable for cost-effective
broadband optical access networks ?

Given the low costs of Ethernet equipment and by capitalizing on the
technology and products of the Ethernet LAN market, EPON appears to
be better than ATM-based PONs.

the use of WDM on the already installed fiber infrastructure provides an
multichannel upgrade path of EPON access networks, leading to WDM
EPONs.
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The 10-Gigabit Ethernet (10GbE) provides an extended maximum
transmission range of 40 kilometers, compared to (GbE) 5 kilometers,.
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Ethernet is the predominant technology in local area networks (LANs).and
low-cost solution also for optical high-speed MANs and WANs.
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10GbE equipment costs are about 80% lower than that of SONET
equipment and 30–60% lower than other managed network services.
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The use of Ethernet technology not only in today’s LANs but also in future
access, metropolitan, and wide area networks will potentially lead to endto-end Ethernet optical networks.
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future access networks are likely to be bimodal .So-called radio-over-fiber
(RoF) networks may be viewed as the final frontier of optical networks
which interface optical access networks with their wireless counterparts
Applications
Throughput-Critical
includes large-size file transfers
require much bandwidth
Latency-Critical
comprises small- to medium
size file transfers
video & image
email attachments
interactive games
backup of files
broadcast television
file downloading
ebooks
video conferencing
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Depending on the application, traffic loads and throughputdelay performance requirements may change over time.
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web browsing has led to rather asymmetric network traffic
loads. it is based on the client–server paradigm, (each client
sends a short request message to a server for downloading
data files).
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In future (optical) networks, the traffic load is expected to
become less asymmetric via so-called peer-to-peer (P2P)
applications.In p2p each client also acts as a server from
which other clients may download files
Services
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Connection-Oriented Services: Connection-oriented services
require a connection to be established before any data can be
sent by executing a specific handshake procedure between
sender and destination.
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sender and destination and possibly also intermediate nodes
need to maintain state information for each established
connection.
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Examples are transmission control protocol (TCP), ATM, and
multi protocol label switching (MPLS).
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By using the state information, connection-oriented services
are able to recover from data loss and provide quality of
service (QoS) to applications.
Services
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Connectionless Services: Connectionless services do not
require the establishment of any connection prior to sending
data.
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Connectionless services are less reliable than their connectionoriented counterparts, but they are well suited for the transfer
of best-effort traffic. As an example is the Internet protocol
(IP) which provides best-effort service to higher protocol
layers and applications.
Some of today’s most important services
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triple-play services :Triple-play offerings comprise
bidirectional voice, bidirectional data, and unidirectional video
services. all three services are delivered to the subscriber via
FTTX networks .
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virtual private network (VPN) services:A VPN supports a
closed community of authorized users, allowing them to access
various network-related services and sources. a VPN is a
virtual topology that is built on a physical network
infrastructure whose resources may be shared by multiple
other VPNs.
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cost-effective
provide privacy by isolating traffic belonging to different VPNs from each
other.
offer services that enable a mobile to remotely access the intranet and/or
extranet of a company.
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As shown , optical switching networks offer services to
applications.
Depending on the traffic demands of various clients optical
switching networks need to be able to provide different types
of connections.
Applications
services
op networks
Opticalswitching
switching
networks
Optical
services
Applications
Fig 2.2 Optical switching networks offering services to applications.
Switching granularity
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connections between source and destination may be switched
at different granularities.
Major switching granularities, ranging from coarse granularity
at the top to fine granularity at the bottom of the table:
Switching granularity
Fiber switching
Waveband switching
Subwavelength switching
Optical circuit switching (OCS)
Optical burst switching (OBS)
Optical packet switching(OPS)
Switching granularity
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Fiber switching : is done by
switching all data arriving on an
incoming fiber to another outgoing
fiber without making any difference
among the various wavelength
channels.
waveband switching : the set of wavelength channels carried
on the fiber is divided into multiple adjacent wavebands.
Wavebands are switched independently from each other;
waveband switching provides a finer switching granularity.
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Wavelength switching : wavelength switching an input WDM
comb signal is first demultiplexed into its individual
wavelength channels and each wavelength channel is then
switched independently. A given wavelength channel may
carry the data of a single client . In OTDM networks, each
time slot carries the data of a different client and may be
switched individually.

OCS: all switching techniques discussed so far are optical
circuit switching (OCS) techniques. circuits – fibers,
wavebands, wavelengths, or time slots –are dedicated to
source–destination node pairs .
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OPS: OPS networks is an example for subwavelength
switching optical networks, where packets on a given
wavelength channel are switched independently from each
other.
packet-switched networks require buffers to resolve
contention. Fiber delay lines (FDLs) are used.FDLs are fiber
loops where the optical signal continues to circulate until it is
forwarded.
the reading/writing of the information is restricted to integer
multiples of the round-trip time of the loop FDLs must be
designed such that the largest packet size can be stored on the
loop.
OPS networks favor cell switching, where each cell has the
same fixed size.
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OCS networks provide circuits with a high level of QoS. At
the downside, they require a two-way reservation to set up
optical circuits.
OPS networks are connectionless and thus avoid any delay due
to circuit set-up. But OPS networks do not necessarily meet
QoS requirements.
OBS: in OBS networks client data is aggregated at the
network ingress and sent as bursts across the network. For
each burst a reservation control packet is sent on a dedicated
control wavelength channel prior to sending the burst on one
of the data Wavelength channels after an offset time.
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The control packet is used to configure intermediate OBS
nodes between the ingress node and egress node of the OBS
network.
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The offset time is set such that the burst can be all-optically
switched at intermediate nodes. so, the burst does not need to
be OEO converted, buffered, and processed at intermediate
nodes, thus avoiding the need for optical RAM and FDL.
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OBS networks deploy a one-way reservation, as is done in
OCS networks .Also, by controlling the offset time, service
differentiation and various QoS levels can be achieved.
Interlayer networking
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The data plane encompasses all switching mechanisms required
to send data from source to destination. the data plane needs to be
controlled by means of a control plane.
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the control plane is responsible for setting up and making the
various switching techniques work in a coordinated and efficient
manner throughout the optical network.
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The control plane can be implemented at the medium access
control (MAC) layer of optical switching networks to avoid
collisions of data frames on each wavelength channel. Also to
route and establish optical connections from source to
destination.
Interlayer networking
Two approaches for control plane:
 new control protocols are designed for the optical switching
networks, taking their respective properties into account.
 already existing control protocols which were successfully
deployed in electronic data networks ,are used.for example IP
centric control plane .
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interlayer networking between AONs and IP clients became
the next evolutionary step in designing flexible optical
switching networks, leading to so-called IP/WDM networks.
interconnection models of IP-over-optical networks:
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Peer Model: IP networks and optical networks are considered an
integrated network with a unified control plane .peer model
implies that optical network nodes need to provide IP routers with
full routing information .
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Overlay Model: IP networks and optical networks operate
completely independently from each other and run their own set
of routing and signaling protocols. In order to interconnect
geographically distributed IP routers across optical networks
appropriate interfaces between both types of network need to be
standardized.
 Interdomain (Augmented) Model:
both IP and optical
networks have their own routing instances, but reachability
information of geographically distributed IP routers is passed
by the underlying optical networks onward to IP clients.
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The standardization will allow end users and network
operators to dynamically control end-to-end connections.
International Telecommunication Union (ITU-T).
ASTN
Internet Engineering Task Force (IETF).
GMPLS
Optical Internet working Forum (OIF).
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The ASON and GMPLS protocols are well suited for
conventional centrally managed optical switching networks .
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in customer-controlled and optical networks customers acquire
their own dark fibers and/or point-to-point wavelengths and are
responsible for protection and restoration, without requiring any
service from a central management.
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Buying one’s own optical network may result in significant cost
savings since ongoing monthly expenditures are replaced with a
one-time initial CAPEX shared by the customers.
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user-controlled optical networks support wavelength switching
and burst switching.
security
Many of the security mechanisms used in electronic networks
can also be applied in optical networks.
Differences:
 for high-speed data rates on multiple WDM channels infrequent
attacks may lead to large amounts of lost information.
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eavesdropping by means of tapping may comprise the privacy of
optical communications.
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malicious signals are harder to detect in transparent optical
networks than in opaque ones with OEO conversion.
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In (EDFAs), a malicious high-power optical signal uses more
of the upper-state photons and reduces the gain of the other
user signals.
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the limited crosstalk of optical devices ,(OXCs) and
(OADMs), can lead to a reduced QoS on one or more
wavelength channels if the power of a malicious optical signal
is sufficiently high.
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the security of optical networks can be improved by means of
preventive measures. bandlimiting optical filters discard
optical signals outside the operational spectrum and mitigate
the gain competition of EDFAs.
Traffic grooming
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Traffic grooming :is the process of grouping many small
telecomunications flows into larger units, which can be
processed as single entities.

In a network using both TDM and WDM , two flows which
are destined for a common node can be placed on the same
wavelength, allowing them to be dropped by a single OADM.
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traffic grooming was done in the area of SONET/ (SDH) ring
networks. it can be extended to optical mesh WDM networks,
where low-rate circuits and data flows are assigned to
wavelength channels which optically bypass OADMs.
objective of grooming
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Often the objective of grooming is minimizing the cost of the
network and line terminating equipment (LTE).
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Thus grooming typically involves minimizing the usage of
ADMs.This is similar to the use of virtual networking and
virtual paths in ATM networks.
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Effective grooming requires consideration of the topology of
the network and the different routes in use. This is especially
useful when dealing with mesh networkes
Components
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Combiner:It collects wavelength channels from all S input
ports and combines them onto the common output port.
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Splitter: It equally distributes all incoming wavelength
channels to all S output ports.
λ
λ
λ
λ
Sx1
λ
Combiner
λ
Splitter
1XS
λ
λ
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Waveband partitioner: It partitions an incoming set of
contiguous wavelength channels into two wavebands A and B,
where 1 ≤ ΛA, Λ B ≤ Λ − 1, and Λ = Λ A + Λ B.
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Waveband departitioner:It collects two different wavebands
consisting of ΛA and Λ B contiguous wavelength channels from
the upper and lower input port, respectively.
ΛA
ΛA
Λ
Λ
λ
Π
λ
ΛBλ
Σ
ΛB
λ
λ
λ
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Passive star coupler (PSC): . it collects wavelength channels
from all D input ports and equally distributes them to all D
output ports.
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Arrayed waveguide grating (AWG):The AWG routes every
second wavelength to the same output port. This period of the
wavelength response is called free spectral range (FSR).
the FSR of a D × D AWG consists of D contiguous
wavelengths, the physical degree of an AWG is identical to the
number of wavelengths per FSR.
FSR FSR
λ
DxD
psc
λ
λ
λ
λ
DxD
AWG
λ
λ
The
arrayed
waveguides
introduce wavelength-dependent
phase delays such that only
frequencies
with
a
phase
difference of integer times2π
interfere constructively in the
output slab waveguide. Thus,
each output port carries periodic
pass frequencies. The spacing of
these periodic pass frequencies is
called FSR.
In put / out put waveguides
N input
ports
N output
ports
Slab
waveguide
Array of M
waveguides
Waveplate
Schematic layout of an NxN AWG
λ8
1
1
λ1…λ4 …λ8
2
λ1
3
λ1
2
λ1…λ4 …λ8
λ1…λ4 …λ8
3
λ4
4
5
INPUT
4
λ4
5
6
λ8
7
8
OUTPUT
6
7
λ8
8
Fig. 3.3
In Fig. 3.3 the routing connectivity of an 8 × 8 AWG is illustrated. λk ’s
routing information is to exit the output port that is (k − 1) ports below the
corresponding input port; that is, λ1 goes from input port 1 to exit port 1 and
from input port 5 to exit port 5.
Transmitters
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A transmitter comprises a light source, a modulator ,and
supporting electronics.
Broadband light sources:
broad spectrum in the range of 10-100 nm.
small bandwidth-distance product LEDs are mainly applied for low data
rates and/or short distances. output powers −10 dBm.
superluminescent diodes with an output power of 18.0 dBm and a 3-dB
bandwidth of 35 nm .
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Lasers:
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To achieve a significantly increased bandwidth-distance product lasers are
deployed. laser is an optical amplifier enclosed within a reflective cavity
that causes the light to oscillate via positive feedback. output powers,
between 0 and 10 dBm.
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Lasers can be either fixed tuned or
tunable.
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a laser can be tuned by controlling the
cavity length and/or the refractive
index of the lasing medium.

instead of tunable lasers one might use
an array of fixed-tuned lasers, or multi
frequency lasers
Tranmitter tuning
Type
range
tuning
time
Mechanically 500nm 1-10ms
Tunable
Acousto-optic 100nm
10μs
Electro-optic 10-15nm 1-10ns
Injection current 30nm
15ns
Receivers (optical filters)
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A receiver is composed of an optical filter, a photodetector,
a demodulator, and supporting electronics.
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Optical filters : are used to select a slice of a broadband
signal or one wavelength out of the (WDM) comb. The
selected wavelength is subsequently optoelectrically
converted by a photodetector.
Fixed tuned filters
Dielectric thin film filters
Diffraction gratings
Fiber bragg gratings(FBGS)
tunable optical filters
Mach-Zehnder interferometer (MZI)
acousto-optic tunable filters (AOTFs)
Electro-optic tunable filters (EOTFs)
Transmission impairments
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Attenuation.
Dispersion
Nonlinearities
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Attenuation :
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The peak in loss in the 1400-nm region is due to hydroxyl
ion (OH−) impurities in the fiber.
this peak is reduced In Lucent’s AllWave fiber. among
0.85, 1.3, and 1.55 µm, the latter band provides the
smallest attenuation of ∼0.25 dB/km.
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dispersion
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different components of the transmitted signal travel at
different velocities in the fiber, arriving at different times at the
receiver. As a result, the pulse widens and causes inter symbol
interference (ISI).
dispersion limits the minimum bit spacing (maximum
transmission rate).
Modal dispersion: arises only in multimode fiber where
different modes travel at different velocities. Clearly, in singlemode fibers (SMFs) modal dispersion is not a problem.

Waveguide dispersion : After entering an SMF, major portion of
an information-carrying light pulse travels within the core, the
rest within the cladding. Both portions propagate at different
velocities since the core and the cladding have different refractive
indices.
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Chromatic (material) dispersion: different frequency
components of a pulse travel at different velocities due to the fact
that the refractive index of the fiber is a function of the
wavelength. Standard (SMFs) have a chromatic dispersion of 17
ps/(nm·km) at 1550 nm.
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By controlling the waveguide dispersion accordingly, NZ-DSFs have a
chromatic dispersion between 1 and 8 ps/(nm·km).
Alcatel’s TeraLight Metro Fiber has a dispersion of 8 ps/(nm·km).
low negative dispersion of Corning’s MetroCor fiber enables the use of
low-cost directly modulated DFB lasers.
Polarization mode dispersion (PMD) :PMD arises because the
fiber core is not perfectly circular, particularly in older
installations.Thus, different polarizations of the signal travel at
different velocities. PMD is proving to be a serious impediment
in very-high-speed systems operating at 10 Gb/s and beyond.
Nonlinearities

when the power levels get fairly high the nonlinearities can
place significant limitations on high-speed systems as well as
WDM systems.
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Nonlinearities owing to the dependence of refractive index on
the optical power : (SPM), (XPM), and four-wave mixing
(FWM).
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Nonlinearities owing to scattering effects in the fiber medium
due to the interaction of light waves with phonons (molecular
vibrations) in the silica medium. stimulated Raman scattering
(SRS) and stimulated Brillouin scattering (SBS).
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Self-phase modulation :variations in the power of an optical
signal changes the phase of the signal. It leads to spectral
broadening. instantaneous variations of the frequency around
the signal’s central frequency.
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Cross-phase modulation :XPM is a shift in the phase of a
signal caused by the change in intensity of a signal
propagating at a different wavelength. XPM can lead to
asymmetric spectral broadening.

Four-wave mixing :FWM occurs when two wavelengths,
operating at frequencies f1 and f2, respectively ,mix to cause
signals at frequencies such as 2 f1 − f2 and 2 f2 − f1. These
extra signals can cause interference if they overlap with
frequencies used for data transmission.

Stimulated Raman scattering :SRS is caused by the
interaction of light with molecular vibrations. Light incident
on the molecules creates scattered light at a longer wavelength
than that of the incident light. The light generated at the lower
frequencies is called the Stokes wave. To reduce the amount of
loss, the power on each channel needs to be below a certain
level.

Stimulated Brillouin scattering : SBS is similar to SRS,
except that the frequency shift is caused by sound waves rather
than molecular vibrations .in SBS the Stokes wave propagates
in the opposite direction of the input light.
Crosstalk

Crosstalk decreases the signal-to-noise ratio (SNR) leading to
an increased BER.

interchannel crosstalk : is caused by signals on different
wavelengths . It may be removed through the use of
appropriate narrowband filters.

intrachannel crosstalk: is caused by signals on the same
wavelength on another fiber due to imperfect transmission
characteristics of components (AWG). it occurs in
switching/routing nodes and it cannot be removed through
filtering.
Noise

Amplified spontaneous emission (ASE) :In optical EDFA the
spontaneous emission is amplified in addition to the incident
light signal.

Shot noise :Is the main complication in recovering the
transmitted bit of photodetector. Shot noise current occurs due
to the random distribution of the electrons generated by the
photodetection even when the input light intensity is constant.

Thermal noise : the photocurrent is amplified by an electrical
amplifier. The electrical amplifier introduces thermal noise
current due to the random motion of electrons at typical
temperatures.
The End
Thank you for your attention