2. Basic Optical Networking Devices
An optical network comprises of the below elements:
optical devices interconnected by optical links (OLs).
Tunable lasers
optical buffers or delay elements
optical amplifiers
optical filters,
wavelength-division multiplexers
optical switches.
Chapter 15 2
3. Tunable Lasers
• Tunable lasers can continuously change their emission wavelengths, or
colors, in a given spectral range.
• These changes work in collaboration with optical switches to select a
particular wavelength for connection establishment.
• A tunable dispersion compensator is used to compensate for fiber-
dispersion losses over the length of the fiber.
3
4. Optical Buffers or Delay Elements
• An optical buffer is a device that is capable of temporarily storing light. Just as in the
case of a regular buffer, it is a storage medium that enables compensation for a
difference in time of occurrence of events.
• Optical buffers or optical delay elements, can be implemented by using a certain
length of fiber to delay signals. No practical optical memory is available with the
current technology.
4
5. Optical Amplifiers
• Regeneration of a signal, requires amplification of the signal or the conversion of the
signal from optical to electronic.
• The fact is that the regeneration of optical signals is very costly compared to optical amplification, since the process of
signal regeneration requires several steps including optical to electrical to optical conversion with electrical signal
processing.
• Theuseofopticalamplifiersenablestheachievementoflarge-distance communicationwithouttheneedofaregenerator.
• Anopticalamplifier canamplify signalsatmany wavelengthssimultaneously.
5
6. Optical Filters
• An optical filter equalizes the gain of transmission
systems and filters the noise or any unwanted
wavelengths.
6
(a)
l1l2l3l4
l2l3l4
l1
7. Optical Filters
The design of an optical filter has a number of challenging factors.
Insertion loss is one and is the loss of power at a filter. A low insertion loss is one of
the specifications for a good optical filter.
Keeping the temperature at the desired level in optical systems, Temperature
variation should not affect the passband of a filter.
Chapter 15 7
8. Wavelength-Division Multiplexer
(WDM)
A WDM mixes all incoming signals with
different wavelengths and sends them to a
common output.
A demultiplexer does the opposite
operation, separating the wavelengths and
dispatching them onto output ports.
On the common link, each channel carries
information at light speed with minimal
loss.
8
Multiplexer
l1
l2
ln
1
2
n
1
2
n
Ch1Ch2 Chn
Demultiplexer
9. Wavelength-Division Multiplexer (WDM)
with n Inputs
9
• With the higher-speed variations of WDM, the number of channels is (dense WDM).
very large, and the wavelengths are as close as 0.1 nm. Such systems are referred to
as DWDM
10. Optical Switches
• An optical switch is the heart of switching and
routing operation in an optical network.
• The objective in using optical switches rather
than semiconductor switches is to increase the
speed and volume of traffic switching in a core
node of a computer communication network.
• larger-scale optical switch can act as an optical
cross connect (OXC) to accept various
wavelengths on network input ports, and route
them to appropriate output ports.
10
(a) (b) (c)
l1l2l3l4
l2l3l4
l1
l1 l2
l1 l1,1 l2,1
l1,2 l2,2
l1,2 l2,1
l1,1 l2,2
l2
11. Functions of Optical Switch
An optical switch performs one or more of the following three main functions:
1. Routes all wavelengths of an incoming port to a different outgoing port.
2. Switches specific wavelengths from an incoming port to multiple outgoing
ports.
3. Takes incoming wavelengths and converts them to another wavelength on
the outgoing port.
11
12. Classification of Switch Elements
1) Non-electro-optical Switches:
- These switches have simple structures.
- For example, a mechanical optical switch uses mirrors at a switch’s
input and output ports. A switch can be controlled by moving the
mirrors and directing a light beam to a desired direction, and
eventually to the desired output port.
- The advantages of mechanical switches are low insertion, low
crosstalk, and low cost. However, they have low speed.
12
14. Classification of Switch Elements
2) Thermo-optic switch:
- These switches are built on a waveguide.
- In this type of switch, a change of temperature in the thermo-optic structure
of the waveguide can change the refractive index of the material, thereby allowing
a switching function to be formed.
14
16. Contention Resolution in Optical Networks
• Designing large-scale switching devices is one of the main challenges in optical
networks.
• Ideally all the functions inside an optical node should be performed in the optical
domain.
• packet contention in switches can be processed only in electronic devices. Several
technological factors bring restrictions to the design of an optical switch.
• The expansion of a fixed-size switch to higher scales is also a noticeable challenge.
• optical switches face the challenge of contention resolution within their structures.
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17. Operations for Contention Resolution
Contention resolution may require one or more of the following three operations:
1) Optical buffering: Optical buffers are implemented using fixed-length delay fibers.
2) Wavelength conversion: By changing wavelengths, signals can be shuffled and
forwarded onto other channels.
3) Deflection routing: If two or more lightpaths need to use the same output link, only
one is routed along the desired output link, the others are deflected onto undesired
paths directed to the destination through a longer route but with higher priorities.
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21. Other Losses in Optical Networks
• Another important factor in optical switches is insertion loss, which is the fraction of
power lost in the forward direction. This factor should be kept as small as possible in
optical switches.
• Crosstalk, is another problem, which is the ratio of output power from a desired input
to the output power from all other inputs. The factor of crosstalk in switches must be
minimized.
• The cost of optical devices, especially WDMs, is high compared to that of electronic
modules.
21
22. Routing in All-Optical Networks
• Routing in all-optical networks is based on establishment of lightpaths (LPs).
• Any two LPs traversing the same fiber optical link (OL) cannot share the same
wavelength on that link.
• When there is more than one feasible wavelength between a source OXC and a
destination OXC, then a wavelength assignment algorithm is required to select a
wavelength for a given lightpath
• routing in all-optical networks can be classified as unicast routing, more
professionally called wavelength routing and broadcasting.
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23. Wavelength Routing Versus Broadcasting
• In the Figure a Wavelength Routing Node, the two lightpaths—OXC1-OXC2-OXC3-OXC4
and OXC6-OXC2-OXC5—do not use any shared optical link and can therefore be assigned
the same wavelength.
• If two or more lightpaths share part of their common paths over an optical link, they must
therefore use a different wavelength.
23
24. Wavelength Routing Versus Broadcasting
• A broadcasting node combines all incoming signals and delivers a fraction of the power
from each signal to each output port, creating a broadcast function of wavelengths.
• To route from OXC1 to OXC4 and OXC5, the information is broadcast at OXC2, so that
one copy of the information is directed to destination OXC5 and the other copy of the
information is headed to destination OXC4.
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25. Blocking Estimation For Lightpaths
• When there are insufficient network resources to set up a lightpath, the requested
connection is blocked.
• Any connection may also be blocked if there is no wavelength available on all the OLs
along the chosen route.
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26. Calculation of Blocking Probability
• Let the probability that a wavelength is used on an optical link (OL) and thus is not
available be p.
• If the network provides n wavelengths on every OL and an LP is constructed by r
OLs, , the probability that an OL is not available denoted by BOL is actually the
probability that all wavelengths on the OL are taken and/or unavailable and can be
derived by
BOL = pn
26
27. Calculation of Blocking Probability
• Note that for BOL , we use .Lee’s method of the parallel link rule.The probability
that a given wavelength is free on any given link is (1-p)
• The probability that a lightpath request is blocked denoted by BLP by can be
developed using Lee’s method rule of series links.
• If the number of available wavelengths on each OL can be different from the ones
on any other optical links so that for optical links 1, 2,……,r the number of available
wavelengths can be n1,n2,……..,nr respectively.
27
28. Calculation of Blocking Probability
• we can also estimate the possibility that all wavelengths of all optical links are taken
leading to the total blocking of a light path (LP) as
28
29. Example
Consider the 5-node optical network shown in below
figure. Suppose for any optical link (OL), the probability
that a wavelength is used on the OL is p = 0.2.
All the available wavelengths on each OL are shown on
the figure. Find individual probability that the
corresponding lightpath over any of the three routes
from OXC1 to OXC3, from OXC3 to OXC4, or from
OXC1 to OXC4 is not available.
29
OCX 1-OXC3
n=2 , r=2
BoL=𝑃𝑛
=0.04
The blocking probability for OCX1-OCX3
BLP1= 1-(1−0.04)2
=0.078
OCX 3-OXC4
n=3 , r=1
BoL=𝑃𝑛=0.008
The blocking probability for OCX3-OCX4
BLP2= 1-(1−0.008)1
=0.008
OCX 1-OXC4
The blocking probability for OCX1-OCX4
BLP= 1-(1−0.04)2(1−0.008)1=0.085
