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Chapter 6 Optical System Design
and Performance
 6.1 Point-to-Point Transmission Systems
 6.1.1 Traditional Single-channel Systems
 6.1.2 Amplified Single-Channel Systems
 6.1.3 WDM Systems Overview
 6.2 Modulation (Making the Light Carry a Signal)
 6.2.1 On-Off Keying (OOK)
• NRZ Coding
• NRZI Coding
• RZ Coding
 6.2.2 Receiving the Signal
 6.2.3 Timing Recovery
• Phase Locked Loops (PLLs)
• Timing Jitter
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 6.2.4
 6.2.5
 6.2.6
 6.2.7
Analogue Amplitude Modulation
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Polarity Modulation (PolSK)
 6.3 Transmission System Limits and
Characteristics
 6.4 Optical System Engineering
 6.4.1 System Power Budgeting
• Connector/Splice Loss Budgeting
• Power Penalties
 6.4.2 Reflections
 6.4.3 Bit Error Rates (BER)
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 6.5 Optical Network
 6.6.1 Optical Networking Technologies
•
•
•
•
•
SONET
Gigabit Ethernet
Fiber Channel
ESCON
FDDL
 6.6.2 PON (Passive Optical Network)
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6.1 Point-to-Point Transmission Systems
6.1.1 Traditional Single-channel Systems
Figure 6.1 Conventional Long Distance
Fiber Transmission System.
 A large number of electronic (digital) signals are
combined using time division multiplexing (TDM)
and presented to the optical transmission system
as a single data stream.
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6.1.1 Traditional Single-Channel System
 This single data stream is carried in an optical channel
at speeds ranging from 155 Mbps to 1.2 Gbps.
 The wavelength used is almost always 1310 nm.
 Every 30-50 km the signal is received at a repeater
station, converted to electronic form, re-clocked and
re-transmitted.
 When such a system needs to be upgraded all of the
equipment in the link must be replaced. This is because
the repeaters are code and speed sensitive devices.
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6.1.1 Traditional Single-Channel System
6.1.1.1 Repeaters
 What we do is extract the digital information
stream from the old signal and then build a new
signal containing the original information. This
function is performed by a repeater.
 As it travels along a wire, any signal (electrical or
optical) is changed (distorted) by the conditions it
encounters along its path. It also becomes weaker
(attenuated) over distance due to energy loss.
Thus it becomes necessary to boost the signal.
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6.1.1 Traditional Single-Channel System
 The signal can be boosted by simply amplifying it. This
makes the signal stronger, but (as is shown in Figure 6.2)
it amplifies a distorted signal.
 A digital signal is received and it is reconstructed in the
repeater. A new signal is passed on (the 2nd example of
Figure 6.2) which is completely free from any distortion
that was present when the signal was received at the
repeater.
Figure 6.2
Repeater Function compared
with Amplification.
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6.1.2 Amplified Single-Channel Systems
Figure 6.3 Amplified Single-Channel Transmission System.
The systems use optical amplifiers (EDFAs) with span lengths
from 110 to 150 km.
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6.1.2 Amplified Single-Channel Systems
 The wavelength used is now 1550nm.
This is done for two reasons:
1. To exploit the low attenuation window of fiber
in the 1500 nm "window“.
2. To allow the use of Erbium Doped Fiber Amplifiers
(EDFAs).
 The distance between amplifiers is now increased to
between 110 and 150 km.
 The speed is generally increased to either 1.2 Gbps
or 2.4 Gbps.
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6.1.2 Amplified Single-Channel Systems
 There are three significant changes: (compare to Fig. 6.3)
1. In older systems, the fiber didn't disperse the signal by
very much because we were using the 1310 nm band.
However, by moving to the 1550 nm band, we have
brought on a dispersion problem.
2. The link may be upgraded to use higher speeds and the
modulation format may be changed without changing
equipment in the field. You only have to change the
equipment at each end!
3. Provided the link has been planned properly it can now
be upgraded to use WDM technology again without
change to the outside plant.
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6.1.2 Amplified Single-Channel Systems
 EDFAs do produce some spontaneous emission noise and
this tends to get amplified along the way (it is called ASE
for Amplified Spontaneous Emission).
 The amplified systems are considered significantly better
than the earlier systems for the following reasons:
1. Amplifiers cost less than repeaters and require less
maintenance.
2. The use of an amplifier enables future upgrades and
changes to take place with minimal impact (read cost)
on the installed link.
3. The use of the amplifier allows for future use of WDM
technology with minimal change to the outside plant.
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6.1.3 WDM Systems Overview
 Figure 6.4 shows a typical first generation long
distance WDM configuration.
 Transmission is point-to-point.
Figure 6.4 WDM Long Distance Fiber
Transmission System.
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6.1.3 WDM Systems Overview
 It should be noted that each optical channel is
completely independent of the other optical channels.
It may run at its own rate (speed) and use its own
encodings and protocols without any dependence
on the other channels at all.
 All of the current systems use a range of wavelengths
between 1540 nm and 1560 nm.
 There are two reasons for this:
1). to take advantage of the "low loss" transmission
window in optical fiber;
2). to enable the use of erbium-dopped fiber amplifiers.
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6.2 Modulation (Making the Light
Carry a Signal)
6.2.1 On-Off Keying (OOK)
 Most current optical transmission systems
encode the signal as a sequence of light pulses
in a binary form. This is called "on-off keying"
(OOK).
 It is like a very simple form of digital baseband
transmission in the electronic world.
 The signal is there or it isn't; beyond this the
amplitude of the signal doesn't matter.
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Non-Return to Zero (NRZ) Coding
Figure 6.6.
NRZ Coding
 NRZ coding:
A one bit is represented as the presence of light and
a zero bit is represented as the absence of light.
 This method of coding is used for some very slow
speed optical links but has been replaced by other
methods for most purposes.
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NRZI Coding
Figure 6.6
NRZI Encoding Example
 In order to ensure enough transitions in the data for the
receiver to operate stably, Most digital communication
systems using fiber optics use Non-Return to Zero
Inverted (NRZI) coding.
 In NRZI coding, a zero bit is represented as a change of
state on the line and a one bit as the absence of a change
of state. This algorithm will obviously ensure that strings
of zero bits do not cause a problem.
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Return to Zero (RZ) Coding
Figure 6.7 Return-to-Zero (RZ) Coding
 In RZ coding the signal returns to the zero state
every bit time. As illustrated a "1"bit is represented
by a "ON" laser state for only half a bit time. Even in
the "1" state the bit is "0" for half of the time.
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Return to Zero (RZ) Coding
 In a restricted bandwidth environment (such as in
most electronic communications) this is not a coding
of choice. The reason is that there are two different
line states required to represent a bit (at least for a
"1" bit).
 In the optical fiber environment, bandwidth is not
a major constraint. RZ coding is proposed as a basis
for some Optical Time-Division Multiplexing
(OTDM) systems.
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6.2.2 Receiving the Signal
Figure 6.8 Digital Receiver Functions.
1. The incoming optical signal is converted to an
electronic one using either a PIN-diode or an APD.
2. The signal is then pre-amplified and passed through
a band-pass filter. There are a number of very low
frequencies that get into the signal and there will be
very high frequency harmonics that we don't need.
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6.2.2 Receiving the Signal
3. Further amplification with feedback control of the gain is
used to provide stable signal levels for the rest of the
process. This control circuit usually controls the bias
current and thus the sensitivity of the photodiode as well.
4. A phase-locked loop is then used to recover a bit stream
and (optionally) the timing information.
6. At this stage the stream of bits needs to be decoded from
the coding used on the line into its data format coding.
This process varies depending on the encoding and is
occasionally integrated with the PLL depending on the
code in use.
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6.2.2 Receiving the Signal
The important issues for the receiver are:
 Noise: In the receiver we typically have a very low level input
signal which requires a high gain and therefore we have the
potential of high noise. Most of the noise in an unamplified optical
link originates in the receiver.
 Decision Point: The signal level at which we say that "all voltages
below this will be interpreted as a 0 and all voltages above this will
be interpreted as a 1". This is a critical parameter and must be
determined dynamically.
 Filtering: There are both low frequency (1000 Hz or so) and very
high frequency components here that are not needed. These
components both add to the noise and can cause malfunction of later
stages in the process. Unwanted harmonics cause the PLL to detect
false conditions (called "aliases").
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6.2.3 Timing Recovery
 There are many situations where a receiver needs
to recover a very precise timing from the received
bit stream in addition to just reconstructing the
bit stream.
 In order to recover precise timing not only must
there be a good coding structure with many
transitions, but the receiver must use a much
more sophisticated device than a DPLL to recover
the timing. This device is an analogue phase
locked loop.
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Phase Locked Loops (PLLs)
 While DPLLs have a great advantage in simplicity
and cost they suffer from three major deficiencies:
1. Even at quite slow speeds they cannot recover a good
enough quality clocking signal for most applications
where timing recovery is important.
2. As link speed is increased, they become less and less
effective. Because circuit speeds have not increased in
the same ratio as have communication speeds.
3. As digital signals increase in speed, they start behaving
more like waveforms and less like "square waves" and
the simplistic DPLL technique becomes less appropriate.
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Phase Locked Loops (PLLs)
What is needed is a continuous-time, analogue PLL
that is illustrated in Figure 6.9.
Figure 6.9 Operating Principle of a Continuous
(Analogue) PLL
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Phase Locked Loops (PLLs)
 The VCO (Voltage Controlled Oscillator) is the key
to the operation.
1. The VCO is designed to produce a clock frequency
close to the frequency being received.
2. Output of the VCO is fed to a comparison device
(a phase detector) which matches the input signal to
the VCO output.
3. The phase detector produces a voltage output which
represents the difference between the input signal
and the output signal.
4. The voltage output is then used to control (change)
the frequency of the VCO.
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Phase Locked Loops (PLLs)
 Properly designed, the output signal will be very close
indeed to the timing and phase of the input signal.
 There are two uses for the PLL output:
1. Recovering the bit stream (that is, providing the
necessary timing to determine where one bit starts
and another one ends).
2. Recovering the (average) timing (that is, providing
a stable timing source at exactly the same rate as
the timing of the input bit stream).
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Timing Jitter
 Jitter is the generic term given to the difference between
the (notional) "correct" timing of a received bit and the
timing as detected by the PLL.
 Some bits will be detected slightly early and others
slightly late. This means that the detected timing will
vary more or less randomly by a small amount either
side of the correct timing - hence the name "jitter".
 Jitter is minimized if both the received signal and the
PLL are of high quality. But although you can minimize
jitter, you can never quite get rid of it altogether.
 Jitter can have many sources. In an optical system the
predominant cause of jitter is dispersion.
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6.2.4 Analogue Amplitude Modulation
(Continuous Wave)
 Lasers have traditionally been very difficult to
modulate using standard amplitude modulation.
 This is caused by the non-linear response typical
of standard Fabry-Perot lasers.
 However, some DFB and DBR lasers have a
reasonably linear response and can be modulated
with an analogue waveform.
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6.2.4 Analogue Amplitude Modulation
(Continuous Wave)
 The major current use of this is in cable TV and HFC
distribution systems. An analogue signal is prepared
exactly as though it was to be put straight onto the coaxial
cable. Instead of putting it straight onto the cable it is
used to modulate a laser.
 At the receiver (often a simple PIN diode) the signal is
amplified electronically and placed straight onto a section
of coaxial cable. In standard (one-way) cable systems the
maximum frequency present in the combined waveform
is 500 MHz. In HFC systems this can increase up to 800
MHz or even 1 GHz.
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6.2.5 Frequency Shift Keying (FSK)
 It is difficult to modulate the frequency of a laser and this
is one of the reasons that FM optical systems are not yet
in general use. However, Distributed Bragg Reflector
(DBR) lasers are becoming commercially available.
 For FSK (or any system using coherent detection) to work,
the real problem is the need for coherent detection.
 The receiver "locks on" to the signal and is able to detect
signals many times lower in amplitude than simple
detectors can use. This translates to greater distances
between repeaters and lower-cost systems.
 In addition, FSK promises much higher data rates than
the pulse systems currently in use.
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6.2.6 Phase Shift Keying (PSK)
 You can't control the phase of a laser's output signal
directly and so you can't get a laser to produce phasemodulated light.
 However, a signal can be modulated in phase by
placing' a modulation device in the light path between
the laser and the fiber.
 At this time PSK is being done in the laboratory but
there are no available commercial devices.
 Again, PSK requires coherent detection and this is
difficult and expensive.
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6.2.7 Polarity Modulation (PolSK)
 Lasers produce linearly polarized light. Another
modulation dimension can be achieved by introducing
polarization changes.
 Unfortunately, this is not an available technique (not
even in the lab) but feasibility studies are being
undertaken to determine if PolSK could be
productively used for fiber communications.
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6.3 Transmission System Limits
and Characteristics
The characteristics of various transmission systems are
summarized in Table 6.1.
Table 6.1 Optical Fiber State of the Art
Medium
Source
Technology
Status
Speed
Distance
Product
2 Mbps
2 km
4M
100Mbps
2 km
200 M
OC-3
155 Mbps
2 km
310 M
Long Distance
10Gbps
50 km
500 G
Amplitude
Modulation
40Gbps
40 km
1.6 T
Coherent
400Mbps
370 km
150 G
Solutions
100Gbps
4000 km
8OOT
Copper
Multimode
FDDI
LED
Single
Mode
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Laser
in use
in Lab
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6.3 Transmission System Limits
and Characteristics
 A universally accepted measure of the capability of a
transmission technology is the product of the maximum
distance between repeaters and the speed of transmission.
 In electrical systems, maximum achievable speed
multiplied by the maximum distance at this speed yields a
good rule of thumb constant. It is not quite so constant in
optical systems, but nevertheless the speed and distance
product is a useful guide to the capability of a technology.
 Note that the Table 1 refers to single channel systems only.
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6.3 Transmission System Limits
and Characteristics
Table 6.2 shows the attenuation characteristics of various
transmission media and the maximum spacing of repeaters
available on that medium.
Table 6.2 Signal Loss in Various Materials
Material
Attenuation
Regenerator
Spacing
Max. 35 dB
Coaxial Cable
25 dB/km
1.5 km
Telephone Twisted Pair
12 -18 dB/km
2- 3 km
Window Glass
5 dB/km
7m
Silica - Installed
0.18 -1 dB/km
50 -150 km
Silica - Development
0.16 dB/km
250 km
Halide - Research
0.01 dB/km
3500 km
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6.3 Transmission System Limits
and Characteristics
 The concept of Table 6.2 is very conceptual. Special
coaxial cable systems exist with repeater spacing of 12 km.
 There exist systems capable of operating at very high
speed (a few Mbps) over telephone twisted pairs for
distances of four to six kilometers without repeaters.
 The technology here is called ADSL (Asymmetric Digital
Subscriber Line) or VDSL (Very fast Digital Subscriber
Line).
 This technology makes use of very sophisticated digital
signal processing to eke the last bit per second from a
very difficult medium. Nevertheless, the advantage of
fiber transmission is obvious.
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6.4 Optical System Engineering
6.4.1 System Power Budgeting
 Attenuation of both multimode and single-mode fiber is
generally linear with distance. The amount of signal loss
due to cable attenuation is just the attenuation per
kilometer multiplied by the distance.
 To determine the maximum distance you can send a
signal (leaving out the effects of dispersion), all you need
to do is to add up all the sources of attenuation along the
way and then compare it with the "link budget".
 The link budget is the difference between the transmitter
power and the sensitivity of the receiver.
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6.4.1 System Power Budgeting
 Thus, if you have a transmitter of power -10 dBm
and a receiver that requires a signal of power -20
dBm (minimum) then you have 10 dB of link budget :
1. 10 connectors at 0.3 dB per connector = 3 dB
2. 2 km of cable at 2 dB/km (Multimode Graded index
fiber at 1300 nm) = 4 dB
3. Contingency of (say) 2 dB for deterioration due to
ageing over the life of the system.
 This leaves us with a total of 9 dB system loss. This is
within our link budget and so we would expect such a
system to have sufficient power.
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6.4.1 System Power Budgeting
Figure 6.10 shows the characteristics of some typical
devices versus the transmission speed (in bits/second).
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6.4.1 System Power Budgeting
1. Every laser has a limit to the maximum speed at which
it can be modulated but up to that limit power output is
relatively constant.
2. LEDs produce less and less output as the modulation rate
is increased. The difference in fiber types only relates to
the amount of power you can couple from an LED into
the different types of fiber.
3. All receivers require higher power as the speed is
increased. To reliably detect a bit a receiver needs a
certain number of photons. Therefore every time we
double the modulation speed we need to also double the
required power for a constant signal-to-noise ratio.
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6.4.1 System Power Budgeting
 If you look in the figure for a given bit rate (vertical line)
there will be a difference between the required receiver
power and the available transmitter power.
 This difference is the amount we have available for losses
in the fiber and connectors (and other optical devices such
as splitters and circulators).
 It is also very important to allow some margin in the
design for ageing of components (lasers produce less
power as they age, detectors become less sensitive, etc.).
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Connector/Splice Loss Budgeting
 The signal loss experienced at a connector or splice
is not a fixed or predictable amount! The problem
is the measured losses in actual splices and actual
connectors vary considerably from each other.
 The good news is that actual measurements form
(roughly) a "normal" statistical distribution about
the mean (average).
 Table 6.3 shows "typical" losses that may be
expected from different connector types.
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Connector/Splice Loss Budgeting
Table 6.3 Connector Characteristics
Connector Type
Insertion Loss
(MM)
Typical
Insertion Loss
(SM) Typical
Return Loss
Typical
ST
0.25 dB
0.2 dB
40 dB
SC
0.25 dB
0.2 dB
40 dB
SMA
1.5 dB
FSD
0.6 dB
FC
0.25 dB
0.2 dB
40 dB
D4
0.25 dB
0.2 dB
35 dB
DIN
0.25 dB
0.2 dB
40 dB
Biconic
0.6dB
0.3 dB
30 dB
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Connector/Splice Loss Budgeting
1. For a connection using almost any modern singlemode connector where both connectors are from
the same supplier you can expect a mean loss of 0.2
dB with a standard deviation of 0.15 dB.
2. If the manufacturers of the two connectors are
different, then you can expect an average loss of
0.35 dB with a standard deviation of 0.25 dB.
3. One type of single-mode connector may have an
"average loss" of 0.2 dB but in practical situations
this loss might vary from perhaps 0.1 dB to 0.8 dB.
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Connector/Splice Loss Budgeting
 If you know the average loss for a single connector and
the standard deviation (s) of the connector loss for a
particular situation then you can calculate these figures
for any given combination.
1. The average (mean) of the total is just the average loss
of a single connector multiplied by the number of
connectors.
2. The standard deviation (s) of the total is just the
standard deviation of a single connector multiplied by
the square root of the number of connectors involved.
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Connector/Splice Loss Budgeting
 In long distance links it is common to regard splices as
part of the fiber loss. So you might get raw SM fiber with
a loss (at 1550 nm) of 0.21 dB/km. After cabling, this will
increase to perhaps 0.23 dB/km.
 For loss budget purposes you might allocate .26 dB/km for
installed cable. Cable is typically supplied in 2 km lengths
so in a100 km link there will be a minimum of 50 splices.
 In the 1310 nm band, a typical cable attenuation might be
0.36 dB/km but it is typical to allocate 0.4 dB/km for fiber
losses in new fiber used in this wavelength band.
 The same piece of installed fiber cable would be budgeted
at 0.4 dB/km when used in the 1310 nm band and at 0.26
dB/km when used in the 1550 nm band.
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Power Penalties
 There are a number of phenomena that occur
within an optical transmission system that can be
compensated for by increasing the power budget.
 In each case the amount of additional power
required to overcome the problem is termed the
"power penalty".
 The three most important issues here for digital
systems are :
1. System noise
2. Effect of dispersion and
3. Extinction ratio
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Power Penalties
Signal-to-Noise Ratio (SNR)
 The quality of any received signal in any communication
system is largely determined by the ratio of the signal
power to the noise power - the SNR.
 In simple systems most of the noise comes from within the
receiver itself and so is usually compensated for by an
adjustment of the receiver sensitivity specification.
 In complex systems with EDFAs, ASE noise becomes
important and to compensate we indulge in power level
planning throughout the system.
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Power Penalties
Inter-Symbol Interference (ISI)
 Dispersion causes bits (really line states or
bauds) to merge into one another on the link.
 We can compensate for this by increasing the
signal power level.
 Thus for certain levels of dispersion we can
nominate a system power budget (allowance)
to compensate.
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Power Penalties
Extinction Ratio
 If a zero bit is represented by a finite power level rather than a
true complete absence of power then the difference between the
power level of a 1-bit and that of a 0-bit is narrowed.
 The power level of the 0-bit becomes the noise floor of every 1bit. The receiver decision point has to be higher and therefore
there is an increased probability of error.
 This can be compensated for by an increase in available power
level at the receiver. An extinction ratio of 10 dB incurs a power
penalty (in either a PIN-diode receiver or an APD) of about 1 dB
over what it would have been with a truly zero value for a 0-bit.
 An extinction ratio of 3 dB causes a power penalty of 5 dB in a
PIN-diode receiver and 7 dB in an APD.
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6.4.2 Reflections
 The reflections discussed here are unintended
ones that occur at connectors, joins and in some
devices.
 The unwanted reflections can have many highly
undesirable effects. Among the most important
of these are:
1. Disruption of laser operation
2. Return Loss
3. Amplifier operation
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6.4.2 Reflections
1. Disruption of laser operation :
Reflections entering a laser disturb its stable operation adding
noise and shifting the wavelength.
2. Return Loss :
Reflections can vary with the signal and produce a random loss
of signal power. This is termed “ return loss ”.
3. Amplifier operation :
Reflections returning into an optical amplifier can have two
main effects:
- In the extreme case of reflections at both ends the amplifier
becomes a laser and produces significant power of its own.
- In lesser cases reflections can cause the amplifier to saturate
taking
away power) and again introduce noise to the signal.
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6.4.2 Reflections
 The major causes of reflections are:
1. Joins between high RI material and fiber (such as at
the junction between a laser or LED and a fiber or
between any planar optical component and a fiber).
2. Joins between fibers of different characteristics.
For example where a Pr-doped amplifier employing
ZBLAN host glass is coupled to standard fiber for
input and output.
3. Any bad connector produces significant reflections.
4. Some optical devices such as Fabry-Perot filters
reflect unwanted light as part of their design.
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6.4.2 Reflections
 Reflections can be controlled by the following measures:
1. Taking care with fiber connectors and joins to ensure that
they are made correctly and produce minimum reflections.
This can be checked using an OTDR.
2. The inclusion of isolators in the packaging of particularly
sensitive optical components (such as DFB lasers and
amplifiers). The isolators attenuate the signal and are
polarization sensitive. Their use should be carefully
planned and minimized.
3. In critical situations a diagonal splice in the fiber or a
connector using a diagonal fiber interface can be employed.
The diagonal join ensures that any unwanted reflections
are directed out of the fiber core.
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6.4.2 Reflections
 On many public network optical networks today error
rates of 10-14 are consistently achieved and so user
expectation is that errors will be very rare events indeed.
 Public network operators seem to consider the minimum
acceptable error rate to be around 10-12.
 The faster the link the lower we need the error rate to be!
But the harder that low error rate becomes to deliver.
 In many standards (such as the ATM recommendations)
the expected error rate performance of links over which
the system will be run are specified in the standard.
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6.6.1 Optical Network Technologies
 There are five technologies to implement the highspeed capabilities of optical networks.
 For each of the five, one or more specific protocol
layers have been defined by appropriate standards
bodies, usually the Institute of Electrical and
Electronics Engineers (IEEE) or the International
Telecommunications Union (ITU).
 These technologies comprise SONET, Gigabit
Ethernet, Fiber Channel, IBM’s Enterprise System
Connection (ESCON), and Fiber Distributed Data
Interface (FDDI).
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SONET
 SONET was the first widely deployed optical network
technology. It is an electronic network technology
that utilizes optical links.
 It is optimized to guarantee extremely reliable
transport of constant bit rate 64 kbps channels or
combinations of them, either between two points, or
in a ring configuration.
 Because of its worldwide standards, the ready
availability of equipment, and its long track record,
SONET is the “gold standard” for connecting devices
such as routers.
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Gigabit Ethernet
 Gigabit Ethernet is currently the fastest
implementation of the Ethernet technology.
 It requires opto-electronic conversions, but does
allow Local Area Networks (LANs) to take
advantage of the high speed and low cost of optical
components.
 Gigabit Ethernet utilizes single-mode fiber and a
wavelength of 1310 nm for distances up to 2 km,
and multi-mode fiber at 850 nm for distances up to
200 m.
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Fiber Channel
 Fiber Channel is an application of optical fiber for
interconnection of computers and peripheral
devices at relatively low speeds (i.e., low compared
to possible speeds on optical networks) using
standardized components.
 It employs lasers or LEDs on single or multi-mode
fiber, and can achieve data rates of up to 100 Mbps,
over distances of up to 10 km. However, the multimode fiber media can only reach up to about 2 km.
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ESCON
 ESCON is an architecture intended for
connecting processors and peripherals .
 It is an electro-optical system, which uses
optical fiber to connect “stations” (computers
or peripherals) to others through an electronic
space-division switch.
 Maximum data rate (as a communications
device) is 160 Mbps over single-mode fiber.
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FDDI
 FDDI was originally proposed as a standard for
input/output (I/O) channels using optical fiber,
but is now more commonly considered as a tokenring LAN technology that utilizes two countercirculating rings for increased reliability.
 The ring may be up to 200 km in total length,
though stations can only be 2 km apart.
A maximum of 500 stations is supported. Current
standards allow a maximum data rate of 100 Mbps.
 Multi-mode fiber is more commonly employed than
single-mode fiber, though in practice, FDDI more
often runs over copper wires.
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6.6.1 Optical Network Technologies
Table 6.4 Optical Network Technologies
and Major Applications
Telecom
Provider
Core
Network
Internet
Core
ISP
Corporate
MAN
Undersea
Cables
Large
LAN
Local
Distribution
WDM







OTDM


SONET


Local
or
Internal
Comm



Gigabit
Ethernet
With
Optical
Medium



Fiber
Channel


ESCON



FDDI
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Or
Home
LAN

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
6.6.2 PON (Passive Optical Network)
 A PON network system is used for delivering broadband
network access service to business and residential
customers.
 PON is an FTTx technology, which opens up the ‘last mile
bottleneck’ present in data services available to the
majority of home and business computer users today.
FTTx : Fiber-to-the-x
Any network which extends fiber optic capacity
directly to the user; x can be replaced with “home”,
“business”, “curb”, “cabinet”, depending on how
close the fiber connection actually is to the end user.
 A PON network is called passive, because from the public
network connection (Central Office), to the end user, there
are no active electronics required.
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6.6.2 PON (Passive Optical Network)
 In Fig. 6.11, PON is a network that looks like a star,
where the center of the star is the service provider’s
central office, and fibers reach out from the C.O.
(Central Office) to all the available subscribers.
 As the fiber gets closer to the end-subscriber, the fiber
is split and divided among many end-points.
 The fiber split is handled by a passive fiber optic
component, a passive optical splitter, and one fiber is
sent from the splitter point to each subscriber.
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6.6.2 PON (Passive Optical Network)
Fig. 6.11 PON network Diagram.
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6.6.2 PON (Passive Optical Network)
 Signals flowing from the subscriber back to the C.O.
are merged onto the same fiber, using the optical
splitter in reverse.
 Signals from each subscriber are processed by the C.O.
receiver in sequence, using timing method called Time
Domain Multiple Access.
 The distance that a PON can reach is just over 12 miles.
 And some description information of each element is
shown in Table 6.6.
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6.6.2 PON (Passive Optical Network)
Table 6.5 A PON network elements description
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6.6.2 PON (Passive Optical Network)
 A PON network reduce the cost of broadband
service, primarily by:
1. sharing the cost of optical terminal equipment
(the electrical-to-optical conversion) in the
Central Office,
2. reducing the cost of servicing active devices
located in the field, and
3. sharing the fiber with multiple subscribers.
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6.6.2 PON (Passive Optical Network)
 There are three types of PON equipment available,
ATM-PON (APON), Ethernet-PON (EPON), and
Gigabit PON (GPON).
 APON has a means for connecting multiple
subscribers to the same receiver in the Central Office,
Time Domain Multiplexed Access.
 This means that each subscriber transmitter talks to
the C.O. receiver in turn, and the scheduling of this is
controlled by the OLT line card in the C.O.
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6.6.2 PON (Passive Optical Network)
 ATM-PON and Gigabit-PON use ATM-switching
protocols to optimize real-time delivery of information,
like carrying on a conversation.
 ATM network conversations frequently focus on
Quality of Service (QoS) as it’s best feature, which is
the guarantee that every subscriber connected to the
network received a certain minimum block of
information to transmit its information. This is why
ATM is best suited for handling voice conversations.
 GPON operates similar to APON, but at higher (1.25 ~
2.5 Gbps) line-rates.
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6.6.2 PON (Passive Optical Network)
 EPON is designed to optimize the transmission
of packets of data information.
 EPON uses the same means of merging each
subscriber’s upstream data onto the same fiber,
Time Domain Multiplexed Access (TDMA).
 The EPON protocol uses Ethernet transmission,
the data networking standard. As such, EPON
can handle traffic better which is more often
data- than is voice-related.
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6.6.2 PON (Passive Optical Network)
 EPON uses Voice over IP (VoIP) to carry phone
conversations, and treats all information equally, as
data.
 A current characteristic of VoIP is the effect that it
has on what could be called “digitizing” the sound of
someone’s voice.
 The common argument between the APON and
EPON network focuses on each standard’s strength,
and which is better for a service provider to
concentrate on, voice quality, or data efficiency.
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