23-08-2012, 01:25 PM
WDM Network Design
[attachment=32504]
Introduction to Optical Design
A network planner needs to optimize the various electrical and optical parameters to ensure
smooth operations of a wavelength division multiplexing (WDM) network. Whether the
network topology is that of a point-to-point link, a ring, or a mesh, system design inherently
can be considered to be of two separate parts: optical system design and electrical or higherlayer
system design. To the networking world, the optical layer (WDM layer) appears as a
barren physical layer whose function is to transport raw bits at a high bit rate with negligible
loss. Most conventional network layer planners do not care about the heuristics of the
optical layer.
However, such lapses can often be catastrophic. Until the bit rate and the transmission
distance is under some bounded constraint (for example, small networks), it is often not
important to consider the optical parameters.
However, as the bit rate increases and transmission length increases, these optical parameters
have the capability of playing truant in the network. A network planner must consider
the affecting parameters and build a network that accommodates the impairments caused
by the optical parameters. This chapter explores some of the design constraints involved in
the WDM network design.
Consider an optical signal as a slowly varying signal of amplitude A (τ, t) (function of
distance 'τ' and time 't'), on which various parameters are acting at all times. An optical
signal, as discussed in Chapter 1, “Introduction to Optical Networking,” propagates through
a silica fiber with propagation constant β, whose value is obtained from the solution of the
basic wave equations. Further, this optical signal is subjected to attenuation, which by
virtue of itself, is a property of the propagating medium—silica fiber in this case. Attenuation
in a fiber is characterized by the attenuation constant α, which gives the loss (in dB)
per traveled km.
Factors That Affect System Design
Initially, fiber loss was considered the biggest factor in limiting the length of an optical
channel. However, as data rates grew and pulses occupied lesser and lesser time slots, group
velocity dispersion and nonlinearities (SPM, XPM, and FWM) became important considerations.
As we will see in the following sections, an optical link is designed by taking into account
a figure of merit, which is generally the bit error rate (BER) of the system. For most
practical WDM networks, this requirement of BER is 10-12 (~ 10-9 to 10-12), which means
that a maximum one out of every 1012 bits can be corrupted during transmission. Therefore,
BER is considered an important figure of merit for WDM networks; all designs are based
to adhere to that quality.
In Chapter 2, we saw the analytical explanation behind BER. It showed BER to be a ratio
of the difference of high and low bit levels (power) to the difference in standard deviation
of high and low bit levels. As can be observed it is quite difficult to calculate BER instantaneously.
Another plausible explanation of BER can be considered as follows. For a photodetector to
detect a 1 bit correctly (assuming nonreturn-to-zero/return-to-zero, or NRZ/RZ modulation;
see Chapter 2), it needs a certain minimum number of photons (Np) falling on it. If NTP is
the number of photons launched at the transmitter and Δp is the number of photons lost
(hypothetically) due to attenuation, absorption, scattering, and other impairments during
transmission, then if NTP - Δp < Np, the receiver will not be able to decode the signals
properly. To sustain good communication, it is imperative that NTP – Δp > Np over 'L' the
desired length of transmission channel. The number of photons translates to the power
(which is a function of intensity) of the optical signal.
Long-Haul Impairments: Nonlinearity
By placing optical amplifiers, we can greatly enhance the power of an optical signal as it
reaches the photodetector. Yet another system design consideration is the net fiber nonlinearity
that is present in silica fibers. The intensity of the electromagnetic wave propagating
through a fiber gives rise to nonlinearities. The refractive index has a strong nonlinear
component that depends on the power level of the signal. Nonlinearity produces a nonlinear
phase shift denoted by φNL. This is shown in Equation 4-3.
Design of a Point-to-Point Link Based on Q-Factor
and OSNR
To design a network, it is imperative to comply the system design with the BER requirement
of the network. If one carefully considers the preceding criteria, it should be evident
that calculating BER instantaneously is an intriguing task given that a designer has tools
such as a spreadsheet and calculator. Chapter 2 briefly discussed the Q-factor of an optical
signal. The Q-factor provides a qualitative description of the receiver performance because
it is a function of the signal to noise ratio (optical). The Q-factor suggests the minimum
SNR required to obtain a specific BER for a given signal. Figure 4-3 shows the relationship
of Q-factor to BER. As we can see, the higher the value of Q-factor, the better the BER.
Figure 4-4 shows the penalty of the Q-factor due to nonlinear effects by increase in input
power.
Calculation of OSNR for a Point-to-Point Link
Consider a physical link AB, as shown in Figure 4-5. Assume this to be a long-haul fiber
WDM link (a link that is several hundred kilometers). Amplifiers are placed periodically at
repeated intervals to boost signal power. Therefore, a signal can reach much farther than the
maximum allowable accumulated loss due to the fiber (∝L). However, in doing so, each
amplifier stage adds its own component of amplified spontaneous emission (ASE) noise
and degrades the OSNR further. Moreover, every amplifier amplifies the already present
noise. Note that noise is omnipresent throughout the spectra and almost impossible to be
removed. Therefore, it is imperative to devise a method to calculate the OSNR (output) at
the end of an N stage-amplified system and see if the value N is still valid.
In an OSNR-based design, we must ensure that OSNR of the final stage is in compliance
with system OSNR requirements and hence the BER requirements. To make the system
support a particular BER, it is necessary to make the OSNR system design compliant.
[attachment=32504]
Introduction to Optical Design
A network planner needs to optimize the various electrical and optical parameters to ensure
smooth operations of a wavelength division multiplexing (WDM) network. Whether the
network topology is that of a point-to-point link, a ring, or a mesh, system design inherently
can be considered to be of two separate parts: optical system design and electrical or higherlayer
system design. To the networking world, the optical layer (WDM layer) appears as a
barren physical layer whose function is to transport raw bits at a high bit rate with negligible
loss. Most conventional network layer planners do not care about the heuristics of the
optical layer.
However, such lapses can often be catastrophic. Until the bit rate and the transmission
distance is under some bounded constraint (for example, small networks), it is often not
important to consider the optical parameters.
However, as the bit rate increases and transmission length increases, these optical parameters
have the capability of playing truant in the network. A network planner must consider
the affecting parameters and build a network that accommodates the impairments caused
by the optical parameters. This chapter explores some of the design constraints involved in
the WDM network design.
Consider an optical signal as a slowly varying signal of amplitude A (τ, t) (function of
distance 'τ' and time 't'), on which various parameters are acting at all times. An optical
signal, as discussed in Chapter 1, “Introduction to Optical Networking,” propagates through
a silica fiber with propagation constant β, whose value is obtained from the solution of the
basic wave equations. Further, this optical signal is subjected to attenuation, which by
virtue of itself, is a property of the propagating medium—silica fiber in this case. Attenuation
in a fiber is characterized by the attenuation constant α, which gives the loss (in dB)
per traveled km.
Factors That Affect System Design
Initially, fiber loss was considered the biggest factor in limiting the length of an optical
channel. However, as data rates grew and pulses occupied lesser and lesser time slots, group
velocity dispersion and nonlinearities (SPM, XPM, and FWM) became important considerations.
As we will see in the following sections, an optical link is designed by taking into account
a figure of merit, which is generally the bit error rate (BER) of the system. For most
practical WDM networks, this requirement of BER is 10-12 (~ 10-9 to 10-12), which means
that a maximum one out of every 1012 bits can be corrupted during transmission. Therefore,
BER is considered an important figure of merit for WDM networks; all designs are based
to adhere to that quality.
In Chapter 2, we saw the analytical explanation behind BER. It showed BER to be a ratio
of the difference of high and low bit levels (power) to the difference in standard deviation
of high and low bit levels. As can be observed it is quite difficult to calculate BER instantaneously.
Another plausible explanation of BER can be considered as follows. For a photodetector to
detect a 1 bit correctly (assuming nonreturn-to-zero/return-to-zero, or NRZ/RZ modulation;
see Chapter 2), it needs a certain minimum number of photons (Np) falling on it. If NTP is
the number of photons launched at the transmitter and Δp is the number of photons lost
(hypothetically) due to attenuation, absorption, scattering, and other impairments during
transmission, then if NTP - Δp < Np, the receiver will not be able to decode the signals
properly. To sustain good communication, it is imperative that NTP – Δp > Np over 'L' the
desired length of transmission channel. The number of photons translates to the power
(which is a function of intensity) of the optical signal.
Long-Haul Impairments: Nonlinearity
By placing optical amplifiers, we can greatly enhance the power of an optical signal as it
reaches the photodetector. Yet another system design consideration is the net fiber nonlinearity
that is present in silica fibers. The intensity of the electromagnetic wave propagating
through a fiber gives rise to nonlinearities. The refractive index has a strong nonlinear
component that depends on the power level of the signal. Nonlinearity produces a nonlinear
phase shift denoted by φNL. This is shown in Equation 4-3.
Design of a Point-to-Point Link Based on Q-Factor
and OSNR
To design a network, it is imperative to comply the system design with the BER requirement
of the network. If one carefully considers the preceding criteria, it should be evident
that calculating BER instantaneously is an intriguing task given that a designer has tools
such as a spreadsheet and calculator. Chapter 2 briefly discussed the Q-factor of an optical
signal. The Q-factor provides a qualitative description of the receiver performance because
it is a function of the signal to noise ratio (optical). The Q-factor suggests the minimum
SNR required to obtain a specific BER for a given signal. Figure 4-3 shows the relationship
of Q-factor to BER. As we can see, the higher the value of Q-factor, the better the BER.
Figure 4-4 shows the penalty of the Q-factor due to nonlinear effects by increase in input
power.
Calculation of OSNR for a Point-to-Point Link
Consider a physical link AB, as shown in Figure 4-5. Assume this to be a long-haul fiber
WDM link (a link that is several hundred kilometers). Amplifiers are placed periodically at
repeated intervals to boost signal power. Therefore, a signal can reach much farther than the
maximum allowable accumulated loss due to the fiber (∝L). However, in doing so, each
amplifier stage adds its own component of amplified spontaneous emission (ASE) noise
and degrades the OSNR further. Moreover, every amplifier amplifies the already present
noise. Note that noise is omnipresent throughout the spectra and almost impossible to be
removed. Therefore, it is imperative to devise a method to calculate the OSNR (output) at
the end of an N stage-amplified system and see if the value N is still valid.
In an OSNR-based design, we must ensure that OSNR of the final stage is in compliance
with system OSNR requirements and hence the BER requirements. To make the system
support a particular BER, it is necessary to make the OSNR system design compliant.