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Technology Overview


The backbone of the telecommunications networks, usually referred to as the core or the transport network, is the heart of all large network providers’ operations. This fiber "highway" is constantly evolving—becoming bigger, faster and more complex. The backbone network is usually split into two "subnetworks": long-haul and metropolitan.

Long-Haul Networks

Long-haul networks connect metropolitan areas to each other or interconnect with other long-haul networks, enabling seamless and efficient intercity and international connectivity. Long-haul networks carry a lot more data than any other type of network, and over much greater distances, which can reach hundreds or thousands of kilometers.

Terrestrial LongHaul Network
Figure 1. Terrestrial optical networks.

 

The typical long-haul network operates at data rates of either 2.5 Gbit/s or 10 Gbit/s. Some trials are currently underway to implement 40 Gbit/s transmission in order to accommodate the extra bandwidth required as a result of high-bandwidth-application deployments, such as IPTV and VoD

Optical carrier (OC-x)

Synchronous transport mode (STM-x)

Line rate (Mbit/s)

OC-3

STM-1

155.52

OC-12

STM-4

622.08

OC-48

STM-16

2488.32 (2.5)

OC-192

STM-64

9953.28 (10)

OC-768

STM-256

39813.12 (40)

Table 1. Standardized transmission line rates.

 

Simpler optical links are point-to-point and don't require optical amplification (figure 1) or signal multiplexing. In long-haul networks using erbium-doped fiber amplifiers (EDFAs), transmitted signals must be regenerated every 400 km or so (depending on the characteristics of the EDFA) to overcome the signal loss due to attenuation, distortion caused by dispersion and nonlinear effects, as well as noise build-up generated within the EDFAs themselves. This regeneration is accomplished through optical-to-electrical-to-optical (O-E-O) conversion, which regenerates the signal during the electrical phase. The signal may then be reamplified, reshaped and retimed (3R). Such regeneration equipment is required on a per-channel basis, which is costly.

Traditional Long-Haul Network
Figure 2. Traditional long-haul network without optical signal amplification.

 

 

However, using a combination of a hybrid distributed Raman amplifier and an EDFA (figure 3) allows the regeneration-site spacing to be extended from 500 km to 2000 km. Yet the low-noise advantage provided by the Raman amplification comes at a price: due to the low efficiency of the Raman process, the pump used must be much more powerful than the one that would be used with EDFAs. This brings constraints related to cost, human safety, connector cleanliness and potential additional non-linear effects. 

A long-haul WDM point-to-point link
Figure 3. A long-haul WDM point-to-point link.

 

SONET (synchronous optical network) or SDH (synchronous digital hierarchy) are the most frequent transport technologies used in long-haul networks. A multitude of those streams can be transported simultaneously through the same fiber over dense wavelength-division multiplexing (DWDM) technology. Since severe impairments can arise with increased DWDM channel counts, careful engineering provisions are required to maintain channel quality over long distances.

High bit rates and synchronized data transmission put utmost importance on detailed testing of the SONET/SDH infrastructure. Parameters such as data integrity, delays, amplification and attenuation become far more critical to control, at every step of the life cycle of the network. SONET/SDH and optical transport layers must be tested concurrently when assessing problems within these networks. Optically speaking, higher bit rates require the most complex testing; therefore, chromatic dispersion (CD) and polarization-mode dispersion (PMD) are some of the parameters to keep in check.

Metropolitan Networks

Metropolitan (or metro) networks are the bridge between first-mile infrastructures in urban areas (access networks) and long-haul networks. They combine various next-generation technologies such as optical transport over DWDM or coarse WDM (CWDM) rings, traditional circuit-switched transport protocols (SONET/SDH/PDH) and data protocols like Ethernet, IP and Fibre Channel. Over the last few years, thanks to the development of carrier-grade optical gigabit Ethernet technology, Ethernet has emerged as a leading solution in metro-area networks (MANs). 

Ethernet-over-SONET/SDH troubleshooting and maintenance in a metro network
Figure 4. Ethernet-over-SONET/SDH troubleshooting and maintenance in a metro network.

 

This technology blend makes interoperability and multiprotocol testing paramount. Testing all layers, from the fiber to the individual applications, is critical to efficiently install, maintain and manage such complex networks. Today’s multiprotocol fabrics are facilitated by the latest advances in protocol standards. However, technologies like next-generation SONET/SDH and 10 Gigabit LAN/WAN Ethernet need to be tested in context with legacy protocols and correlated with all the layers they put together.

Wavelength-Division Multiplexing: Multiplying the Network’s Capacity

Wavelength-division multiplexing (WDM) has been used since 1996 and has brought forth a remarkable evolution. Its ability to exponentially increase fiber bandwidth on existing fiber networks significantly reduced the need for new cable deployments, while paving the way for future all-optical network designs and installations.

Representation of contiguous WDM channels from a single fiber, as measured and displayed by an optical spectrum
Figure 5. Representation of contiguous WDM channels from a single fiber, as measured and displayed by an optical spectrum.

 

WDM systems are based on the ability of an optical fiber to carry many different wavelengths of light simultaneously without mutual interference. Each wavelength represents an optical channel within the fiber. Proven optical methods are available to combine individual channels within a fiber and to extract them at appropriate points along a network. Channel wavelength separations can be very small—a fraction of a nanometer or 10-9 m—giving rise to DWDM systems.

Bandwidth capacity increases rapidly with the multiplication of channels
Figure 6. Bandwidth capacity increases rapidly with the multiplication of channels (from DWDM guide,  p.3).

 

WDM networks carrying up to 160 independent optical channels, each at speeds of up to 10 Gbit/s, in a single fiber are commercially available. Even bidirectional traffic can be sent over the same fiber. Still, the majority of WDM systems deployed today typically run 8 to 16 channels at speeds of 2.5 and 10 Gbit/s.

Acronym

Full Name

Channel Spacing

WWDM

Wide wavelength-division multiplexing

≥50 nm

CWDM

Coarse wavelength-division multiplexing

<50 nm

DWDM

Dense wavelength-division multiplexing


≥1000 GHz

Table 2. Rules for classification of WDM systems—ITU-T G.671/IEC 62074-1.

 

The success of DWDM is largely due to the development of the EDFA, which uses energy from a laser pump to optically amplify all the signal wavelengths presented to its input (within its narrow bandpass centered at 1550 nm) without requiring that they be converted into electrical signals and back again into optical signals (O-E-O conversion).

DWDM Enablers and Key Parameters

The many advantages of DWDM systems bring stringent requirements, both in terms of design and testing:

  • Optical component properties and cable characteristics, which could safely be neglected in systems using simpler transmission techniques, must be addressed
  • The new spectral dimension that is inherent to these systems implies new criteria for network design and for the selection of components, thus leading to different, often much tighter, specifications than those applicable to single-wavelength systems
  • All the parameters relevant to transmission efficiency and accuracy must be measured at each channel wavelength, especially when wavelengths are very closely spaced. Verification and multilevel testing is needed throughout the DWDM network: components, subsystems, optical media etc. 

Network Element

Technology

Benefits/Purpose

Critical Parameters

Cabled optical fiber
  • Non-dispersion-shifted singlemode fiber (NDSF)
  • Non-zero dispersion-shifted singlemode fiber (NZDSF)
  • Dispersion-shifted fiber (DSF)
  • Cut-off shifted fiber
  • Non-zero dispersion-shifted fiber for wideband transport
  • Bending-loss-insensitive singlemode fiber for the access network
  • NDSF has zero dispersion at 1310 nm; it has also a large core preventing non-linear effects; it has been deployed in hundreds of million of kilometers since its introduction, making it the least expensive fiber.
  • The dispersion of NZDSF ranges between -10 and +10 (ps/nm•km) and can be adjusted for DWDM transmissions with a large number of channels and long distances; however, it is expensive.
  • Attenuation (spectral) spontaneous emission
  • Chromatic dispersion (CD)
  • Zero-dispersion wavelelength
  • Slope of the dispersion at zero-dispersion wavelength
  • Polarization mode dispersion (PMD)
  • Cabled fiber PMDQ coefficient
  • System DGDmax
  • Second-order PMD for very high bit rates
  • Optical power handling and damage threshold
  • Non-linear effects (NLE)
Transmitter / modulator
  • Temperature-controlled, frequency-stabilized distributed-feedback (DFB) laser
  • May contain high-power (booster) amplifier
  • Digital modulator (direct modulation for low bit rates, external modulation for high bit rates)
  • Modulation format generator
  • Analog/digital O-E-O converter
  • Service network interface (if at the service subscriber premises)
  • DFB lasers provide narrow-frequency spectra for broadband high-bit-rate transmissions
  • High-output optical power for long-distance transmissions
  • High stability for very narrow-spaced high-count DWDM channel transmissions
  • Complex modulation formats at very high bit rates
  • Frequency spectrum/spectral width
  • Output power
  • Power/frequency stability (drift, chirp)
  • Source spontaneous emission noise
  • BER
  • Optical power handling and damage threshold
Receiver
  • User network interface or transmitter if at regeneration node
  • Temperature-controlled, frequency-stabilized avalanche photodiode (APD) InGaAs-based detector (for high bit rates
  • May contain low-noise pre-amplifier
  • Digital modulation format demodulator
  • Digital/analog O-E-O converter
  • PIN detectors are inexpensive and applicable for low bit rates and short distances
  • APD detectors are more expensive but have lower noise and are better suited for higher bit rates and longer distances
  • PIN and APD have both wide spectral response
  • APD responds faster and has higher sensitivity and lower noise
  • BER
  • Sensitivity
  • Overload
  • Response time
  • Noise
Multiplexer/ demultiplexer (mux/demux) wavelength-selective branching device
  • Narrowband filters
  • Common types: thin-film filter, fiber Bragg grating, array waveguide grating, fused biconic tapered fiber
  • Infrequent types: bulk-optic grating, liquid crystal
  • Combination and separation of narrow-spaced high-count channels at both ends of the network
  • Optical power handling and damage threshold
  • Channel attenuation/Insertion loss, ripple and uniformity
  • Number of channels
  • Channel frequency range
  • x-dB passband width
  • Chromatic dispersion
  • PDL
  • PMD
  • Adjacent, non-adjacent and total channel isolation
  • Optical power handling and damage threshold
  • Directivity
  • Return loss
Optical amplifiers
  • Erbium-doped fiber amplifier (EDFA)
  • Raman fiber amplifier (RFA)
  • Semiconductor optical amplifier (SOA)
  • Single-stage amplifier
  • Multistage amplifier
  • Single-pump amplifier
  • Multipump amplifier
  • Backward-pumping amplifier
  • Forward-pumping amplifier
  • Boost the power level of all lambdas transmitted in the fiber without the need for electrical signal conversion
  • Gain, small-signal and saturation
  • Amplified spontaneous emission
  • Noise figure
  • Gain slope
  • Channel addition/removal in steady state
  • Input-output optical power
  • Gain cross saturation
  • Polarization dependent gain
  • Multichannel power tilt transient gain response
  • Multichannel gain tilt (dB) at min/max input conditions
Attenuators
  • Fixed-plug attenuator
  • Variable optical attenuator
  • Optical power-limiter attenuator
  • Optical power-fuse attenuator
  • Balances the power level for each wavelength
  • Prevents excessive power
  • Minimum attenuation
  • Insertion loss
  • Attenuation range
  • Accuracy-repeatability of the attenuation set value
  • Return loss
  • PDL
  • PMD
  • Optical power handling and damage threshold
  • Wavelength-dependence
Switches
  • Electro-optic or Lithium Niobate waveguide
  • Liquid-crystal waveguide
  • Bubble waveguide
  • Thermo-optical, silica on silicon waveguide
  • MEMS 2D, 3D
  • Electroholography
  • SOA and EDFA switches
  • Electrically switchable fiber grating
  • Opto-mechanical
  • Acousto-optical
  • Traffic routing
  • Provide protection
  • Insertion and coupling loss
  • Return loss
  • PDL
  • Crosstalk and isolation
  • Protocol/bit rate/wavelength dependence
  • Manufacturability-reliability
  • Switching time
  • Stability
  • Operating spectral range
  • Directivity
  • Optical power handling and damage threshold
  • Latching time
Dispersion-compensating module
  • Dispersion-compensating fibers (DCF)
  • Chirped fiber Bragg grating (CFBG)
  • Virtual image phased array (VIPA)
  • Higher-order mode fiber (HOM)
  • Spectral inversion: invert spectral components typically at mid system range
  • Nonlinearities (amplifiers)
  • Pre-chirping
  • Dispersion mapping
  • Holey fiber
  • Solitons
  • FEC
  • Phase correction
  • Decision level feedback
  • Reduction at an acceptable level of the pulse broadening due to chromatic dispersion
  • Wavelength range
  • Group delay
  • Slope of the dispersion
  • Figure of merit
  • Group delay - phase ripple
  • Insertion loss
  • PMD
  • Return loss
  • Multipath interference (MPI)
  • Optical power handling and damage threshold
Non-wavelength-selective fiber-optic branching devices
  • Thin-film
  • Planar lightwave circuit (AWG)
  • Fiber Bragg Grating (FBG)
  • Bulk optic grating
  • Fused biconic tapered fiber
  • Separate (split) or combine (couple) the power of broadband signals depending on the branching ratio
  • Insertion loss
  • Return loss
  • Directivity
  • Excess loss
  • Uniformity
  • Optical power handling and damage threshold
  • Operating wavelength range
Isolators
 
  • Used when backscattered light can degrade the performance of a sensitive component such as a DFB laser
  • Wavelength-dependence
  • Insertion loss (both ways)
  • Return loss
  • PMD
  • PDL
  • Optical power handling and damage threshold
  • Multipath interference
Circulators
 
  • Used when backscattered light can degrade the performance of a sensitive component such as a DFB laser
  • Wavelength-dependence
  • Insertion loss (both ways)
  • Return loss
  • PMD
  • PDL
  • Isolation
  • Optical power handling and damage threshold
  • Multipath interference

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