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": the long-haul and the metropolitan.
The Long-Haul Network
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.
|
|
Figure 1. Terrestrial fiber-optic 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.
|
STM-1
|
155.52
|
|
STM-4
|
622.08
|
|
STM-16
|
2488.32 (2.5)
|
|
STM-64
|
9953.28 (10)
|
|
STM-256
|
39813.12 (40)
|
Table 1. Standardized transmission line rates
The simpler optical links are point-to-point, without optical amplification (Figure 1) or signal multiplexing. In long-haul networks using of EDFAs, transmitted signals must be regenerated every 400 km or so (depending on the EDFA’s characteristics) to overcome the signal loss due to fiber attenuation, distortion caused by dispersion and nonlinear effects, as well as the 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.
Figure 2. Traditional long-haul network without optical signal amplification
However, using a combination of a hybrid distributed Raman amplifier and an EDFA (Figure 2) allows the regeneration-site spacing to be extended from 500 km to 2000 km. However, 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
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 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 prime 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 networks’ life cycle. 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—chromatic dispersion (CD) and polarization-mode dispersion (PMD) being some of the parameters to keep in check.
The Metropolitan Network
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 CWDM (coarse WDM) rings, traditional circuit-switched transport protocols (SONET/SDH/PDH) and data protocols such as Ethernet, IP and Fibre Channel. Over the last few years, Ethernet has emerged as a leading solution in metro-area networks (MANs), thanks to the development of carrier-grade optical Gigabit Ethernet technology.
Figure 4. Ethernet-over-SONET/SDH Troubleshooting and Maintenance in a Metro Metwork
This technology blend makes interoperability and multiprotocol testing paramount. Testing all layers, from the fiber to the individual applications, is critical in order 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 brought forth a remarkable evolution. Its ability to increase fiber bandwidth exponentially on existing fiber networks significantly reduced the need for new cable deployments, while paving the way for future all-optical network designs and installations.
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 dense wavelength-division multiplexing (DWDM)
systems.
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.
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 erbium-doped fiber amplifier (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.
|
Cabled optical fiber
|
|
Transmitter/ modulator
|
|
Receiver
|
|
Multiplexer/ demultiplexer (mux/demux)
wavelength-selective branching device
|
|
Optical amplifiers
|
|
Attenuators
|
|
Switches
|
|
Dispersion- compensating module
|
|
Non-wavelength-selective fiber- optic branching devices
|
|
Isolators
|
|
Circulators
|