Driving Optical Network Evolution, TELEKOMUNIKACJA, eng, --Światłowody (koksa)
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Driving Optical Network Evolution
Overview
Over the years, advancement in technologies has improved transmission
limitations, the number of wavelengths we can send down a piece of fiber,
performance, amplification techniques, and protection and redundancy of the
network. When people have described and spoken at length about optical
networks, they have typically limited the discussion of optical network technology
to providing physical-layer connectivity. When actual network services are
discussed, optical transport is augmented through the addition of several
protocol layers, each with its own sets of unique requirements, to make up a
service-enabling network. Until recently, transport was provided through specific
companies that concentrated on the core of the network and provided only point-
to-point transport services. A strong shift in revenue opportunities from a service
provider and vendor perspective, changing traffic patterns from the enterprise
customer, and capabilities to drive optical fiber into metropolitan (metro) areas
has opened up the next emerging frontier of networking. Providers are now
considering emerging lucrative opportunities in the metro space. Whereas
traditional or incumbent vendors have been installing optical equipment in the
space for some time, little attention has been paid to the opportunity available
through the introduction of new technology advancements and the economic
implications these technologies will have.
Specifically, the new technologies in the metro space provide better and more
profitable economics, scale, and new services and business models. The current
metro infrastructure comprises this equipment, which emphasizes voice traffic; is
limited in scalability; and was not designed to take advantage of new
technologies, topologies, and changing traffic conditions. Next-generation
equipment such as next-generation Synchronous Optical Network (SONET),
metro core dense wavelength division multiplexing (DWDM), metro-edge
DWDM, and advancements in the optical core have taken advantage of these
limitations, and they are scalable and data optimized; they include integrated
DWDM functionality and new amplification techniques; and they have made
improvements in the operational and provisioning cycles.
This tutorial provides technical information that can help engineers address
numerous Cisco innovations and technologies for Cisco Complete Optical
Multiservice Edge and Transport (Cisco COMET). They can be broken down into
five key areas: photonics, protection, protocols, packets, and provisioning.
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Network Flexibility
Networks today must support a variety of traffic types, including legacy traffic
based on regional SONET ring structures that require multiple traffic adds/drops
(that is, voice, asynchronous transfer mode [ATM], frame relay) but must also
support high-speed Internet backbones that are typically express lanes that
require little add/drop multiplexing. Deploying the hybrid Raman amplifier and
erbium-doped fiber amplifier (EDFA) amplification application in the L-band
enables extended long-haul reach for this express Internet traffic, while still
allowing deployment of the C-band as traditional long haul for legacy-type traffic,
a deployment that requires multiple traffic add/drop sites. This mix of traditional
long haul in the C-band and extended long haul in the L-band allows for better
network flexibility.
Amplification Extended to Metro, Long Haul
The key drivers for this application include a reduction in the cost of bandwidth
(that is, a reduction in price/performance and distance, an increase in network
capacity, higher network availability, and better network flexibility).
Reduction in Cost of Bandwidth
In conventional long-haul (EDFA) technology, the transmission signals must be
regenerated every 500 km or so to overcome signal distortion due to dispersion
and nonlinear effects and to overcome the build-up of noise generated within the
EDFA amplifiers. This regeneration is accomplished through optical-to-electrical-
to-optical (O–E–O) conversion, the signal being regenerated during the electrical
phase. This regeneration equipment is required on a per-channel basis and is,
therefore, very expensive, and it also requires a large equipment footprint and
high electrical power consumption and subsequent site climatic control. If a
hybrid distributed Raman amplifier plus EDFA technology is used, the
regeneration-site spacing can be extended from 500 km to 2,000 km. This
extended–long-haul application, therefore, introduces significant cost savings
and reduces the dollar cost of transmission capacity for digital signal (DS3) per
kilometer.
Network Capacity
A limiting factor in DWDM systems that restricts the minimum channel spacing
and, therefore, the capacity of the system lies in pulse distortions and
interference that arises from nonlinear effects. Four-wave mixing (FWM) and
cross-phase modulation (XPM) are two such nonlinear effects that are channel-
spacing dependent and, therefore, restrict the minimum channel spacing and
ultimate fiber capacity. However, the efficiency of these nonlinear effects is
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dependent on the channel signal power. Using the Raman amplification
effectively reduces the "apparent" loss of the transmission fiber that the signal
sees. Therefore, the "per-channel" power launched by the EDFA can be reduced,
and this reduction in per-channel power reduces nonlinear effects in the fiber
and allows closer channel spacing and greater system capacity.
Network Availability
The network availability is determined from the failure in time (FIT) rates of the
components that make up the network. The regeneration sites that are placed
every 500 km in conventional EDFA–based networks are "heavy" in high-speed
electronics and optical components and, therefore, have the highest FIT rate and
thus the highest failure rate in the network. Using hybrid distributed Raman
amplifiers plus EDFA amplification in extended–long-haul systems dramatically
reduces the number of regeneration sites, yielding significantly higher network
availability.
Channel Spacing
With enhancements in demultiplexing technology, it is now possible to deploy
DWDM systems with 50-GHz channel spacing at 10-Gbps rates. This scenario
allows for greater channel counts and, therefore, higher capacities. Previously in
the C-band with 100-GHz spacing, it was possible to deploy 40 channels; with
50-GHz spacing, this figure has been doubled to 80 channels.
Improved transmitter wavelength stability is required to achieve 50-GHz channel
spacing. "Wavelength locking" of transponder transmitter lasers has been
introduced to achieve improved wavelength stability. The local feedback loop
ensures long-term accuracy of the transmitter laser wavelength over the
operating temperature range of the system.
With the closer channel spacing, multichannel, nonlinear effects such as FWM
and XPM become more critical. To control these nonlinear effects, automatic
power provisioning (APP) of the amplifiers is required to control and maintain
channel launch powers below nonlinear thresholds. To maintain span distances
with the greater channel counts and with the requirement to maintain per-
channel launch power below nonlinear thresholds, greater sensitivity is required
in the receivers. This (change increased to greater) increased sensitivity has been
achieved through the introduction of out-of-band forward error correction (OOB
FEC) transponders. The 7-dB FEC gain, in fact, allows for enhanced span
distances, even with this increased capacity.
Until recently, the EDFA gain bandwidth was restricted to the so-called C-band, a
wavelength band of about 35 nm spanning from just below 1530 nm to just over
1560 nm. However, by optimizing the erbium fiber doping composition and fiber
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design and implementing an improved pumping scheme, it has been possible to
extend the gain bandwidth out past 1600 nm, the L-band.
The introduction of amplifiers for the L-band has allowed for increased system
capacity over the installed fiber plant. This additional bandwidth allows for
growth of up to 80 additional long-haul channels at 50-GHz spacing.
Alternatively, this bandwidth can be used with a hybrid of L-band EDFA
amplifiers and Raman amplification for extended-long-haul applications,
allowing greater reach between costly regeneration sites.
Topics
1. Error Correction, Threshold Control
2. Protection
3. Protocols and Packets
4. Provisioning
5. Provisioning Services
6. Summary
Self-Test
Correct Answers
Glossary
1. Error Correction, Threshold Control
Transmission fiber dispersion, fiber nonlinear effects, and amplifier noise limit
the number of channels and the unregenerated transmission distance of DWDM
systems. These factors can be overcome with OOB FEC transponders to enable a
70 percent increase in the number of channels or a 60 percent increase in the
transmission distance. Additionally, the OOB FEC allows an improvement in the
quality of service (QoS) by guaranteeing a received data channel bit-error rate
(BER) of better than 1.0E—15 OOB FEC coding relies on Reed-Solomon
algorithms to add redundancy bits to the data stream, enabling the identification
and correction of corrupted data bits. These redundant bits take the optical
carrier (OC)–192 data rate from 9.953 Gbps to 10.663 Gbps and yield a 7-dB
improvement in optical signal-to-noise ratio (OSNR) margin compared to non-
FEC transmission. This 7-dB OSNR improvement allows for the improved
channel capacity, transmission distance, and QoS.
To further enhance performance, the 10-Gbps OOB FEC transponders utilize
optimized threshold crossing control in the receiver side of the transponder to set
the decision circuit threshold to the in the received data "eye." When multiple
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traces of the data stream are superimposed on top of each other, the 0s and 1s
form an "eye." The more open the eye, the more reliably the 0s and 1s will be
detected and the better the BER. However, amplitude noise from the EDFA
amplifiers and electronics, phase noise, dispersion effects, and interference
resulting from conversion of phase into amplitude modulation start to close the
eye. As the eye closes, the decision circuit that determines if a bit is a 0 or 1 gives
fewer bit errors if the decision threshold level can adaptively change to the
optimum level. The optical receiver of the OOB FEC line extender modules
(LEMs) and receive transponders (RXTs) feature adaptive threshold crossing
control driven by the number of errored 0s and 1s determined in the bit stream.
The result is improved receiver sensitivity and a resultant improvement in BER
performance.
2. Protection
As mentioned previously, traditional networks have been optimized for voice
traffic, from both transport and protection levels. Many network topologies exist,
from point-to-point, ring, and hub-and-spoke to fully meshed networks.
Meshed networks fall outside the common Telcordia specified protection
schemes of Bidirectional Line-Switched Ring (BLSR) and Universal Path-
Switched Ring (UPSR). As a result, legacy SONET equipment manufacturers
have not offered viable solutions for meshed networks. With its path-protected
meshed network (PPMN) capability, Cisco has extended the simple concept of
path protection on a SONET ring to meshed networks, offering service providers
a new degree of flexibility in designing their networks.
Meshed Networks
"Meshed networks" refers to any number of sites arbitrarily connected together
with at least one loop. For this discussion, the connections between sites are
SONET, at various line rates. Sites within the meshed network that can be
reached from other sites through at least two distinct routes form the mesh,
whereas the remaining sites are spurs off of this mesh. Meshed networks are
often large rings with numerous sub-rings, as shown in
Figure 1
.
Figure 1. Sample Meshed Network
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