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.

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 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 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.