Broadband

August 4, 2003
Trends in metro DWDM networks

INDIA -- The telecommunications industry in India is growing at a tremendous rate. It is also important to keep in mind that to successfully harness this growth, network operators in India are faced with unique challenges that sets them apart from carriers elsewhere in the world. Two of these challenges are the uncertain timing of growth in demand and the difficulty in accessing trenching rights within cities. A unique model for deploying metropolitan networks in India, which could provide an economical solution to these challenges, is the deployment of a hybrid cable with a mix of ITU G.652.C and G.655-compliant fibers. This solution prepares the network for possible high demand growth by using some G.655 fibers optimized for 1,550nm DWDM (dense wavelength division multiplexing) applications, minimising the need for re-trenching, all the while keeping the up-front investment manageable.

WDM and optical fiber advancements
Commonly referred to as ITU G.652 compliant fibers, the first single-mode fibers were designed for the 1,310nm window. They offered near-zero chromatic dispersion at 1,310nm with lower loss than existing copper solutions. Throughout the early adoption of optical networks, 1,310nm technologies experienced evolutionary change. This change could not keep up with network/carrier requirements resulting in fiber and transmission experts looking to the 1,550nm window to take advantage of the nearly 40 percent attenuation reduction available in the low-loss 1,550nm region of silica. In 1985, Corning addressed this opportunity by developing a fiber with low chromatic dispersion at 1,550nm: dispersion shifted fiber (or ITU G.653).

The development of erbium-doped fiber amplifiers (EDFA) for 1,550nm operation provided the ability to amplify light signals at the exact wavelengths where silica fiber has its lowest attenuation, enabling propagation in the optical domain for more than 500km. The development of erbium amplifiers, which can amplify multiple channels simultaneously, in addition to advances in optical filter technology, provided the capability to multiplex channels (or wavelengths) onto one fiber commonly referred to as WDM.

Since adjacent channels interact and create non-linear noise such as four-wave mixing (FWM) in a near-zero dispersion fiber, the move to WDM posed a challenge for G.653 fibers. The advent of WDM systems created the need for new fiber types that considered the impact of dispersion on non-linear inter-channel cross-talk penalties in the 1,550nm window. Fiber designers shifted the zero dispersion point out of the operating band, ensuring that sufficient chromatic dispersion would be present to prevent non-linear penalties. This led to the invention of non-zero dispersion shifted fiber (NZ-DSF or ITU G.655).

Evolution of metro networks
Legacy metropolitan networks rely heavily on operation at 1,310nm. Consequently, this typically led to deployment of standard, single-mode type fibers. Over the past few years, many network operators have looked to WDM applications to increase network capacity. Based on a variety of technology choices, these applications require careful consideration while evaluating optical fiber type. WDM applications present several solutions to meet network requirements (see Fig. 1). WDM, coarse WDM (CWDM) and dense WDM (DWDM) aggregate more channels onto a single fiber, optimizing the fiber's efficiency.

The network's optical topology can also evolve from a point-to-point solution through ring architecture, finally arriving at a mesh network. Each topology provides efficient use of electronics in the network while reducing the overall amount of O-E-O (optical-to-electrical-to-optical) sites. The need to increase service and efficient use of equipment for providing the lowest cost per bit to the carrier and customer has always driven WDM network evolution.

As bandwidth service and distance demands continue to rise adding pressure to metro networks, carriers are increasingly looking to the 1,550nm window and DWDM for relief. ITU G.655 compliant NZ-DSFs, such as Corning's LEAF optical fiber offer excellent, proven performance in the 1,550nm region. Offering chromatic dispersion that is one-fourth of standard single-mode fibers, LEAF fiber delivers longer reach with respect to dispersion penalties, especially at higher data rates. Fig. 2 demonstrates this dispersion reach advantage over standard single-mode fibers. Longer dispersion reach eliminates the need for costly electronic regeneration or dispersion compensation in metro WDM applications, making it an attractive alternative when considering network design.

For a metropolitan or regional network operator, the ability to fully exploit existing and emerging technology trends is critical. This becomes more evident as metropolitan networks grow in service offerings, bandwidth, and distance. Typical metropolitan networks are currently in the 75-150 km range. Such distances can technologically stress standard, single-mode (or G.652) networks. Fig. 3 compares what standard single-mode fiber, low water peak standard single-mode fiber (G.652.C) and NZ-DSF (G.655) deliver today.

For most legacy technologies on shorter optical path networks, standard single-mode fibers offer a suitable solution. However, legacy standard single-mode fibers are not optimized for some WDM applications due to the high attenuation around the water peak region. ITU G.652.C-compliant fibers, such as Corning's SMF-28e fiber, are optimized for networks where transmission occurs across a broad range of wavelengths from 1,285nm to 1,625nm.
While G.652.C compliant fibers offer excellent capabilities for shorter, unamplified metro and access networks, they do not fully address the needs for 1,550nm transmission. As Fig. 3 clearly points out, an attractive option that delivers the best of both technologies is installing composite cables containing both standard single-mode (ITU G.652.C) and non-zero dispersion shifted fibers (ITU G.655). Commonly referred to as "hybrid cables," this option provides network planners the flexibility to take advantage of the current technologies, while ensuring their networks are best suited to take advantage of WDM when network demands warrant its use.

Dependent on several demand criteria, metropolitan networks pose an array of technology choices for carriers. In isolation, single- and multi-channel solutions can address the aforementioned criteria. Part of that solution is WDM, allowing efficient use of optical fiber and electronics and the ability to provide a low-cost alternative to the carrier. The combination of ITU G.652.C and G.655 fibers in a single cable optimises the 1,310nm and full-spectrum capabilities of today, while enabling the DWDM platform solution for the carrier's evolving metropolitan network.

For inquires or questions regarding this article, please contact Joel Orban, product line manager, High Data Rate Products, at orbanja@corning.com, Sandeep Sharma, sales engineering manager, India at sharmasa@corning.com or Michael Kunigonis, market development manager, Asia at kunigonimp@corning.com