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TECHNOLOGY OPTIONS

l SDH/SONET: Synchronous digital hierarchy (SDH) and synchronous optical network (SONET) refer to a group of fiber-optic transmission rates that can transport digital signals of different capacities. Since their emergence from standards bodies around 1990, SDH and its variant, SONET, have helped revolutionize the performance and cost of telecommunications networks based on optical fibers. SDH has provided transmission networks with a vendor-independent and sophisticated signal structure that has a rich feature set. This has resulted in new network applications, the deployment of new equipment in new network topologies, and management by operations systems of much greater power than previously seen in transmission networks.

It was widely accepted that the new multiplexing method should be synchronous and based not on bit interleaving as was the PDH, but on byte interleaving, as are the multiplexing structures from 64 kbps to the primary rates of 1,544 kbps (1.5 Mbps) and 2,048 kbps (2 Mbps). By these means the new multiplexing method was to give a similar level of switching flexibility both above and below the primary rates (though most SDH products do not implement flexibility below primary rate). In addition, it was to have comprehensive management options to support new services and more centralized network control.

In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic–based transmission hierarchy. SONET, a fiber optic transmission system for high-speed digital traffic, is a North American standard. SONET is widely used in carrier networks to aggregate lower speed T1 and T3 lines and transport their traffic on self-healing ring architectures that have advanced network management and restoration capabilities. SONET is an intelligent system that provides advanced network management and a standard optical interface.

l Dense Wavelength Division Multiplexing (DWDM): It enables a single optical fiber to simultaneously carry multiple traffic-bearing signals, thereby increasing the capacity of fiber many times over. DWDM systems can support more than 150 wavelengths, each carrying up to 10 Gbps. Such systems provide more than a terabit per second of transmission on one optical strand. SDH and DWDM are complimentary in nature with DWDM at the backbone and SDH at the access.

l Coarse Wavelength Division Multiplexing (CWDM): It’s an older WDM technology and is emerging as a low cost alternative to DWDM especially in metro network access and enterprise applications. As they need to serve smaller bandwidth applications than DWDM systems, CWDM systems are characterized by wider channel spacing than DWDM optical networks.

The frequency separation between each individual color of light on the actual fiber is significantly further apart, which allows the system designers to use lasers that have looser tolerances on spectral width and thermal drift, therefore less expensive.

l Optical Ethernet: Two alternative architectures based on the successful fusion of optical and Ethernet technologies—collectively referred to as Optical Ethernet—have recently emerged to address the shortcomings of legacy SONET/SDH in today’s metro networks.

Purposely built for data transport, Ethernet over fiber and next generation SONET/SDH-based metropolitan-area networks (MANs) combine the familiarity and ubiquity of Ethernet networking with the speed of optical transport to overcome capacity bottlenecks and alleviate opex and capex constraints.

Optical Ethernet is the technology that extends Ethernet beyond the local-area network (LAN) and into MANs and wide-area networks (WANs). While Ethernet LANs are almost exclusively used within the enterprise, optical Ethernet technology can be used as a service provider offering. Optical Ethernet supports the delivery of a full suite of carrier class Ethernet services up to 1 Gbps. It also provides for integrated optical transport, switching, and statistical multiplexing to help reduce the number of devices and capital expenses in the MAN. Finally, optical Ethernet provides bandwidth-on-demand service-provisioning capabilities.

l Automatically-switched Optical Network/Intelligent Optical Network (ASON/ION): Legacy optical networks suffer from inflexibility and high operating expenses due to the inherent limitations built into network equipment and network management architectures. Provisioning an end-to-end connection is therefore a manually intensive, slow process that may take weeks or months to complete and, additionally, may also require coordination between several vendor-specific management systems.

The solution is to make the network itself smarter and automate these processes by adding intelligence. A possible first step is to upgrade the network management systems with more sophisticated provisioning capabilities. Such a centralized approach to intelligent networking, however, has limitations of both functionality and scalability.

The alternative approach, which is being widely endorsed by the industry, is to distribute intelligence to every network element and link the elements at the control plane level so that they communicate and provide bottom-up data and functions for management.

The network becomes the database of record rather than the management system. This is the approach being embodied now in industry standards efforts such as the ITU-T’s G.8080 (formerly G.ason) standards, the IETF’s GMPLS specifications and the OIF’s Optical UNI or user-network interface.