Metro optical networks channel traffic within the metro domain (e.g., inter-business, inter-office, metropolitan connectivity), as well as to and from large long-haul points of presence (POPs). To address the increasing customer demand for high-bandwidth applications, several key transport architectures have been deployed. Specific to this tutorial are the two main transport technologies: SONET/SDH and dense wavelength division multiplexing (DWDM).

Today's metro optical networks are facing a bottleneck due to the increasing demand for broadband services. Traditionally, carriers have addressed the bottleneck by scaling their existing SONET/SDH infrastructure. Despite the significant capital and operational investment, this approach typically yields only a four-fold increase in capacity (e.g., from optical carrier [OC]–12 to OC–48). A better alternative has been to apply WDM technology to the metro network.

Many metro WDM solutions are available today; however, all of them are either ring-centric architectures, based upon elements called optical add/drop multiplexers (OADMs), or simple point-to-point systems. For a carrier to deploy wavelength networking, these solutions require separate switching elements, such as SONET/SDH-centric optical cross-connects (OXCs). The problem with SONET/SDH OXCs is that they can be very costly, take up valuable power and space, and are not fully transparent. While SONET/SDH will continue to have its role in the network, it is becoming clear that other "Layer 1" technologies (e.g., Gigabit Ethernet and SAN interfaces) are also emerging in the metro. As a result, transparency will be a key attribute for the efficient networking and transport of multiple Layer-1 technologies. (See section 2, Metro Operational Requirements)

An OADX combines the efficiency of WDM transmission with the flexibility and transparency of a wavelength switch. The result is a cost- and space-efficient optical networking platform that allows existing services to grow while also enabling transparent wavelength services.

SONET/SDH With more than 150,000 SONET systems in the United States and more than 150,000 SDH systems in the rest of the world, SONET/SDH is the dominant metro transport and access technology. SONET is a standard for optical telecommunications transport and is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades. The increased configuration flexibility and bandwidth availability of SONET provides significant advantages over the older telecommunications system.1 DWDM DWDM is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed and transmitted on a single optical fiber. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered per second by the optical fiber. DWDM is also sometimes called WDM. Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. Each signal carried can be at a different rate (OC–3/12/48, etc.) and in a different format (SONET, asynchronous transfer mode [ATM], data, etc.).2

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¹IEC Web ProForums, "SONET" tutorial, www.iec.org/online/tutorials

²IEC Web ProForums, "DWDM" tutorials, www.iec.org/online/tutorials