During the “telecom bubble” era of the late 1990s early 2000s, there were high hopes and speculation that all-optical networks would quickly become prevalent. Many envisioned a relatively simple backbone networks where client signals were optically (wavelength division) multiplexed and switched without the core optical network elements having to do any electrical (and hence client signal dependent) processing of the client signals. In many ways, this appeared to be the ultimate integrated network. In response, the ITU-T Study Group 15 (SG15) developed a series of Optical Transport Network (OTN) standards for wavelength division multiplexed (WDM) networks that covered the physical layer, signal rate and format specification, and equipment functional requirements.
Three factors slowed the initial adoption of OTN. First, carriers had huge capital investments in their existing SONET/SDH networks and lacked money to replace or over-lay them with a new network layer and its associated new network management systems. Second, a number of SONET/SDH-based proprietary WDM solutions had already been developed that, while not ideal, were adequately serving the needs of many carriers. In fact, the ITU-T Rec. G.709 standard discussed in this white paper is very similar to SONET/SDH in many ways. Third, carriers had only recently seen bandwidth demand rise beyond what was offered by the combination of the existing WDM equipment and the large amount of fiber deployed in the backbone networks.
Since the mid 2000s, however, compelling reasons to deploy OTN have emerged worldwide, thus making OTN a fundamental component of carrier RFPs for optical metro network equipment. Initially, the most compelling application for OTN was point-to-point links where the enhanced forward error correction (FEC) capability standardized for OTN allowed longer spans of optical cable, higher data rates, or both. Today, OTN is being demanded by carriers worldwide as not just a point-to-point technology but as an entirely new network layer to transition away from SONET/SDH and enable “video-ready” metro optical networks for high bandwidth service delivery to subscribers over broadband access networks. OTN enables carriers to fully utilize each wavelength by building transparent, scalable and cost-optimized networks where client traffic like video and Ethernet is mapped into OTN at the edge of the transport network. In this model, SONET/SDH becomes another client. Another important application is providing cost-effective wide area network (WAN) connectivity for enterprise Ethernet and storage area network (SAN) signals.
This white paper provides an overview of the OTN standards, with primary focus on ITU-T G.709. This second issue of the white paper includes the new material that was incorporated into the 2009 revision of G.709, its subsequent first two amendments, and a brief description of the new G.7044 standard on hitless resizing of ODUflex(GFP) signals.
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