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Optical Networks - _6_2 Optical Transport Network_75

Optical Networks - _6_2 Optical Transport Network_75 - 6.2...

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6.2 Optical Transport Network 389 DS1 grooming DS0 grooming DS3 grooming STS-48 grooming Wavelength, waveband, fiber grooming Narrowband Wideband Broadband Optical All-optical SONET layer Optical layer Figure 6.9 Different types of crossconnect systems. traffic at the DS1 level. At the bottom of the hierarchy is a narrowband DCS, which grooms traffic at the DS0 level. Next up is a wideband DCS, which grooms traffic at DS1 rates, and then a broadband DCS, which grooms traffic at DS3/STS-1 rates. These DCSs typically have interfaces ranging from the grooming rate to much higher- speed interfaces. For instance, a wideband DCS will have interfaces ranging from DS1 to OC-12, while a broadband DCS will have interfaces ranging from DS3 to OC-768. There are also DCSs that groom at DS3 rates and above, with primarily high-speed optical interfaces. While such a box could be called broadband DCS, it is more commonly called an optical crossconnect. However, we also have other types of optical crossconnects that groom traffic at STS-48 rates, and yet others that use purely optical switch fabrics and groom traffic in units of wavelengths or more. Instead of having this hierarchy of crossconnect systems, why not have a single DCS with high-speed interfaces, which grooms at the lowest desired rate, say, DS0? This is not possible due to practical considerations of scalability, cost, and footprint. For instance, it is difficult to imagine building a crossconnect with hundreds to thousands of 10 Gb/s OC-192 ports that grooms down to the DS1 level. In general, the higher the speed of the desired interfaces on the crossconnect, the higher up it will reside in the grooming hierarchy of Figure 6.9. DCSs can also incorporate ADM functions and perform other network functions such as restoration against failures, the topic of Chapter 9. 6.2 Optical Transport Network The Optical Transport Network (OTN), sometimes referred to as G.709, was de- signed to transport data packet traffic such as IP and Ethernet over fiber optics, as well as legacy traffic and in particular SONET/SDH. It is called the digital wrapper
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390 Client Layers of the Optical Layer Table 6.4 OTN line rates compared with SONET/SDH line rates. OTN (G.709) Line Rates SONET/SDH Line Rates OTU1: 2.666 Gb/s STS-48/ STM-16: 2.488 Gb/s OTU2: 10.709 Gb/s STS-192/ STM-64: 9.953 Gb/s OTU3: 43.018 Gb/s STS-786/STM-128: 39.813 Gb/s technology because it wraps any client signal in overhead information for opera- tions, administration, and management. Its line rates, OTU1, OTU2, and OTU3, are shown in Table 6.4. It builds on SONET/SDH concepts, and it features the following capabilities. 1. Forward error correction (FEC): OTN has been designed for high data trans- mission rates, as shown in Table 6.4. At very high data rates or over very long distances, noise is significant and becomes a problem when ensuring low bit error rates. Forward error correction (FEC) as we discussed in Section 4.5 is critical to achieving these low bit error rates. FEC had already been used in implementations of SDH. These are proprietary coding schemes that rely on making use of unused section overhead bytes to carry the redundant FEC bytes.
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