 data stream from one telephone subscriber or computer to another, a failure only impacts one customer. Because of the large volumes transmitted by fiber, failures in these networks can put hundreds of businesses, police stations, or hospitals out of service. For this reason, the majority of carriers deploy bidirectional ring topology in long-haul and Metropolitan Area Networks (MANs), where each fiber ring, multiplexer, and power supply is duplicated. Ring topology is costly because the fiber and the multiplexers are all duplicated, even though this combined capacity is not used on a day-to-day basis when there are no failures. The spare fiber in ring topology is known as the protect ring. It reroutes traffic in the other direction, as is illustrated in Figure 4-5. Read the section "Mesh Configuration Backups," later in this chapter, to learn about new, lower-cost mesh technologies that are being deployed. Programmable www.TechnicalPDF.com Protection path Ring topolgy One primary path One pre-provisioned protection Wasted capacity Limited protection against multiple failures www TechicaIDDECOm Egress Alternate Ingress Routes with outages Resilient mesh Automated provisioning Dynamic path choices for multiple failures High network availability Bandwitth optimization Provisioning refers planning design installation network equipment routes. www.TechnicalPDF.com Figure 4-5 A comparison of ring to mesh topology. (Adapted figure from Ciena Corporation) OPTICAL TRANSPORT NETWORKS-CARRYING MULTIPLE TYPES OF TRAFFIC Optical Transport Network (OTN) is an International Telecommunications Union (ITU) G.709 standard for transmitting, monitoring, and managing traffic on fiber- optic cabling. OTN is an OSI (Open Systems Interconnections) Layer 1 service. As described in Chapter 1, "Computing and Enabling Technologies," Layer 1 is the physical and wireless medium over which traffic travels. OTN is used for asynchronous protocols such as Gigabit Ethernet as well as synchronous protocols such as Synchronous Optical Network (SONE