s against an extended range of fault conditions compared to other BLSRs that support line analysis only. Path analysis also enables the use of SONET systems on the inter-ring link, permitting, for example, wide separation between inter- connecting gateways, as illustrated in Figure 3-60. Traditional SONET Architectures Primary Inter-Ring Gateways Figure 3-60 Interconnecting STS-1s Ring 2, remote secondary UPSR or BLSR gateways using a Collocated linear SONET system. Shelves Ring 1, UPSR or BLSR STS-1s Secondary Inter-Ring STS-1s Gateway (Ring 2) Secondary Inter-Ring Gateway (Ring 1) Linear System For many types of failure situations, end-to-end service is maintained through a combination of RPS and duplicate inter-ring gateways. In the event of a primary gateway failure, for instance, RPS bypasses the faulty node, and inter-ring services pass through the secondary gateway to the adjacent ring. 3.4.6 Wavelength Division Multiplexing (WDM) WDM multiplies the capacity of existing fiber spans by combining two or more optical signals of different wavelengths on a single fiber. Because it defers-or avoids altogether-the large capital outlays and long lead times associated with deploying new fiber cable, WDM is an ideal solution for crit- ical high-growth routes having an immediate need for more bandwidth. An external coupling device performs the actual mixing of the different optical signals. In unidirectional WDM, multiple wavelengths travel in the same direction on an optical fiber, while they pass in opposite directions in bidi- rectional WDM arrangements. Bidirectional WDM is often the preferred approach, especially in applications employing two wavelengths due to the one-to-one association of an individual transport system to an individual optical fiber. WDM can be further classified as either wideband (also called cross- band), narrowband, or dense depending on the wavelengths involved. Wideband WDM This WDM technique doubles the capacity of a fiber span by combining the 1,310-nm wavelength with a second waveleng