th level, which provides an additional level of switching capability in comparison to a path-switching ring. To accomplish this, one pair of fiber is idle but available for instantaneous utilization. A second difference between path and line-sharing rings concerns the number of fibers supported by each topology. A path-sharing ring sup- ports two fibers and simply selects the best signal. A line-switching ring supports the use of two and four fibers. When a two-fiber line-switched ring is employed, only a single pair is used to form the ring. In this con- figuration the capacity of the ring is partitioned into two equal portions, with each treated as if it were a separate line, and line-switching occurs on a path basis. However, when four fibers are used, it becomes possible to compensate for multiple fiber failures and switch on a line basis. Now that we understand the general topology supported by SONET and SDH, we will discuss WDM and DWDM operations in the WAN Wavelength Division Multiplexing (WDM) and Dense WDM (DWDM) In the previous section we became acquainted with SONET and SDH operation. In that section we noted that both technologies use time divi- sion multiplexing on a byte interleaving basis to transport circuit- switched voice and leased-line services in the form of T- and E-carrier facilities. Because SONET and SDH are limited to the static mapping of T- and E-carrier transmission facilities, they are not well suited for transporting bursty traffic, such as LAN-to-LAN transmission. For example, the con- nection of two 10BASE-T Ethernet LANs via a WAN may on average be satisfied by the use of a T1 line operating at 1.544 Mbits/s, which can be easily accommodated within a SONET or SDH transport. However, if the customer wanted the ability to burst transmission to 10 Mbits/s, the operator of the SONET or SDH facility would need to map the Ether- net signal to the next largest payload in the optical transport, which would be the 51.8-Mbit/s STS-1 frame in SONET. To illustrate how inefficient SONET coul