in within a customer's router). The original Protocol Data Unit (PDU), including a Layer 2 header and Layer 2 payload (ATM header/payload), which in turn includes a Layer 3 header and Layer 3 payload (Internet Pro- tocol [IP] header/payload), is carried from SONET node to SONET node inside the SONET envelope. Multiple SONET links may be involved in one data link layer (DLL) link; in turn, multiple DLL links are involved in an end-to-end path (for example, an IP route). Because a standard SONET/SDH ring is limited to transmitting one wavelength (for example, 1,310 mm), there are capacity limitations in terms of what you can achieve at the fiber level. Each time the capacity of a stan- dard ring is filled, the service provider is required to lay more fiber and add a set of SONET/SDH devices. This is costly in long-haul applications. For metro access and metro core applications, the jury is still out on the advan- tages of highly packed wavelengths (such as 120); however, lower-density Traditional SONET Architectures systems (such as 8, 16, or 32) may be appropriate in the metro core space, and 2 to 4 wavelengths may be fine for metro access applications, if the cost of the devices is within specific ranges. In the metro network case, there a traditional transmission cost versus multiplexing cost trade-off for the architecture. In long haul, the transmission cost is high; therefore, multi- plexing costs are easily justified. In metro applications, the transmission cost could be bounded so the cost of electronics supporting multiplexing, particularly if this is high, may or may not always be justified. 3.1.2 Second-Generation Optical Networks The second generation of optical networking brought the introduction of DWDM technologies to the existing SONET ring systems. In this architec- ture, as seen in Figure 3-2, DWDM terminals are inserted into segments of fiber rings where more than just single wavelength capacity is required. By multiplying up to 80 channels in the second generation and more in the third, ring capa