3, is aligned by column directly below the first path overhead byte. Similarly, the third path overhead byte is aligned under the second one, and SO on. OVERHEAD BYTES One of the key advantages of SONET and SDH as well as a key disadvantage of each is their structured hierarchy. The key advantages of the structured hierarchy facilitates OAM&P operations. Other advantages include the fact that an upgrade from one hierarchy to another as well as the multiplexing of lower layers into a higher layer are relatively simple time division multiplexing operations. Unfortunate- ly, the price of these advantages is in the overhead associated with the need for section, line, and path overhead bytes. As we noted earlier, each STS-1 frame of 810 bytes has only 774 bytes available for the actual pay- 90 columns Figure 6.6 Transport Positioning of the Overhead synchronous payload envelope (SPE) within H1 H2 H3 an STS-1 frame. 9 rows Frame 1 Overhead Start of STS-1 SPE Column STS-1 SPE H1 H2 H3 9 rows Frame 2 Fiber in the WAN load. For this reason, work commenced on transporting data directly over individual wavelengths provided by WDM and DWDM systems when this book was prepared. In the wonderful world of the Internet Protocol (IP), the technology being developed is referred to as IP over DWDM. Now that we have an appreciation for the advantages and disad- vantages associated with the use of the three types of STS-1 overhead bytes, let's see how those bytes are used. In SO doing, we will examine those bytes with respect to the area in which they are contained. Section Overhead Bytes The term section overhead bytes refers to the path between repeaters or other locations where the optical signal is transmit- ted or received. Figure 6.7 illustrates the basic positioning of the STS-1 overhead bytes associated with an STS-1 payload. Note that 9 bytes repre- sent section overhead, while 18 and 9 bytes represent line overhead and path overhead, respectively. Also note that while the section and line overhead bytes are in fixed position