, with their exact position denoted by the H1 and H2 pointers that were described earlier. The first level of buildup of the STS-1 signal is obtained by byte interleaving of three such signals. This action results in the creation of an STS-3 frame, which can be viewed as a matrix of 9 rows and 270 columns. Similar to an STS-1 frame, an STS-3 frame contains section and line overhead bytes. While an engineer or communications carrier tech- nician will need information about the function of those bytes and the placement of low-order signals, such as DS1, DS1C, and DS2s, within an STS-1 signal, we will not go that deep into the signal structure. Instead, we will observe the general structure of an STS-N frame, which is shown in Figure 6.8. Note that N can be 1, 3, 12, 48, 192, or even 768 and provides a two-dimensional structure such that the sequence of N X 3 columns for 9 rows represents the transport overhead. Thus, when 3, there will be 9 columns (3 X 3) by 9 rows or a total of 81 available trans- port overhead bytes for section and line overhead. However, only a sub- set of the available transport overhead bytes are actually used in upper- hierarchy signals. Now that we have an appreciation for SONET, let's discuss its interna- tional counterpart, SDH. The SDH Hierarchy As we noted earlier in this chapter, the Synchronous Digital Hierarchy (SDH) represents an international standard that is very similar to that of SONET. The key difference between the two resides in the fact that SONET begins with an STS-1 signal operating at 51.840 Mbits/s while SDH commences with an STM-1 (Synchronous Transport Module 1) sig- nal which corresponds to the SONET STS-3 signal. In other words, SDH begins at 155.52 Mbits/s. Team-Fly® Fiber in the WAN N X 90 columns Figure 6.8 The general structure N X 3 of the STS-N frame. columns STS-N Envelope Capacity (SPE floats here) The SDH transmission hierarchy is listed in Table 6.2; however, one item in that table requires a bit of elaboration. That item is the STM-0 entry, since under SDH