ed by the TU-n pointer. (a) STM-1 with one AU-4 (b) STM-1 with three AU-3s AU-n pointer AU-n AU-n pointer + VC-n Figure 19.22 Administrative units in an STM-1 frame. [From ITU-T Rec. Q.708, Figure 3-2/G.708 (Ref. 6).] ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS = 1, 2, 3 (a) STM-1 with one AU-4 containing TUs (b) STM-1 with three AU-3s containing TUs AU-n pointer TU-n pointer AU-n AU-n pointer + VC-n TU-n TU-n pointer + VC-n Figure 19.23 Two-stage multiplex. [From TU-TG.708, Figure 3-3/G.708 (Ref. 6).] The AU-3 may be used to carry, via the VC-3, a number of TU-ns (n = 1, 2) form- ing a two-stage multiplex. An example of this arrangement is shown in Figure 19.22 and 19.23b. The VC-n associated with each TU-n does not have a fixed phase relation- ship with respect to the start of the VC-3. The TU-n pointer is in a fixed location in the VC-3, and the location of the first octet of the VC-n is indicated by the TU-n pointer (Ref. 7). 19.3.3.3 Interconnection of STM-1s. SDH has been designed to be universal, allow- ing transport of a large variety of signals including those specified in ITU-T Rec. G.702 (Ref. 8), such as the North American DS1 hierarchy and the European E-1 hierarchy. However, different structures can be employed for the transport of virtual containers. The following interconnection rules are used. 1. The rule for interconnecting two AUGs based on two different types of adminis- trative unit, namely, AU-4 and AU-3, is to use the AU-4 structure. Therefore the AUG based on AU-3 is demultiplexed to the TUG-2 or VC-3 level according to the type of payload and is remultiplexed within an AUG via the TUG-3/VC-4/AU- 4 route. 2. The rule for interconnecting VC-11s transported via different types of tributary unit, namely, TU-11 and TU-12, is to use the TU-11 structure. VC-11, TU-11, and TU-12 are described in ITU-T Rec. G.709 (Ref. 7). REVIEW EXERCISES What was the principal driving force for the development of SONET? 2. Being that the frame period of SONET/SDH is 125 usec, what service are these formats opti