nd are sim- ilar to the STS's payload. Within an STS-1 frame, each VT occupies a num- Figure 3-10 SONET topological layers. SONET SONET Regenerator SONET Terminal or Switch Section Section Path (End-to-End) Traditional SONET Architectures ber of columns (see Figure 3-11). The VTs are grouped in distinct groups. Within the STS-1, a number of VT groups can be mixed together to form an STS-1 payload. To accommodate different mixes of VTs in a bandwidth- efficient manner, the STS-1 SPE is divided into seven groups. A VT group may contain one VT6, three VT2s, or four VT1.5s. A VT group contains only one size of VTs, but different kinds of VT groups can be mixed into one STS- 1 SPE (see Figure 3-12). Because multiplexing is synchronous, low-speed tributaries can be muxed together, but are still visible at higher rates. An individual VT containing a DS1 can be extracted without demultiplexing the entire STS-1.7 Pointers SONET uses pointers to compensate for frequency and phase variations. Pointers enable the transparent transport of SPEs (either STS or VT) across plesiochronous boundaries, that is, between nodes with sep- arate network clocks having almost the same timing. The pointer is an off- set value that points to the byte where the SPE begins. The use of pointers avoids the delays and loss of data associated with the use of large (125- microsecond frame) slip buffers for synchronization. Pointers provide a means of dynamically and flexibly phase-aligning STS and VT payloads, thereby permitting the ease of dropping, inserting, and crossconnecting these payloads in the network. Transmission signal wander and jitter can also be readily minimized with pointers. Figure 3-13 contains an STS-1 pointer that enables the SPE to be separated from the transport overhead. The diagram depicts a typical case of the SPE straddling over two STS-1 frames. If there are any frequency or phase variations between the STS-1 frame and its SPE, the pointer value will be increased or decreased accord- ingly to maintain synchronization.6 F