t of the N STS-1s that make up the STS-Nc (Ref. 3). Figure 19.7 shows the transport overhead assignment of an OC-3 carrying an STS- 3c SPE. STS-3C Serial signal 125 us stream 155.52 Mbps 2430 bytes [19440 bits]/frame STS-3C 9 rows Transparent overhead Payload capacity = 149.76 Mbps Designed for tributaries > 50 Mbps 9 columns 260 columns 1 column Figure 19.6 STS-3c concatenated SPE. [Courtesy of Hewlett-Packard (Ref. 2).] 40C-N stands for optical carrier at the N level. This has the same electrical signal as STS-N and the same bit rate and format structure. ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS Transport Overhead (9 columns) Section Overhead Overhead = Undefined overhead byte (all-zeros pattern as an objective) = Concentration Indication H1* = 10010011 H2* = 11111111 Figure 19.7 Transport overhead assignment showing OC-3 carrying an STS-3c SPE. (Based on Ref. 1, Figure 3-8 and updated to current practice.) STS-1 Serial Signal 125 us Stream 51.64 Mbps 9 rows Transparent Overhead Virtual Tributary Frame STS-1 SPE Payload Capacity 86 columns Figure 19.8 The virtual tributary (VT) concept. (From Ref. 2, courtesy of Hewlett-Packard.) 19.2.2.4 Structure of Virtual Tributaries (VTs). The SONET STS-1 SPE with a channel capacity of 50.11 Mbps has been designed specifically to transport a DS3 tribu- tary signal. To accommodate sub-STS-1 rate payloads such as DS1, the VT structure is used. It consists of four sizes: VT1.5 (1.728 Mbps) for DS1 transport, VT2 (2.304 Mbps) for E1 transport, VT3 (3.456 Mbps) for DS1C transport, and VT6 (6.912 Mbps) for DS2 transport. The virtual tributary concept is illustrated in Figure 19.8. The four VT configu- rations are shown in Figure 19.9. In the 87-column by 9-row structure of the STS-1 SPE, the VTs occupy 3, 4, 6, and 12 columns, respectively. 19.2 SONET 3 Columns 4 Columns 6 Columns 12 Columns 9 Rows VT 1.5 1.728 Mbps 2.304 Mbps 3.456 Mbps 6.912 Mbps Optimized for Optimized for Optimized for Optimized for DS1 transport 2-Mbps transport DS1C transport DS2 transport Figure 19.9 