Optical Network Design and Implementation

Virtual Tributaries

The STS SPE can be subdivided into smaller structures, known as virtual tributaries (VTs). VTs are used for transporting and switching payloads smaller than the STS-1 rate. All services below the DS3, such as DS1 signals, are transported in the VT structure. The various sub-DS3 TDM payloads that can be carried over VTs are shown in Table 5-6. An individual VT containing a DS1 can be added or dropped without multiplexing or demultiplexing the entire STS-1. This improved accessibility improves switching and grooming at VT or STS levels. The STS-1 payload can be subdivided into VTs that are synchronous signals used to transport lower-speed transmissions. There are four sizes of VTs: VT1.5 (1.728 Mbps), VT2 (2.304 Mbps), VT3 (3.456 ...