load envelope protection switching information line alarm indication signal (AIS) line far end receive failure (FERF) indication STS Path Overhead: performance monitoring of the STS SPE signal label (equipped or unequipped) path status path trace VT Path Overhead: performance monitoring (virtual tributary level) signal label (equipped or unequipped) path status pointer (depending on VT type) The STS-1 payload has the capacity to transport up to 28 DS1s 1 DS3 21 E1s (2.048-Mbps ITU-type signal) or combinations of the above Chapter 3 SONET carries substantial management information, expanding OAM&P capabilities compared with previous systems such as DS1 and DS3. The management information has several layers, as indicated in Fig- ure 3-10. Path-level overhead is carried end to end; it is added to DS1 sig- nals when they are mapped into VTs and for STS-1 payloads that travel end to end. Line overhead is used for the STS-n signal between STS-n multi- plexers. Section overhead is used for communications between adjacent net- work elements, such as regenerators. Transport overhead is comprised of section and line overhead. The STS-1 path overhead is part of the SPE. Enough information is contained in the overhead to allow the network to operate and enable OAM&P communications between an intelligent net- work controller and the individual nodes.6 Virtual Tributaries (VTs) A rather elaborate mechanism exists in SONET for it to be able to carry (multiplex) subrate lower-speed T1, E1, and T2 channels. This mechanism is called a virtual tributary (VT). Although this has limited interest in a discussion on broadband, it is worth a quick review because it has implications on the complexity/cost of SONET equipment. The STS-1 payload may be subdivided into VTs, which are syn- chronous signals used to transport signals, as follows: VT1.5 One DS1 (1.544 Mbps), carried in 1.728 Mbps One CETP 1 (2.048 Mbps), carried in 2.304 Mbps VT6 One DS2 (6.312 Mbps), carried in 6.912 Mbps VT path overhead and VT payloads comprise the VT's SPE a