Optical Network Design and Implementation

SONET Framing

The STS level 1 (STS-1) is the basic building block of SONET optical interfaces with a rate of 51.84 Mbps. SDH does not have an STS-1 equivalent, because the STM-1 frame is functionally equivalent to the STS-3c. The STS consists of two parts: the STS payload and the STS overhead used for signaling and protocol information. The term payload refers to the user data within the STS-N SONET frame. Payloads could be sub-STS-1 level, such as T1 or E1 payloads. As shown in Figure 5-5, the SONET envelope is the portion of the frame that carries the payload and path overhead. The POH and payload constitute the SPE. The SPE floats within the SONET envelope and could begin anywhere within the envelope from columns 4 to 90 of the STS-1. This ...