ayload itself, it travels closely with the payload. The first byte of the payload is in fact the first byte of the Path Overhead. A word about nomenclature: Two distinct terms are used, often (incor- rectly) interchangeably. The terms are synchronous transport signal (STS) and optical carrier level (OC). They are used interchangeably because, while an STS-1 and an OC-1 are both 51.84 Mbps signals, one is an electrically framed signal (STS) but the other describes an optical sig- nal (OC). Keep in mind that the signals that SONET transports usually originate at an electrical source such as a T1. These data must be collected and multiplexed at an electrical level before being handed over to the opti- cal transport system. The optical networking part of the SONET system speaks in terms of OC. The SONET frame is transmitted serially on a row- by-row basis. The SONET multiplexer transmits (and therefore receives!) the first byte of row one, all the way to the ninetieth byte of row one, then wraps to transmit the first byte of row two, all the way to the ninetieth byte of row two, and so on, until all 810 bytes have been transmitted. Because the rows are transmitted serially, the many overhead bytes do not all appear at the beginning of the transmission of the frame- instead, they are peppered along the bitstream like highway markers. For example, the first two bytes of overhead in the Section Overhead are the framing bytes, followed by the single-byte signal identifier. The next 87 bytes are user payload, followed by the next byte of section overhead -in other words, there are 87 bytes of user data between the first three Section Overhead bytes and the next one! The designers of SONET were clearly thinking the day they came up with this, because each byte of data appears just when it is needed. Truly a remarkable thing! Because of the unique way that the user's data is mapped into the SONET frame, the data can actually start pretty much anywhere in the payload envelope. The payload is always the same number of bytes, 