XRT75R03D_06 Datasheet, PDF (97/135 Pages) Exar Corporation – EXAR DATA SHEET FORMAT TEMPLATES

XRT75R03D
REV. 1.0.4
THREE CHANNEL E3/DS3/STS-1 LINE
An STS-1 frame is a data-structure that consists of 810 bytes (or 6480 bits). A given STS-1 frame can be
viewed as being a 9 row by 90 byte column array (making up the 810 bytes). The frame-repetition rate (for an
STS-1 frame) is 8000 frames/second. Therefore, the bit-rate for an STS-1 signal is (6480 bits/frame * 8000
frames/sec =) 51.84Mbps.
A simple illustration of this SONET STS-1 frame is presented below in Figure 32.
FIGURE 32. A SIMPLE ILLUSTRATION OF THE SONET STS-1 FRAME
90 Bytes
9 Rows
STS-1 Frame (810 Bytes)
First Byte of the STS-1 Frame
Last Byte of the STS-1 Frame
Figure 32 indicates that the very first byte of a given STS-1 frame (to be transmitted or received) is located in
the extreme upper left hand corner of the 90 column by 9 row array, and that the very last byte of a given STS-
1 frame is located in the extreme lower right-hand corner of the frame structure. Whenever a Network Element
transmits a SONET STS-1 frame, it starts by transmitting all of the data, residing within the top row of the STS-
1 frame structure (beginning with the left-most byte, and then transmitting the very next byte, to the right). After
the Network Equipment has completed its transmission of the top or first row, it will then proceed to transmit
the second row of data (again starting with the left-most byte, first). Once the Network Equipment has
transmitted the last byte of a given STS-1 frame, it will proceed to start transmitting the very next STS-1 frame.
The illustration of the STS-1 frame (in Figure 32) is very simplistic, for multiple reasons. One major reason is
that the STS-1 frame consists of numerous types of bytes. For the sake of discussion within this data sheet,
the STS-1 frame will be described as consisting of the following types (or groups) of bytes.
• The Transport Overheads (or TOH) Bytes
• The Envelope Capacity Bytes
10.2.1.1.1
The Transport Overhead (TOH) Bytes
The Transport Overhead or TOH bytes occupy the very first three (3) byte columns within each STS-1 frame.
Figure 33 presents another simple illustration of an STS-1 frame structure. However, in this case, both the
TOH and the Envelope Capacity bytes are designated in this Figure.
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