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SONET Tutorial: Basic Frame Structure

By K. Surya Prakash



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Basic SONET Frame Structure: 

          The basic SONET frame is as shown in figure 2. This signal is known as Synchronous Transport Signal Level-1 (STS-1). It consists of 9 rows of 90 bytes i.e. a total of 810 bytes. It is transmitted from left to right and top to bottom. The two dimensional figure is just for convenience. Actual transmission takes place serially i.e. the left most byte in the top row is transmitted, then the second byte in the first row and so on. After the 90th byte in the first row the left most byte in the second row is transmitted and it goes on. One more point to be noted is that msb is transmitted first and the numbering of  bits in a byte is as shown in figure-3. The frame length is 125s (i.e. 8000 frames per second). The STS-1 has a bit rate of 51.84Mbps. The frame for the lowest SDH rate STM-1 contains 270 columns by 9 rows. We will learn more about it later.






The first 3 columns of SONET frame are called Transport Overhead (TOH). The remaining 87 columns are called Synchronous Payload Envelope (SPE). The first column of SPE is called Payload Overhead (POH). A point to be noted here is that every SONET frame repeats every 125s no matter how fast the line speed gets. As line rate goes up SONET frame gets bigger, just sufficient to keep the frame rate at 8000 frames per second.

SONET and SDH Interleaving:

If three STS-1s are byte interleaved, the resulting frame is called STS-3. Byte interleaving means 1st byte of 1st STS-1 (called A1) is transmitted, then A1 byte of 2nd STS-1, then A1 byte of 3rd STS-1 and so on. Now the resultant rate of the frame would be 3 times more. This multiplexing is carried out for all levels of SONET and SDH. Because of this, SONET/SDH maintains a frame rate of 125s. See figure-4 in which different colors for bytes of each frame are  used to have clear understanding of multiplexing and how higher order frames are generated. Note that not all the overhead bytes in STS-1 frame are used in higher order frames. These are represented as X in the figure-4.






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