STM-1 Frame Structure & Section Over head

STM-1 Frame Structure

 STM-1 frame contains 2430 bytes of information. Each byte contains 8 data bits (i.e. a 64kbit/s channel). Duration of STM-1 transport frame is 125ms. The number of frames per second is 1 second / 125ms = 8000 Frames per second.

So, rate of STM-1 frame is calculated as follows: -

8 bits x 2430 bytes x 8000 per second = 155,520,000 bits/s or 155 Mbit/s.

 STM-1  frame chopped up into 9 segments, stacked on top of each other as shown in the diagram below. The bits start at the top left with byte number one and are read from left to right and top to bottom. They are arranged as 270 columns across and 9 rows down.

STM-1 Section Overheads

      The STM-1 Section Overhead (SOH) consists of nine columns by nine rows as shown below. It forms the start of the STM-1 frame.The SOH contains control and status messages at the optical fibre level.

First three rows are RSOH ( Regenerator Section Overhead), Fourth row is AU-4 pointer. Fifth to Ninth row are MSOH ( Multiplexer section Overhead).

A1 & A2 - STM-1 Frame Alignment. These 6 bytes are used for STM-1 frame alignment. They are the first bytes transmitted. Frame alignment takes place over three STM-1 frames.

J0 - STM-1 Section Path Trace. This byte is used to provide a fixed length user configurable string, which can be used to verify network topology connections.

B1 - Byte Interleaved Parity Check 8 (BIP-8). This byte provides an error monitoring function for the entire STM-1 frame after encoding.

B2 - Byte Interleaved Parity Check 24 (BIP-24). These 3 bytes provide an error monitoring function for the STM-1 frame before encoding.A comparison between the BIP-8 and BIP-24 checks reveal if there were any encoding errors.

D1 to D12 - Data Communications Channel (DCC). These bytes provide a data channel for the use of network management systems.

K1 - Automatic protection Switching (APS). This byte is used to perform automatic protection switching of the optical fibre.

X - Reserved. These bytes are reserved for national use.

All unmarked bytes are reserved for future international standardisation.