FAQ's & Guides

Synchronous Transport Module (STM) in Telecommunications

November 21, 2018



In order to understand exactly what STM is and how it works, it is important to know some information regarding the background as to why it was first developed.


As telecommunications networks developed and bandwidth demand increased, a technology called PDH (Plesiochronous Digital Hierarchy) was developed. This technology joined the basic primary multiplexer 2.048Mb/s trunks together by adding bits (bit stuffing), which synchronised the trunks at each level of the PDH. 2.048Mb/s was named ‘E1’ and the hierarchy was based on 4 E1’s:

  • E1 = 2.048Mb/s
  • E2 = 4 x E1 (8Mb/s)
  • E3 = 4 x E2 (34Mb/s)
  • E4 = 4 x E3 (140Mb/s)
  • E5 = 4 x E4 (565Mb/s)

E1 Frame Structure

An E1 link operates over two separate sets of wires, usually unshielded twisted pair or coaxial cable. The line data rate (2.048Mb/s) is split into 32 timeslots, each of which is allocated 8 bits, resulting in each timeslot sending and receiving an 8-bit PCM (Pulse-Code Modulation) sample. These 8-bit samples are usually encoded using A-law algorithm, which is 8,000 times per second (8 x 8,000 x 32 timeslots = 2,048,000). The timeslots are numbered from 0 to 31.

Time-slot 0 is used for transmission management and time-slot 16 is used for signalling. The rest are for voice/data transport.


As management of PDH is very inflexible, SDH was developed to allow for extended features and capabilities, including improved management performance, optical interface as standard and bandwidth on demand (you can read more about SDH here). In the same way as PDH, SDH is also made up of multiple E1’s.

  • STM-1 = 63 x E1 (155Mb/s)
  • STM-4 = 4 x STM-1 (622Mb/s)
  • STM-16 = 4 x STM-4 (2.5Gb/s)

STM Frame Structure

An STM-1 frame structure consists of 9 rows and 270 columns of bytes, totalling 2,430 bytes. The byte-orientated arrangement has a bit-rate of 155.52Mb/s. The frame is transmitted at 125 µs (microseconds), resulting in 8,000 frames per second on the circuit.

The frame of STM-1 consists of payload blocks, overhead blocks and pointers. The ratio of these components can vary and depends on the initial payload that needs to be transmitted.

The last 261 columns of the frame provide the information payload, the first 9 contain the overhead and administrative pointers.  The administrative pointers can contain one or more virtual containers which have the path overhead or the virtual container payload information.

An STM-1 signal is divided into two categories: the regenerator section overhead and the multiplex section overhead.

Regenerator Section Overhead (RSOH)

The Regenerator Section Overhead uses the initial three rows and nine columns of the STM-1 frame. It monitors the network sections that present on the fiber-optic cable network.

Multiplex Section Overhead (MSOH)

The Multiplex Section Overhead uses the rows 5 to 9 and the first 9 columns of the STM-1 frame. It contains the information that allows data packets to be transmitted on the same network compared to other data packets.

Both overheads provide information on the transmission system and its management function, such as failure detection, service channels and monitoring of transmission quality.


At Carritech, we supply and repair a wide range of products that utilise Synchronous Transport Modules for data transmission. For more information about specific products and how they use STM view our products or contact us at


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