SONET, which stands for Synchronous Optical Networking is a protocol designed to transfer digital data over fiber optic channels. It finds its application in communication systems where data needs to be transmitted at a high speed, like telephone. SONET protocol transmits data that are synchronized in time.  A main advantage of SONET is that it can bind signals from different protocols and transmit the combined resultant signal through its channel. This protocol is mainly designed for use in North America and Japan. SONET has a base transmission rate of 51.84Mbit/s with speeds up to 2488.32Mbit/s.

STM stands for Synchronous Transport Module. STM-1 is the base signal of the Synchronous Digital Hierarchy (SDH) protocol. SDH is a synchronous transmission protocol used in high speed communication systems and is primarily designed for use in Europe. The base transmission rate of SDH protocol is 155.52Mbit/s and this transmission rate is designated as STM-1. STM-1 is the framing unit of SDH.

Synchronous Transport Signal (STS) is the base signal of SONET protocol. SONET has an initial transmission rate of 51.84Mbit/s, which is designated as STS-1 level. SDH has a lowest bit rate of 155.52 Mbps which is three times that of the lowest bit rate of SONET. Hence STM-1 signal of SDH protocol is equivalent to the STS-3 signal of SONET. This STS-3 signal is obtained by multiplexing 3 STS-1 signals.

SONET protocol mainly comprises of four layers. These layers are as follows-photonic layer, section layer, line layer and path layer. Each layer can communicate with the layers directly connected to it. This ensures smooth flow of data between the layers. SDH protocol has the same layers as SONET but uses the terms path, multiplex section and regen section to describe the layers.

Photonic layer is concerned with the conversion of the signal from electrical to optical format. The section layer takes care of framing, scrambling and detection of errors in the signal to be transmitted. Line layer takes care of multiplexing and synchronization functions while the path layer is concerned with end-to-end transmission of the signal from the sending end to receiving end.

The data is being transmitted through the network in the form of frames. Each frame is divided into header and payload. Header consists of the overhead added at different layers and Payload contains the actual information to be transmitted through the network. Path layer overhead is added to the payload of the frame, whereas section and line layer overheads are added to the header.

A STS-1 frame has 810 octets arranged in 9 rows of 90 octets. They are divided such that there are 3 octets of overhead and 87 octets of payload and are transmitted. A STM-1 frame on the other hand can transmit data three times faster than STS-1 frame. It consists of 2430 octets arranged as 270 octets in 9 rows, with 9 octets of overhead and 261 octets of payload. It takes 125 microseconds to transmit both STS-1 and STM-1 frame.

Thus both SONET which uses STS signals and SDH which uses STM signals are protocols which are developed for providing interface for optical networks. They mainly differ in the transmission rates they offer-SDH can transmit data three times faster than SONET. Both protocols can support signals from different vendors and transmit them through the network, while efficiently managing the network. They monitor the performance of various devices connected to the network like multiplexers, repeaters etc. They can not only transport signals from different protocols but also have provisions whereby it can accommodate signals from upcoming services like Asynchronous Transport Module (ATM) and Broadband Integrated Services Digital Network (B-ISDN). Both have very slight differences between them and they are being widely used all over the world.