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What Is STM-1?

STM-1, or Synchronous Transport Module level 1, is a foundational building block of the SDH network, which transmits digital signals at 155.52 Mbps. It's crucial for seamless, high-speed telecommunications, ensuring data integrity and synchronization. Wondering how STM-1 impacts your daily internet experience and the evolution of digital communication? Let's explore its significance in our connected world.
Geisha A. Legazpi
Geisha A. Legazpi

Synchronous transport module level-1 (STM-1) is the designation of the second to the lowest level in the synchronous digital hierarchy (SDH). The STM-1 line rate is 155.520 megabits per second (Mbps), while the actual payload is 150.336 Mbps. The STM-0 is at the lowest SDH level, and the STM-0 line rate is 51.840 Mbps. Other SDH rates are STM-4, STM-16, and STM-64 with 4, 16, and 64 times the rate of STM-1, respectively. The US counterpart standard of SDH is the synchronous optical network (SONET), which is a fiber optic transmission standard, while the International Telecommunication Union’s (ITU) Telecommunication Standardization Sector (ITU-T) manages the SDH standard.

The payload rate is lower than the line rate by a difference known as the overhead rate. This is analogous to sending a package, and there is an overhead weight used for the packaging material. The actual payload is the item that is actually sent, while the total package weight equals the payload plus the overhead. The actual payload rate is important to the services that SDH will be supporting. Instead of setting focus on the line rate, focus is on the payload rate of the STM-1 due to the complexity of STM-1 as a whole.

Woman doing a handstand with a computer
Woman doing a handstand with a computer

A typical basic payload subrate is one voice channel. An ordinary voice channel is sampled 8,000 times per second. The given rate is a standard rate derived from the rule that if the maximum voice frequency of interest is 4,000 cycles per second, the sampling rate should be 8,000 times per second.

The voice channel level uses 8 bits for satisfactory reproduction. Eight digital bits will be capable of encoding a total of 256 levels, or 128 positive levels and 128 negative levels. There will be more samples in lower levels of voice than in higher levels. The latter is referred to as a companding algorithm.

The resulting rate for one voice channel is 64,000 bits per second (bps). An STM-1 is therefore able to carry 2,349 voice channels, and 2,349 is the payload rate 150,336 kilobytes per second (kbps)/64 kbps. By using multiplexers and demultiplexers, communication networks are able to utilize small clusters of subrate channels as needed.

For example, if a cellular phone service provider were to interconnect to other networks with 1,000 x 64 kbps channels for voice and another 1,000 x 64 kbps for data, a single STM-1 link would suffice. This results in a spare of 349 x 64 kbps channels. There are many ways to implement that same connection depending on predicted future needs and extent of existing channel-saving equipment, called transcoders.

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Discussion Comments


@MrMoody - I don’t think that the bandwidth mentioned in the article is even the maximum. I’ve heard that SONET can be boosted into multiple Gigabytes per second, which I suppose is what makes smooth transfer of video over the Internet possible.

I don’t know what technology they use to do the boosting but I know it can be done; I have a friend who worked in the telecommunications industry and he worked in their video on demand services department.


@David09 - I’m not in telecom myself, but I was surprised at how easily voice transported over the Internet, considering that voice calls from what I understand tend to be chopped up into little packets and then sent on their way.

I always wondered how those packets reached their destination safely and reassembled in the correct order.

It appears from the article that the line rate for STM-1 is 155.520 Mbps and the payload takes up the bulk of this capacity at 150.336 Mbps. This means there is a little left over, which I would guess is used for managing information about the packets to make sure they come together in the right order.


I worked in the telecommunications industry for ten years. While I was not an engineer myself, I knew the company was always looking for ways to slice and dice our capacity for maximum usage, whether we are talking about STM1 bandwidth or some other protocol.

One of the things that first surprised me was how little bandwidth that voice occupied. Voice is easily sampled and converted into digital bits, and it can be compressed and recompressed over long distances with no loss of data integrity.

The article is correct when it mentions how many voice channels you can pack into a single STM1 line. It’s quite a lot when you think about it, and this has made the phenomenon of voice over Internet protocol far easier to implement on a wide scale.

The only balancing act for us was to determine how much of our bandwidth we’d allocate to voice, and how much to data.

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