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.
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.