To understand how multiplexing of SONET protocol works, one needs first to understand the notion of the width of a bit of data in time.

If we assume that the bandwidth (i.e., data rate) of a data link is 1Mbps, it means that this link can transmit $10^6$ bits of data in a second. If we imagine the length of 1 second as a ruler, then the length of 1 bit on that ruler is $\frac {1}{10^6}s$, or $1\mu s$. The more sophisticated is the capacity of the link, the narrower each bit becomes. For a $x$-Mbps link, the length of 1 bit data is $\frac{1}{x} \mu s$. Therefore for data of $y$ byte in size, its length in time is:

$\frac{8y}{x}\mu s$

A given SONET link runs at one of a finite set of possible rates, ranging from 51.84Mbps (STS-1) to 39,813,120Mbps (STS-768). Note that all of these rates are integer multiples of STS-1.

SONET data frame can contain frames of low-rate channels.

Each frame of SONET has a fixed length: $125 \mu s$. Therefore for STS-1, which has a bandwidth of 51.84Mbps, the size of the frame is:

$125 \times 51.84 / 8 = 810 \text{bytes}$

While for STS-n, the size of the frame is $810 * n$ bytes, Meaning that n STS-1 frames fit exactly in a single STS-n frame. The bytes from these frames are interleaved, so that the bytes in each STS-1 frame are evenly paced as they show up at the receiver at a smooth 51 Mbps speed.

The payloads from STS-1 are concatenated to form a larger STS-N payload, dentoed STS-Nc.