#  [NetWork] Multiplexing of SONET Protocol

By [Aulee](https://paragraph.com/@aulee) · 2024-01-19

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Preparation: How long is a bit of data, in time?
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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:

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

How does SONET multiplex low-speed links onto one high-speed link?
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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.

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*Originally published on [Aulee](https://paragraph.com/@aulee/network-multiplexing-of-sonet-protocol)*