What is SONET?

This blog post introduces the term SONET and briefly defines the Basic STS-1 frame in its most simplist of terms.

Mapping/Multipllexing Lower-Speed ODUj Tributary Signals into a Higher-Speed ODUk Server Signal

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SONET is short for “Synchronous Optical NETwork.”

SONET is primarily a North American Standard.  The International (or the “Rest of the World”) version of SONET is called SDH, or Synchronous Digital Hierarchy.

SONET is an Optical Transport Standard that preceded OTN.  Just like OTN supports multiple data rates (e.g., OTU1, OTU2, etc.).  SONET supports a variety of rates, as I show below.

Table 1, List of Standard SONET and SDH Bit-Rates

SONET and SDH Bit Rate Table

Table 1 shows that the SONET/SDH Standards define bit rates up to almost 40Gbps.

This table also includes a column labeled STS-N Rates, where N can be integer numbers 1, 3, 12, 48, 192, and 768. We will talk a little bit about the STS-1 frame in this blog post. We will discuss STS-N frames and data rates in other blog posts.

What is the STS-1 Signal and Frame?

We will start this blog post by looking at the basic STS-1 Frame I show in Figure 1.

A Simple SONET STS-1 Frame with the Transport Overhead and Envelope Capacity shown

Figure 1, Illustration of the Basic STS-1 Frame (Simplified Drawing).

NOTE: STS stands for Synchronous Transport Signal.

The STS-1 frame consists of 9 rows and 90-byte columns (for a total of 810 bytes).

The frame repetition rate is 8000 frames/s.


90 bytes/row x 9 rows/frame x 8000 = 51.84Mbps (which matches the STS-1 data rate in Table 1).

Of these 810 bytes, we refer to 27 bytes as the Transport Overhead (TOH) bytes

We call the remaining 783 bytes the “Envelope Capacity.” 

NOTE: The Envelope Capacity (for an STS-1 frame) is large enough to carry the following:

  • 1 DS3 Signal
  • 28 DS1 Signals
  • 21 E1 Signals

Whenever we transport an STS-1 signal over a copper medium (such as a coaxial cable), we sometimes call this signal an EC-1 (Electrical Carrier – 1) signal. Likewise, whenever we transport an STS-1 signal over optical fiber, we can call it an OC-1 (Optical Carrier 1) signal

In other blog posts, we will show that we can further subdivide the TOH (Transport Overhead) within the STS-1 frame into the Section Overhead (SOH) and the Line Overhead (LOH).

Figure 2 shows that the basic STS-1 frame consists of 3 basic portions.

Basic Structure of a SONET STS-1 Frame with Section Overhead (SOH) and Line Overhead (LOH)

Figure 2, The SOH (Section Overhead), LOH (Line Overhead), and the Envelope Capacity within an STS-1 Frame.

In other blog posts, I will discuss the SOH, LOH, and Envelope Capacity in greater detail. 

I also present the SONET Reference Model in another blog post.

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Author: Darrell Smith

Darrell Smith has more than 30 years of experience as an Electrical Engineer. He has about 20 years of experience as an Applications Engineer and the remainder of his time was spent in Hardware Design and Product Marketing. He will now be sharing his wealth of knowledge on this blog.

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