SONET Basics - STS level and OC Specificaitons

SONET and SDH are technically comparable standards.

SONET defines optical signals and a synchronous frame structure for multiplexed digital traffic. It is a set of standards that define the rates and formats for optical networks specified in ANSI T1.105, ANSI T1.106, and ANSI T1.117.

Synchronous Digital Hierarchy (SDH), is used in Europe by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). SONET equipment is generally used in North America, and SDH equipment is generally accepted everywhere else in the world.


Both SONET and SDH are based on a structure that has a basic frame format and speed. The frame format used by SONET is the Synchronous Transport Signal (STS), with STS-1 as the base-level signal at 51.84 Mbps. An STS-1 frame can be carried in an OC-1 signal. The frame format used by SDH is the Synchronous Transport Module (STM), with STM-1 as the base-level signal at 155.52Mbps. An STM-1 frame can be carried in an OC-3 signal.

Both SONET and SDH have a hierarchy of signaling speeds. Multiple lower-level signals can be multiplexed to form higher-level signals. For example, three STS-1 signals can be multiplexed together to form an STS-3 signal, and four STM-1 signals multiplexed together to form an STM-4 signal.

TThe OC specification is determined by the STS level. The STS level is the rate at which SONET can multiplex multiple sources of data to a single fiber optic line. OC specifications are measured in multiples of 3, with a base of 1.

STS Level	OC Specification	Data Rate (Mbps)
1	OC-1	51.84
3	OC-3	155.52
9	OC-9	466.56
12	OC-12	622.08
18	OC-18	933.12
24	OC-24	1244.16
36	OC-36	1866.23
48	OC-48	2488.32
96	OC-96	4976.64
192	OC-192	9953.28

SONET Framing

A standard STS-1 frame is nine rows by 90 bytes. The first three bytes of each row represent the Section and Line overhead. These overhead bits comprise framing bits and pointers to different parts of the SONET frame.
There is one column of bytes in the payload that represents the STS path overhead. This column frequently "floats" throughout the frame. Its location in the frame is determined by a pointer in the Section and Line overhead.
The combination of the Section and Line overhead comprises the transport overhead, and the remainder is the SPE.
For STS-1, a single SONET frame is transmitted in 125 microseconds, or 8000 frames per second. 8000 fps * 810 B/frame = 51.84 Mbs, of which the payload is roughly 49.5 Mbs, enough to encapsulate 28 DS-1s, a full DS-3, or 21 CEPT-1s.
An STS-3 is very similar to STS-3c. The frame is nine rows by 270 bytes. The first nine columns contain the transport overhead section, and the rest is SPE. For both STS-3 and STS-3c, the transport overhead (Line and Section) is the same.
For an STS-3 frame, the SPE contains three separate payloads and three separate path overhead fields. In essence, it is the SPE of three separate STS-1s packed together, one after another.
In STS-3c, there is only one path overhead field for the entire SPE. The SPE for an STS-3c is a much larger version of a single STS-1 SPE.
STM-1 is the SDH (non-North American) equivalent of a SONET (North American) STS-3 frame (STS-3c to be exact). For STM-1, a single SDH frame is also transmitted in 125 microseconds, but the frame is 270 bytes long by nine rows wide, or 155.52 Mbs, with a nine-byte header for each row. The nine-byte header contains the Multiplexer and Regenerator overhead. This is nearly identical to the STS-3c Line and Section overhead. In fact, this is where the SDH and SONET standards differ.

SONET Transport Hierarchy

Each level of the hierarchy terminates its corresponding fields in the SONET payload, as such:

Section

A section is a single fiber run that can be terminated by a network element (Line or Path) or an optical regenerator.
The main function of the section layer is to properly format the SONET frames, and to convert the electrical signals to optical signals. Section Terminating Equipment (STE) can originate, access, modify, or terminate the section header overhead. (A standard STS-1 frame is nine rows by 90 bytes. The first three bytes of each row comprise the Section and Line header overhead.)

Line

Line-Terminating Equipment (LTE) originates or terminates one or more sections of a line signal. The LTE does the synchronization and multiplexing of information on SONET frames. Multiple lower-level SONET signals can be mixed together to form higher-level SONET signals. An Add/Drop Multiplexer (ADM) is an example of LTE.

Path

Path-Terminating Equipment (PTE) interfaces non-SONET equipment to the SONET network. At this layer, the payload is mapped and demapped into the SONET frame. For example, an STS PTE can assemble 25 1.544 Mbps DS1 signals and insert path overhead to form an STS-1 signal.
This layer is concerned with end-to-end transport of data.