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| 10.3 |  | Examining Fiber Optic Transports |
| 10.3.1 |  | Defining SONET and SDH |

SONET and SDH are standards for interfacing Operating Telephone Company (OTC) optical networks. Together they are a set of global standards that interface equipment from different vendors. SDH is basically the international version of SONET, and SONET can be thought of as the North American version of SDH. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation. The popularity of SONET is not shrinking, however SDH is gaining more and more popularity. There are some slight differences between SONET and SDH.

Both SONET and SDH can transport signals for all the networks in existence today and they have the flexibility to accommodate any networks defined in the future. They are used in three traditional telecommunications areas, such as long-haul networks, local networks, and loop carriers. They can also be used to carry CATV video traffic.

The main differences are in the basic SDH and SONET frame formats, but SDH and SONET are essentially identical beyond the STS-3 signal level. The base signal for SONET is STS-1 and the base signal for SDH is STM-1. STS-3c is equivalent to STM-1 and the lower tributaries can be mapped interchangeably between the two formats from that point on.

The Synchronous Transport Signal level 1 (STS-1) is the basic signal rate of SONET. SONET adopts a frame length of 125 microns (µs) or frame rate of 8000 octets per frame. Each frame has 9 rows by 90 columns of octets, or bytes, forming a total of 810 octets per frame. This gives SONET a basic transmission rate of 51.840 Mbps. Of the 810 total octets, 27 are dedicated overhead. The first 3 columns make up the transport overhead where 9 bytes are used for section overhead, and 18 are used for line overhead. Columns 4 to 90 contain the Synchronous Payload Envelope (SPE) where the STS-1 Path Overhead is found. Because the first 3 columns are overhead, the actual data rate is 50.112 Mbps rather than 51.840 Mbps. The STS-1 frame is transmitted starting from the byte in row 1 column 1 to the byte in row 9 column 90. The most significant bit of a byte is transmitted first.

Higher line rates are obtained by synchronous multiplexing of the lower line rates. The STS-1 signal can be repeated N times, where valid values of N are 1, 3, 12, 48, and 192. Therefore, STS-192 has a line rate as follows:

> 51.84 Mbps * 192 = 9953.28 Mbps

The Synchronous Transport Module level 1 (STM-1) is the basic signal rate of SDH. SDH also uses a frame length of 125 microns (ms) or frame rate of 8000 octets per frame. The STM-1 frame consists of overhead plus a virtual container capacity. Each frame has 9 rows by 270 columns of octets, or bytes, structure totaling 2430 octets per frame. Of the 2430 total octets, 81 are dedicated section overhead. The first 9 columns make up the section overhead and columns 10 to 270 make up the Virtual Container (VC). The VC plus the pointers, H1, H2, and H3 bytes, is called the Administrative Unit (AU).

Carried within the VC capacity, which has its own frame structure of 9 rows and 261 columns, is the Path Overhead and the Container. The first column is for Path Overhead and the payload container follows it.

The STM frame is transmitted in a byte-serial fashion, row-by-row, like STS frames, and is scrambled immediately prior to transmission to ensure adequate clock timing content for downstream regenerators.

Regenerators extend the maximum distance and quality of a line by decomposing it into multiple sections.