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SONET Timing

SONET is based on synchronous transmission, meaning the average frequency of all the clocks in the network are the same (synchronous) or nearly the same (plesiochronous). As a result of this approach, the clocks are referenced to a stable reference point. Therefore, the need to align the data streams directly is less necessary. As we stated earlier, the user payloads, such as DS1, are directly accessible so demultiplexing is not necessary to access the bit streams. Also, the synchronous signals can be stacked together without bit stuffing.

The Benefits of Byte Alignment

Byte multiplexing (also called octet multiplexing) is more efficient and less error-prone than the bit multiplexing operation explained earlier. Most hardware and software today are designed to process data in chunks of eight bits, often called byte-aligned processing. In addition, bit aligned processing is more error-prone than its byte-aligned counterpart, because of the use of smaller buffers and shorter timing increments. There is less tolerance for errors in the bit aligned operation (and we show examples of bit processing in Chapters 3 and 4).

SONET requires byte alignment operations, and any timing adjustments that are performed in the SONET network are done on a byte basis, not on a bit basis.

Floating Payloads

Another major aspect of synchronous systems (in general), and SONET (specifically), pertains to how payload, such as DS1 or DS3 signals, is inserted into the SONET channel. For those situations in which the reference clocking signal may vary (even if only slightly), SONET uses pointers to allow the payload streams to "float" within the payload envelope (the term envelope is used to describe the SONET signal on the channel; the term frame is also used). Indeed, synchronous clocking is the key to pointers; it allows a flexible allocation and alignment of the payload within the transmission envelope. Thus, SONET's payload is called a synchronous payload envelope (SPE).

The concept of a synchronous system is elegantly simple. By holding specific bits in a silicon memory buffer for a defined and predictable period of time, it is possible to move information from one part of a payload envelope to another part. It also allows a system to know where the bits are located at all times. Of course, this idea is "old hat" to software engineers, but it is a different way of thinking for other designers. As one person has put it, "Since the bits are lined up in time, we now know where they are in both time and space. So, in a sense, we can now move information in four dimensions, instead of the usual three."

The U.S. implementation of SONET uses a central clocking source—for example, from a telephone company's end office. This office must use an accurate clocking source known as stratum 3. Stratum 3 clocking requires an accuracy of 1.6 parts in 1 billion elements. Chapter 3 provides more detailed information on synchronization and clocking operations as well as the accuracy levels of the stratum 1, 2, 3, and 4 clocks.

Table 1–1 Typical SONET payloads.


Digital Bit Rate

Voice Circuits



System Name

North American multiplexing hierarchy


1.544 Mbit/s





3.152 Mbit/s





6.312 Mbit/s





44.736 Mbit/s






274.176 Mbit/s





European multiplexing hierarchy


2.048 Mbit/s






8.448 Mbit/s






34.368 Mbit/s






139.264 Mbit/s






565.148 Mbit/s





Japanese multiplexing hierarchy


1.544 Mbit/s






6.312 Mbit/s






34.064 Mbit/s






97.728 Mbit/s






397.20 Mbit/s






1588.80 Mbit/s





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