SONET Overhead
| In the SPE, 36 bytes (roughly 4 percent) of the STS-1 frame are used to transport overhead information. This overhead is multiplexed with the payload by byte interleaving. The SONET overhead channels provide transport of the information required to manage and maintain the SONET network. Without the use of the overhead channels, SONET networks would be unreliable and very difficult to maintain. You can't understand how the network operates, how to isolate troubles, or what information is available for network tests without understanding SONET overhead. Embedded in the SONET frame is the SPE. The SPE contains the client signal that is being transported. The first column of the SPE contains the path overhead, is one of three fields containing overhead for operations and management purposes. The section and line overhead maintain a fixed position in the first three columns of the matrix. The location of the path overhead within an individual frame does not have a fixed position. Section, line, and path overhead are multiplexed with the payload using byte interleaving. The section and line overhead are inserted in the frame/signal 3 bytes at a time, and the path overhead is inserted 1 byte at a time. SONET/SDH Transmission SegmentsWhen referencing SONET overhead, it is imperative to have an understanding of the transmission segments. For management purposes, SONET/SDH transmission systems are divided into different segments. In SONET, these segments are called sections, lines, and paths. SectionsThe lowest-level transmission segment is the section. The section exists between any adjacent O-E-O processing points on the transmission facility. All the SONET network elements you've learned about so far perform O-E-O processing; they also all terminate sections. It's not shown in Figure 2-19, but on long spans in which there are several consecutive regenerators, even the segment between adjacent regenerators is considered a section. The section is the shortest transmission segment that is visible from a management perspective. Figure 2-19. Examples of Sections On long-haul systems that employ optical amplifiers, section overhead is not terminated at the optical amplifier. In SDH, the equivalent terminology is a regenerator section. In Figure 2-20, you can see the 9 bytes of section overhead. Recall that a section is the transmission segment between adjacent O-E-O processing points. The overhead represented in this figure is thus terminated and processed at essentially every SONET network element, whether it is a regenerator, cross-connect, add/drop mux, or terminal multiplexer. The section overhead provides the lowest level of granularity for management visibility of the SONET network. Figure 2-20. Section Overhead Bytes Several functions are supported by these bytes of overhead information. The A1 and A2 bytes are used for framing; they indicate the beginning of the STS-1 frame. All other byte positions are determined by counting from these bytes. The J0/Z0 byte has different meanings, depending on whether the STS-1 is the first STS-1 of an STS-N or one of the second through N STS-1s. In the first STS-1, the J0 byte, called the trace byte, is transmitted. A unique character is transmitted in the J0 byte; at any downstream location, the identity of the STS-1/STS-N can be verified by comparing the J0 byte received to the one that was transmitted. The B1 byte carries the result of a parity check. The parity check occurs over all bits of the entire STS-N signal of the previous frame. Because of this, the use of the B1 byte is defined for only the first STS-1 of an STS-N. The E1 byte is a section orderwire. It can be used for voice communications between regenerator section locations. Recall that each byte of overhead provides a 64-Kbps channel so that standard pulse code modulation (PCM) voice communications are possible over this channel. F1 is a "user" byte. Its use is not standardized. Vendors can use this byte to provide special features within a single vendor network. The D1, D2, and D3 bytes form a 192-Kbps data-communications channel. It is used for communication between network-management system elements. LinesThe next SONET transmission segment is the line. The line exists between consecutive network elements that process the signal at the STS level. Any SONET node that does multiplexing or cross-connecting terminates the line. As discussed, these nodes also terminate a section. Management overhead at the line level is used for functions such as protection switching, error detection, synchronization status, and functions related to the position of the SONET payload within the SONET frame. Figure 2-21 defines the endpoints of the line segment. In SDH, this segment is called a multiplexer section. Figure 2-21. Line Example In Figure 2-22, you can see 18 bytes of line overhead. Table 2-1 documents the functions of the bytes in the line and section overhead. Figure 2-22. Line Overhead Bytes
PathsThe final transmission segment is the path, the end-to-end trail of the signal. The path exists from wherever the payload is multiplexed into the SONET/SDH format to wherever demultiplexing of the same payload takes place. As the name implies, this function is generically performed at the PTE. A PTE can function as a terminal multiplexer or an add/ drop multiplexer. Unlike sections and lines, which deal with the composite signal, paths are associated with the client signal that is mapped into the SONET/SDH payload. In addition, when supporting subrate multiplexing, such as transporting multiple DS1s, each individual DS1 has an associated path overhead. Figure 2-23 shows an example of a path. The use of the term path is common in SONET and SDH. Figure 2-23. Path Example In Figure 2-24, you can see 9 bytes of path overhead. Each is described as follows:
Now that you've seen all the bytes for the SONET overhead, Figure 2-25 serves as a visual reminder of the structure of the SONET overhead and SPE. Figure 2-25. SONET Overhead |
EAN: 2147483647
Pages: 140