progress bar code in vb.net 2008 The SDH Frame in Software

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The SDH Frame
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To understand the SDH frame structure, it is first helpful to understand the relationship between SDH and SONET. Functionally, they are identical: In both cases, the intent of the technology is to provide a globally standardized transmission system for high-speed data. SONET is indeed optimized for the T1-heavy North American market, while SDH is more applicable to Europe; beyond that, however, the overhead and design considerations of the two are virtually identical. There are, however, some key differences. Perhaps the greatest difference between the two lies in the physical nature of the frame. A SONET STS-1 frame has 810 total bytes, for an overall aggregate bit rate of 51.84 Mbps perfectly adequate for the North American 44.736 Mbps DS3. An SDH STM-1 frame, however, designed to transport a 139.264 Mbps European E4 or CEPT-4 signal, must be larger if it is to accommodate that much bandwidth it clearly won t fit in the limited space available in an STS-1. An STM-1, then, operates at a fundamental rate of 155.52 Mbps, enough for the bandwidth requirements of the E4. This should be where the d j vu starts to kick in: Perceptive readers will remember that 155.52 Mbps number from our discussions of the SONET STS-3, which offers exactly the same bandwidth. An STM-1 frame is shown in Figure 3-60. It is a byte-interleaved, 9-rowby-270-column frame, with the first nine columns devoted to overhead and the remaining 261 devoted to payload transport. A comparison of the bandwidth between SONET and SDH systems is also interesting. The fundamental SDH signal is exactly three times the bandwidth of the fundamental SONET signal, and this relationship continues all the way up the hierarchy.
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Figure 3-60 An SDH STM-1 frame
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9 rows
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261 columns
STM Frame Overhead
The overhead in an STM frame is very similar to that of an STS-1 frame, although the nomenclature varies somewhat. Instead of Section, Line, and Path Overhead to designate the different regions of the network that the overhead components address, SDH uses Regenerator Section, Multiplex Section, and Path Overhead, as shown in Figure 3-61. The Regenerator Section (RSOH) occupies the first three rows of nine bytes, the Multiplex Section (MSOH) the final five. Row four is reserved for the pointer. As in SONET, the Path Overhead floats gently on the payload tides, rising and falling in response to phase shifts. Functionally, these overhead components are identical to their SONET counterparts.
Overhead Details
Because an STM-1 is three times as large as an STS-1, it has three times the overhead capacity nine columns instead of three (plus Path Overhead). The first row of the RSOH is its SONET counterpart, with the exception of the last two bytes, which are labeled as being reserved for national use and are specific to the PTT administration that
Figure 3-61 SDH Overhead
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3
implements the network. In SONET they are not yet assigned. The second row is different from SONET in that it has three bytes reserved for media-dependent implementation (differences in the actual transmission medium, whether copper, coaxial, or fiber) and the final two reserved for national use. As before, they are not yet definitively assigned in the SONET realm. The final row of the RSOH also sports two bytes reserved for mediadependent information, while they are reserved in SONET. All other regenerator section/section overhead functions are identical between the two. The MSOH overhead in the SDH frame is almost exactly the same as that of the SONET Line Overhead, with one exception: Row nine of the SDH frame has two bytes reserved for national administration use. They are reserved in the SONET world. The pointer in an SDH frame is conceptually identical to that of a SONET pointer, although it has some minor differences in nomenclature. In SDH the pointer is referred to as an AU pointer, referring to the standard naming convention described earlier. SONET and SDH were originally rolled out to replace the T1 and E1 hierarchies, which were suffering from demands for bandwidth beyond what they were capable of delivering. Their principal deliverable was voice and lots of it. Let s take a moment now to describe the process of voice digitization, still a key component of network transport.
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