vb.net qr code Figure 2-22 SONET payload pointer. in Software

Creation Code 39 Full ASCII in Software Figure 2-22 SONET payload pointer.

Figure 2-22 SONET payload pointer.
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Of course, it would be too easy to make the pointer s behavior so simple. The 10 bits of the actual pointer are divided into five increment bits and five decrement bits. The odd bits are the increment bits; the even bits are the decrement bits. Whenever a pointer adjustment takes place, the absolute value of the pointer changes dramatically. In the event of a negative adjustment of the pointer, the decrement bits are inverted, telling the receiver that the adjustment has occurred (in concert with the inversion of the New Data Flag). In the event of a positive adjustment, the increment bits are flipped. This should sound a little strange to the reader. After all, this results in the conversion of the pointer value from a binary number that indicated the absolute location of the beginning of the SPE to a nonsensical number that now indicates nothing about the actual location of the payload. Remember, though, that the accuracy of SONET is such that the pointer value will never need to wander more than a single byte in either direction. Thus, it doesn t matter that the value of the pointer changes drastically; if the receiver sees the NDF and then notices that at least three of the five pointer bytes changed, then it knows that either a positive or negative justification took place and that it should modify its search accordingly. We don t know about you, but we believe that somebody was practicing better life through chemistry when they thought this stuff up. It is truly amazing. H3: pointer action byte: The pointer action byte is the byte that actually compensates for the movement of the SPE within the STS frame. As we saw in our loading dock example, the payload may actually arrive faster than the train cars do or may not arrive in time to align the payload with a passing train car. In SONET systems, the pointer action byte compensates for this. In the event that the payload rate exceeds the frame capacity (in other words, more than 783 bytes are ready to be transmitted within a single 125 _s period), a single excess byte can be carried in the pointer action byte position, thus expanding the SPE from 783 bytes to 784. This technique is called negative timing justification. As the reader might expect, an opposite technique is available called positive timing justification, which is used whenever the SPE is short a byte in which case the H3 byte pushes itself forward in the frame, providing the missing byte to the system as it attempts to build a 783-byte payload. One thing that should be noted: SONET timing systems are extremely accurate and thus only allow the payload to shift by as much as a single byte in either direction. However, this technique, known in SONET parlance as floating mode, is complex and expensive, and many service providers do all they can to avoid it because it requires the deployment of sophisticated pointer processing. The alternative to floating mode is called
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2
locked mode, in which case the payload pointer points to the first available payload byte in the next frame, which is row one, column four, as shown in Figure 2-23. However, this technique also has its downside; it requires buffering ability and complex network-management techniques, and it assumes that network-wide timing sources are operating in synchronous lock-step an assumption that may not be valid in today s network environment, as stable as it is. So both techniques are employed and will continue to be until network-wide timing becomes a reality. Timing will be discussed in more detail in a later section of the book. A final note about the H3 byte: it represents an integral part of the payload pointer function and therefore, like the pointer itself, must be provided for every STS within an STS-N frame. However, unless it carries justification information, it is ignored. B2: BIP-8: Sound familiar B2 is another bit-interleaved parity byte and is used to carry error-checking information. It calculates its value based on the Line Overhead (it does not include the Section Overhead) and payload of the previous frame before it is scrambled for transmission. The information is then placed in the current frame before it is scrambled. The BIP-8 is required for all STS in an STS-N. K1/K2: automatic protection switching: One of the principal advantages of SONET is its ability to detect a failure in the network and switch traffic to a backup transmission span automatically if one is available, ensuring protection of customer data. This technique is called automatic protection switching (APS). If the network is deployed across a ring architecture, the K1 and K2 bytes are used to switch from one ring to another. Consider, for example, the ring shown in Figure 2-24. Although many companies use four-fiber rings, for our purposes this one has two. The outer ring, in this
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