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XRT75L00D Datasheet(PDF) 65 Page - Exar Corporation |
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XRT75L00D Datasheet(HTML) 65 Page - Exar Corporation |
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65 / 92 page  XRT75L00D REV. 1.0.2 E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCHRONIZER 60 What does this mean at the Destination PTE? In this case, this DS3 signal (which has now been mapped into an STS-1 data-stream) will be transported across the SONET network. As this STS-1 signal arrives at the "Destination" PTE, this PTE will extract (or de- map) this DS3 data from each incoming STS-1 SPE. Now, in this case, most (e.g., 177/178.8) of the incoming STS-1 SPEs will contain 5592 DS3 data-bits. Therefore, the nominal data rate of the DS3 signal being de- mapped from SONET will be 44.736Mbps. However, in approximately 1 out of every 178 incoming STS-1 SPEs, the SPE will carry only 5591 DS3 data bits. This means that (during these times) the data rate of the de- mapped DS3 signal will have an instantaneous frequency that is less than 44.736Mbps. These "excursions" of the de-mapped DS3 data-rate, from the nominal DS3 frequency can be viewed as occurrences of mapping/de- mapping jitter with an amplitude of approximately 1UI-pp. Since this occurrence of "de-mapping" jitter is periodic and occurs once every 22.35ms, we can state that this jitter has a frequency of 44.7Hz. We talked about De-Mapping Jitter, What about De-Mapping Wander? The Telcordia and Bellcore specifications define "Wander" as "Jitter with a frequency of less than 10Hz". Based upon this definition, the DS3 signal (that is being transported by SONET) will cease to contain jitter and will now contain "Wander", whenever the frequency offset of the DS3 signal being mapped into SONET is less than 0.2ppm. 9.3 Jitter/Wander due to Pointer Adjustments In the previous section, we described how a DS3 signal is asynchronously-mapped into SONET, and we also defined "Mapping/De-mapping" jitter. In this section, we will describe how occurrences within the SONET network will induce jitter/wander within the DS3 signal that is being transported across the SONET network. In order to accomplish this, we will discuss the following topics in detail. • The concept of an STS-1 SPE pointer • The concept of Pointer Adjustments • The causes of Pointer Adjustments • How Pointer Adjustments induce jitter/wander within a DS3 signal being transported by that SONET network. 9.3.1 The Concept of an STS-1 SPE Pointer As mentioned earlier, the STS-1 SPE is not aligned to the STS-1 frame boundaries and is permitted to "float" within the Envelope Capacity. As a consequence, the STS-1 SPE will often times "straddle" across two FIGURE 40. AN ILLUSTRATION OF THE STS-1 SPE TRAFFIC THAT WILL BE GENERATED BY THE SOURCE PTE, WHEN MAPPING A DS3 SIGNAL THAT HAS A BIT RATE OF 44.736MBPS - 1PPM, INTO AN STS-1 SIGNAL Source PTE Source PTE 44.736Mbps - 1ppm 5592 DS3 Data Bits 5592 DS3 Data Bits 5592 DS3 Data Bits 5592 DS3 Data Bits SPE # N SPE # N+1 5592 DS3 Data Bits 5592 DS3 Data Bits SPE # N+177 5591 DS3 Data Bits 5591 DS3 Data Bits 5592 DS3 Data Bits 5592 DS3 Data Bits SPE # N+178 SPE # N+179 DS3 Data Bit Excluded Here STS-1 SPE Data Stream |
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