XRT75R03D_06 Datasheet, PDF (107/135 Pages) Exar Corporation – EXAR DATA SHEET FORMAT TEMPLATES

XRT75R03D
REV. 1.0.4
THREE CHANNEL E3/DS3/STS-1 LINE
Whenever these "positive-stuffing" events occur then (for these particular STS-1 SPEs) the SPE will carry only
5591 DS3 data bits (e.g., in this case, only 2 Stuff Opportunity bits will be carrying DS3 data-bits, and the
remaining 7 Stuff Opportunity bits are "stuff" bits).
Figure 41 presents an illustration of the STS-1 SPE traffic that will be generated by the "Source" PTE, during
this condition.
FIGURE 41. 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
DS3 Data
Bit Excluded Here
SSoouurrccee
PPTTEE
SPE # N SPE # N+1
55559922
55559922
DDSS33DDaatata DDSS33DDaatata
BBitists
BBitists
SPE # N+177
SPE # N+179
55559922
55559911
55559922
DDSS33DDaatata DDSS33DDaatata DDSS33DDaatata
BBitists
BBitists
BBitists
SPE # N+178
44.736Mbps - 1ppm
STS-1 SPE Data Stream
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.
10.3 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
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