XRT75VL00D datasheet(64/92 Pages) EXAR

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XRT75VL00D

E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCHRONIZER

REV. 1.0.3

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 5593 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 greater than 44.736Mbps. These "excursion"

of the de-mapped DS3 data-rate, from the nominal DS3 frequency can be viewed as occurrences of "mapping/

de-mapping" jitter. Since each of these "bit-stuffing" events involve the insertion of one DS3 data bit, we can

say that the amplitude of this "mapping/de-mapping" jitter is approximately 1UI-pp. From this point on, we will

be referring to this type of jitter (e.g., that which is induced by the mapping and de-mapping process) as "de-

mapping" jitter.

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.

9.2.2.3

The 44.736Mbps - 1ppm Case

In this case, let us assume that a DS3 signal, with a bit-rate of 44.736Mbps - 1ppm is being mapped into an

STS-1 signal with a bit-rate of 51.84Mbps + 0ppm. In this case, the following this will occur.

• In general, most of the STS-1 SPEs will each transport 5592 DS3 data bits.

• However, within a "one-second" period a DS3 signal that has a bit-rate of 44.736Mbps - 1ppm will deliver

approximately 45 too few bits below that of a DS3 signal with a bit-rate of 44.736Mbps + 0ppm. This means

that this particular signal will need to "positive-stuff" or exclude a DS3 data bit from mapping every (1/44.736)

= 22.35ms.

In other words, we will need to avoid mapping this DS3 data-bit about one in every

(22.35ms*8000) = 178.8 STS-1 SPEs.

What does this mean at the "Source" PTE?

All of this means that as the "Source" PTE maps this DS3 signal, with a data rate of 44.736Mbps - 1ppm into

an STS-1 signal, most of the resulting "outbound" STS-1 SPEs will transport 5592 DS3 data bits (e.g., 3 Stuff

Opportunity bits will be carrying DS3 data bits, the remaining 6 Stuff Opportunity bits are "stuff" bits). However,

in approximately one out of 178.8 "outbound" STS-1 SPEs, there will be a need for a "positive-stuffing" event.

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 40 presents an illustration of the STS-1 SPE traffic that will be generated by the "Source" PTE, during

this condition.

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