XRT75VL00D datasheet(62/92 Pages) EXAR

62 / 92 page

XRT75VL00D

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

REV. 1.0.3

• The 44.736MHz - 1ppm Case

Throughout each of these cases, we will discuss how the resulting "bit-stuffing" (that was done when mapping

the DS3 signal into SONET) affects the amount of intrinsic jitter and wander that will be present in the DS3

signal, once it is ultimately de-mapped from SONET.

9.2.2.1

The Ideal Case for Mapping DS3 data into an STS-1 Signal (e.g., with no Frequency

Offsets)

Let us assume that we are mapping a DS3 signal, which has a bit rate of exactly 44.736Mbps (with no

frequency offset) into SONET. Further, let us assume that the SONET circuitry within the PTE is clocked at

exactly 51.84MHz (also with no frequency offset), as depicted below.

Given the above-mentioned assumptions, we can state the following.

• The DS3 data-stream has a bit-rate of exactly 44.736Mbps

• The PTE will create 8000 STS-1 SPE’s per second

• In order to properly map a DS3 data-stream into an STS-1 data-stream, then each STS-1 SPE must carry

(44.736Mbps/8000 =) 5592 DS3 data bits.

Is there a Problem?

According to Figure 37, each STS-1 SPE only contains 5589 bits that are specifically designated for "DS3 data

bits". In this case, each STS-1 SPE appears to be three bits "short".

No there is a Simple Solution

No, earlier we mentioned that each STS-1 SPE consists of nine (9) "Stuff Opportunity" bits. Therefore, these

three additional bits (for DS3 data) are obtained by using three of these "Stuff Opportunity" bits.

As a

consequence, three (3) of these nine (9) "Stuff Opportunity" bits, within each STS-1 SPE, will carry DS3 data-

bits. The remaining six (6) "Stuff Opportunity" bits will typically function as "stuff" bits.

In summary, for the "Ideal Case"; where there is no frequency offset between the DS3 and the STS-1 bit-rates,

once this DS3 data-stream has been mapped into the STS-1 data-stream, then each and every STS-1 SPE will

have the following "Stuff Opportunity" bit utilization.

3 "Stuff Opportunity" bits will carry DS3 data bits.

6 "Stuff Opportunity" bits will function as "stuff" bits

In this case, this DS3 signal (which has now been mapped into STS-1) 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-

stream from each incoming STS-1 SPE. Now since each and every STS-1 SPE contains exactly 5592 DS3

data bits; then the bit rate of this DS3 signal will be exactly 44.736Mbps (such as it was when it was mapped

into SONET, at the "Source" PTE).

FIGURE 38. A SIMPLE ILLUSTRATION OF A DS3 DATA-STREAM BEING MAPPED INTO AN STS-1 SPE, VIA A PTE

PTE

44.736MHz + 0ppm

DS3_Data_In

STS-1_Data_Out

51.84MHz + 0ppm

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