XRT75VL00D datasheet(61/92 Pages) EXAR

61 / 92 page

XRT75VL00D

REV. 1.0.3

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

Figure 37 presents an alternative illustration of Telcordia GR-253-CORE’s recommendation on how to

asynchronously map DS3 data into an STS-1 SPE. In this case, the STS-1 SPE bit-format is expressed purely

in the form of "bit-types" and "numbers of bits within each of these types of bits". If one studies this figure

closely he/she will notice that this is the same "87 byte column x 9 row" structure that we have been talking

about when defining the STS-1 SPE. However, in this figure, the "user-data" field is now defined and is said to

consist of five (5) different types of bits. Each of these bit-types play a role when asynchronously mapping a

DS3 signal into an STS-1 SPE. Each of these types of bits are listed and described below.

Fixed Stuff Bits

Fixed Stuff bits are simply "space-filler" bits that simply occupy space within the STS-1 SPE. These bit-fields

have no functional role other than "space occupation". Telcordia GR-253-CORE does not define any particular

value that these bits should be set to. Each of the 9 rows, within the STS-1 SPE will contain 59 of these "fixed

stuff" bits.

DS3 Data Bits

The DS3 Data-Bits are (as its name implies) used to transport the DS3 data-bits within the STS-1 SPE. If the

STS-1 SPE is transporting a framed DS3 data-stream, then these DS3 Data bits will carry both the "DS3

payload data" and the "DS3 overhead bits". Each of the 9 rows, within the STS-1 SPE will contain 621 of these

"DS3 Data bits". This means that each STS-1 SPE contains 5,589 of these DS3 Data bit-fields.

Stuff Opportunity Bits

The "Stuff" Opportunity bits will function as either a "stuff" (or junk) bit, or it will carry a DS3 data-bit. The

decision as to whether to have a "Stuff Opportunity" bit transport a "DS3 data-bit" or a "stuff" bit depends upon

the "timing differences" between the DS3 data that is being mapped into the STS-1 SPE and the timing source

that is driving the STS-1 circuitry within the PTE.

As will be described later on, these "Stuff Opportunity" Bits play a very important role in "frequency-justifying"

the DS3 data that is being mapped into the STS-1 SPE. These "Stuff Opportunity" bits also play a critical role

in inducing Intrinsic Jitter and Wander within the DS3 signal (as it is de-mapped by the remote PTE).

Each of the 9 rows, within the STS-1 SPE consists of one (1) Stuff Opportunity bit. Hence, there are a total of

nine "Stuff Opportunity" bits within each STS-1 SPE.

Stuff Control/Indicator Bits

Each of the nine (9) rows within the STS-1 SPE contains five (5) Stuff Control/Indicator bits. The purpose of

these "Stuff Control/Indicator" bits is to indicate (to the de-mapping PTE) whether the "Stuff Opportunity" bits

(that resides in the same row) is a "Stuff" bit or is carrying a DS3 data bit.

If all five of these "Stuff Control/Indicator" bits, within a given row are set to "0", then this means that the

corresponding "Stuff Opportunity" bit (e.g., the "Stuff Opportunity" bit within the same row) is carrying a DS3

data bit.

Conversely, if all five of these "Stuff Control/Indicator" bits, within a given row are set to "1" then this means

that the corresponding "Stuff Opportunity" bit is carrying a "stuff" bit.

Overhead Communication Bits

Telcordia GR-253-CORE permits the user to use these two bits (for each row) as some sort of

"Communications" bit. Some Mapper devices, such as the XRT94L43 12-Channel DS3/E3/STS-1 to STS-12/

STM-1 Mapper and the XRT94L33 3-Channel DS3/E3/STS-1 to STS-3/STM-1 Mapper IC (both from Exar

Corporation) do permit the user to have access to these bit-fields.

However, in general, these particular bits can also be thought of as "Fixed Stuff" bits, that mostly have a "space

occupation" function.

9.2.2

DS3 Frequency Offsets and the Use of the "Stuff Opportunity" Bits

In order to fully convey the role that the "stuff-opportunity" bits play, when mapping DS3 data into SONET, we

will present a detailed discussion of each of the following "Mapping DS3 into STS-1" scenarios.

• The Ideal Case (e.g., with no frequency offsets)

• The 44.736Mbps + 1 ppm Case

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