XRT75VL00D datasheet(63/92 Pages) EXAR

63 / 92 page

XRT75VL00D

REV. 1.0.3

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

As a consequence, no "Mapping/De-Mapping" Jitter or Wander is induced in the "Ideal Case".

9.2.2.2

The 44.736Mbps + 1ppm Case

The "above example" was a very ideal case. In reality, there are going to be frequency offsets in both the DS3

and STS-1 signals. For instance Bellcore GR-499-CORE mandates that a DS3 signal have a bit rate of

44.736Mbps ± 20ppm. Hence, the bit-rate of a "Bellcore" compliant DS3 signal can vary from the exact correct

frequency for DS3 by as much of 20ppm in either direction. Similarly, many SONET applications mandate that

SONET equipment use at least a "Stratum 3" level clock as its timing source. This requirement mandates that

an STS-1 signal must have a bit rate that is in the range of 51.84 ± 4.6ppm. To make matters worse, there are

also provisions for SONET equipment to use (what is referred to as) a "SONET Minimum Clock" (SMC) as its

timing source. In this case, an STS-1 signal can have a bit-rate in the range of 51.84Mbps ± 20ppm.

In order to convey the impact that frequency offsets (in either the DS3 or STS-1 signal) will impose on the bit-

stuffing behavior, and the resulting bit-rate, intrinsic jitter and wander within the DS3 signal that is being

transported across the SONET network; 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

things will occur.

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

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

approximately 44.7 additional bits (over and above that of a DS3 signal with a bit-rate of 44.736Mbps + 0

ppm). This means that this particular signal will need to "negative-stuff" or map in an additional DS3 data bit

every (1/44.736 =) 22.35ms. In other words, this additional DS3 data bit will need to be mapped into about

one in every (22.35ms · 8000 =) 178.8 STS-1 SPEs in order to avoid dropping any DS3 data-bits.

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, as in the

"Ideal" case). However, in approximately one out of 178.8 "outbound" STS-1 SPEs, there will be a need to

insert an additional DS3 data bit within this STS-1 SPE. Whenever this occurs, then (for these particular STS-

1 SPEs) the SPE will be carrying 5593 DS3 data bits (e.g., 4 Stuff Opportunity bits will be carrying DS3 data

bits, the remaining 5 Stuff Opportunity bits are "stuff" bits).

Figure 39 presents an illustration of the STS-1 SPE traffic that will be generated by the "Source" PTE, during

this condition.

FIGURE 39. AN ILLUSTRATION OF THE STS-1 SPE TRAFFIC THAT WILL BE GENERATED BY THE "SOURCE" PTE,

WHEN MAPPING IN 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

5593

DS3 Data

Bits

5593

DS3 Data

Bits

5592

DS3 Data

Bits

5592

DS3 Data

Bits

SPE # N+178

SPE # N+179

Extra DS3 Data

Bit Stuffed Here

STS-1 SPE Data Stream

XRT75VL00D Datasheet(PDF) 63 Page - Exar Corporation

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