XRT75VL00D datasheet(70/92 Pages) EXAR

70 / 92 page

XRT75VL00D

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

REV. 1.0.3

Figure 45 presents an illustration of the bit-format within the 16-bit word (consist of the H1 and H2 bytes) with

the "I" bits designated.

NOTE: At this time the "I" bits are inverted in order to denote that an "incrementing" pointer adjustment event is currently

occurring.

During the STS-1 frame that follows the "Byte-Stuffing" event

The "I" bits (within the "pointer-word") will be set back to their normal value; and the contents of the H1 and H2

bytes will be incremented by "1".

If f1 > f2

If frequency f1 is greater than f2, then this means that the STS-1 data is being clocked into the "Slip Buffer" at

a faster rate than is being clocked out. In this case, the "Slip Buffer" will start to fill up. Whenever this occurs,

a typical strategy is to delete (e.g., negative-stuff) a byte from the Slip Buffer. The purpose of this "negative-

stuffing" is to compensate for the frequency differences between f1 and f2; and to attempt to keep the "Slip

Buffer" at a somewhat constant fill-level.

NOTE: This byte, which is being "un-stuffed" does carry valuable information for the user (e.g., this byte is typically a

payload byte). Therefore, whenever this negative stuffing occurs, two things must happen.

a. The "negative-stuffed" byte must not be simply discarded. In other words, it must somehow also be

transmitted to the remote PTE with the remainder of the SPE data.

b. The remote PTE must be notified of the occurrence of these "negative-stuffing" events. Further, the

remote PTE must know where to obtain this "negative-stuffed" byte.

Negative-Stuffing and Pointer-Decrementing in a SONET Network

Whenever this "byte negative-stuffing" occurs then the following other things occur within the STS-1 data-

stream.

During the STS-1 frame that contains the "Negative Byte-Stuffing" Event

a. The "Negative-Stuffed" byte will be inserted into the H3 byte position. Whenever an SPE data byte is

inserted into the H3 byte position (which is ordinarily an unused byte), the number of bytes that will exist

between the H3 byte and the J1 byte within the very next SPE will be reduced by 1 byte.

As a

consequence, in this case, the J1 byte (and in-turn, the rest of the SPE) will now be "byte-shifted"

towards the H3 byte position.

b. The "Transmitting" Network Element will notify the remote terminal of this "negative-stuff" event by

inverting certain bits within the "pointer word" (within the H1 and H2 bytes) that are referred to as "D"

bits.

FIGURE 45. AN ILLUSTRATION OF THE BIT FORMAT WITHIN THE 16-BIT WORD (CONSISTING OF THE H1 AND H2

BYTES

) WITH THE "I" BITS DESIGNATED

LSB

MSB

H2 Byte

H1 Byte

10 Bit Pointer Expression

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