XRT75VL00D datasheet(69/92 Pages) EXAR

69 / 92 page

XRT75VL00D

REV. 1.0.3

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

In the "Slip Buffer, the "input" STS-1 data (labeled "STS-1 Data_IN") is latched into the FIFO, upon a given

edge of the corresponding "STS-1 Clock_f1" input clock signal. The STS-1 Data (labeled "STS-1 Data_OUT")

is clocked out of the Slip Buffer upon a given edge of the "STS-1 Clock_f2" input clock signal.

The behavior of the data, passing through the "Slip Buffer" is now described for each possible relationship

between frequencies f1 and f2.

If f1 = f2

If both frequencies, f1 and f2 are exactly equal, then the STS-1 data will be "clocked" into the "Slip Buffer" at

exactly the same rate that it is "clocked out". In this case, the "Slip Buffer" will neither fill-up nor become

depleted. As a consequence, no pointer-adjustments will occur in this STS-1 data stream. In other words, the

STS-1 SPE will remain at a constant location (or offset) within each STS-1 envelope capacity for the duration

that this STS-1 signal is supporting this particular service.

If f1 < f2

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

a faster rate than it is being clocked in.

In this case, the "Slip Buffer" will eventually become depleted.

Whenever this occurs, a typical strategy is to "stuff" (or insert) a "dummy byte" into the data stream. The

purpose of stuffing this "dummy byte" is to compensate for the frequency differences between f1 and f2, and

attempt to keep the "Slip Buffer, at a somewhat constant fill level.

NOTE: This "dummy byte" does not carry any valuable information (not for the user, nor for the system).

Since this "dummy byte" carries no useful information, it is important that the "Receiving PTE" be notified

anytime this "dummy byte" stuffing occurs. This way, the Receiving Terminal can "know" not to treat this

"dummy byte" as user data.

Byte-Stuffing and Pointer Incrementing in a SONET Network

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

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

a. The "stuff-byte" will be inserted into the byte position immediately after the H3 byte. This insertion of the

"dummy byte" immediately after the H3 byte position will cause the J1 byte (and in-turn, the rest of the

SPE) to be "byte-shifted" away from the H3 byte. As a consequence, the offset between the H3 byte posi-

tion and the STS-1 SPE will now have been increased by 1 byte.

b. The "Transmitting" Network Equipment will notify the remote terminal of this byte-stuffing event, by invert-

ing certain bits within the "pointer word" (within the H1 and H2 bytes) that are referred to as "I" bits.

FIGURE 44. AN ILLUSTRATION OF AN STS-1 SIGNAL BEING PROCESSED VIA A SLIP BUFFER

SLIP BUFFER

STS-1 Data_IN

STS-1 Clock_f1

STS-1 Data_OUT

STS-1 Clock_f2

Clock Domain operating

At frequency f1

Clock Domain operating

At Frequency f2.

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