XRT75R12D_06 Datasheet, PDF (108/134 Pages) Exar Corporation – TWELVE CHANNEL E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET

XRT75R12D
TWELVE CHANNEL E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCHRONIZER REV. 1.0.1
FIGURE 51. AN ILLUSTRATION OF AN STS-1 SIGNAL BEING PROCESSED VIA A SLIP BUFFER
Clock Domain operating
At frequency f1
STS-1 Data_IN
STS-1 Clock_f1
SSLLIIPPBBUUFFFFEERR
STS-1 Data_OUT
STS-1 Clock_f2
Clock Domain operating
At Frequency f2.
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.
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