PMC-Sierra, Inc.
PRELIMINARY
PM5381 S/UNI-2488
DATASHEET
PMC-2000489
ISSUE 1
SATURN USER NETWORK INTERFACE FOR 2488 MBIT/S
10.7.5 POS Overhead Removal
The overhead removal consists of stripping SONET/SDH overhead bytes from the data stream.
Once overhead bytes are removed, the data stream consists of PPP/HDLC frame octets that can
be fed directly to the descrambler or the PPP/HDLC Frame Delineation block.
10.7.6 POS Descrambler
When enabled, the self-synchronous descrambler operates on the PPP Frame data,
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descrambling the data with the polynomial x +1. Descrambling is performed on the raw data
stream, before any PPP frame delineation or byte destuffing is performed. Data scrambling can
provide for a more robust system preventing the injection of hostile patterns into the data stream.
10.7.7 POS PPP/HDLC Frame Delineation
The PPP/HDLC Frame Delineation is performed on the descrambled data and consists of
arranging the framed octets. Frame boundaries are found by searching for the Flag Character
(0x7E). Flags are also used to fill inter-packet spacing. This block removes the Flag and Idle
Sequences and passes the data onto the Byte Destuffing block. The PPP/HDLC Frame format is
shown in the figure below.
Figure 13: PPP/HDLC Over SONET Frame Format
Flag
Packet (PPP or other)
POS Frame
FCS
Flag
Flag
In the event of a FIFO overflow caused by the FIFO being full while a packet is being received,
the packet is marked with an error so it can be discarded by the system. Subsequent bytes
associated with this now aborted frame are discarded. Reception of PPP/HDLC data resumes
when a Start of Packet is encountered and the FIFO level is below a programmable Reception
Initialization Level.
10.7.8 POS Byte Destuffing
The byte destuffing algorithm searches the Control Escape character (0x7D). These characters,
listed in the table below are added for transparency in the transmit direction and must be
removed to recover the user data. When the Control Escape character is encountered, it is
removed and the following data byte is XORed with 0x20. Therefore, any escaped data byte will
be processed properly by the S/UNI-2488.
Proprietary and Confidentail to PMC-Sierra Inc., and for its Customer’s Internal Use
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PMC [ PMC-SIERRA, INC ]
