S/UNI®-8x155 ASSP Telecom Standard Product Data Sheet
Released
Figure 8 Packet Over SONET/SDH Frame Format
Flag
Packet (PPP or other)
POS Frame
FCS
Flag
Flag
In the event of a FIFO underflow caused by the FIFO being empty while a packet is being
transmitted, the packet is aborted by transmitting the Abort Sequence. The Abort Sequence
consists of an Escape Control character (0x7D) followed by the Flag Sequence (0x7E). Bytes
associated with this aborted frame are still read from the FIFO but are discarded and replaced
with the Flag Sequence in the outgoing data stream. Transmission of data resumes a start of the
next packet is encountered in the FIFO data stream.
The POS Frame Generator also performs inter-packet gaping. This operation consists of
inserting a programmable number of Flag Sequence characters between each POS frame
transmission. This feature allows to control the system effective data transmission rate if
required.
For correct operation, the TXFP only supports packets ranging in size from 2 bytes to 65534
bytes in length.
10.13.2FCS Generator
The FCS Generator performs a CRC-16.ISO-3309 or CRC-32 calculation on the whole POS
frame, before byte stuffing and data scrambling. A parallel implementation of the CRC
polynomial is used. The CRC algorithm for the frame checking sequence (FCS) field is either a
CRC-16.ISO-3309 or CRC-32 function. The CRC-16.ISO-3309 is two bytes in size and has a
5
12
2
16
generating polynomial g(x) = 1 + x + x + x . The CRC-32 is four bytes in size and has a
4
5
7
8
10
11
12
16
22
generating polynomial g(x) = 1 + x + x + x + x + x + x + x + x + x + x + x
+
23
26
32
x
+ x + x . The first FCS bit transmitted is the coefficient of the highest term. When
transmitting a packet from the Transmit FIFO, the FCS Generator appends the result after the
last data byte, before the closing flag. Note that the Frame Check Sequence is the one's
complement of the CRC register after calculation ends. FCS calculation and insertion can be
disabled.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC- 2010299, Issue 2
84
PMC [ PMC-SIERRA, INC ]
