
PRELIMINARY
XRT86SH221
46
REV. P1.0.5
SDH-TO-PDH FRAMER/MAPPER WITH INTEGRATED 21-CHANNEL E1 SH LIU
Once a frame has been located, the rxstoh_proc block outputs the current row, column, and time slot of the de-
scrambled data. Note that for the 8-bit version of rxstoh_proc, a time slot is the same as an STS slot where as
for the 32-bit version of rxstoh_proc, each time slot contains 4 STS slots (byte lanes). The software can also
force the SEF condition which causes the rxstoh_proc to re-find frame.
Alternatively, the SDH framer also monitors a frame pulse input. When the frame pulse input is asserted, the
framer automatically assumes the current byte on the input data bus is the first A1 byte.
The software can force the SEF condition in the framer in the rxstoh_proc block by writing a "1" to the
appropriate register file bit. This causes the rxstoh_proc block to declare an SEF alarm and re-find frame. The
bit is cleared after the rxstoh_proc block has rediscovered frame alignment (after at least 1 frame) and the SEF
alarm has been removed.
De-scrambling
The rxstoh_proc de-scrambles all bytes of the incoming stream except for theA1, A2, and J0/Z0 bytes. The
following SDH standard rule is observed by rxstoh_proc. De-scrambling can be disabled via software.
SDH interface signals shall be scrambled (i.e., scrambled at the transmitter and de-scrambled at the receiver)
using a frame synchronous scrambler of sequence length 127, operating at the line rate. The generating
polynomial for the scrambler shall be 1+x6+x7. The scrambler shall be reset to '1111111' on the most-significant
bit of the byte following the Z0 byte in the Nth STM-0 (i.e., the byte following the last Z0 byte). That bit and all
subsequent bits to be scrambled shall be added, modulo 2, to the output from the x7 position of the scrambler.
The scrambler shall run continuously from that bit on throughout the remainder of the STM-N frame.
BIP Processing
The rxstoh_proc block calculates BIP-8 over the pertinent bytes of the incoming stream for comparison with
both the B1 and B2 fields of the transport overhead. The B1 and B2 values are calculated according to the
following SDH standard rules:
I
The B1 byte in a line-side signal shall carry a BIP-8 code, using even parity. The section BIP-8 shall be
calculated over all bits of the previous STM-N frame after scrambling and placed in the B1 byte of the
current STM-N frame before scrambling.
I
The B2 byte shall be provided in all STM-0s within an STM-N to carry a Line BIP-8 code, using even
parity. The Line BIP-8 shall be calculated over all bits of the Line Overhead and the envelop capacity of
the previous STM-0 frame before scrambling, and placed in the B2 byte of the current STM-0 frame
before scrambling.
The rxstoh_proc block outputs an error mask to the rxstoh_stat block after each comparison with B1/B2. Note
that only the first B1 byte of an STM-N stream contains the BIP-8 bits. The second through the Nth B1 bytes
are all undefined. The number of B2 bytes is equal to the number of STM-0's within the STM-N signal. Two
memories are used in the B2 error code calculations. One memory is used to store the running value of the B2
error calculations. This memory is a 12x8/32 dual port RAM with one port for reads and one port for writes.
This is necessary because as the hardware is calculating the B2 code for each STM-0, it needs to store the
new value into the memory and at the same time fetch the current code for the next STM-0 from memory. The
second memory is a single port 12x8/32 RAM used to store the final B2 codes for all the STM-0's. This memory
is read at the B2 byte locations for comparisons and written into the A1 byte locations to store the final B2
codes for the previous frame. For the 8 bit version of the rxstoh_proc block, the B2 RAM widths are 8 bits. For
the 32 bit version of rxstoh_proc block, the B2 RAM widths are 32 bits.
LINE RDI AND AIS DETECTION
The rxstoh_proc block monitors the 3 least significant bits of the first K2 byte for RDI_L and AIS_L detection.
The AIS_L detection algorithm follows the following SDH standard rules:
I
LTE shall detect an AIS-L defect on the incoming signal when bits 6, 7, and 8 of the K2 byte contain the
'111' pattern in 5 consecutive frames
I
LTE shall terminate the AIS-L defect on the incoming signal when bits 6, 7, and 8 of the K2 byte have
any pattern other than '111' in five consecutive frames.