
XRT86SH221
PRELIMINARY
47
SDH-TO-PDH FRAMER/MAPPER WITH INTEGRATED 21-CHANNEL E1 SH LIU
REV. P1.0.5
N
OTE
:
The number of consecutive observations is 5. Also, bits 6, 7 and 8 in the SDH standard refer to the least significant
3 bits. The RDI_L detection algorithm follows the following SDH standard rules:
I
LTE shall detect an RDI-L defect on the incoming signal when bits 6, 7, and 8 of the K2 byte contains the
'110' pattern in 5 to 10 consecutive frames
I
LTE shall terminate the RDI-L defect on the incoming signal when bits 6, 7, and 8 of the K2 byte have
any pattern other than '110' in five to 10 consecutive frames.
DOWNSTREAM AIS INSERTION
The rxstoh_proc block will insert path AIS in the downstream data when prompted by the rxstoh_stat block.
RXSTOH_STAT BLOCK
The rxstoh_stat block has an 8 bit internal bus. The rxstoh_stat block contains the status and control registers
for the transport overhead blocks. In addition, the rxstoh_stat block monitors the S1, REI-L, and K1/K2 fields of
the transport overhead. The rxstoh_stat block also monitors the B1 and B2 error masks and accumulates the
bit or word error events.
SYNCHRONIZATION STATUS (S1) MONITOR
The rxstoh_stat block extracts the synchronization status (S1) byte from the line overhead and monitors it for
change. The S1 byte is only defined for the first STM-0 in an STM-1 signal. The extraction algorithm is defined
by the SDH standard as:
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A change in the S1 synchronization status message shall be detected if at least 8 consecutive samples
(these may or may not be consecutive SDH frames) of bits 5-8 (least significant 4 bits) of the S1 byte
have the same (new) value. The sampling rate shall be such that the maximum time to detect a change
(assuming no transmission errors) is 1 second.
I
For S1 messages, if no validated synchronization status message is detected (e.g., due to transmission
errors or the receipt of an undefined message) for a period of greater than 10 seconds, the NE shall
consider the reference failed.
The rxstoh_stat block monitors the S1 byte for 8 consecutive identical values after which the new value is
stored in a register for access by software. The rxstoh_stat block also implements a counter for an unstable
S1. This counter is incremented for each byte that differs from the previously received byte. An invalid S1
condition is declared when the S1 counter reaches 32. The S1 counter is cleared to 0 when 8 consecutive
identical S1 bytes are received. Upon detection of an invalid S1 condition, the software can then set up a 10s
timer to detect for S1 reference failure.
REI-L (M0/M1) MONITOR
The M0/M1 field contains the line remote error indicator (REI-L) count. For an STM-0 stream, the count is
contained in the first REI-L byte (M0 byte) whereas for an STM-1 or higher stream, the count is contained in the
third REI-L byte (M1 byte). The rxstoh_stat block interprets the REI-L field according to the following SDH
standard rules:
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LTE terminating an OC-1 or STM-0 electrical signal shall set bits 5 through 8 of the M0 byte to indicate
(to the upstream LTE) the count of the interleaved-bit block errors that it has detected based on the BIP-
8 (B2) byte. The error count shall be a binary number from zero (i.e., 0000) to 8 (i.e., 1000). The
remaining seven values represented by the four REI-L bits (i.e., 1001 through 1111) shall not be
transmitted, and shall be interpreted by receiving LTE as zero errors.
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LTE terminating an OC-N or STM-N electrical signal (N greater than or equal to 3) shall set the M1 byte
to indicate (to the upstream LTE) the count of the interleaved-bit block errors that it has detected using
the B2 bytes. For values of N below 48, the error count shall be a binary number from zero to 8xN. The
remaining possible values [i.e., 255-(8xN)] represented by the eight REI-L bits shall not be transmitted
and shall be interpreted by the receiving LTE as zero errors. For N equal to 48, the count shall be
truncated at 255.
The rxstoh_stat accumulates the REI-L counts in a 20 bit saturation counter. The count is transferred to a
holding register and reset by software request. The rxstoh_stat also flags any non zero REI-L counts. The