
PRELIMINARY
XRT86SH221
78
REV. P1.0.5
SDH-TO-PDH FRAMER/MAPPER WITH INTEGRATED 21-CHANNEL E1 SH LIU
Z6/N2 : VT PATH GROWTH (TANDEM CONNECTION, TC)
N2 is allocated for Tandem Connection Monitoring for the VC2, VC-12 and VC-11 level. The structure of the N2
byte is given in
Table 8
.
Bits 1-2 are used as an even BIP-2 for the Tandem Connection.
Bit 3 is fixed to "1". This guarantees that the contents of N2 are not all zeroes at the TC source. This enables
the detection of an unequipped or supervisory unequipped signal at the Tandem Connection sink without the
need for monitoring further OH-bytes.
Bit 4 operates as an "incoming AIS" indicator.
Bit 5 operates as the TC-REI of the Tandem Connection to indicate errored blocks caused within the Tandem
Connection.
Bit 6 operates as the OEI to indicate errored blocks of the egressing VC-n.
Bits 7-8 operate in a 76 multiframe as:
I
the access point identifier of the Tandem Connection (TC-APId); it complies with the generic 16-byte
string format of J0;
I
the TC-RDI, indicates to the far end that defects have been detected within the Tandem Connection at
the near end Tandem Connection sink; An 8-bit ounter is provided for counting the number of REI bits
received as 1 in bit 5 of N2. An REI indicates that the distant end has detected one or two errors
between the BIP-2 calculation of the previous frame (all the bytes) and the BIP-2 value carried in the N2
byte in the current frame.
I
the ODI, indicates to the far end that TU-AIS has been inserted at the TC-sink into the egressing TU-n
due to defects before or within the Tandem Connection; An 8-bit counter is provided for counting the
number of OEI bits received as equal to 1 in bit 6 of N2. An OEI indication (a1) indicates that the distant
end has detected one or two errors when, the BIP-2 calculated for the previous frame is compared
against the BIP-2 value carried in the V5 byte in the current frame.
I
reserved capacity (for future standardization).
I
The structure of the multiframe is given in
Table 9
and
Table 10
.
TCM Functionality - Source
If no valid TU-n is entering the Tandem Connection at the TC-source, a valid pointer is inserted. This results
in a VC-AIS signal and bit 4 is set to "1". Even BIP-2 parity is calculated over the inserted VC-AIS signal and
written into bits 1-2 of N2.
If a valid TU-n is entering the Tandem Connection at the TC source, then even BIP-2 parity is calculated over
the incoming valid VC-n or the inserted VC-AIS signal and written into bits 1-2 of N2.
I
The bits TC-REI, TC-RDI, OEI, ODI are set to "1" if the corresponding anomaly or defect is detected at
the associated TC-sink of the reverse direction.
I
The original BIP-2 is compensated according to the algorithm described below.
N
OTE
:
In an unequipped or supervisory unequipped signal entering a Tandem Connection, the N2 and V5 bytes are
overwritten with values not equal to all zeroes.
T
ABLE
8: N2
BYTE
STRUCTURE
B
1
B
2
B
3
B
4
B
5
B
6
B
7
B
8
BIP-2
"1"
"Incoming AIS"
TC-REI
OEI
TC-APId, TC-RDI ODI, reserved