Page 1187 - 5G Basics - Core Network Aspects

Transport aspects 2

The payload FCS (a CRC-32) is appended to the end of each GFP frame and is calculated across the payload
information field of the GFP frame as per [ITU-T G.7041]. The purpose of the payload FCS is to provide
visibility of bit errors occurring anywhere in the GFP payload information field and thus augments the
coverage provided by the per-superblock CRC-24 (which only provides coverage for the "control" overhead
in each superblock). The payload FCS is only for the purposes of gathering statistics.
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All octets in the GFP payload area are scrambled using the X + 1 self-synchronous scrambler, again as per
[ITU-T G.7041].

17.9 Mapping a supra-2.488 Gbit/s CBR signal into OPUflex using BMP

Mapping of a supra-2.488 Gbit/s CBR client signal (with up to 100 ppm bit-rate tolerance) into an OPUflex
is performed by a bit-synchronous mapping procedure (BMP). Table 17-14 specifies the clients defined by
this Recommendation.

The bit-synchronous mapping process deployed to map constant bit rate client signals into an OPUflex does
not generate any justification control signals.

The OPUflex clock for the bit-synchronous mapping is derived from the client signal. During a signal fail
condition of the incoming client signal (e.g., in the case of a loss of input signal), this failed incoming signal
is replaced by the appropriate replacement signal as defined in Table 17-15. The OPUflex payload signal bit
rate shall be within the limits specified in Table 7-3 and the limits for the ODCb clock defined in [ITU-T
G.8251] and no frame phase discontinuity shall be introduced in this case and when resynchronization on
the incoming client signal.
During a signal fail condition of the incoming ODUflex/OPUflex signal (e.g., in the case of an ODUflex-AIS,
ODUflex-LCK, ODUflex-OCI condition), the failed client signal is replaced by the appropriate replacement
signal as defined in Table 17-15.
The OPUflex overhead for this mapping consists of:
– A payload structure identifier (PSI) including the payload type (PT) as specified in Table 15-9, the
client signal fail (CSF) and 254 bytes plus 7 bits reserved for future international standardization
(RES);
– Three justification control (JC) bytes, consisting of two bits for justification control (with fixed 00
value) and six bits reserved for future international standardization;
– One negative justification opportunity (NJO) byte (carrying a justification byte); and
– Three bytes reserved for future international standardization (RES).
NOTE – To allow the use of a common asynchronous/bit-synchronous de-mapper circuit for CBR client signals into
ODUk (k=1,2,3 and flex), JC, NJO and PJO fields are assumed to be present in the OPUflex frame structure for the
mapping of a supra-2.488G CBR client signal (Figure 17-18). This OPUflex frame structure is now compatible with the
OPUk frame structure for the mapping of a CBR2G5, CBR10G or CBR40G signal (Figure 17-1). As a CBR signal is mapped
into the OPUflex without justification, the NJO field contains a justification byte (stuff), the PJO field contains a data
byte (D), and the JC bits are fixed to 00.
The OPUflex payload for this mapping consists of 4  3808 bytes (Figure 17-18). Groups of eight successive
bits (not necessarily being a byte) of the client signal are mapped into a data (D) byte of the OPUflex
payload area under control of the BMP control mechanism. Each data byte in the OPUflex payload area
carries 8 client bits.

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