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Transport aspects 2

Annex G

Mapping ODU0 into a low latency OTU0 (OTU0LL)
(This annex forms an integral part of this Recommendation.)

G.1 Introduction
Within the optical transport network client signals with a bit rate up to 1.25 Gbit/s are transported within
an ODU0 and the ODU0 is transported within a server ODUk and OTUk (k = 1,2,3,4). This annex specifies a
low latency 1.25G OTU0 (OTU0LL) frame format in which one ODU0 is transported which carries a client
(e.g., 1G Ethernet) signal using an [ITU-T G.698.3] application code. This OTU0LL may be used at the edge of
the optical transport network.

G.2 Optical transport unit 0 low latency (OTU0LL)
The OTU0LL conditions the ODU0 for transport over a multi-vendor optical network interface at the edge of
the optical transport network. The OTU0LL frame structure, including the OTU0LL FEC is completely
standardized. The optical aspects of the multi-vendor optical network interface at the edge of the optical
transport network are outside the scope of this Recommendation.
NOTE 1 – Transport of the OTU0LL over the SOTU, MOTU, SOTUm and MOTUm interfaces specified in this
Recommendation is not supported.
NOTE 2 – An ODU0 which is transported within an OTU0LL may be passed through the OTN and terminated at the far
end edge of the OTN.
G.2.1 OTUk frame structure

The OTU0LL frame structure is based on the ODU0 frame structure and extends it with a distributed
forward error correction (FEC) as shown in Figure G.1. Sixteen times 16 columns are added to the ODU0
frame for the FEC and the reserved overhead bytes in row 1, columns 8 to 14 of the ODU0 overhead are
used for an OTU0LL specific overhead, resulting in an octet-based block frame structure with four rows and
4080 columns. The MSB in each octet is bit 1, the LSB is bit 8.

The OTU0LL overhead is the same as the OTUk overhead.
The bit rate of the OTU0LL signal is 255/239 × 1 244 160 kbit/s (~1 327 451.046 kbit/s). The frame period of
the OTU0LL signal is approximately 98.354 μs.
The sixty-four 16-byte RS(255,239) FEC fields in the OTL0LL frame contain the Reed-Solomon RS(255,239)
FEC code. Each RS(255,339) FEC is computed over the previous 239 OTU0LL bytes. Transmission of the
OTU0LL FEC is mandatory.
NOTE – The distribution of the RS(255,239) FEC fields over the OTU0LL frame minimizes the transfer delay introduced
by the processing of this FEC and the number of codecs to compute this FEC.

The RS(255,239) FEC code shall be computed as specified in Annex A with the notion that each FEC is
computed over the previous 239 OTU0LL bytes instead of over a sub-row as described in this annex for the
case of an OTUk (k=1,2,3,4).

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