Function Implementation

The OTN standard system has defined all kinds of overhead bytes. This enables the OTN network to have the following advantages: effective monitoring function, flexible grooming capability at both the optical and electrical layers, and carrier-class, manageable, and operable network. The OTN processing module inside the board processes the OTN overheads and thus help the equipment monitor signals and report the performance events of the signals.

Electrical-Layer Overhead of the OTN

The OTN protocol has defined the hierarchy and interfaces of the OTN. An overhead of the OTN frame consists of optical-layer overhead and electrical-layer overheads.

The OptiX OSN 1800(NA) does not support monitoring of the optical-layer overhead.

Figure 1 illustrates the electrical-layer overhead of the OTN in addition to the overhead frame structure.

The optical-layer overheads of the OTN consist of OPUk overhead, ODUk overhead, OTUk overhead, and frame alignment overhead. Table 1 provides the definition of each overhead byte.

Figure 1 Electrical-layer overheads of the OTN

OTUk: complete standardized optical channel transport unit-k	OTUkV: functionality-standardize optical channel transport unit-k
ODUk: optical channel data unit-k	OPUk: optical channel payload unit-k

NOTE:

The index "k" represents a supported bit rate of the OTN signals. ITU-T G.709 has defined three levels of bit rate:

k = 1 represents an approximate bit rate of 2.5 Gbit/s.
k = 2 represents an approximate bit rate of 10 Gbit/s.
k = 3 represents an approximate bit rate of 40 Gbit/s.

For detailed information, see ITU-T G.709.

The nominal OPU1 Payload rate is approximately: 2 488 320.000 kbit/s. The nominal OPU1-xV (x is natural number) rate is approximately: 2.48 * x Gbit/s, the WDM device supports x = 2 at present. The rate of OTUkV is 5 Gbit/s at present.

**Table 1** Definitions of the OTN overhead bytes and their relationship between equipment
Overhead Type	Overhead Byte	Meaning of the Overhead Byte	Relationship Between Equipment
OPUk overhead	PSI	Payload structure identifier	The OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A support monitoring the OPUk overhead and reporting the performance events of the payload. The OptiX OSN 1800(NA) does not supports monitoring the OPUk overhead and reporting the performance events of the payload.
	JC	Justification control bytes	Not in use at present.
	NJO	Negative justification opportunity bytes	Not in use at present.
	RES	Reserved for future international standardization	Not in use at present.
ODUk overhead	TCMACT	TCM activation/deactivation coordination protocol control channel	The OptiX OSN 1800(NA) does not support monitoring the TCM overhead. The OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A support monitoring the overhead of a maximum of six TCM sublayers and reporting the performance events of the payload.
	TCMn (n = 1-6)	Tandem connection monitoring sublayer n overhead
	FTFL	Fault type and fault location reporting communication channel	The OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A support the FTFL function.
	PM	Path monitoring	The OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A support monitoring the PM overhead and reporting the performance events of the payload. The OptiX OSN 1800(NA) supports monitoring the PM overhead and reporting the performance events of the payload.
	Ingress PW Priority	Experimental overhead	Used for the experiment purpose only.
	GCC1/GCC2	Generic communication channel 1/2	In the case of the OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A, either the GCC1 or GCC2 byte can be used as the communication channel of the management information. In the case of the OptiX OSN 1800(NA), either the GCC1 or GCC2 byte can be used as the communication channel of the management information.
	APS/PCC	Automatic protection switching and protection communication control channel	The OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A use the APS/PCC overhead to realize the ODU protection function defined by the OTN standard. The OptiX OSN 1800(NA) uses the APS/PCC overhead to realize the ODU protection function defined by the OTN standard.
OTUk Overhead	SM	Section monitoring	The OptiX OSN 1800(NA) supports monitoring the SM overhead and reporting the performance events of the payload. The OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A support monitoring the SM overhead and reporting the performance events of the payload.
	GCC0	Generic communication channel 0	In the case of the OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A, the GCC0 byte is used for realizing communication between the OTUk terminals. In the case of the OptiX OSN 1800(NA), the GCC0 byte is used for realizing communication between the OTUk terminals.
	RES	Reserved for future international standardization	Not in use at present.
Frame alignment overhead	FAS	Frame alignment signal	The OptiX OSN 8800 T64(NA)/8800 T32(NA)/8800 T16(NA)/6800A/3800A support monitoring the frame alignment overhead and reporting the performance events of the payload. The OptiX OSN 1800(NA) supports monitoring the frame alignment overhead and reporting the performance events of the payload.
Frame alignment overhead	MFAS	Multiframe alignment signal

Optical-Layer Overheads of the OTN

The OTN network interface determines the OTN signal structure. The optical transport module (OTM) is generated after the optical overhead is inserted to the OTUk (v) signal. The OTM overhead includes:

OTS overhead
OMS overhead
OCh overhead
Generic management information

Figure 2 shows the basic information structure of the OTM. For details, see recommendation G.798.

Figure 2 Basic information structure of the OTM

One optical supervisory channel (OSC) is used for transporting the OTM overhead signals (OOS).