G.709
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ITU-T Recommendation G.709 "Interfaces for the optical transport network (OTN))" is a standardized method for transporting optical wavelengths in the optical transport hierarchy (OTH). G.709 allows the use of completely optical switches (Optical Cross-Connect) without requiring expensive optical/electrical/optical {O/E/O} conversions.
The frame structure defined in G.709 includes an area that is reserved for Forward Error Correction (FEC) codes. The standard FEC code (described in G.975) is a Reed-Solomon coding calculated across the payload. This allows detection and correction of bit errors due to impurities in optical equipment, problems with marginal equipment and fibers. The FEC code also extends the distance the optical signal can travel before requiring regeneration. The OAM&P overhead added by G.709 enhances the QoS of WDM and DWDM equipment.
G.709 is positioned as a server layer for several client signals, e.g. SDH/SONET, ATM, IP, Ethernet, and OTN ODUk. Because G.709 is an evolution of SDH (G.707) its frame structures and frame repetition rates are based on that technology.
The basic frame structure is the ODUk that provides the payload area of the OPUk that is transported by the OTUk.
| SDH Line Signal (G.707) | SONET Line Signal (T1.105) | OTH Line Signal (G.709) | G.709 Line Rate (kbit/s) | OTUk Payload Rate (kbit/s) | OTUk frame period (µs) |
|---|---|---|---|---|---|
| STM-16 | OC-48 (STS-48 SPE) | OTU1 | 2 666 057 | 2 488 320 | 48.971 |
| STM-64 | OC-192 (STS-192 SPE) | OTU2 | 10 709 225 | 9 953 280 | 12.191 |
| STM-256 | OC-768 (STS-768 SPE) | OTU3 | 43 018 41 | 39 813 120 | 3.035 |
Unlike SDH/SONET, the line rate is increased by maintaining the G.709 frame structure (4 rows x 4080 columns) and decreasing the frame period (in SDH/SONET the frame structure is increased and the frame period of 125 µs is maintained).
