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ITU-T Recommendation G.709 "Interfaces for the optical transport network (OTN))" describes a means of communicating data over an optical network. It is a standardized method for transparent transport of services over optical wavelengths in DWDM systems. It also known as Optical Transport Hierarchy (OTH) standard.
G.709 is positioned as a server layer for various client signals, e.g. SDH/SONET, ATM, IP, Ethernet, and OTN ODUk (where k=1, 2 or 3)[1]
The frame structure defined in G.709 is constructed of 4 areas:
- OPUk[1] is the area in which payload is mapped.
- ODUk[1] contains the OPUk with additional overhead bytes (e.g. TTI, BIP8, GCC1/2, TCM etc).
- OTUk[1] is the section and includes framing , TTI, BIP8 and GCC0 bytes.
- FEC - The standard FEC code (described in G.975) is a Reed-Solomon coding calculated across the payload (OPU)columns. This allows detection and correction of bit errors due to impurities in optical equipment, and problems with marginal equipment and fibers. The FEC code also extends the distance the optical signal can travel before requiring regeneration.
G.709 offers rich OAM&P capabilities such as TCM (Tandem Connection Monitoring), End to End performance monitoring, misconnection detection and General Communication Channel (GCC).
SDH/SONET and G.709 rates
| SDH Line Signal (G.707) | SONET Line Signal (T1.105) | OTH Line Signal (G.709) | G.709 Line Rate (kbit/s) | OPUk 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 | 42,836,900 | 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).
