Optical transport network
An optical transport network is a digital wrapper that encapsulates frames of data, to allow multiple data sources to be sent on the same channel. This creates an optical virtual private network for each client signal.
ITU-T defines an optical transport network as a set of optical network elements connected by optical fiber links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals. An ONE may re-time, re-Amplify, re-shape but it does not have to be 3R it can be purely photonic. Unless connected by optical fibre links, it shall not be OTN. Mere functionality of switching, management, supervision shall not make it OTN, unless the signals are carried through optical fibre. Unlike SONET/SDH, OTN provides a mechanism to manage multiplexed wavelengths in a DWDM system.
Comparing OTN and SONET/SDH
| OTN | SONET/SDH | |
| Scaling | 400Gbit/s | 40Gbit/s |
| Error correcting | ||
| Timing | Does not require | Requires |
| Octet-based block frame structure | Fixed, 16300 Byte | Variable, 2430- 622 080 Byte |
| Frame rate | Variable | 125 μs |
Standards
OTN was designed to provide higher throughput than its predecessor SONET/SDH, which stops at 40 Gbit/s, per channel.ITU-T Recommendation G.709 is commonly called Optical Transport Network . As of December 2009, OTN has standardized the following line rates.
The OTUk is an information structure into which another information structure called ODUk is mapped. The ODUk signal is the server layer signal for client signals. The following ODUk information structures are defined in ITU-T Recommendation G.709
| Signal | Data Rate | Typical Applications |
| ODU0 | 1.24416 | Transport of a timing transparent transcoded 1000BASE-X signal or a stream of packets using Generic Framing Procedure |
| ODU1 | 2.49877512605042 | Transport of two ODU0 signals or a STS-48/STM-16 signal or a stream of packets using Generic Framing Procedure. |
| ODU2 | 10.0372739240506 | Transport of up to eight ODU0 signals or up to four ODU1 signals or a STS-192/STM-64 signal or a WAN PHY or a stream of packets using Generic Framing Procedure |
| ODU2e | 10.3995253164557 | Transport of a 10 Gigabit Ethernet signal or a timing transparent transcoded Fibre Channel 10GFC signal |
| ODU3 | 40.3192189830509 | Transport of up to 32 ODU0 signals or up to 16 ODU1 signals or up to four ODU2 signals or a STS-768/STM-256 signal or a timing transparent transcoded 40 Gigabit Ethernet signal or a stream of packets using Generic Framing Procedure |
| ODU3e2 | 41.7859685595012 | Transport of up to four ODU2e signals |
| ODU4 | 104.794445814978 | Transport of up to 80 ODU0 signals or up to 40 ODU1 signals or up to ten ODU2 signals or up to two ODU3 signals or a 100 Gigabit Ethernet signal |
| ODUflex | x client bit rate | Transport of a constant bitrate signal such as Fibre Channel 8GFC, InfiniBand or Common Public Radio Interface |
| ODUflex | any configured rate | Transport of a stream of packets using Generic Framing Procedure |
Equipment
At a very high level, the typical signals processed by OTN equipment at the Optical Channel layer are:- SONET/SDH
- Ethernet/FibreChannel
- Packets
- OTN
- Protocol processing of all the signals:-
- *Mapping and de-mapping of non-OTN signals into and out of OTN signals
- *Multiplexing and de-multiplexing of OTN signals
- *Forward error correction on OTN signals
- Packet processing in conjunction with mapping/de-mapping of packet into and out of OTN signals
Switch Fabric