OTN (Optical Transport Network) Is an optical transport network that provides transmission and multiplexing of digital data over DWDM wavelength channels.
OTN protocol stack
The OTN protocol stack consists of 4 layers:
- Optical Channel (Och) – the lower level of the protocols;
- OPU (Optical Channel Payload Unit) – user data unit;
- ODU (Optical Channel Data Unit) protocol – optical channel data unit;
- OTU (Optical Channel Transport Unit) is an optical channel transport unit.
Optical channel
The lower layer of the protocols is the optical channel; usually a DWDM spectral channel. This corresponds to the photonic level of SDH technology.
OPU protocol
The OPU protocol is responsible for delivering data between network users. It provides:
- encapsulation of user data such as Ethernet and SDH frames in OPUs;
- alignment of user data rates and OPUs;
- on the receiving side, extracts user data and transfers it to the user.
The OPU1, OPU2, OPU3 and OPU4 units correspond to this protocol depending on the baud rate. To perform its functions, the OPU protocol adds its own OPU OH (OverHead) header to user data. OPUs are not network modified.
ODU protocol
ODU protocol runs between the end nodes of the OTN network. Its functions include:
- multiplexing and demultiplexing of OPUUs. (For example, multiplexing four OPU1s into one OPU2;
- monitoring the quality of connections in the OTN network.
This protocol forms ODUs of the required speed by adding its own header to the corresponding OPUs. The ODU protocol is analogous to the SDH line protocol.
OTU protocol
The OTU protocol works between adjacent nodes of the OTN network, which support electrical regeneration of the optical signal, also called 3R (retiming, reshaping and regeneration) functions. The main purpose of the protocol is to control and correct errors using FEC codes. This protocol adds its trailer to the ODUk block containing the FEC code, forming an OTUk block. The OTU protocol corresponds to the SDH section protocol. OTUk blocks are placed directly on the optical channel.
OTN frame
An OTN frame is usually represented as a matrix of 4080 column-bytes and four rows.
The frame consists of a user data field (Payload) and service fields of OPU, ODU and OTU. The frame format depends on the OTN rate level.
The user data field is located from 17th to 3824th columns and occupies all four lines of the frame, and the OPU header occupies columns 15 and 16 also in four lines. If necessary, the OPU OH can occupy several frames in a row (for example, this option occurs when it is necessary to describe the structure of the user data field, multiplexing several lower-level OPUs).
The ODU is represented by the ODU OH only. The OTU consists of an OTU OH header and an trailer containing the FEC error correction code.
The frame starts with a small frame alignment field, which is necessary to recognize the start of the frame.
OTN speed hierarchy
Correction of errors
Correction (FEC) uses Reed-Solomon codes RS (255, 239). In this self-correcting code, the data is encoded in blocks of 255 bytes, of which 239 bytes are user-defined and 16 bytes are the correcting code. Reed-Solomon codes can correct up to 8 erroneous bytes in a 255 byte block.The use of the Reed-Solomon code improves the signal-to-noise power ratio by 5 dB while reducing the bit error rate from 10-3 (without using FEC) to 10-2 (after applying FEC). This effect makes it possible to increase the distance between the network generators by 20 km or to use less powerful signal transmitters.
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