OTN Protocol Stack

The OTN protocol stack consists of 4 layers:

• Optical Channel (Och) - the lower level of protocols;
• Protocol OPU (Optical Channel Payload Unit) - block of user data of the optical channel;
• Protocol ODU (Optical Channel Data Unit) - data block of the optical channel;
• OTU protocol (Optical Channel Transport Unit) is an optical channel transport unit.

Optical Channel

The lower level of the protocols is the optical channel; this is usually a DWDM spectral channel. This level roughly corresponds to the photon level of SDH technology.

OPU Protocol

The OPU protocol is responsible for delivering data between network users. It provides:

• Encapsulating user data, such as Ethernet and SDH frames, in OPUs
• alignment of user data rates and OPUs;
• on the receiving side, retrieves user data and passes it to the user.
  

Depending on the data transfer rate, the OPU1, OPU2, OPU3 and OPU4 units correspond to this protocol. To perform its functions, the OPU protocol adds its OPU OH (OverHead) header to user data. OPUs are not network modified.

ODU Protocol

The ODU protocol works between the end nodes of the OTN network. Its functions include:

• multiplexing and demultiplexing of OPUU units. (For example, multiplexing four OPU1 blocks into one OPU2 block;
• monitoring the quality of connections in the OTN network.
  

This protocol generates ODUs of the required speed, adding its header to the corresponding OPUs. The ODU protocol is an analog of the SDH line protocol.

OTU Protocol

The OTU protocol works between neighboring nodes of the OTN network, which support the functions of 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 the OTUk block. The OTU protocol conforms to the SDH section protocol. OTUk units are placed directly in the optical channel.

An OTN frame is usually represented as a matrix of 4080 byte columns and four rows.
The frame consists of a user data field (Payload) and service fields of OPU, ODU, and OTU blocks. The frame format depends on the OTN speed level.
The user data field is located from the 17th to 3824th column and occupies all four rows of the frame, and the OPU block header occupies columns 15 and 16 also in four rows. If necessary, the OPU OH header can occupy several frames in a row (for example, this option occurs when you need to describe the structure of a user data field that multiplexes several lower-level OPUs).
The ODU is represented only by the ODU OH header. The OTU block consists of the OTU OH header and the OTU FEC trailer containing the FEC error correction code.
The frame begins with a small frame alignment field, necessary to recognize the beginning of the frame.

The direct error correction (FEC) procedure uses the Reed-Solomon RS code (255, 239). In this self-correcting code, data is encoded in blocks of 255 bytes, of which 239 bytes are user-defined, and 16 bytes are a correction code. Reed-Solomon codes allow you to correct up to 8 erroneous bytes in a block of 255 bytes.
The use of the Reed-Solomon code allows to improve the ratio of signal power to noise power by 5 dB while reducing the level of bit errors 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.

• Optical Transport Network Tutorial [1]

V. Olifer, N. Olifer, "Computer Networks", 5th edition