DWDM networks are not initially digital networks because they provide users with only a few spectral channels, which are only data transmission medium. Also originally DWDM multiplexers perform SDH multiplexer functionality, in consequence of which inherited a number of shortcomings that became apparent with increasing data rates.
Optical Transport Network (OTN)
Disadvantages DWDM systems inherited from SDH systems:
- Insufficient effectiveness of FEC codes, taken as SDH standard. This prevents further increase the spectral density of channels in DWDM multiplexers.
- Too "small" switching units for backbone networks operating at speeds of 10 and 40 Gb / s. Even the maximum size of the containers VC-4 (140 Mbit / s) are large enough for the unit of STM-multiplexer 256, which must commute up to 256 containers for each of its ports.
- Not taken into account peculiarities of different types of traffic. SDH technology developers took into account only the voice traffic.
To overcome these drawbacks, a new technology aimed optical transport networks (Optical Transport Network, OTN)
Hierarchy speeds
Currently three standardized OTN rate, are selected so as to transparently transmit frames STM-16, STM-64 and STM-256, along with service headings.
Hierarchy OTN speeds
The protocol stack OTN
The protocol stack includes four layers:
- Protocol OPU (Optical Channel Payload Unit - Custom Unit Optical Channel Data);
- Protocol ODU (Optical Channel Data Unit - Optical Channel Data Unit);
- Protocol OTU (Optical Channel Transport Unit - transport block optical channel;
- Optical Channel (Optical Channel, Och).
The protocol stack OTN Optical Channel, Och - This spectrum DWDM channel.
OPU (Optical Channel Payload Unit ) - Responsible for the delivery of data between network users. Engaged encapsulated user data, such as Ethernet or SDH frames, in units OPU, alignment transmission rates of user data blocks and OPU, and on the receiving side extracts user data and transmits them to the user.
ODU (Optical Channel Data Unit) - Just as the OPU protocol operates between the end OTN network nodes. Its functions include multiplexing and demultiplexing unit OPU.
OTU (Optical Channel Transport Unit) - It works between two adjacent nodes OTN network that supports electrical regeneration of the optical signal function, also called function 3R (retiming, reshaping and regeneration).
Кадр OTN
Leveling of speed
As with other technologies based on the synchronous multiplex the TDM, OTN technology to solve the problem of equalizing rates of user streams to the multiplexer data rate. Alignment mechanism OTN speeds is some hybrid of the mechanism of bit-stuffing PDH technology and mechanism of the positive and negative justification on the basis of indicators used in SDH technology.
The Working of OTN alignment mechanism depends on the load display mode - Synchronous or Asynchronous mode.
Synchronous load mode - OTN multiplexer synchronizes reception and transmission of synchronization pulses which are in the received user data stream.
Asynchronous load mode - OTN multiplexer is synchronized by its own clock source which is independent from the user data.
To equalize the velocities in the OTN frame, two bytes are used:
- Positive Justification Opportunity, PJO;
- Negative Justification Opportunity, NJO.
Multiplexing units
ODU multiplexing user data field OPUk is divided block units on the so-called tributary slots (Tributary Slot, TS), which are placed in the data block OPUk-1.
OTN multiplexing units
Error correction
In the procedure OTN forward error correction (FEC), which uses RS Reed-Solomon code (255,239). This self-correcting code data is encoded in blocks of 255 bytes, 239 bytes of which are user-defined, and 16 bytes are correction code. Reed-Solomon codes allow to correct up to 8 erroneous bytes in a block of 255 bytes, which is a very good performance for the self-correcting code. The use of Reed-Solomon code allows to improve the signal-to-noise power by 5 dB. This effect makes it possible to increase the distance between regenerators network by 20 km or use a less powerful signal transmitters.
