The Optical Transport Hierarchy OTH is a new transport technology for the Optical Transport Network OTN developed by the ITU. It is based on the network architecture defined in ITU G.872 “Architecture for the Optical Transport Network (OTN)” G.872 defines an architecture that is composed of the Optical Channel (OCh), Optical Multiplex Section (OMS) and Optical Transmission Section (OTS). It then describes the functionality that needed to make OTN work. However, it may be interesting to note the decision made during G.872 development as noted in Section 9.1/G.872 : “During the development of ITU-T Rec. G.709, (implementation of the Optical Channel Layer according to ITU-T Rec. G.872 requirements), it was realized that the only techniques presently available that could meet the requirements for associated OCh trace, as well as providing an accurate assessment of the quality of a digital client signal, were digital techniques….” “For this reason ITU-T Rec. G.709 chose to implement the Optical Channel by means of a digital framed signal with digital overhead that supports the management requirements for the OCh. Furthermore this allows the use of Forward Error Correction for enhanced system performance. This results in the introduction of two digital layer networks, the ODU and OTU. The intention is that all client signals would be mapped into the Optical Channel via the ODU and OTU layer networks.” Currently there are no physical implementations of the OCh, OMS and OTS layers.
Why use OTN
OTN offers the following advantages relative to SONET/SDH:
- Stronger Forward Error Correction
- More Levels of Tandem Connection Monitoring (TCM)
- Transparent Transport of Client Signals
- Switching Scalability
OTN has the following disadvantages:
- Requires new hardware and management system
We will discuss the advantages and disadvantages in the following sections.
Forward Error Correction (FEC)
Forward error correction is a major feature of the OTN.
Already SDH has a FEC defined. It uses undefined SOH bytes to transport the FEC check information and is therefore called a in-band FEC. It allows only a limited number of FEC check information, which limits the performance of the FEC.
For the OTN a Reed-Solomon 16 byte-interleaved FEC scheme is defined, which uses 4×256 bytes of check information per ODU frame. In addition enhanced (proprietary) FEC schemes are explicitly allowed and widely used.
FEC has been proven to be effective in OSNR limited systems as well as in dispersion limited systems. As for non-linear effects, reducing the output power leads to OSNR limitations, against which FEC is useful. FEC is less effective against PMD, however.
G.709 defines a stronger Forward Error Correction for OTN that can result in up to 6.2 dB improvement in Signal to Noise Ratio (SNR). Another way of looking at this, is that to transmit a signal at a certain Bit Error Rate (BER) with 6.2 dB less power than without such an FEC.
The coding gain provided by the FEC can be used to:
– Increase the maximum span length and/or the number of spans, resulting in an extended reach. (Note that this assumes that other impairments like chromatic and polarization mode dispersion are not becoming limiting factors.)
– Increase the number of DWDM channels in a DWDM system which is limited by the output power of the amplifiers by decreasing the power per channel and increasing the number of channels. (Note that changes in non-linear effects due to the reduced per channel power have to be taken into account.)
– Relax the component parameters (e.g launched power, eye mask, extinction ratio, noise figures, filter isolation) for a given link and lower the component costs.
– but the most importantly the FEC is an enabler for transparent optical networks: Transparent optical network elements like OADMs and PXCs introduce significant optical impairments (e.g. attenuation). The number of transparent optical network elements that can be crossed by an optical path before 3R regeneration is needed is therefore strongly limited. With FEC a optical path can cross more transparent optical network elements.
This allows to evolve from today’s point-to-point links to transparent, meshed optical networks with sufficient functionality.
Note: There is additional information on FEC in Section 11 of sup.dsn Also Appendix 1 of G.975.1 lists some additional Enhanced FEC schemes.
OPUk Overhead and Processing
The OPUk (k = 1,2,3) frame structure is shown in Figure 15. It is organized in an octet-based block frame structure with four rows and 3810 columns.
The two main areas of the OPUk frame are:
• OPUk overhead area;
• OPUk payload area;
Columns 15 to 16 of the OPUk are dedicated to OPUk overhead area.
Columns 17 to 3824 of the OPUk are dedicated to OPUk payload area.
NOTE – OPUk column numbers are derived from the OPUk columns in the ODUk frame
OPUk OH information is added to the OPUk information payload to create an OPUk. It includes information to support the adaptation of client signals. The OPUk OH is terminated where the OPUk is assembled and disassembled.
OPUk Overhead Byte Descriptions
The OPUk Overhead bytes are shown in Figure 16
Payload Structure Identifier (PSI)
The 256-byte PSI signal is aligned with the ODUk multiframe (i.e. PSI is present at ODUk multiframe position 0000 0000, PSI at position 0000 0001, PSI at position 0000 0010, etc.). PSI contains a one-byte Payload type. PSI to PSI are mapping and concatenation specific. Click here for more infromation http://www.phpini.in/otn/psi-payload-structure-identifier.html
Payload Type (PT)
A one-byte payload type signal is defined in the PSI byte of the payload structure identifier to indicate the composition of the OPUk signal. The code points are defined in Table 5.
Mapping Signals into an OPUk
There are a number of Payload Types defined in Table 5..