Optical Transport Network (OTN) Tutorial

Transcription

1 Optical Transport Network (OTN) Tutorial Disclaimer: This is a Tutorial. This is NOT a Recommendation! This tutorial has no standards significance. It is purely for educational purposes. In case of conflict between the material contained in the tutorial and the material of the relevant Recommendation the latter always prevails. This tutorial should NOT be used as a reference; only the relevant Recommendations can be referenced. Summary This document provides a tutorial for Optical Transport Network standards and their applications. The objective is to provide the telecommunications engineers with a document that forms the basis for understanding OTN. Contact: Timothy P. Walker AMCC USA Tel: Fax: twalker@amcc.com

2 - - Scope...5 Abbreviations...5 What is OTN/OTH...7. OTN Standards...7 Where is it standardized for Why use OTN Forward Error Correction (FEC) Theoretical Description Coding Gain Tandem Connection Monitoring Transparent Transport of Client Signals Switching Scalability OTN Hierarchy Overview OTUk, ODUk, OPUk Frame Structure OPUk Overhead and Processing OPUk Overhead Byte Descriptions Payload Structure Identifier (PSI) Payload Type (PT) Mapping Signals into an OPUk Frequency Justification Mapping a CBRG5 signal (e.g. STM-6) into OPU Mapping a CBR0G signal (e.g. STM-6) into OPU Mapping a CBR0G signal (e.g. STM-56) into OPU ODUk Overhead and Processing Path Monitoring (PM) Byte Descriptions Trail Trace Identifier (TTI) BIP Backward Defect Indication (BDI) Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE) Path Monitoring Status (STAT) Tandem Connection Monitoring (TCM) Trail Trace Identifier (TTI) BIP Backward Defect Indication (BDI)...

3 Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE) TCM Monitoring Status (STAT) Tandem Connection Monitoring ACTivation/deactivation (TCM-ACT) General Communication Channels (GCC, GCC) Automatic Protection Switching and Protection Communication Channel (APS/PCC) Fault Type and Fault Location reporting communication channel (FTFL)... 0 OTUk Overhead and Processing Scrambling Frame Alignment Overhead Frame alignment signal (FAS) Multiframe alignment signal (MFAS) SM Byte Descriptions Trail Trace Identifier (TTI) BIP Backward Defect Indication (BDI) Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE) Incoming Alignment Error (IAE) General Communication Channel 0 (GCC0)...9 ODUk Multiplexing...9. Multiplexing Data Rates ODU to ODU Justification Rate ODU to ODU Justification Rate ODU to ODU Justification Rate...0. x ODU to ODU Multiplexing..... x ODU to ODU Multiplexing Structure..... x ODU to ODU Justification Structure..... OPU Payload Structure Identifier (PSI)..... OPU Multiplex Structure Identifier (MSI) Frequency Justification...5. ODU/ODU to ODU Multiplexing ODU/ODU to ODU Multiplexing Structure ODU/ODU to ODU Justification Structure OPU Payload Structure Identifier (PSI) OPU Multiplex Structure Identifier (MSI) Frequency Justification Maintenance Signal Insertion...5

4 - -.. Client source ODUk-AIS Client source ODUk-OCI Line source ODUk-AIS Defect detection and correlation dplm (Payload Mismatch) cplm dmsim (Multiplex Structure Identifier Mismatch supervision) cmsim dloflom (Loss of Frame and Multiframe) cloflom aais (AIS insertion) SSF (Server Signal Fail)...5 ODUk Virtual Concatenation / OTN over SONET...55 Synchronisation Introduction Network requirements Mapping and Multiplexing Equipment requirements...57 OTN Maintenance Signals OTUk maintenance signals OTUk alarm indication signal (OTUk-AIS) ODUk maintenance signals ODUk Alarm Indication Signal (ODUk-AIS) ODUk Open Connection Indication (ODUk-OCI) ODUk Locked (ODUk-LCK) Client maintenance signal Generic AIS for constant bit rate signals OTN Defects Acknowledgements Bibliography Open Issues ODUk Virtual Concatenation / OTN over SONET...6

5 - 5 - Scope This document provides a tutorial for Optical Transport Network standards and their applications. The objective is to provide the telecommunications engineers with a document that forms the basis for understanding OTN. Abbreviations This tutorial uses the following abbreviations: 0xYY R ACT AI AIS APS BDI BEI BIAE BIP CBR CI CM CRC DAPI EXP ExTI FAS FDI FEC GCC IaDI IAE IrDI JOH LSB MFAS MFI MS YY is a value in hexadecimal presentation Reamplification, Reshaping and Retiming Activation (in the TCM ACT byte) Adapted Information Alarm Indication Signal Automatic Protection Switching Backward Defect Indication Backward Error Indication Backward Incoming Alignment Error Bit Interleaved Parity Constant Bit Rate Characteristic Information Connection Monitoring Cyclic Redundancy Check Destination Access Point Identifier Experimental Expected Trace Identifier Frame Alignment Signal Forward Defect Indication Forward Error Correction General Communication Channel Intra-Domain Interface Incoming Alignment Error Inter-Domain Interface Justification Overhead Least Significant Bit MultiFrame Alignment Signal Multiframe Indicator Maintenance Signal

6 - 6 - MSB Most Significant Bit MSI Multiplex Structure Identifier NNI Network Node Interface OCh Optical channel with full functionality OCI Open Connection Indication ODU Optical Channel Data Unit ODUk Optical Channel Data Unit-k ODTUjk Optical channel Data Tributary Unit j into k ODTUG Optical channel Data Tributary Unit Group ODUk-Xv X virtually concatenated ODUk's OH Overhead OMS Optical Multiplex Section OMS-OH Optical Multiplex Section Overhead OMU Optical Multiplex Unit ONNI Optical Network Node Interface OOS OTM Overhead Signal OPS Optical Physical Section OPU Optical Channel Payload Unit OPUk Optical Channel Payload Unit-k OPUk-Xv X virtually concatenated OPUk's OSC Optical Supervisory Channel OTH Optical Transport Hierarchy OTM Optical Transport Module OTN Optical Transport Network OTS Optical Transmission Section OTS-OH Optical Transmission Section Overhead OTU Optical Channel Transport Unit OTUk Optical Channel Transport Unit-k PCC Protection Communication Channel PM Path Monitoring PMI Payload Missing Indication PMOH Path Monitoring OverHead ppm parts per million PRBS Pseudo Random Binary Sequence PSI Payload Structure Identifier PT Payload Type

7 - 7 - RES RS SAPI Sk SM SMOH So TC TCM TS TxTI UNI VCG VCOH vcpt Reserved for future international standardization Reed-Solomon Source Access Point Identifier Sink Section Monitoring Section Monitoring OverHead Source Tandem Connection Tandem Connection Monitoring Tributary Slot Transmitted Trace Identifier User-to-Network Interface Virtual Concatenation Group Virtual Concatenation Overhead virtual concatenated Payload Type What is OTN/OTH. OTN Standards There are many standards that fall under the umbrella of OTN. This document focuses on Layer standards. Therefore, it does not describe the Physical or Optical layers. Furthermore, it doesn t describe any layers implemented in Software. Thus this document only describes the digital layer that could be implemented in ASIC. 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.87 "Architecture for the Optical Transport Network (OTN)" G.87 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.87 development as noted in Section 9./G.87 : During the development of ITU-T Rec. G.709, (implementation of the Optical Channel Layer according to ITU-T Rec. G.87 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 listed in clause 6. 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. As they are defined and implemented, they will be included in this tutorial.

8 - 8 - Thus the main implementation of OTH is that described in G.709 and G.798. Where is it standardized for The optical transport network architecture as specified in ITU-T G.87 defines two interface classes: Inter-domain interface (IrDI); Intra-domain interface (IaDI). The OTN IrDI interfaces are defined with R processing at each end of the interface. This would be the interface between Operators. It could also be thought of as the interface between different Vendors within the same Operator (see Figure ). User Inter-Domain IrDI Inter-Domain IrDI Inter-Domain IrDI Operator A Operator B User R R R R R R Intra-Domain IaDI Intra-Domain IaDI Intra-Domain IaDI Intra-Domain IaDI Figure IaDI vs. IrDI The IaDI is the interfaces within an Operator or a Vendor domain. G.709 applies to information transferred across IrDI and IaDI interfaces. G.709 compliance by itself is not sufficient to guarantee mid-span meet, since it doesn t specify the electrical or optical interfaces. It is important to note that G.709 only defines logical interfaces. 5 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. 5. 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.

9 - 9 - For the OTN a Reed-Solomon 6 byte-interleaved FEC scheme is defined, which uses x56 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. 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. 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 R 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 of sup.dsn Also Appendix of G.975. lists some additional Enhanced FEC schemes. 5.. Theoretical Description G.709 FEC implements a Reed-Solomon RS(55,9) code. A Reed-Solomon code is specified as RS(n,k) with s-bit symbols where n is the total number of symbols per codeword, k is the number of information symbols, and s is the size of a symbol. A codeword consists of data and parity, also known as check symbols, added to the data. The check symbols are extra redundant bytes used to detect and correct errors in a signal so that the original data can be recovered. For G.709: s = Size of the symbol = 8 bits n = Symbols per codeword = 55 bytes k = Information symbols per codeword = 9 bytes A typical system is shown in Figure :

10 - 0 - Figure FEC Block Diagram This means the encoder takes k information symbols of s bits, each, and adds check symbols to make an n-symbol codeword. There are n-k check symbols of s bits, each. A Reed-Solomon decoder can correct up to t symbols that contain errors in a codeword, where t = n-k. The following Figure shows a typical Reed-Solomon codeword: Figure Reed-Solomon Codeword For the standard ITU recommended RS (55,9) code: t = n - k = 55-9 = 6 t = 8 Hence, the decoder can correct any 8 symbols in a codeword Reed-Solomon codes treat errors on a symbol basis; therefore, a symbol that contains all bits in error is as easy to detect and correct as a symbol that contains a single bit-error. That is why the Reed-Solomon code is particularly well suited to correcting burst errors (where a series of bits in the codeword are received in error by the decoder.) Given a symbol size s, the maximum codeword length (n) for a Reed-Solomon code is n = s = 55 Interleaving data from different codewords improves the efficiency of Reed-Solomon codes because the effect of burst errors is shared among many other codewords. By interleaving, it spreads the impact of a noise burst over multiple symbols, which come from several codewords. As long as each deinterleaved codeword has fewer errors than it can correct, the interleaved group of codewords will be corrected. It is possible that some codewords will be corrected and some not if excessive errors are encountered. Interleaving actually integrates the error correction powers of all of the codewords included in the interleaved group, that is the depth of the interleaver. This allows a higher rate of code and channel efficiency and still protects against an occasional very long error. For example, if 6 codewords that can correct 8 errors each are interleaved, the interleaved group can correct almost any combination of symbol errors that total less than 5. It does not matter if all 5 are in one long burst, there are 5

11 - - one-symbol errors, or anywhere in between. Both ITU-T G.709 and ITU-T G.975 specify interleaving as part of the transport frame to improve error-correction efficiency. 5.. Coding Gain The advantage of using FEC is that the probability of an error remaining in the decoded data is lower than the probability of an error if an FEC algorithm, such as Reed-Solomon, is not used. This is coding gain in essence. Coding Gain is difference in Input SNR for a given Output BER. The Input SNR is measured either as Q factor or as E b /N 0 (Section 5...), or OSNR (). The Net Coding Gain takes into effect that there was a 7% rate expansion due to the FEC. What this means is that the data rate had to increase by 7% in order to transmit both the data and the FEC Coding Gain measured via Q Factor The widely used technique of measuring coding gain is the Q-factor (Quality factor) measurement. This technique estimates the OSNR at the optical amplifier or receiver by measuring BER vs. voltage threshold at voltage levels where BER can be accurately determined (see Figures and 5). In reality, however, Q-factor is derived from the measurement of the eye-pattern signal. It is defined as the ratio of peak-to-peak signal to total noise (conventionally electrical): Figure Eye Diagram u = ON level average value σ = ON level noise standard deviation u 0 = OFF level average value σ 0 = OFF level noise standard deviation A system that requires an operating BER of 0-5 has a Q-factor measurement of 8 db without FEC If RS(55, 9) FEC is employed, the Q-factor measurement decreases to.8 db, yielding 6. db of coding gain.

12 - - BER vs Q for R-S 55 Code (t = 8).0E-0 BER in.0e-06 BER out No Overhead.0E-09 BER.0E-.0E-5.0E-8.0E Q (db) Figure 5 BER vs. Q Factor The 6. db gained through FEC would allow a transmission with longer span while maintaining the original BER. Thus the transmission distance is improved with relatively small increase in semiconductor content Coding Gain measured via E b /N 0 Another way to measure coding gain is with a plot of BER vs. E b /N 0. E b is the bit energy and can be described as signal power (S) times the bit time T b. N 0 is the noise power spectral density and can be described as the noise power (N) divided by the bandwidth (W). Thus E b /N 0 is equal to SNR * (Bandwidth/Bit Rate). For a more thorough discussion see [Sklar]

13 Uncorrected BER G db db Eb/No Figure 6 BER vs E b /N 0 What Figure 6 shows is that for a given input SNR (E b /N 0 ), what the BER out of the FEC decoder would be. Thus if one wanted to operate their system at 0 - BER, then they would need over db SNR without FEC or only 8.5 db with FEC Coding Gain measured via OSNR Figure 7 shows the FEC net coding gain (NCG) of various FEC schemes. These are theoretical and real measurements results from running systems. Coding gain is the reduction of signal-to-noise ratio due to the use of the FEC at a reference BER The Net Coding Gain (NCG) takes into account the fact that the bandwidth extension needed for the FEC scheme is associated with increased noise in the receiver. For example consider a reference BER of 0-5. The SDH in-band FEC provides a NCG of db. The standard OTN FEC a NCG of 6. db and a enhanced FEC a NCG of 9.5 db.

14 - - Unencoded SDH in-band FEC OTN standard FEC OTN EFEC (measured) OTN EFEC (theoretical) FEC margins 5. Tandem Connection Monitoring Figure 7 Coding Gain measured via OSNR SONET/SDH monitoring is divided into Section, Line and Path monitoring. A problem arises when you have Carrier s Carrier situation as shown in Figure 8, where it is required to monitor a segment of the path that passes another carrier network.

15 - 5 - Border of the OTN Client mapping into ODU Operator A Operator B Operator A User Working User Start of the OTN Client mapping into ODU Protection protection supervision (TCM) domain and domain interconnect supervision (TCM lead operator QoS supervision (TCM) user QoS supervision (TCM) end-to-end path supervision (PM) Figure 8 Tandem Connection Monitoring Here Operator A needs to have Operator B carry his signal. However he also needs a way of monitoring the signal as it passes through Operator B s network. This is what a Tandem connection is. It is a layer between Line Monitoring and Path Monitoring. SONET/SDH was modified to allow a single Tandem connection. G.709 allows six. TCM is used by the User to monitor the Quality of Service (QoS) that they see. TCM is used by the first operator to monitor their end-to-end QoS. TCM is used by the various domains for Intra domain monitoring. Then TCM is used for protection monitoring by Operator B. There is no standard on which TCM is used by whom. The operators have to have an agreement, so that they don t conflict. TCM s also support monitoring of ODUk (G.709 w/0 FEC) connections for one or more of the following network applications (refer to ITU-T G.805 and ITU-T G.87): optical UNI to UNI tandem connection monitoring; monitoring the ODUk connection through the public transport network (from public network ingress network termination to egress network termination); optical NNI to NNI tandem connection monitoring; monitoring the ODUk connection through the network of a network operator (from operator network ingress network termination to egress network termination); sublayer monitoring for linear +, : and :n optical channel subnetwork connection protection switching, to determine the signal fail and signal degrade conditions; sublayer monitoring for optical channel shared protection ring (SPRing) protection switching, to determine the signal fail and signal degrade conditions;

16 - 6 - Monitoring an optical channel tandem connection for the purpose of detecting a signal fail or signal degrade condition in a switched optical channel connection, to initiate automatic restoration of the connection during fault and error conditions in the network; Monitoring an optical channel tandem connection for, e.g., fault localization or verification of delivered quality of service. A TCM field is assigned to a monitored connection as described in /G.709. The number of monitored connections along an ODUk trail may vary between 0 and 6. Monitored connections can be nested, overlapping and/or cascaded. Nesting and cascading is shown in Figure 9. Monitored connections A-A/B-B/C-C and A-A/B-B are nested, while B-B/B-B are cascaded. TCM6 TCM6 TCM6 TCM6 TCM6 TCM6 TCM6 TCM5 TCM5 TCM5 TCM5 TCM5 TCM5 TCM5 TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM A B C C B B B A C-C B-B B-B A-A TCMi TCMi TCM OH field not in use TCM OH field in use Figure 9 ODUk monitored connections (Figure 5-6/G.709) T560-0 Overlapping monitored connections as shown in Figure 0 (B-B and C-C) are also supported.

17 - 7 - TCM6 TCM6 TCM6 TCM6 TCM6 TCM5 TCM5 TCM5 TCM5 TCM5 TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM TCM A B C B C A C-C B-B A-A TCMi TCMi TCM OH field not in use TCM OH field in use T Figure 0 Overlapping ODUk monitored connections (Figure 5-7/G.709) 5. Transparent Transport of Client Signals G.709 defines the OPUk which can contain the entire SONET/SDH signal. This means that one can transport four STM-6/OC-8 signals in one OTU and not modify any of the SONET/SDH overhead. Thus the transport of such client signals in the OTN is bit-transparent (i.e. the integrity of the whole client signal is maintained). It is also timing transparent. The asynchronous mapping mode transfers the input timing (Asynchronous mapping client) to the far end (Asynchronous demapping client). It is also delay transparent. For example if four STM-6/OC-8 signals are mapped into ODU s and then multiplexed into an ODU, their timing relationship is preserved until they are demapped back to ODU s. 5. Switching Scalability When SONET/SDH was developed in the mid eighties its main purpose was to provide the transport technology for voice services. Two switching levels were therefore defined. Lower order switching at.5/ Mbit/s to directly support the T/E voice signals and a higher order switching level at 50/50 Mbit/s for traffic engineering. Switching levels at higher bit rates were not foreseen. Over time the line rate increased while the switching rate was fixed. The gap between line rate and switching bit rate widened. Furthermore new services at higher bit rates (IP, Ethernet services) had to be supported. Contiguous and virtual concatenation were introduce in order to solve part of the services problem as they allow to support services above the standard SONET/SDH switching bit rates. The gap between line or service bit rate and switching bit rate however still exists as even with concatenation switching is performed at the STS-/VC- level.

18 - 8 - For a x0g to 0G SONET/SDH multiplexer this means processing of 56 VC- in the SDH case and even worse, processing of 768 STS--SPEs in the SONET case. This will result not only in efforts in the equipment hardware, but also in management and operations efforts. For efficient equipment and network design and operations, switching at higher bit rates has to be introduced. One could now argue that photonic switching of wavelengths is the solution. But with photonic switching the switching bit rate is bound to the bit rate of the wavelength and as such would be the service. A independent selection for service bit rates and DWDM technology is not possible. A operator offering.5 Gbit/s IP interconnection would need a Nx.5G DWDM system. When adding 0 G services he has to upgrade some of its wavelengths to 0G. This would lead to inefficient network designs. OTN provides the solution to the problem by placing no restrictions on switching bit rates. As the line rate grows new switching bit rates are added. A operator can offer services at various bit rates (.5G, 0G, ) independent of the bit rate per wavelength using the multiplexing and inverse multiplexing features of the OTN. 6 OTN Hierarchy Overview G.709 defines a number of layers in the OTN hierarchy (Figure ). Clients (e.g. STM-N, ATM, IP, Ethernet, OTN ODUk) OPUk ODUk ODUkP ODUkT OTUkV OTUk OTUkV OTUk OCh substructure OCh OChr OMSn OTSn OPSn OTM-n.m Full functionality OTM interface OTM-0.m, OTM-nr.m Reduced functionality OTM interface T580-0 Figure Full OTN Hierarchy Figure shows how they are envisioned being used in a network.

19 - 9 - optical sub-network domain IaDIs optical sub-network OTN IrDI domain optical sub-network IaDIs OTS OTS OTS OMS OMS OCh OCh OTU OTU OPS OChr OTU ODU OTS OTS OTS OTS OMS OMS OCh OTU Optical line amplifier (OTS termination) -R regeneration (OCh, OTU termination) Optical cross connect/add -drop/terminal mux (OMS termination) Figure OTN Network Layers However for all intents and purposes there are only four layers OCh Client access (ODU termination) OTUk ODUk OTN Hierarchy OPUk SONET/SDH CBR Signals Figure OTN Hierarchy The OPUk, ODUk, and OTUk are in the electrical domain. The OCh is in the Optical domain. There are more layers in the Optical domain than just the OCh, but they are not being used now. The OPUk encapsulates the Client signal (e.g. SONET/SDH) and does any rate justification that is needed. It is analogous to the Path layer in SONET/SDH in that it is mapped at the source, demapped at the sink, and not modified by the network. The ODUk performs similar functions as the Line Overhead in SONET/SDH. The OTUk contains the FEC and performs similar functions as the Section Overhead in SONET/SDH. After the FEC are added, the signal is then sent to a SERDES (Serializer/Deserializer) to be converted to the Optical Domain. The data rates were constructed so that they could transfer SONET/SDH signal efficiently. The bit rates are shown in the following tables:

20 - 0 - OTU OTU OTU Table OTU types and capacity (Table 7-/G.709) OTU type OTU nominal bit rate OTU bit rate tolerance 55/ kbit/s 55/ kbit/s 55/ kbit/s ±0 ppm NOTE The nominal OTUk rates are approximately: kbit/s (OTU), kbit/s (OTU) and kbit/s (OTU). ODU ODU ODU Table ODU types and capacity (Table 7-/G.709) ODU type ODU nominal bit rate ODU bit rate tolerance 9/ kbit/s 9/ kbit/s 9/ kbit/s ±0 ppm NOTE The nominal ODUk rates are approximately: kbit/s (ODU), kbit/s (ODU) and kbit/s (ODU). OPU OPU OPU Table OPU types and capacity (Table 7-/G.709) OPU type OPU Payload nominal bit rate OPU Payload bit rate tolerance 88 0 kbit/s 8/ kbit/s 8/ kbit/s ±0 ppm NOTE The nominal OPUk Payload rates are approximately: kbit/s (OPU Payload), kbit/s (OPU Payload) and kbit/s (OPU Payload). Table OTUk/ODUk/OPUk frame periods (Table 7-/G.709) OTU/ODU/OPU type OTU/ODU/OPU/OPU-Xv 8.97 µs OTU/ODU/OPU/OPU-Xv.9 µs OTU/ODU/OPU/OPU-Xv.05 µs NOTE The period is an approximated value, rounded to digits. Period (Note) 7 OTUk, ODUk, OPUk Frame Structure Figure shows the overall Frame format for an OTUk signal. The various fields will be explained in the following subsections.

21 - - Column Row PM and TCMi (i=..6) TTI BIP-8 ODUk Overhead OPUk Overhead OPUk Payload ( x 808 bytes) BIP8 Parity Block SAPI DAPI 5 BEI BDI OPUk OH Row# Frame Alignment overhead RES TCM ACT TCM6 TCM TCM TCM Column # TCM5 OTUk overhead PM TCM EXP FTFL OPUk overhead 6 Operator Specific 5 6 Mapping specific PSI GCC GCC APS/PCC RES PM: TCM: SAPI: DAPI: RES: ACT: Path Monitoring FTFL: Tandem Connection Monitoring EXP: Source Access Point Identifier GCC: Destination Access Point Identifier APS: Reserved for future international standardisation PCC: Activation/deactivation control channel Fault Type & Fault Location reporting channel Experimental General Communication Channel Automatic Protection Switching coordination channel Protection Communication Control channel TTI: Trail Trace Identifier BIP8: Bit Interleaved Parity - level 8 BEI: Backward Error Indication BDI: Backward Defect Indication STAT: Status PSI: Payload Structure Identifier PT: Payload Type 0 55 PT RES Figure OTN Frame Format (Figure 5-/G.709) 8 OPUk Overhead and Processing The OPUk (k =,,) frame structure is shown in Figure 5. It is organized in an octet-based block frame structure with four rows and 80 columns. Row Column OPUk overhead area OPUk payload area ( 808 bytes) T50-00 Figure 5 OPUk frame structure (Figure -/G.709) The two main areas of the OPUk frame are: OPUk overhead area; OPUk payload area;