Agilent makes light work in theThe next generation optical transport network (OTN)enables the interconnection of network elements for thetransport of different types of client signals, for exampleSONET/SDH, ATM, IP/Ethernet.

The network node interface of the OTN is defined in ITU-TG.709 and includes a rich overhead and forward errorcorrection.

OTU type and capacity

OTN

OPUk OH

The Optical Channel Payload Unit overhead includes information to support the adaptation of clientsignals. It is terminated where the OPUk is assembled and disassembled, that is, at the edges ofthe OTN.

Mapping Specific Overhead

Seven bytes are reserved for mapping overhead. The use of these bytes depends on the specificclient signal mapping. For example, mapping SONET/SDH uses four of these bytes for justification.

Payload Structure Identifier (PSI)

This is a 256 byte multiframe signal contained within an OPUk OH byte.PSI [0] contains the payload type (PT) byte.PSI [1] to PSI [255] are reserved for future international standardization.

Payload Type (PT)

Payload Type code points:

Mapping of SONET/SDH client signals

Mapping may be performed using asynchronous or bit-synchronous modes.

Justification Control (JC)

There are three justification control bytes, each containing twojustification control bits. The two bits are used to indicatewhether the negative or positive justification opportunity bytescontain data. A majority vote (two out of three JC bytes) are usedfor error protection.

The value contained in the Negative Justification Opportunity(NJO) and Positive Justification Opportunity (PJO) when they areused as justification bytes is all zeros. In bit-synchronous modeall JC bits are set to “00” and PJO is always a data byte.

Payload

The OPUk payload area is 4 ´ 3808 bytes, including the PJO byte. Groups of eight successive bits (not necessarily being a byte) of the SONET/SDHsignal are mapped into payload bytes of the OPUk.

In the case of OPU2 and OPU3 fixed stuff bytes are added in specific columns in order to allow for future ODU multiplexing. For example fourODU2s into an ODU3.

The Optical Transport Unit overhead is terminated where the OTUk signal is assembled anddisassembled. It is similar to the section overhead in SONET or the Regenerator Sectionoverhead in SDH.

Section Monitoring contains the following subfields:

Trail Trace Identifier (TTI)

64 byte multi-frame signal, contained in one overhead byte. Contains the source and destinationaccess point identifiers (API) for the current section. The API are made of Country Code (CC), ITUCarrier Code (ICC) and Unique Access point code (UAPC).

The TTI also contains 32 operator-specific bytes.

Error Detection Code (BIP-8)

This byte is a Bit Interleaved Parity computed over all the bits in the OPUk (col 15 - 3824), twoframes perviously. It is used to determine error performance across a section.

Backward Defect Indication (BDI)

This one bit is high when a signal fail statushas been detected in the backward direction.

Backward Error Indication (BEI)

Four bit signal to convey the number of BIP errors in the reverse direction. Only binary values 0 to 8 are valid. Any othervalue is interpreted as zero.

Incoming Alignment Error (IAE)

One bit signal used to indicate a framealignment error in the incoming OTUk.

This may be used to suppress the counting of bit errors during a frame phase change.

Reserved (RES)

Two bits are reserved for future standardization. They are set to “00”.

General Communications Channel 0 (GCC0)

Two bytes are allocated as a communications channel between section end points. It is a clear channel and any formatcan be used. The rate is dependent upon the k value.

Frame Alignment Signal (FAS)

The six-byte frame alignment signal is Hex codes F6 F6 F6 28 28 28.

This contains end-to-end path overhead signals and tandem connection overhead signals. The ODUk path OH is terminated where theODUk is assembled and disassembled. The ODUk TC OH is added and terminated at the source and sink of the corresponding tandemconnection.

It is similar to the Path Overhead in both SONET and SDH.

Path Monitoring

The three path monitoring bytes are very similar to the Section Monitoringbytes except that they apply to the ODUk path.

• Trail trace identifier (TTI)• Bit-interleaved parity (BIP-8) refer to OTUk Overhead• Backward defect indication (BDI) for explanation• Backward error indication(BEI)• Status bits indicating presence of a maintenance signal (STAT).

Path Monitoring Status (STAT)

These three bits replace the IAE and RES bits in the OTUk section monitoringarea. They are used to indicate the presence of a maintenance signal.

ODUk PM status interpretationTandem Connection Monitoring Activation/Deactivation coordination protocol (TCM ACT)

This one-byte signal is presently undefined.

General Communication Channels (GCC1, GCC2)

Two fields of two bytes are allocated to support two GCCs between any two network elements with access to the ODUk frame.

Automatic Protection Switching and Protection Control Path (APS/PCC)

These four bytes are currently undefined.

Fault Type and Fault Location reporting communication channel (FTFL)

This is a 256-byte multiframe signal contained within one ODUk OH byte.

Fault Indication Codes

The FTFL field is used to report a fault type (signal fail or signal degrade) and fault location (country and operator) in the forward and backwarddirections.

Experimental Overhead (EXP)

These two bytes are for experimental use by a network operator. There is no requirement to forward the EXP overhead beyond the (sub)network.

Reserved overhead (RES)

A total of nine bytes are reserved in the ODUk for future international standardization. These bytes are set to all zeros.

Tandem Connection Monitoring Overhead

An optical channel can support up to six layers of tandem connection. Six sets of three bytes are used to monitor them: TCM1-6.

Each set of three bytes is very similar to the path monitoring bytes above, except that they apply to the tandem connection.

• Trail Trace Identifier (TTI);• Bit Interleaved Parity 8 (BIP-8);• Backward Defect Indication (BDI);• Backward Error Indication (BEI);• Status bits indicating the presence of TCM overhead, Incoming Alignment Error, or a maintenance signal (STAT).

Tandem Connection Monitoring Status (STAT)

These three bits are used to indicate the presence of a maintenance signal, if there is an incoming alignment error, or if there is noactive source.

ODUk TCM status interpretation

OTUk Alarm Indication Signal (OTUk-AIS)

This is a generic signal sent downstream when a defecthas been detected in order to suppress defects or faults.It consists of a pseudo-random bit sequence (PRBS)211–1, which replaces the entire OTUk.

ODUk Alarm Indication Signal (ODUk-AIS)

The ODUk alarm indication signal consists of all ones inthe entire ODUk signal, excluding FTFL byte. This doesnot overwrite the Frame Alignment or OTUk overhead

ODUk Open Connection Indication (ODUk-OCI)

An open connection indication is sent downstream whenno signal is detected from upstream. It consists of arepeating “0110 0110” pattern in the entire ODUk signal,which excludes the Frame Alignment overhead and theOTUk overhead.

ODUk Locked (ODUk-LCK)

The locked maintenance signal is generated on operatorrequest in order to lock the signal from user access. Itconsists of a repeating “0101 0101” pattern in the entireODUk signal, which excludes the Frame Alignmentoverhead and the OTUk overhead.

Client Maintenance Signal (Generic AIS)

The generic alarm indication signal for constant bit rateclient signals is a PRBS 211–1 repeating sequence.

Multi Frame Alignment Signal (MFAS)

This byte is a binary count from 0 to 255, incrementing once per frame.Some overhead signals span multiple frames and use this signal to lockto a common multi-frame.

Forward Error Correction (FEC) can be used to provide more system margin if the span length remains constant or to increase span lengthwith a given BER objective and optical power. FEC detects and corrects errors to effectively deliver a 7 to 8 dB improvement in signal-to-noise ratio i.e. margin. For ITU G.709 the FEC code used is a Reed-Solomon RS(255,239). This is byte interleaved to increase burst errorperformance.

The FEC check parity bytes are added when the OTUk structure is generated and they are located in columns 3825 to 4080.

Before FEC processing, each OTU row is separated into 16 sub-rows using byte-interleaving. Each FEC encoder/decoder processes one ofthese sub-rows. The FEC parity check bytes are calculated over the 239 information bytes of each sub-row and transmitted in the last16bytes of the same sub-row.

Frame Alignment OPUk OverheadOTUk Overhead

ODUk Overhead

Forward Error Correction (FEC)Maintenance Signals

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As telecom technology continues to evolve,there’s every reason to choose OmniBEROTN now. Not only can it solve today’sSONET/SDH problems, it’s ideal for testingdevices and modules for the optical transportnetwork (OTN) to ITU-T G.709. It has beendesigned to evolve as transmissiontechnology evolves, providing an upgradepath to jitter, gigabit ethernet and 40 Gb/s.In short, it’s the platform of the future, today.

G.709

Product specification and descriptions in this document subject to change without notice.

© Agilent Technologies, Inc. 2001 • Printed in UK, 5 November, 2001 • 5988-4004EN

Data are based on information, diagrams and figures taken from ITU-T Recommendations with thekind permission of the International Telecommunication Union (ITU) as copyright holder. Whereappropriate, please consult the full text of the relevant ITU Recommendations for the authoritativeversion. These may be obtained from the ITU Sales Section, Place des Nations, CH-1211 Geneva 20,Switzerland; Tel.: +41.22.730.61.41, Fax: +41.22.730.51.94, E-mail: sales@itu.int. www.agilent.com/comms/otn