Agilent makes light work in the OTN
G.709
The next generation optical transport network (OTN)
enables the interconnection of network elements for the
transport of different types of client signals, for example
SONET/SDH, ATM, IP/Ethernet.
The network node interface of the OTN is defined in ITU-T
G.709 and includes a rich overhead and forward error
correction.
Frame Alignment
Frame Alignment Signal (FAS)
Multi Frame Alignment Signal (MFAS)
The six-byte frame alignment signal is Hex codes F6 F6 F6 28 28 28.
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 lock
to a common multi-frame.
OTUk Overhead
The Optical Transport Unit overhead is terminated where the OTUk signal is assembled and
disassembled. It is similar to the section overhead in SONET or the Regenerator Section
overhead in SDH.
OPUk Overhead
The Optical Channel Payload Unit overhead includes information to support the adaptation of client
signals. It is terminated where the OPUk is assembled and disassembled, that is, at the edges of
the OTN.
OPUk OH
Mapping Specific Overhead
Section Monitoring contains the following subfields:
Trail Trace Identifier (TTI)
ODUk Overhead
Seven bytes are reserved for mapping overhead. The use of these bytes depends on the specific
client signal mapping. For example, mapping SONET/SDH uses four of these bytes for justification.
64 byte multi-frame signal, contained in one overhead byte. Contains the source and destination
access point identifiers (API) for the current section. The API are made of Country Code (CC), ITU
Carrier Code (ICC) and Unique Access point code (UAPC).
Payload Structure Identifier (PSI)
The TTI also contains 32 operator-specific bytes.
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.
Error Detection Code (BIP-8)
This byte is a Bit Interleaved Parity computed over all the bits in the OPUk (col 15 - 3824), two
frames perviously. It is used to determine error performance across a section.
Payload Type (PT)
Payload Type code points:
Backward Defect Indication (BDI)
This contains end-to-end path overhead signals and tandem connection overhead signals. The ODUk path OH is terminated where the
ODUk is assembled and disassembled. The ODUk TC OH is added and terminated at the source and sink of the corresponding tandem
connection.
This one bit is high when a signal fail status
has 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 other
value is interpreted as zero.
Incoming Alignment Error (IAE)
One bit signal used to indicate a frame
alignment error in the incoming OTUk.
This may be used to suppress the counting of bit errors during a frame phase change.
Reserved (RES)
It is similar to the Path Overhead in both SONET and SDH.
Two bits are reserved for future standardization. They are set to “00”.
Path Monitoring
General Communications Channel 0 (GCC0)
The three path monitoring bytes are very similar to the Section Monitoring
bytes except that they apply to the ODUk path.
Two bytes are allocated as a communications channel between section end points. It is a clear channel and any format
can be used. The rate is dependent upon the k value.
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ü
Trail trace identifier (TTI)
ï refer to OTUk Overhead
Bit-interleaved parity (BIP-8)
ý
Backward defect indication (BDI) ï for explanation
þ
Backward error indication(BEI)
Status bits indicating presence of a maintenance signal (STAT).
Mapping of SONET/SDH client signals
Mapping may be performed using asynchronous or bit-synchronous modes.
Path Monitoring Status (STAT)
These three bits replace the IAE and RES bits in the OTUk section monitoring
area. They are used to indicate the presence of a maintenance signal.
ODUk PM status interpretation
Tandem 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
Justification Control (JC)
Tandem Connection Monitoring Overhead
There are three justification control bytes, each containing two
justification control bits. The two bits are used to indicate
whether the negative or positive justification opportunity bytes
contain data. A majority vote (two out of three JC bytes) are used
for error protection.
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.
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OTU type and capacity
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).
The value contained in the Negative Justification Opportunity
(NJO) and Positive Justification Opportunity (PJO) when they are
used as justification bytes is all zeros. In bit-synchronous mode
all JC bits are set to “00” and PJO is always a data byte.
Tandem Connection Monitoring Status (STAT)
Payload
These three bits are used to indicate the presence of a maintenance signal, if there is an incoming alignment error, or if there is no
active source.
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/SDH
signal are mapped into payload bytes of the OPUk.
ODUk TCM status interpretation
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 four
ODU2s into an ODU3.
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 backward
directions.
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.
Maintenance Signals
OTUk Alarm Indication Signal (OTUk-AIS)
ODUk Locked (ODUk-LCK)
This is a generic signal sent downstream when a defect
has 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.
The locked maintenance signal is generated on operator
request in order to lock the signal from user access. It
consists of a repeating “0101 0101” pattern in the entire
ODUk signal, which excludes the Frame Alignment
overhead and the OTUk overhead.
ODUk Alarm Indication Signal (ODUk-AIS)
The ODUk alarm indication signal consists of all ones in
the entire ODUk signal, excluding FTFL byte. This does
not overwrite the Frame Alignment or OTUk overhead
ODUk Open Connection Indication (ODUk-OCI)
An open connection indication is sent downstream when
no signal is detected from upstream. It consists of a
repeating “0110 0110” pattern in the entire ODUk signal,
which excludes the Frame Alignment overhead and the
OTUk overhead.
Forward Error Correction (FEC) can be used to provide more system margin if the span length remains constant or to increase span length
with a given BER objective and optical power. FEC detects and corrects errors to effectively deliver a 7 to 8 dB improvement in signal-tonoise 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 error
performance.
Client Maintenance Signal (Generic AIS)
The generic alarm indication signal for constant bit rate
client signals is a PRBS 211–1 repeating sequence.
Forward Error Correction (FEC)
Data are based on information, diagrams and figures taken from ITU-T Recommendations with the
kind permission of the International Telecommunication Union (ITU) as copyright holder. Where
appropriate, please consult the full text of the relevant ITU Recommendations for the authoritative
version. 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.
As telecom technology continues to evolve,
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OTN now. Not only can it solve today’s
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In short, it’s the platform of the future, today.
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 of
these sub-rows. The FEC parity check bytes are calculated over the 239 information bytes of each sub-row and transmitted in the last16
bytes of the same sub-row.
www.agilent.com/comms/otn
Product specification and descriptions in this document subject to change without notice.
© Agilent Technologies, Inc. 2001 • Printed in UK, 5 November, 2001 • 5988-4004EN