Optical Transport Network (OTN) ITU-T standard G.709 Paper: Andreas Schubert: ”G.709 – The Optical Transport Network (OTN)” http://www.jdsu.com/product-literature/g709otn_wp_opt_tm_ae.pdf

Overview Literature Motivation for an optical transport network standard Properties of OTN G.709 Interface standard OTN framing structure - whats the point of all this overhead. Tandem connection monitoring Forward Error Correction (FEC) OTN switching

Literature ITU-T Andreas Schubert “White paper” G.709 “Interface for the optical transport network (OTN)” The most referenced standard in OTN Describes Intra and inter –domain interfaces Framing and framing structure G.872 - Architecture of optical transport networks Described from a network level viewpoint Describes network layers Andreas Schubert “White paper” What is a white paper? G.709 - The Optical Transport Network (OTN)

Why not IP directly over WDM? IP-WDM, a buzzword from the late 90’s Can you transport an IP-packet directly on a fiber? Which functionality is required? 1……….. 2…. 3.. 4..

Why not IP directly over WDM? IP-WDM, a buzzword from the late 90’s Can you transport an IP-packet directly on a fiber? Which functionality is required? 1 Clock information 2 Start/stop indication of packet 3 Error checking? 4 Error correction?

Functionality added to IP for WDM transport Framing for the physical layer transport May e.g. be Ethernet Monitoring of errors and signal quality Management for provisioning of data-paths Fault handling, fast protection Management of wavelengths Management of optical network elements Functionality at the physical layer

Why OTN? Standard for optical networks required Optical interconnection between equipment from different vendors Optical interconnection between different operators Once called “digital wrapper” Framing of client signal of different protocols for transport over the physical optical layer E.g. IP/Ethernet or IP/ATM or SDH Takes SDH/SONET further, enabling optical functionality From a single to multiple wavelengths Forward Error Correction (FEC) What is FEC?

Properties of OTN Protocol transparency Handles “any” protocol-stack and gives a physical layer to higher layer protocols like e.g. IP Backward compatibility for existing protocols Handles both SDH and Ethernet FEC Reduces cost, improves performance Reduction of 3R regeneration Allows management of all-optical network elements

Interfaces Inter-domain interfaces (IrDI) Location between networks of two operators Location between sub-networks of two vendors in the same operator domain The location within the sub-network of one vendor 3R regeneration – Why? Intra-Domain interfaces (IaDI) The location between the equipment of an individual manufacturer’s sub-network Transparent network

Inter and intra -domain Intra- IaDI within e.g. an operator Inter- IrDI between operators or connection to e.g. a customer (client)

Transport of a client signal Over-Head (OH) added to form optical channel payload unit (OPU) OH to OPU forming optical channel data unit (ODU) Additional OH plus FEC are added to form the optical channel transport unit (OTU) Adding further OH creates an OCh which is carried by one color (wavelength)

OTN layer structure OCh = Optical channel OMS = Optical Multiplex section OTS = Optical Transmission Section

OTN hierarchy Client Client OPU OH Client OH OPUk ODU OH ODUk FEC OTU OCh payload OChannel Associated overhead Non OCCp OCCp OCCp OCCp OCCp OCC OMS payload OTS payload

In-band and out of band OH Non-associated OH (out of band) may be added to OCh enabling management of multiple colours in OTN. Optical Multiplex Section (OMS) Multiplex of OCh’s Optical Transmission Section (OTS) The transmission medium, I.e. the fibre

Optical channel structure (OCh) 1 7 8 14 15 16 17 3824 3825 4080 1 FAS OTU-OH Client FEC 2 ODU-OH OPU-OH 3 4 PSI RES TCM/ACT TCM6 TCM5 TCM4 FTFL PT TCM3 TCM2 TCM1 PM EXP RES GCC1 GCC2 APS/PCC RES

Optical Payload Unit (OPU) PSI Framing of client signal May be of any protocol, e.g. SONET/SDH, GFP, IP, GbE Payload Structure Identifier (PSI) 256 Byte message Justification bits for asynchronous mapping of client signal PSI0 contains Payload Type (PT) (single byte) identifying type of payload RES – Bytes Reserved for future use PT RES

Optical channel Data Unit (ODU) functionality Tandem Monitoring (TCM) Hierarchical error checking using parity bytes Allows up to six tandem connections, nesting and overlapping Path Monitoring (PM) Monitoring of particular sections Fault location General Communication Channels (GCC) Typically Management communication Automatic Protection Switching (APS) Protection switching at one or more levels What is protection switching?

ODU-k: Defined for several bitrates -k is a number defining the bitrate Originally defined bitrates ODU1: 2.5 Gb/s ODU2: 10 Gb/s ODU3: 40 Gb/s Recently defined/to be defined ODU0: 1 Gb/s (Matches Gigabit Ethernet) ODU4: 100 Gb/s (Matches 100 Gigabit Ethernet)

ODU in OTN hierarchy Client Client OPU OH Client OH OPUk ODU OH ODUk FEC OTU OCh payload OChannel Associated overhead Non OCCp OCCp OCCp OCCp OCCp OCC OMS payload OTS payload

ODU OH PM - Path Monitoring, contains three sub-fields TCM1-TCM6 RES TCM/ACT TCM6 TCM5 TCM4 FTFL TCM3 TCM2 TCM1 PM EXP GCC1 GCC2 APS/PCC RES PM - Path Monitoring, contains three sub-fields TCM1-TCM6 OH for six independent TCM’s Contains similar sub-fields as PM TCM/ACT Activation/deactivation of TCM GCC Communication between network elements (management), two channels APS/PCC Automatic Protection Switching Protection Communication Channel RES Reserved for future use EXP Experimental use FTFL - Fault Type and fault location Channel Fault status, type and location Related to TCM span

Tandem Connection monitoring Six levels, nested or cascaded connections Nested: A1-A2/B1-B2/C1-C2 and A1-A2/B3-B4 Cascaded: B1-B2/B3-B4 Carriers may maintain their own service level agreement (SLA) What is a SLA? Figure from G.709 standard

TCM and PM overhead structure Trail-Trace Identifier TTI BIP-8 Maintenance Signal present Parity Source access Point Identifier SAPI BEI BDI STAT PM Destination access Point Identifier DAPI Operator specific BEI BDI STAT TCM Backward Error Indication Info. Signal failure Upstream Single bit - Country of origin - Network operator - Administrative details

FTFL sub-fields Fault status information Type and location of fault Related to TCM span Codes specified: No fault, Signal fail, signal degrade

Optical channel transport unit (OTU) Support transport of OTU via one or more optical channel connections Frame alignment Signal (FAS) Multi Frame Alignment Signal (MFAS) ODU frames may span multiple OTU frames TTI and TCM-ACT (activation) signals requires multiple ODU-frames for being processed Forward Error Correction (FEC)

OTU OH structure FAS OTU Comm. OTU term. Points (management) FAS MFAS GCC RES Section Monitoring = PM + IAE bit IAE: Ingress inform egress Incoming Alignment error on signal REServed For future use

Forward Error Correction (FEC) 1 7 8 14 15 16 17 3824 3825 4080 1 FAS OTU-OH Client FEC 2 ODU-OH OPU-OH 3 4 Added at the end of the OTU-frame Very important property of OTN, why? Detection of errors Correction of errors through redundant information Allows lower signal quality I.e. allows larger degradation by physical impairments

Physical impairments to be compensated Linear (explain these) Attenuation Noise Dispersion: Chromatic, mode and polarisation Non-linear Four wave mixing Self phase modulation Cross phase modulation

FEC gain Bit Error Rate (BER) improvement example: 10-4 -> 10-15 How can BER be defined, and how is it measured? Gain in power level approx 5 dB for 7 % FEC Reduce amount of 3R regenerators Use existing 2.5 Gb/s links for 10 Gb/s Early warning when degradation of link quality start to appear Longer spans between amplifiers in subsea-systems (lower cost)

FEC algorithms Performance increase depends on algorithm and amount of overhead (redundancy information) Standardized algorithms, G.709. Reed-Solomon based

OTN switching Client signals of different bitrates can be multiplexed into higher bitrate OTN signal. Switching down to Gigabit granularity (ODU-0)

Summary OTN Management to the high bandwidth WDM network SDH/SONET single wavelength, OTN – multiple wavelengths Builds on management functionality from SDH/SONET Monitoring functionality GCC channels for management communication Transparency to other protocols, e.g. IP Wrap whatever you like FEC compensates physical impairments, increases cost-efficiency OTN switching is being deployed OTN transmission and switching market is increasing rapidly