Optical Transport Network The ITU-T Recommendations on OTN Transport Plane

1. Optical Transport NetworkThe ITU-T Recommendations on OTN Transport Plane Achille Pattavina, Politecnico di Milano (Polimi), pattavina@elet.polimi.it Guido Maier, Corecom, maier@corecom.it

2. Outline • Layered view of the optical network • Optical transmission network • OTN design Optical transfer layer

3. Outline • Layered view of the optical network • Evolution of protocol stack • Optical transmission network • OTN design Optical transfer layer

4. Layered model“Everything over WDM” • Optical networks are developing as transport platforms common to different electronic clients

5. Optical network protocolsThe layered approach Electronics L1 L2 … Logical layers Optical layer Optics Physical layers Fiber layer • The WDM network is a complex system comprising • Physical transport resources and transmission equipment • Switching devices (OADMs, OXCs) • Interfaces to electronic upper layers • A set of protocols and techniques is needed to control and manage the system • Layered approach: independency of layers, except for client-server relation • Electronic layers (logical layers) are clients of optical layers (physical or better optical transport layers) • Purpose • Easy design and standardization • Easy management and control • Flexibility of the optical layer • A future-proof approach, protected against changes of upper layers

6. Lightpath switched optical networksBasic facts • Lightpath-switched optical networks (or Optical Transport Networks - OTN) are the main wide-spread application of optical communications today • Main area of employment • High-capacity transport network • Backbone interconnection • Both wide-area and metropolitan (WAN, MAN), international networks, transoceanic links, etc. • Main characteristics • Transmission based on digital intensity modulation • Optical connections • Basic entities, switched and routed on the basis of their wavelength • Switching equipment • Optical add-drop multiplexer (OADM) • Optical cross-connect (OXC)

7. Lightpath switched optical networksBasic facts • Started as a technique to upgrade capacity of large telephone national companies which were able to support huge investments in research and testing • Exploits all the WDM-multiplexed link bandwidth and all the advantages of optical fibers • Flexibility, adaptable to any physical topology • Very well matched to existing electronic physical and transport protocols (mainly SDH / Sonet, but also IP, ATM, etc.) • Protocol (and bit-rate) transparency • An intense activity of standardization in the last years led to a quite stable scenario • Interoperability of systems from different vendors fostered competition and lowered costs

8. OTNEvolution of physical architecture • POINT-TO-POINT LINKS • Electronic switching • Main challenge: increase capacity(# of wavelengths, channel bit-rate) • Commercial products available • RING MANs / WANs • Elec. switch., optical add & drop • Main challenge: fast optical protection • Commercial products available, test-bed experiments • MESH TRANSPORT NETWORKS • Elec. traffic grooming, optical path switching • Main challenge: network and node management, protection • Commercial products available, test-bed experiments = wavelength mux/demux

9. OTNEvolution of protocols and traffic ATM IP ... POTS SDH, PDH, ... Opt. transmission (WDM) ATM IP ... POTS SDH WDM Opt. transmission ATM ATM IP IP ... ... SDH SDH OTN - ASON OTN Opt. transmission Opt. transmission • WDM AS A PURE PHYSICAL LAYER • Non-reconfigurable optical point-to-point connections • SDH protection • Traffic characteristics do not affect the optical layer • SDH / SONET OVER WDM • Reconfigurable optical paths • WDM protection schemes: dedicated and shared • Static traffic, dynamic management of shared protection paths • MULTI-CLIENT OPTICAL TRANSPORT LAYER • Reconfigurable optical paths • Complex protection schemes, provisioning • Static traffic • A POSSIBLE FUTURE SCENARIO: Automatic Switched Optical Network (ASON) • On-demand lightpath provisioning • Dynamic resource allocation

10. Optical connections in the OTNLightpaths in the layered approach Electronic layers ... SDH ATM IP ... OTN Layer Optical layers Optical transmission Lightpath connectionrequest Lightpath connectionprovisioning • The WDM network is circuit-switched • An optical (point-to-point) circuit is called LIGHTHPATH • Sequence of WDM channels, one for each link, from a source to a destination node • Each lightpath corresponds 1:1 to a virtual (logical) connection • OTN layer basic functions • Lightpath provisioning for the electronic layers (not “on-demand”) • Common transport platform for a multi-protocol electronic-switching environment

11. Competing views of optical networkThe starting point ATM IP ... POTS SDH WDM Opt. transmission • Inefficient protocol stack for IP • Large protocol overhead • Issue of mapping datagram traffic over a circuit-switched network • Control and management issues • Duplication of functions IP+SDH+WDM (e.g. protection) • Unanswered questions (or better questions with many answers) • Will WDM-layer functions migrate to electronic layers or viceversa? • Will SDH disappear or will it become one of the “multi-protocols” supported by WDM?

12. Competing views of optical network Setting-up connections CR1 CR3 CR2 CR4 • Four connections are requested by the client IP layer to the optical layer • Solutions depend on • The “intelligence” of the underlying optical network • The availability of physical transport resources and transmission equipment • The availability of switching devices (OADMs, OXCs) Electronic-layer connection request Electronic switching node(DXC, IP router, ATM switch, etc.) Optical network access point WDM network

13. Competing views of optical network Optical layer: fat and dumb IP Traffic Relation Optical Wavelength Channels l l l l IP Router 1 2 3 4 Optical Path Termination Optical Cross Connect Optical Fiber

14. Competing views of optical network Optical layer: fat and dumb IP WDM Opt. transmission • Optical network: fat and dumb • Networking functions only at the electronic higher layers • IP restoration; NO optical layer protection/restoration • Point-to-point unprotected connectivity is the only requirement to the OTN layers • It requires IP Giga-/Tera-routers • Resource utilization of the WDM layer is not optimized • Mainly sponsored by IP-router vendors (CISCO) • Very good matching to legacy technology • Switching technology is already there (off-the-shelf)

15. Competing views of optical network Optical layer: smart and agile CR1 CR3 CR2 CR4 VWP1 l l 1 2 VWP3 VWP2 VWP4 VWP = Virtual Wavelength Path Electronic-layer ConnectionRequest (traffic relation) Optical wavelength channels Electronic switching node(DXC, IP router, ATM switch, etc.) WDM-layer access-point(optical path termination) Optical Cross Connect (OXC) /Optical Add/Drop Multiplexer (OADM) Optical fiber(WDM transmission system) Wavelength converter

16. Competing views of optical network Optical layer: smart and agile ATM IP ... SDH OTN Opt. transmission • Optical network: smart and agile • Networking Functions at both the electronic and WDM Layers • [IP restoration] + optical layer protection/restoration • It requires IP routers, OXCs and OADMs • Resource utilization of the WDM layer is optimized • Mainly sponsored by optical-technology vendors • OTN protocol • Optical switching equipment has still to prove its reliability and cost effectiveness (advanced devices)

17. Outline • Layered view of the optical network • Optical transmission network • Protocol layering • Signalling • Transport hierarchy • OTN design Optical transfer layer

18. OTN protocol layersFunctions needed to support OTN • The OTN layer is itself a layered set of protocols which perform control and management basic functions regarding • Lightpaths as switched optical-circuit entities • Propagation and maintenance of wavelength optical signals • Fault detection on links and switching equipment • Fault recovery (protection)

19. OTN protocol layersITU-T G-872: Architecture of OTN • The OTN protocol layer has been standardized by ITU-T • Basic architecture definition: G-872 recommendation (1999 – conclusion of a preliminary work started in 1997 by G-otn study-group) • Physical layer interfaces (parameters and testing points): G-959.1 recommendation • Many other recommendations specifying details have been issued(G-709, G-870 to G-879, G-957, G-652, G-653, G-655, G-692, etc.) • A work in progress: some aspects are still under discussion • Optical Transport Network (OTN) definition • “Optical networks are comprised of functionality providing transport, multiplexing, routing, supervision and survivability of client signals that are processed predominantly in the photonic domain”. • The protocol architecture follows the principles of client-service layering

20. OTN protocol layersOTN sublayers • The OTN layer-stack is composed of four layers • Optical channel sublayer (OCh) • Optical multiplex section (OMS) • Optical transmission section (OTS) • Physical media layer • Fiber-type specification, developed in other Recommendations Electronic Layers OCh- Optical Channel OMS- Optical Multiplex Section OTN OTS- Optical Transmission Section Physical media (optical fiber)

21. OTN protocol sublayersSimplified scheme of an OTN connection Tx Rx Electronic switch Optical switch DXC OXC Mux Demux EDFA Electronic layers OCh OMS OTS Physical media

22. OTN protocol sublayers - OChSimplified scheme of an OTN connection Tx Rx Electronic switch Optical switch DXC OXC Mux Demux EDFA OCh trail Electronic layers OCh OMS OTS Physical media

23. OTN protocol sublayersOptical channel sublayer • Optical channel layer network: This layer network provides end-to-end networking of optical channels for transparently conveying client information of varying format (e.g. SDH STM-N, PDH 565 Mbit/s, cell based ATM, etc.) • To provide end-to-end networking, the following capabilities are included in the layer network • optical channel connection rearrangement for flexible network routing • optical channel overhead processing for ensuring integrity of the optical channel adapted information • optical channel supervisory functions for enabling network level operations and management functions, such as connection provisioning, quality of service parameter exchange and network survivability • Typical involved devices: switching subsystems of OXCs and OADMs • Optical channel entity • The lightpath (or optical circuit)

24. OTN protocol sublayersOptical channel sublayer (II) • Optical channel trail terminations • Validate connectivity integrity • Assess transmission quality (defect detection) • E.g. by BER measurements • Transmit defect indications • Network connection functions • Routing • Adaptation to the client electronic layers • Protection and restoration • Management capabilities • Continuity supervision: loss of continuity detection • Lightpath trace identification • Defect indication • Performance monitoring • Adaptation management: payload type indication

25. OTN protocol sublayers - OMSSimplified scheme of an OTN connection Tx Rx OMS trail OMS trail OMS trail Electronic switch Optical switch DXC OXC Mux Demux EDFA OCh trail Electronic layers OCh OMS OTS Physical media

26. OTN protocol sublayersOptical multiplex sectionsublayer • Optical multiplex section layer network: This layer network provides functionality for networking of a multi-wavelength optical signal • Note that a "multi-wavelength" signal includes the case of just one optical channel • The capabilities of this layer network include • optical multiplex section overhead processing for ensuring integrity of the multi-wavelength optical multiplex section adapted information • optical multiplex section supervisory functions for enabling section level operations and management functions, such as multiplex section survivability • These networking capabilities performed for multi-wavelength optical signals provide support for operation and management of optical networks • Typical involved devices: de/multiplexing subsystems of OXCs OADM

27. OTN protocol sublayersOptical multiplex sectionsublayer (II) • Multiplex section entity • Set of optical channels of specified aggregated bandwidth • Each channel has a defined carrier wavelength • Multiplex section trail terminations • Assess the transmission quality (defect detection) • E.g. by monitoring wavelength values in the WDM signal to detect deviations from the nominal values • Transmit defect indications • Management capabilities • Continuity supervision: loss of continuity detection • Defect indication

28. OTN protocol sublayers - OTSSimplified scheme of an OTN connection Tx Rx OMS trail OMS trail OMS trail Electronic switch Optical switch DXC OXC Mux Demux EDFA OCh trail Electronic layers OCh OMS OTS trails OTS trail OTS trails OTS Physical media

29. OTN protocol sublayersOptical transmission sectionsublayer • Optical transmission section layer network: This layer network provides functionality for transmission of optical signals on optical media of various types (e.g. G.652, G.653 and G.655). • The capabilities of this layer network include: • optical transmission section overhead processing for ensuring integrity of the optical transmission section adapted information; • optical transmission section supervisory functions for enabling section level operations and management functions, such as transmission section survivability. • Typical involved devices: optical amplifiers (e.g. EDFA gain-control, etc.), transponders, all-optical regenerators

30. OTN protocol sublayersOptical transmission sectionsublayer (II) • Transmission section entity • Identical to the optical multiplex section entity (set of wavelength channels): one-to-one mapping between OMS and OTS • OTS also manages the optical supervisory channel (OSC) carrying information both for OTS and OMS • Optical transmission section trail terminations • Validate connectivity • Assess transmission quality (defect detection) • E.g. by monitoring power levels of the WDM channels to drive the equalization systems of optical amplifiers and transponders • Transmit defect indications • Management capabilities • Continuity supervision: loss of continuity detection • Trail trace identification and defect indication • Performance monitoring • Management communications: message-based channel

31. OTN protocol sublayersLightpaths with regeneration and -conversion OCh trail   Rx Tx Tx 2    Rx Tx Tx 1  Rx   Rx   Rx Tx Tx  OA   Rx • Transponders of transit nodes should not be OCh trail terminations • Following this rule, the 1 to 1 correspondence between lightpath and OCh-trail is preserved even if the lightpath undergoes regeneration and/or wavelength conversion in middle points • This issue appears still not clear in the recommendations

32. Outline • Layered view of the optical network • Optical transmission network • Protocol layering • Signalling • Transport hierarchy • OTN design Optical transfer layer

33. OTN protocol sublayersOpen issues • Items still under study • Protection control: automatic protection-switching protocol • Connection supervision • Signal quality supervision and performance monitoring in OMS • Signaling format on the OSC • …

34. OTN-protocol signalingOptical Overhead (OH) transmission • OMS and OTS-OH are carried onto the OSC (Optical Supervisory Channel), corresponding to one dedicated wavelength channel in the multiplex (prescribed by G.872) • OCh-OH transmission under debate for a long time • Two possible implementation technologies proposed: • Associated Channel: one independent supervisory channel for each OCh • Common Channel: a single supervisory channel shared by plenty OChs • Advantages of the associated-channel overhead solution • Direct trail-tracing capability • Scalability • Lowest latency communications between OCh trail terminations (important for protection switching)

35. Optical channel overhead transmissionAssociated Channel Overhead • An implementations proposed in the past: • Sub-Carrier Modulation (SCM) • Subcarrier modulated signal added to each  Overhead sequence Freq. converter Transmitted sequence Data sequence High-frequency modulator Low-frequency modulator Low-pass filter Detector High-pass filter

36. Optical channel overhead transmissionAssociated Channel Overhead • The actual implementation accepted in the standard: • Digital (or TDM) Wrappers (DW) • Overhead bytes digitally added to the signal Bo = payload bitrate B = transmission bitrate Lp = frame-payload length Lh = overhead-seq. length Overhead sequence Transmitted frame Data sequence (frame payload) TDM multiplexer (digital wrapper) Modulator Detector TDM demultiplexer (digital unwrapper)

37. Optical channel overhead transmissionAssociated Channel Overhead (II) • Pros and cons • Sub-Carrier Modulation (SCM) • (+) Simple detection by filtering • (-) Adds noise  Limited OH bit-rate (<<1%) • (-) Frequency SC depend on client signal • (-) Large distances between regenerators imply its elimination ? • Digital (or TDM) Wrappers (DW) • (+) Synchronized with data traffic • (+) OH capacity < 10% • (+) Same signal degradation as the regular WDM payload information • (-) Required OEO (bitrate dependent) • Functions that can be supported • Sub-Carrier Modulation (SCM) • Only OCh trace and identification • Digital (or TDM) Wrappers (DW) • Latency sensitive OAM functions • Forward Error Correction (FEC) Digital wrapperproposed by Lucent, Itachi and many others; finally selected forstandardization

38. Outline • Layered view of the optical network • Optical transmission network • Protocol layering • Signalling • Transport hierarchy • OTN design Optical transfer layer

39. Optical transport interface definition Interface for the OTN • A follow-up of OTN standardization (first documents: 02/2000) • Standard activity halted for 2 year due to the OCh-signaling issue: how to perform preserving optical transparency? A pragmatic solution: do it in electronics (the digital wrapper) • Optical Transport (OT) • An adaptation sublayer (or a layer interface) of the WDM layer • All-electronic processing • Performs FEC related functions and OCh-overhead processing • Possible clients: SDH flows STM-16 (2.5 Gbit/s), STM-64 (10 Gbit/s),STM-256 (40 Gbit/s) • TDM multiplexing in the OTN layer: 4 x 2.5, 4 x 10 and 16 x 2.5 Gbit/s • Still under discussion • Overhead standardization: only some fields are standard now • But OTN now really looks like SDH or PDH! What is the need for it? • SDH has to be considered just one of the possible OTN clients • Flows usually managed by SDH equipment (2 – 140 Mbit/s) are too fine-granular for OTN

40. The optical transport hierarchy OCh substructure • Optical Transport Hierarchy (OTH) • OCh sublayer defined in G-872 is further subdivided • Optical technology dependent aspects are omitted on purpose • Transverse compatibility • Terminology • OTU: optical channel transport unit • ODU: optical channel data unit • OPU: optical channel payload unit • TDM multiplexing • k identifies the TDM multiplexing level: 3 values are allowedk = {1, 2, 3} Physical layer

41. The optical transport hierarchy OCh sublayer functions • OPU (Optical Payload Unit) sublayer functions • Adaptation of tributary signals coming from the OTN client layers • ODU (Optical Digital Unit) sublayer functions • End-to-end connection supervision • TDM multiplexing (demultiplexing) of ODU j frames in ODU k frames with j < k (j > k) . • Tandem connection management • Several (up to 6) connection monitoring levels, mainly to allow independent management of the same optical circuit by different network administrative domains • OTU (Optical Transport Unit) sublayer functions • Signal monitoring between digital regeneration points • Forward Error Correcting (FEC) code processing

42. The optical transport hierarchy OTHOCh data units: mapping and encapsulation Client OPUk OPUk Payload OPUk OH ODUk OPUk ODUk Path PMOH ODUk ODUk TC L1 TCMOH 1 to 6 levels of ODUk Tandem ODUk ODUk Tandem Connection ODUk TC Lm Connection TCMOH Monitoring ODUk TCMOH OTUk[V] OTUk[V] OTUk[V] FEC OH Section OCh OCh OCh Payload OH OCG-n.m OCCo OCCo OCCo OCCo OCCo OCCp OCCp OCCp OCCp OCCp OMSn OMSn Payload OMU-n.m OH OTM COMMs OTSn OTM-n.m OTSn Payload OH OOS Electronic mappingand encapsulation Separate (non-associated) supervisory channel(management wavelength) WDM multiplexing WDM transmission

43. The optical transport hierarchy OCh frame format 1 3824 1 2 3 4 ODUk Column 1 4080 1 14 15 3824 3825 4080 Row FA OH OTUk OH 1 1 OTUk FEC RS(255,239) 2 2 or all-0's 3 3 (4 x 256 bytes) 4 4 OTUk MSB LSB T1542400-00 T1542410-00 (114739) (114739) 1 2 3 4 5 6 7 8 • OCh frame and physical transmission order (byte units)

44. Frame structureOPUk Column # 15 16 17 3824 1 2 OPUk payload area Row # area OPUk overhead ´ (4 3808 bytes) 3 4 G.709/Y.1331_F13-1

45. Frame structureODUk Column # 1 14 15 3824 1 2 OPUk area Row # ODUk overhead ´ (4 3810 bytes) 3 area 4 G.709/Y.1331_F12-1 Area reserved for FA and OTUk overhead.

46. Frame structureODUk and OPUk overhead Column PM and TCMi (i=1..6) 1 14 15 16 17 3824 Row 1 2 3 1 TTI BIP-8 2 Overhead OPUk Payload OPUk 0 (4 x 3808 bytes) 1 2 3 4 5 6 7 8 3 ODUk Overhead BDI SAPI BEI STAT 4 15 16 DAPI BIP8 Parity Block 31 OPUk OH 32 15 16 Column # Operator 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Specific Mapping Frame Alignment overhead OTUk overhead 1 2 specific 63 TCM RES TCM6 TCM5 TCM4 2 FTFL 3 Row# OPUk ACT overhead EXP TCM3 TCM2 TCM1 PM 3 PSI 4 GCC1 GCC2 APS/PCC RES 4 PT 0 1 TTI: Trail Trace Identifier FTFL: Fault Type & Fault Location reporting channel PM: Path Monitoring BIP8: Bit Interleaved Parity - level 8 EXP: Experimental TCM: Tandem Connection Monitoring RES BEI: Backward Error Indication GCC: General Communication Channel SAPI: Source Access Point Identifier BDI: Backward Defect Indication APS: Automatic Protection Swiching coordination channel DAPI: Destination Access Point Identifier 255 STAT: Status PCC: Protection Communication Control channel RES: Reserved for future international standardisation PSI: Payload Structure Identifier ACT: Activation/deactivation control channel PT: Payload Type • Detailed frame structure Multiframeoverheadmessage

47. Frame structureOTUk Column 1 7 8 14 15 3824 3825 4080 Row FA OH OTUk OH 1 2 OTUk FEC ´ (4 256 bytes) 3 4 SM 1 2 3 TTI BIP-8 Column # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 1 FAS SM GCC0 RES 0 MFAS BDI IAE SAPI RES BEI/BIAE 15 16 GCC General Communication Channel BDI Backward Defect Indication DAPI BEI Backward Error Indication IAE Incoming Alignment Error 31 BIAE Backward Incoming Alignment Error MFAS MultiFrame Alignment Signal 32 RES Reserved for future international BIP8 Bit Interleaved Parity – level 8 DAPI Destination Access Point Identifier standardization Operator FA Frame Alignment SAPI Source Access Point Identifier specific FAS Frame Alignment Signal SM Section Monitoring TTI Trail Trace Identifier 63 G.709/Y.1331_F15-2

48. Example of multiplexing 4 ODU1 signals into an ODU2 Alignm Client layer signal OPU1 OH ODU1 (e.g., STM-16, ATM, GFP) ODU1 OH 4x Alignm Alignm Alignm Alignm OPU2 OH ODU2 Client layer signal ODU2 OH OPU1 OH ODU1 OH (e.g., STM-16, ATM, GFP) OTU2 Alignm Alignm OH Alignm Alignm OTU2 OTU2 Alignm OPU2 OH FEC Client layer signal OPU1 OH ODU2 OH ODU1 OH (e.g., STM-16, ATM, GFP) G.709/Y.1331_FIII.1 NOTE – The ODU1 floats in ¼ of the OPU2 payload area. An ODU1 frame will cross multiple ODU2 frame boundaries. A complete ODU1 frame (15296 bytes) requires the bandwidth of (15296/3808 = ) 4.017 ODU2 frames. This is not illustrated.

49. Fixed stuff locations when mapping 16 × ODU1 into OPU3 Column 1904 1905 1919 1920 1921 3808 3809 3823 3824 31 32 33 17 1 16 Row 1 OPU3 payload OPU3 payload 2 FIXED STUFF JOH OPU3 TribSlot 1 OPU3 TribSlot 1 OPU3 TribSlot 1 OPU3 TribSlot 15 OPU3 TribSlot 16 OPU3 TribSlot 15 OPU3 TribSlot 16 OPU3 TribSlot 15 OPU3 TribSlot 16 transporting transporting ´ ´ 16 ODU1 16 ODU1 3 PSI 4 G.709/Y.1331_FIV.1

50. OTU type OTU/ODU/OPU type OTU nominal bit rate Period (note) OTU bit rate tolerance The optical transport hierarchy Bitrate and frame period OTU1 OTU1/ODU1/OPU1 255/238 * 2 488 320 kbit/s 48.971 s  20 ppm OTU2 OTU2/ODU2/OPU2 255/237 * 9 953 280 kbit/s 12.191 s OTU3 OTU3/ODU3/OPU3 255/236 * 39 813 120 kbit/s 3.035 s NOTE - The nominal OTUk rates are approximately: 2 666 057.143 kbit/s (OTU1), 10 709 225.316 kbit/s (OTU2) and 43 018 413.559 kbit/s (OTU3). NOTE - The period is an approximated value, rounded to 3 digits. OPU Payload Nominal Bit Rate OPU Type 2.488.320.000 bit/s OPU1 9.995.276.962 bit/s OPU2 OPU3 40.150.519.322 bit/s • In the hierarchy from k = 1 to k = 3 • The frame structure is the same • The frame periods decreases • The opposite of SDH • In the hierarchy from OPU to ODU to OTU • The frame length (in byte) increases, due to the overhead • The frame period is the same • The bitrate increases OPU k bitrates OTU k bitrates OTUk/ ODUk/OPUk frame periods