1. Optical Transport Network (OTN)
ITU-T standard G.709
Paper: Andreas Schubert: ”G.709 – The Optical Transport Network (OTN)”

2. 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

3. 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 Architecture of optical transport networks
Described from a network level viewpoint
Describes network layers
Andreas Schubert “White paper”
What is a white paper?
G The Optical Transport Network (OTN)

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………..
2….
3..
4..

5. 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?

6. 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

7. 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?

8. 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

9. 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

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

11. 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)

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

13. 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

14. 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

15. 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

16. 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

17. 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?

18. 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)

19. 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

20. 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

21. 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

22. 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

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

24. 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)

25. 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

26. 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

27. 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

28. 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)

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

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

31. 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