Optical Transport Network

ITU-T defines an Optical Transport Network (OTN) as a set of Optical Network Elements (ONE) connected by optical fibre links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals.^[1] An ONE may Re-time, Re-transmit, Re-shape (3R) but it does not have to be 3R— it can be purely photonic.

Standards

OTN was designed to provide support for optical networking using wavelength-division multiplexing (WDM) unlike its predecessor SONET/SDH.

ITU-T Recommendation G.709 is commonly called Optical Transport Network (OTN) (also called digital wrapper technology or optical channel wrapper). As of December 2009 OTN has standardized the following line rates.

• OTU1 has a line rate of approximately 2.66 Gbit/s and was designed to transport a SONET OC-48 or synchronous digital hierarchy (SDH) STM-16 signal.
• OTU2 has a line rate of approximately 10.70 Gbit/s and was designed to transport an OC-192, STM-64 or wide area network (WAN) physical layer (PHY) for 10 Gigabit Ethernet (10GBASE-W).
• OTU2e has a line rate of approximately 11.09 Gbit/s and was designed to transport a 10 Gigabit Ethernet local area network (LAN) PHY coming from IP/Ethernet switches and routers at full line rate (10.3 Gbit/s). This is specified in G.Sup43.
• OTU3 has a line rate of approximately 43.01 Gbit/s and was designed to transport an OC-768 or STM-256 signal or a 40 Gigabit Ethernet signal.^[2]
• OTU3e2 has a line rate of approximately 44.58 Gbit/s and was designed to transport up to four OTU2e signals.
• OTU4 has a line rate of approximately 112 Gbit/s and was designed to transport a 100 Gigabit Ethernet signal.

The OTUk (k=1/2/2e/3/3e2/4) is an information structure into which another information structure called ODUk (k=1/2/2e/3/3e2/4) is mapped. The ODUk signal is the server layer signal for client signals. The following ODUk information structures are defined in ITU-T Recommendation G.709

Signal	Data Rate (Gbit/s)	Typical Applications
ODU0	1.24416	Transport of a timing transparent transcoded (compressed) 1000BASE-X signal[3] or a stream of packets (such as Ethernet, MPLS or IP) using Generic Framing Procedure
ODU1	2.49877512605042	Transport of two ODU0 signals or a STS-48/STM-16 signal or a stream of packets (such as Ethernet, MPLS or IP) using Generic Framing Procedure.
ODU2	10.0372739240506	Transport of up to eight ODU0 signals or up to four ODU1 signals or a STS-192/STM-64 signal or a WAN PHY (10GBASE-W) or a stream of packets (such as Ethernet, MPLS or IP) using Generic Framing Procedure
ODU2e	10.3995253164557	Transport of a 10 Gigabit Ethernet signal or a timing transparent transcoded (compressed) Fibre Channel 10GFC signal
ODU3	40.3192189830509	Transport of up to 32 ODU0 signals or up to 16 ODU1 signals or up to four ODU2 signals or a STS-768/STM-256 signal or a timing transparent transcoded 40 Gigabit Ethernet signal or a stream of packets (such as Ethernet, MPLS or IP) using Generic Framing Procedure
ODU3e2	41.7859685595012	Transport of up to four ODU2e signals
ODU4	104.794445814978	Transport of up to 80 ODU0 signals or up to 40 ODU1 signals or up to ten ODU2 signals or up to two ODU3 signals or a 100 Gigabit Ethernet signal
ODUflex(CBR)	239/238 x client bit rate[3]	Transport of a Constant bitrate signal such as Fibre Channel 8GFC, InfiniBand or Common Public Radio Interface
ODUflex(GFP)	any configured rate[3]	Transport of a stream of packets (such as Ethernet, MPLS or IP) using Generic Framing Procedure

Equipment

At a very high level the typical signals that OTN equipment at the Optical Channel layer processes are:

• OTN
• SONET/SDH
• Ethernet/FibreChannel
• Packets

A few of the key functions performed on these signals are:

• Protocol processing of all the signals . Some of the more complex processes are:
  • Forward error correction (FEC) on OTN signals
  • Multiplexing and de-multiplexing of OTN signals
  • Mapping and de-mapping of non-OTN signals into and out of OTN signals
• Packet processing in conjunction with mapping/de-mapping of packet into and out of OTN signals

Switch fabric

The OTN signals at all data-rates have the same frame structure but the frame period reduces as the data-rate increases. As a result, the Time-Slot Interchange (TSI) technique of implementing SONET/SDH switch fabrics is not directly applicable to OTN switch fabrics. OTN switch fabrics are typically implemented using Packet Switch Fabrics.

FEC Latency

On a point-to-point OTN link there is latency due to forward error correction (FEC) processing.

See also

• ITU-T Study Group 15: Optical transport networks and access network infrastructures
• ITU-T G.872: Architecture of optical transport networks
• ITU-T G.664: Optical safety procedures and requirements for optical transport systems
• ITU-T G.692: Optical interfaces for multichannel systems with optical amplifiers
• ITU-T G.709: Interfaces for the Optical Transport Network (OTN)
• ITU-T G.798: Characteristics of optical transport network hierarchy equipment functional blocks
• ITU-T G.871: Framework for optical transport network recommendations
• ITU-T G.874: Management aspects of the optical transport network element
• ITU-T G.959.1: Optical transport network physical layer interfaces

References

1. ^ ITU-T OTN definitions
2. ^ OTN offers transparent service delivery, Retrieved June 2, 2007
3. ^ ^{a b c} "ODU0 and ODUflex — A Future-Proof Solution for OTN Client Mapping". TPACK A/S. February 2010. http://squiz.informatm.com/__data/assets/pdf_file/0007/194623/ODU0_ODUflex_White_Paper_2010_02_15_v1_web.pdf. Retrieved 25 March 2011. 

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