Optical Transport Network

Posted on Mar 17, 2009 in Knowledge Base

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


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.

Signal Approximate data rate (Gbit/s) Applications
OTU1 2.66 Transports SONET OC-48 or Synchronous Digital Hierarchy (SDH) STM-16 signal
OTU2 10.70 Transports an OC-192, STM-64 or Wide Area Network (WAN) physical layer (PHY) for 10 Gigabit Ethernet (10GBASE-W)
OTU2e 11.09 Transports 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 43.01 Transports an OC-768 or STM-256 signal or a 40 Gigabit Ethernet signal.
OTU3e2 44.58 Transports up to four OTU2e signals
OTU4 112 Transports 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 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 Ethernetsignal 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) 239238 x client bit rate Transport of a constant bitrate signal such as Fibre Channel 8GFC, InfiniBand orCommon Public Radio Interface
ODUflex (GFP) any configured rate Transport of a stream of packets (such as Ethernet, MPLS or IP) using Generic Framing Procedure


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

  • OTN
  • 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.