FOT Lessons: Optical Transport Network (OTN)

Optical Transport Network

  ITU-T defines an Optical Transport Network (OTN) as a set of Optical Network Elements (ONE) connected by optical 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.

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.

SignalApproximate data rate (Gbit/s)Applications
OTU12.66Transports SONET OC-48 or synchronous digital hierarchy (SDH) STM-16 signal
OTU210.7Transports an OC-192, STM-64 or wide area network (WAN) physical layer (PHY) for 10 Gigabit Ethernet (10GBASE-W)
OTU2e11.09Transports 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.
OTU2f11.32Transports a 10 Fibre Channel.
OTU343.01Transports an  OC-768  or  STM-256  signal or a  40 Gigabit Ethernet  signal.
OTU3e244.58Transports up to four OTU2e signals
OTU4112Transports 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

SignalData Rate (Gbit/s)Typical Applications
ODU01.24416Transport 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
ODU12.4987751260504Transport 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.
ODU210.037273924051Transport 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
ODU2e10.399525316456Transport of a 10 Gigabit Ethernet signal or a timing transparent transcoded (compressed) Fibre Channel 10GFC signal
ODU340.319218983051Transport 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
ODU3e241.785968559501Transport of up to four ODU2e signals
ODU4104.79444581498Transport 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 rateTransport of a constant bitrate signal such as Fibre Channel 8GFC, InfiniBand or Common Public Radio Interface
ODUflex (GFP)any configured rateTransport 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:

  1. 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
  2. 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.

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