Optical transport network

An optical transport network (OTN) is a digital wrapper that encapsulates frames of data, to allow multiple data sources to be sent on the same channel. This creates an optical virtual private network for each client signal.

Function diagram 200Gbit/s transponder/muxponder, aggregating 4x40Gbit/s and 4x10Gbit/s in to into a single 200Gbit/s /OTU2C standard OTN trunk.

ITU-T defines an optical transport network 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.[1] An ONE may re-time, re-Amplify, re-shape (3R) but it does not have to be 3R – it can be purely photonic. Unless connected by optical fibre links, it shall not be OTN. Mere functionality of switching, management, supervision shall not make it OTN, unless the signals are carried through optical fibre.

Comparing OTN and SONET/SDH
OTN SONET/SDH
Scaling 400Gbit/s (2021) 40Gbit/s
Error correcting Yes, Forward Error Correction, 64b/66b encoding, 512B/513B encoding, 1024B/1027B encoding Yes, Forward Error Correction, BCH code
Timing Does not require Requires
Octet-based block frame structure Fixed, 16300 Byte Variable, 2430- 622 080 Byte
Frame rate Variable (98.354 - 1.163 μs) 125 μs

Standards
OTN multiplexing and mapping structures. Show the relationship between ODU Clients, ODU, ODU-Groups and OTU

OTN was designed to provide higher throughput (currently 400G) than its predecessor SONET/SDH, which stops at 40Gbit/s, per channel.

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 Marketing data Rate (Gbit/s) True Signal rate (OTU) (Gbit/s) Applications Maximum number of signals per channel
# of ODU0, 1.2G # of ODU1, 2.5G # of ODU2, 10G # of ODU2e, 10.4G # of ODU25, 26.4G # of ODU3, 40.3G # of ODU50, 52.8G # of ODU4, 104G
OTU1 2.5 2.66 Transports SONET OC-48 or synchronous digital hierarchy (SDH) STM-16 signal 2 1 0 0 0 0 0 0
OTU2 10 10.7 Transports an OC-192, STM-64 or wide area network (WAN) physical layer (PHY) for 10 Gigabit Ethernet (10GBASE-W) 8 4 1 0 0 0 0 0
OTU2e[2] 10.5 11.1 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. 8 4 1 1 0 0 0 0
OTU25 25 26.4 Transports a 25 Gigabit Ethernet signal 20 10 2 2 1 0 0 0
OTU3 40 43 Transports an OC-768 or STM-256 signal or a 40 Gigabit Ethernet signal.[3] 32 16 4 3 1 1 0 0
OTU3e1/2[4] 41 44.5 develop for transport of 10G LAN PHY, and one for 10G WAN PHY, over SDH and OTN. 32 16 4 3 1 1 0 0
OTU50 50 52.8 Transports a 50 Gigabit Ethernet signal 40 20 5 5 2 1 1 0
OTU4 100 111.8 Transports a 100 Gigabit Ethernet signal 80 40 10 10 2 2 2 1
OTUCn n x 100 n x 105.2 n instances of a logically interleaved 100G (C=100) frame format Total bandwidth / ODU size. e.g. 200G Channel support 4xODU3 and 4xODU2[5]

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[6] or a stream of packets (such as Ethernet, MPLS or IP) using Generic Framing Procedure
ODU12.49877512605042Transport 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.
ODU210.0372739240506Transport 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
ODU2e10.3995253164557Transport of a 10 Gigabit Ethernet signal or a timing transparent transcoded (compressed) Fibre Channel 10GFC signal
ODU340.3192189830509Transport 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
ODU3e241.7859685595012Transport of up to four ODU2e signals
ODU4104.794445814978Transport 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[6]Transport of a constant bitrate signal such as Fibre Channel 8GFC, InfiniBand or Common Public Radio Interface
ODUflex (GFP)any configured rate[6]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 processed by OTN equipment at the Optical Channel layer are:

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

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

  • Protocol processing of all the signals:-
    • Mapping and de-mapping of non-OTN signals into and out of OTN signals
    • Multiplexing and de-multiplexing of OTN signals
    • Forward error correction (FEC) on 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. Hamming distance of the RS(255,239) code is 17

See also

References
  1. ITU-T OTN definitions
  2. "G.Sup43 : Transport of IEEE 10GBASE-R in optical transport networks (OTN)". www.itu.int. Retrieved 31 October 2021.
  3. OTN offers transparent service delivery Archived 2008-05-10 at the Wayback Machine, Retrieved June 2, 2007
  4. "ITU-T G Suppl. 43 (02/2011)". ITU. Retrieved 31 October 2021.
  5. "200G OTN Optical Transponder/Muxponder for DCI Network". FS.com. FS.com. Retrieved 31 October 2021.
  6. "ODU0 and ODUflex — A Future-Proof Solution for OTN Client Mapping" (PDF). TPACK A/S. February 2010. Archived from the original (PDF) on 12 March 2012.
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