Anoptical 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 opticalvirtual private network for each client signal.
ITU-T defines an optical transport network 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.[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. Unlike SONET/SDH, OTN provides a mechanism to manage multiplexed wavelengths in aDWDM system.[2]
| OTN | SONET/SDH | |
|---|---|---|
| Scaling | 400Gbit/s (2021)[3] | 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-basedblockframe structure | Fixed, 16300Byte | Variable, 2430- 622 080Byte |
| Frame rate | Variable (98.354 - 1.163 μs) | 125 μs |
OTN was designed to provide higher throughput (currently 400G) than its predecessorSONET/SDH, which stops at 40 Gbit/s, per channel.
ITU-T RecommendationG.709 is commonly called Optical Transport Network (OTN) (also calleddigital wrapper technology oroptical 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 | TransportsSONETOC-48 or synchronous digital hierarchy (SDH)STM-16 signal | 2 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
| OTU2 | 10 | 10.7 | Transports anOC-192,STM-64 orwide area network (WAN) physical layer (PHY) for10 Gigabit Ethernet (10GBASE-W) | 8 | 4 | 1 | 0 | 0 | 0 | 0 | 0 |
| OTU2e[4] | 10.5 | 11.1 | Transports a 10 Gigabit Ethernetlocal 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 a25 Gigabit Ethernet signal | 20 | 10 | 2 | 2 | 1 | 0 | 0 | 0 |
| OTU3 | 40 | 43 | Transports anOC-768 orSTM-256 signal or a40 Gigabit Ethernet signal.[5] | 32 | 16 | 4 | 3 | 1 | 1 | 0 | 0 |
| OTU3e1/2[6] | 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 a50 Gigabit Ethernet signal | 40 | 20 | 5 | 5 | 2 | 1 | 1 | 0 |
| OTU4 | 100 | 111.8 | Transports a100 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[7] | |||||||
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 RecommendationG.709
| Signal | Data Rate (Gbit/s) | Typical Applications |
|---|---|---|
| ODU0 | 1.24416 | Transport of a timing transparent transcoded (compressed)1000BASE-X signal[8] or a stream of packets (such as Ethernet,MPLS or IP) usingGeneric 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) usingGeneric 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 aWAN PHY (10GBASE-W) or a stream of packets (such as Ethernet, MPLS or IP) usingGeneric Framing Procedure |
| ODU2e | 10.3995253164557 | Transport of a10 Gigabit Ethernet signal or a timing transparent transcoded (compressed) Fibre Channel10GFC 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 transcoded40 Gigabit Ethernet signal or a stream of packets (such as Ethernet, MPLS or IP) usingGeneric 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 a100 Gigabit Ethernet signal |
| ODUflex (CBR) | 239⁄238 x client bit rate[8] | Transport of aconstant bitrate signal such as Fibre Channel8GFC,InfiniBand orCommon Public Radio Interface |
| ODUflex (GFP) | any configured rate[8] | Transport of a stream of packets (such as Ethernet, MPLS or IP) usingGeneric Framing Procedure |
At a very high level, the typical signals processed by OTN equipment at the Optical Channel layer are:
A few of the key functions performed on these signals are:
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, theTime-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.
On a point-to-point OTN link there is latency due toforward error correction (FEC) processing. Hamming distance of the RS(255,239) code is 17
The aim of the optical transport network (OTN) is to combine the benefits of SONET/SDH technology with the bandwidth expandability of DWDM.
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