OTN Definition

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.

ITU-T Recommendation G.709 is commonly called Optical Transport Network (OTN) (also called digital wrapper technology or optical channel wrapper).

The ITU standard G.709, Interface for the Optical Transport Network, provides an industry-wide frame structure and overhead definition for the photonic layer. This standard defines a hierarchy among optical network elements and provides for performance management of complex Dense Wavelength Division Multiplexing (DWDM) networks.

One of the key benefits of single-channel SONET systems was their extensive performance monitoring and error detection capabilities, provided by the SONET Section, Line, and Path overhead bytes. As networks grew from single-channel SONET systems to multi-wavelength DWDM networks, there was a need to provide a similar frame structure; performance monitoring, error correction, and management function at the optical (i.e. wavelength) layer so that carriers could manage their DWDM networks.

The ITU standard G.709 “Interface for the Optical Transport Network” provides this industry-wide frame structure and overhead definition for the photonic layer; G.709 is sometimes referred to as the “digital wrapper.”

The G.709 OTN standards define:

  1. An optical transport hierarchy among optical network elements
  2. A frame structure for mapping client signals
  3. A definition of the overhead bytes for optical layer performance management and FEC

In a similar manner to SONET definitions, the optical network hierarchy is divided into Optical Path, Optical Transport, and Optical Multiplex layers, as shown below.

Signal Approximate data rate (Gbit/s) Applications
OTU1 2.66 Transports SONET OC-48 signal
OTU2 10.70 Transports an OC-192 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.
OTU2f 11.32 Transports a 10 Fibre Channel.
OTU3 43.01 Transports an OC-768 signal or a 40 Gigabit Ethernet signal.
OTU3e2 44.58 Transports up to four OTU2e signals
OTU4 112 Transports a 100 Gigabit Ethernet signal

A client signal is mapped into an OPU payload along with OPU overhead bytes, and this entire package gets mapped into an OTU frame along with additional ODU/OTU overhead information and FEC data.

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 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 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 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) 239238 x client bit rate

Transport of a constant bitrate signal such as Fibre Channel 8GFC, InfiniBand or Common Public Radio Interface

ODUflex (GFP) any configured rate

Transport of a stream of packets (such as Ethernet, MPLS or IP

SONET provides an excellent network infrastructure for all types of mission-critical traffic.

SONET Definition

Synchronous Optical Network (SONET), a standard for connecting fiber-optic transmission systems. SONET was proposed by Bellcore in the middle 1980s and is now an ANSI standard. SONET defines interface standards at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (OC-1) to 9.95 Gbps (OC-192). Prior rate standards used by different countries specified rates that were not compatible for multiplexing. With the implementation of SONET, communication carriers throughout the world can interconnect their existing digital carrier and fiber optic systems.

Since the late 1980s, SONET has been the workhorse of the telecommunications industry. Over the last 20 years, line rates and capacities have gone up, types of interfaces have increased and manufacturers continue to improve their products with new features and lower costs. Whether your network is carrying a single-voice call or a 10 Gbps data pipe, SONET provides the same transport dependability that the industry has come to rely on.

Although with much reduced revenues in recent years on new deployments, there exists a huge installed base of equipment still operating flawlessly, and significant need for growth and support as SONET still provides an excellent network infrastructure for all types of mission-critical traffic.

Why SONET?

  • Standards-Based – ensures compatibility across spans and between vendors
  • Deterministic and Predictable – robust, voice-centric heritage extends high quality of service to all traffic
  • Multi-service Capable – equally effective at carrying TDM and packet-based traffic including ATM, Ethernet and MPLS
  • Fault Tolerant – protected rings provide 50 msec recovery from node and span failures
  • Mature Technology – well-known technology and provisioning model
  • Price/Performance – one of the most cost-effective architectures up to 10 Gbps

SONET and other physical layer transmission rates

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