OTN - Optical Transport Network overview

What is the OTN (Optical Transport Network)?

ITU-T (Telecommunication Standardization Sector) 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.

ITU-T Recommendation G.709 is commonly called Optical Transport Network (OTN).  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.[2]
OTU3e2	44.58	Transports up to four OTU2e signals
OTU4	112	Transports a 100 Gigabit Ethernet signal

Why use OTN?

OTN offers the following advantages relative to SONET/SDH:

·         Stronger Forward Error Correction

·         More Levels of Tandem Connection Monitoring (TCM)

·         Transparent Transport of Client Signals

·         Switching Scalability

OTN Equipment Type

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

·         OTN

·         SONET/SDH

·         Ethernet/Fiber Channel

·         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

OTN Layer

The basic OTN Layer is consisting of the following information.

·         OCh:  Optical Channel

OCh is consist of the end-to-end transparent transmission functions, including re-arranging OCh connection to implement flexible network routing, processing OCh overheads to achieve the completeness of OCh adaption information, and monitoring OCh to implement the running and management of the network section.

·         OMS: Optical Multiplex Section

It provides multi-wavelength signals with network functions, including re-arranging OMS connection to implement flexible multi-wavelength network routing, processing OMS overheads to achieve the completeness of multi-wavelength OMS adaption information, and monitoring OMS to implement the running and management of the section.

·         OTS: Optical Transmission Section
It transmits optical signals over various fibers as well as detects and controls the optical amplifiers.

What is OTM / OADM / ROADM / FOADM ?

OTN - Optical Transmission Network Node Types

There are 3 types of the Optical Transmission Network Node as the following details

1. OTM - Optical Terminal Multiplexer

The terminal multiplexer contains the following functions:

- It can be support for both of the wavelength converting transponder for each wavelength signal it will carry and to receive the input optical signal.- To convert that signal into the electrical domain.- To re-transmit the signal 

2. OADM - Optical add-drop multiplexer

"Optical add-drop multiplexer (OADM)" is a device used in WDM systems (wavelength-division multiplexing). "Add" and "drop" is a capability device to add one or more new wavelength channels to an existing multi-wavelength WDM signal or to drop (remove) one or more channels, passing those signals to "another network path". 

There are 2 types of OADM node as the following details.

2.1 ROADM - Re-configurable optical add-drop multiplexer 

Before the development of device as a ROADM, we need to convert the optical signals to electrical signals and rout those signals by using conventional electronic switches then convert back again to optical signals. But now ROADM is no need to convert like before, the configuration can be done as required without affecting traffic. ROADM also allows for remote configuration / reconfiguration via the NMS (Network Management System).

The ROADM is a dynamic wavelength arrangement scheme, allows for dynamic wavelength arrangement scheme using a Wavelength Selective Switch (WSS). The WSS provides an 8-dimensional cross-connect and enables quick service start-up, remote cross-connect and WDM mesh networking. The ROADM scheme also allows inputting / outputting a single wavelength or wavelength group via the fixed port.

2.2 FOADM - Fixed Optical Add/Drop Multiplexer

The FOADM is a traditional wavelength arrangement scheme that can only input / output a single wavelength via the fixed port.

The distinctness of ROADM is flexible for configuration but the FOADM is Fixed.

3. OA - Optical  Amplifier

Incase of the transmit signals over long distances (>100 km), it is necessary to compensate for attenuation losses within the fiber. The amplifier could be used as a splitter the carrying signal in fiber iptic.

Description for the connection between each types of the particular node:

What is WDM and DWDM ?

WDM & DWDM

Wavelength-division multiplexing (WDM) 

is a method of network transmission along the fiber optic line to carry different wavelengths by a laser light. WDM can transmit data as 2.5 - 10 Gbps at 32 - 64 channels in the same time. Basically, WDM system uses "Multiplexer (mux)" to join the signals together before transmit out and a "De-multiplexer (demux)" at the receiver to split signals apart.

See the following picture for explanations.

Figure 1: mux and demux flow

Dense Wavelength-division multiplexing (DWDM) 

is higher performance transmission method than WDM. DWDM can transmit data as 2.5 - 10 Gbps at 160 channels in the same time. Moreover, DWDM is developed channel spacing up to 25-50 GHz then it can support transmission more than 1 Tbps.