Optical Transport Network Technology

Optical Transport Network Technology

Optical Transmission Network is about converting an electrical signal into an optical signal to send into the optical fiber. It needs amplifiers equipment and  and an optical receiver to recover the signal as an electrical signal. So it containing of part as the following.

Transmitters

Normally used  light-emitting diodes (LEDs) and laser diodes for optical communications but it must be efficient and reliable for operating in an optimal wavelength range and directly modulated at high frequencies.

LEDs are suitable primarily for local-area-network applications with bit rates of 10-100 Mbit/s and transmission distances of a few kilometers. Today, LEDs have been largely superseded by VCSEL (Vertical Cavity Surface Emitting Laser) devices, which offer improved speed, power and spectral properties, at a similar cost. Common VCSEL devices couple well to multi mode fiber. 

Laser diodes are need to be modulated, it is the light output is controlled by a current applied directly to the device. For very high data rates or very long distance links, a laser source may be operated continuous wave, and the light modulated. 

Receivers

Commonly used the photodetector to convert light into electricity. The photodetector is a semiconductor-based photodiode. The photodiodes  used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.

Fiber cable types

An optical fiber cable consists of a core, cladding, and a buffer. The core and cladding are made of high-quality silica glass, although they can both be made of plastic as well. 

There are two types of optic communication:

1. Single-mode optical fibers:

fiber is smaller (<10 micrometers) and requires more expensive components and interconnection methods, but allows much longer, higher-performance links. These modes define the way the wave travels through space, how the wave is distributed in space. Waves can have the same mode but have different frequencies. This mode is mostly used for communication over short distances between node by node of the service provider. 

2. Multi-mode optical fibers: 

has a larger core (≥ 50 micrometers), allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. However, a multi-mode fiber introduces multimode distortion, which often limits the bandwidth and length of the link. Furthermore, because of its higher dopant content, multi-mode fibers are usually expensive and exhibit higher attenuation. 

This mode is mostly used for communication over short distances, such as within a building or on a campus. Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters (2000 feet) — more than sufficient for the majority of premises applications.

Amplifier

Amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. Normally the fiber optic is limited by fiber attenuation distortion then they use the amplifier for doping a length of fiber and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.

Wavelength-division multiplexing

Wavelength-division multiplexing (WDM) is multiplexing and de-multiplexing the fiber optic signal. 

The multiplexing is required in the transmitting equipment to transmit the signal out and then de-multiplexing at the receiving equipment.

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.