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DWDM introduction FT92222EN01GLA1 © 2009 Nokia Siemens Networks 1 Contents 1 General 3 1.1 Signal multiplexing 4 1.2 Transmission windows 6 1.3 ITU wavelength plan 8 1.4 Applicable ITU recommendations 10 2 OTN introduction 19 2.1 OTN introduction 20 2.2 Enhanced forward error correction 30 2.3 Client Signals 32 2.4 Applicable ITU recommendations 34 DWDM introduction

Transcript of 03 Ft92223en01gla1 Dwdm Introduction Nsn

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Contents 1 General 3 1.1 Signal multiplexing 4 1.2 Transmission windows 6 1.3 ITU wavelength plan 8 1.4 Applicable ITU recommendations 10 2 OTN introduction 19 2.1 OTN introduction 20 2.2 Enhanced forward error correction 30 2.3 Client Signals 32 2.4 Applicable ITU recommendations 34

DWDM introduction

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1 General

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1.1 Signal multiplexing Since the very beginning of telecommunications, the need to extend the capacity of a communications channel has been present. The purpose could be to allow the use of this channel by multiple users, to perform a better management of the available resources or just to boost the transmission capacity. Development of these systems was always limited by the maximum capacity allowed by the communications channel and the available technology. Several techniques can be used to improve the use of a communication channel. The principals are e.g.: • Time Division Multiplexing (TDM) • Frequency Division Multiplexing (FDM) These are commonly used techniques in radio and copper transmission systems. With TDM each user is assigned to a certain time slot thus making the transmission time shared by all the users. With FDM each user is assigned to a certain frequency slot transmitting only with the corresponding carrier. This way the available bandwidth is shared. In the case of optical systems the available bandwidth can exceed several Terahertz (1012Hz). TDM could not be used to take advantage of this tremendous bandwidth due to limitations on electrical technology. Electrical circuits simply cannot work on these frequencies. Typical FDM was also a problem in the same way, as it was not possible to use frequency multiplexing at the electrical level. The solution was to use frequency multiplexing at the optical level or Wavelength Division Multiplexing. The basic idea is to use different optical carriers or colors to transmit different signals in the same fiber. A distinction is made between WDM and DWDM (Dense Wavelength Division Multiplexing). With WDM the spacing between channels can be relatively large. In Dense multiplexing the frequency spacing between channels can be as small as 50GHz or less, increasing the overall spectral density of the transmitted signal.

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TDMMux

#1

#2

#3

#4 #1 #2 #3 #4

FDMMux

#1

#2

#3

#4

f1

f2

f3

f4f1 f2 f3 f4

WDMMux

#1

#2

#3

#4

λ1

λ2

λ3

λ4λ1 λ2 λ3 λ4

Time

Frequency

Wavelength

Fig. 1 Comparison between TDM, FDM and WDM techniques

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1.2 Transmission windows If one looks at the possible wavelengths for the transmission of signals one has to look at the fiber properties. Optical fibers are not suitable for transmission at all wavelengths but only in certain windows. Today, usually the second transmission window (around 1300nm) and the third and fourth transmission windows from 1530 to 1565nm (also called Conventional Band) and from 1565 to 1620nm (also called Long Band).are used. Technological reasons limit DWDM applications at the moment to the third and fourth window. The losses caused by the physical effects on the signal due by the type of materials used to produce fibers limit the usable wavelengths to between 1280nm and 1650nm. Within this usable range the techniques used to produce the fibers can cause particular wavelengths to have more loss so we avoid the use of these wavelengths as well.

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Communications radiationIR radiationUV radiationCosmic radiation X ray radiation

SWVHFTVMicrowave, radar

106108101010121014101610181020Frequency (Hz)

Visible light

Wavelength (m) 10-12 10-9 10-6 10-3 100

(1 mm) (1 m)

μm

(1 nm)(1 pm)

C0C

==

300 000 km/sλ x f

Visiblelight

Fiber transmissionwavelength range

λf

==

wavelengthfrequency

102

(1 THz) (1 GHz) (1 MHz)

(100 m)

15001410

1.31.21.11.00.90.80.70.60.50.4

165015501310850 nmO E S C L

1600 nm

U Band

Electromagnetic spectrum

Fig. 2 Electromagnetic spectrum

12001000800 1400 1600

Multimode fiber

Single mode fiber

IR absorption

Rayleighscattering 1/λ4

0.1

1

10

Wavelength [nm]

Atte

nuat

ion

coef

ficie

nt [d

B]

1st window 2nd window 3rd window 4th window

C-Band L-Band

Attenuation coefficient of silica fibers

Fig. 3 Transmission windows

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1.3 ITU wavelength plan ITU-T Recommendation G.694.1 provides the definition of a frequency grid to support dense wavelength division multiplexing (DWDM) applications. A frequency grid is a reference set of frequencies used to denote allowed nominal central frequencies that may be used for DWDM applications. Dense Wavelength Division Multiplexing (DWDM), a WDM technology, is characterized by narrower channel spacing than Coarse WDM (CWDM) as defined in ITU-T Rec. G.671. In general the transmitters employed in DWDM applications require a control mechanism to enable them to meet the application's frequency stability requirements, in contrast to CWDM transmitters which are generally uncontrolled in this respect. The frequency grid defined by this Recommendation supports a variety of channel spacing ranging from 12.5 GHz to 100 GHz and wider (integer multiples of 100 GHz). Uneven channel spacing is also allowed.

1.3.1 Nominal central frequencies for dense WDM systems • For channel spacing of 12.5 GHz on a fiber, the allowed channel frequencies (in

THz) are defined by: 193.1 + n × 0.0125 where n is a positive or negative integer including 0;

• For channel spacing of 25 GHz on a fiber, the allowed channel frequencies (in

THz) are defined by: 193.1 + n × 0.025 where n is a positive or negative integer including 0;

• For channel spacing of 50 GHz on a fiber, the allowed channel frequencies (in

THz) are defined by: 193.1 + n × 0.05 where n is a positive or negative integer including 0;

• For channel spacing of 100 GHz or more on a fiber, the allowed channel

frequencies (in THz) are defined by: 193.1 + n × 0.1 where n is a positive or negative integer including 0;

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

„ C Blue Band“ „C Red Band“

The endpoints shown are illustrative, not normative !!!The endpoints shown are illustrative, not normative !!!

Example nominal central frequencies of the DWDM 50 GHz gridExample nominal central frequencies of the DWDM 50 GHz grid

λ

50GHz~ 4nm Gap

1528

.77

nm19

6.1

THz

1544

.13

nm19

4.1

THz

1548

.11

nm19

3.6

THz

1563

.86

nm19

1.7

THz

Fig. 4 Example nominal central frequencies of the DWDM 50 GHz grid

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1.4 Applicable ITU recommendations The International Telecommunication Union ITU has laid down various definitions and recommendations related to transmission over optical media. The applicable recommendations are: • ITU-T Recommendation G.652

"Characteristics of a single-mode optical fiber and cable" • ITU-T Recommendation G.653

"Characteristics of a dispersion-shifted single-mode optical fiber and cable" • ITU-T Recommendation G.655

"Characteristics of a non-zero dispersion-shifted single-mode optical fiber and cable"

• ITU-T Recommendation G.661 "Definition and test methods for the relevant generic parameters of optical amplifier devices and subsystems"

• ITU-T Recommendation G.662 "Generic characteristics of optical amplifier devices and subsystems"

• ITU-T Recommendation G.663 "Application related aspects of optical amplifier devices and sub-systems"

• ITU-T Recommendation G.664 "Optical safety procedures and requirements for optical transport systems"

• ITU-T Recommendation G.671 "Transmission characteristics of optical components and subsystems"

• ITU-T Recommendation G.694.1 "Spectral grids for WDM applications: DWDM frequency grid"

• ITU-T Recommendation G.694.2 "Spectral grids for WDM applications: CWDM wavelength grid"

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1.4.1 ITU-T recommendation G.652 This Recommendation describes the geometrical, mechanical, and transmission attributes of a single mode optical fiber and cable with zero-dispersion wavelength shifted into the 1310 nm wavelength region. This Recommendation describes a single mode optical fiber cable which has the zero dispersion wavelength around 1310 nm and which is optimized for use in the 1310 nm wavelength region, and which can also be used in the 1550 nm region (where this fiber is not optimized). Both analogue and digital transmission can be used with this fiber. The geometrical, optical, transmission and mechanical parameters are described in recommendation in three categories of attributes: • Fiber attributes are those attributes that are retained throughout cabling and

installation; • Cable attributes that are recommended for cables as they are delivered; • Link attributes that are characteristic of concatenated cables, describing estimation

methods of system interface parameters based on measurements, modeling, or other considerations.

1.4.2 ITU-T Recommendation G.653 This Recommendation describes the geometrical, mechanical, and transmission attributes of a single mode optical fiber and cable with zero-dispersion wavelength shifted into the 1550 nm wavelength region. This Recommendation describes a dispersion-shifted single-mode optical fiber and cable which has a nominal zero-dispersion wavelength close to 1550 nm, and a dispersion coefficient which is monotonically increasing with wavelength. This fiber is optimized for use in the 1550 nm region, but may also be used at around 1310 nm subject to the constraints outlined in this Recommendation. Some provisions are made to support transmission at higher wavelengths – up to 1625 nm. The geometrical, optical, transmission and mechanical parameters are described below in three categories of attributes: • Fiber attributes are those attributes that are retained throughout cabling and

installation; • Cable attributes that are recommended for cables as they are delivered; • Link attributes that are characteristics of concatenated cables, describing

estimation method of system interface parameters based on measurements, modeling, or other considerations.

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1.4.3 ITU-T Recommendation G.655 This Recommendation describes the geometrical, mechanical, and transmission attributes of a single-mode optical fiber which has the absolute value of the chromatic dispersion coefficient greater than some non-zero value throughout the wavelength range from 1530 nm to 1565 nm. This dispersion reduces the growth of nonlinear effects which are particularly deleterious in dense wavelength division multiplexing systems. These fibers are optimized for use at wavelengths in a prescribed region between 1530 nm and 1565 nm. Some provisions are made to support transmission at higher wavelengths of up to 1625 nm. Extensions are possible, in the future, to wavelengths lower than 1530 nm (to be determined). The geometrical, optical, transmission and mechanical parameters are described below in three categories of attributes: • Fiber attributes are those attributes that are retained throughout cabling and

installation; • Cable attributes that are recommended for cables as they are delivered; • Link attributes that are characteristic of concatenated cables, describing estimation

methods of system interface parameters based on measurements, modeling, or other considerations.

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ITU-T Recommendation G.652„Characteristics of a single-mode optical fiber and cable“

ITU-T Recommendation G.653“Characteristics of a dispersion-shifted single-mode optical fiber and cable”

ITU-T Recommendation G.655“Characteristics of a non-zero dispersion-shifted single-mode optical fiber and cable”

Fig. 5 Applicable ITU Recommendations: fibers and cables

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1.4.4 ITU-T Recommendation G.661 This Recommendation intends to provide the definitions of the relevant parameters, common to the different types of optical amplifiers and the test methods of said parameters to be followed, as far as applicable, for optical amplifier devices and subsystems covered by ITU-T Recommendations. This Recommendation applies to Optical Amplifier (OA) devices and subsystems to be used in transmission networks. It covers both Optical Fiber Amplifiers (OFAs) and Semiconductor Optical Amplifiers (SOAs).

1.4.5 ITU-T Recommendation G.662 This Recommendation intends to provide those generic characteristics needed for the specification of Optical Amplifiers as devices and subsystems, primarily for applications in digital transmission. This Recommendation applies to Optical Amplifier (OA) devices and subsystems to be used in transmission networks. It covers both Optical Fiber Amplifiers (OFAs) and Semiconductor Optical Amplifiers (SOAs). The object of this Recommendation is to identify those generic characteristics specifiable for the use of OA devices (as power amplifiers, pre-amplifiers or line amplifiers) and OA subsystems (as optically amplified transmitters or optically amplified receivers).

1.4.6 ITU-T Recommendation G.663 This Recommendation covers application related aspects of OA devices and sub systems, primarily used in digital systems. Applications include both single-channel and multi-channel systems used in point-to-point and point-to-multipoint configurations for use in long-distance networks and optical access networks. The purpose of this Recommendation is to identify which aspects should be considered for each application and to specify appropriate parameter values and ranges for each type of OA device. This Recommendation covers application related aspects of Optical Amplifier (OA) devices and subsystems, primarily used in digital systems. Optical Amplifiers operating in the 1550 nm region, or 1310 nm region, or other wavelength regions are included. Important topics contained in this Recommendation include transmission aspects, maintenance aspects, and optical safety.

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ITU-T Recommendation G.661“Definition and test methods for the relevant generic parameters of optical amplifier devices and subsystems”

ITU-T Recommendation G.662“Generic characteristics of optical amplifier devices and subsystems”

ITU-T Recommendation G.663 “Application related aspects of optical amplifier devices and sub-systems”

Fig. 6 Applicable ITU Recommandations: Optical amplifier

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1.4.7 ITU-T Recommendation G.664 This Recommendation provides guidelines and requirements for techniques to enable optically safe working conditions (for the human eye and skin and for avoiding ignition) on optical interfaces of the Optical Transport Network, in particular, for systems employing high-power Raman amplification techniques, for equipment in restricted and controlled locations. Furthermore, this Recommendation provides new guidelines on APR procedures for systems employing high-power Raman amplification techniques.

1.4.8 ITU-T Recommendation G.671 This Recommendation covers the transmission related aspects of all types of optical components used in long haul networks and access networks. A broad range of types of optical components is included in this Recommendation. This Recommendation also includes transmission characteristics of optical components under the full range of operating conditions. This Recommendation covers the transmission characteristics in the various operating conditions of the following optical components (as an example): • Optical add drop multiplexer (OADM) subsystem; • Optical attenuator; • Optical connector; • Dynamic channel equalizer (DCE); • Optical filter; • Optical isolator; • Optical splice; • Optical switch; • Optical termination; • Optical wavelength multiplexer/demultiplexer;

1.4.9 ITU-T Recommendation G.694.1 The purpose of this Recommendation is to provide the definition of a frequency grid to support dense wavelength division multiplexing (DWDM) applications. This Recommendation provides a frequency grid for dense wavelength division multiplexing (DWDM) applications. The frequency grid, anchored to 193.1 THz, supports a variety of channel spacing ranging from 12.5 GHz to 100 GHz and wider.

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1.4.10 ITU-T Recommendation G.694.2 The purpose of this Recommendation is to provide the definition of a wavelength grid to support coarse wavelength division multiplexing (CWDM) applications. This grid is designed to allow simultaneous transmission of several wavelengths with sufficient separation to permit the use of uncooled sources. This Recommendation provides the wavelength grid for coarse wavelength division multiplexing (CWDM) applications. This wavelength grid supports a channel spacing of 20 nm.

ITU-T Recommendation G.664“Optical safety procedures and requirements for optical transport systems”

ITU-T Recommendation G.671“Transmission characteristics of optical components and subsystems”

ITU-T Recommendation G.694.1 “Sprectral grids for WDM applications: DWDM frequency grid”

ITU-T Recommendation G.694.2“Spectral grids for WDM applications: CWDM wavelength grid”

Fig. 7 Applicable ITU Recommendations: safety, transmission and frequency grid characteristics

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2 OTN introduction

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2.1 OTN introduction In order to implement a fully functional optical network, a series of specifications were created by ITU-T that suggests a transmission scheme, associated with a frame construction protocol. ITU-T Standard G.709, "Interface for the Optical Transport Network (OTN)" builds upon the worldwide use of SONET/SDH to meet the demands of optical networks as they continually evolve to higher bandwidths, speeds, and performance. In other words, G.709 standardizes the evolution necessary from SONET/SDH to advanced, next-generation all-optical networks. Many of the concepts in G.709 are similar to SONET/SDH, i.e., well-defined layers, protection switching, and performance monitoring, but there are important new concepts as discussed below. Comparable to SDH/SONET, in the electrical domain client data are associated with overheads to form a defined frame. Afterwards, the electrical transport unit of client data and overhead is translated into an optical signal of a defined wavelength, the optical channel.

2.1.1 OTN frame The structure of the G.709 Frame consists of 4 rows of 4080 bytes. Note that, similar to SONET/SDH, the G.709 frame contains the following: • Payload area for customer data; • Overhead areas for OAM functions; In addition, the G.709 frame includes a block for Forward Error Correction (FEC). Client signals are mapped from a digital format to an optical format to create an OCh). An OCh has the following three sub-layers, each with its own functions and overhead: • OPU (Optical Payload Unit) consists of the client's payload and the OPU

Overhead (OPU-OH), which is necessary to map the client signal into the OPU. • ODU (Optical Data Unit) is the structure used to transport the OPU. The ODU

consists of the OPU and the ODU Overhead (ODU-OH). The ODU-OH provides path-layer connection monitoring functions.

• OTU (Optical Transport Unit) conditions the ODU for optical transmission. The OTU consists of the ODU, plus the OTU Overhead (OTU-OH), plus the Forward Error Correction (FEC) block. The OTU-OH provides section-layer connection monitoring functions.

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OverheadOverhead Payload Payload

4 ro

ws

4080 bytes

FECFEC

Fig. 8 OTN frame

OTU overhead

OTU overhead

ODU overhead

ODU overhead

OPU payloadOPU payload+

OPU

ove

rhea

d

= OPU (Optical Payload Unit)

OPU (Optical Payload Unit)+ = ODU (Optical Data Unit)

ODU (Optical Data Unit)

+=+ FE

C

OTU (Optical Transport Unit)

Fig. 9 OTN frames

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2.1.2 OTN overheads 2.1.2.1 OPU overhead The functions of the OPU overhead mainly consists of a mapping appropriate overhead (like stuffing bits and control bits for bit rate adjustment) and the payload structure identifier PSI, specifying which type of signal the OPU contains, e.g. STM-64, ATM, etc.

2.1.2.2 ODU overhead The ODU overhead itself contains a wide range of different functionalities, e.g.:

General Communication Channels General communication channels (GCC) serve to connect two net-work elements, a bit like the DCC channels in SDH. They are used to transmit control information between the elements, originating for example in a network management system.

Path Monitoring Path Monitoring (PM) includes a Trail Trace Identifier (PM-TTI), a name that accompanies the signal through the network and that makes is easy to identify malfunctions in switching optical signals. Further on, so called BIP-8 code is generated over the OPU before it is transmitted, this code is then transmitted in the ODU overhead. On the receive side, the code is once more calculated and compared with the value which is received. If differences occur the signal has suffered errors during transmission. Backward Defect and Backward Error Indication (BDI and BEI) are also included, sending information about problems in backward direction.

Tandem Connection Monitoring Tandem Connection Monitoring (TCM) feature is already known from the SDH/SONET, but in the ODU we find it in a much more sophisticated form. Basically, TCM is a refined way of bit error monitoring. There are six different layers on which monitoring can be done available.

2.1.2.3 OTU overhead The framing information to identify the start of a new frame (FAS, MFAS, and FRA) belongs in fact to the framing overhead, just the second part of the first row of bytes is truly the OTU overhead. The OTU overhead contains almost the same functionalities as in the ODU overhead, allowing for the supervision of individual 3R spans. The most interesting part of the OTU frame though is the part for the forward error correction (FEC).

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Fig. 10 OTN Overhead Structure

Section Monitoring In these bytes, we find the section monitoring (SM) overhead which contains e.g. data for trail trace identifier (TTI), bit error check (BIP), backward defect indication (BDI) and backward error indication (BEI), in the same way the ODU overhead does.

General Communication Channel Further on, there is room for a general communication channel (GCC), like in ODU overhead, it is used to transmit control information between the elements, originating for example in a network management system. TIP In fact the functionalities of the OTU overheads bytes are practically identical to those of the ODU overhead. The difference lies in the monitored section: While in the case of the ODU the whole path is supervised, in the case of the OTU only single 3R spans are monitored.

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2.1.3 Properties of the OTN The aim of the OTN is to enable the multi service transport of packet based data and legacy traffic, whilst Digital Wrapper technology accommodates non intrusive management and monitoring of each optical channel assigned to a particular wavelength. The “wrapped” overhead (OH) would therefore make it possible to manage and control client signal information.

Optical Channel (OCh): Several overhead sections are added to the client signal which together with the FEC forms the Optical Transport Unit (OTU). This is then carried by a single wavelength as an Optical Channel (OCh).

Non-associated Overhead: As multiple wavelengths are transported over the OTN, an overhead must be added to each to enable the management functionality of the OTN. Once the optical channel is formed, additional non associated OH is added to individual OCh wavelengths, which then form the Optical Multiplexing Sections (OMS) and Optical Transmission Sections (OTS).

Optical Multiplex Section (OMS): In the Optical Multiplex Section (OMS) layer, both the OMS payload and non-associated overhead (OMS OH) are transported. The OMS payload consists of multiplexed OChs. The OMS-OH, although undefined at this point, is intended to support the connection monitoring and assist service providers in troubleshooting and fault isolation in the OTN.

Optical Transmission Section (OTS): The Optical Transmission Section (OTS) layer transports the OTS payload as well as the OTS Overhead (OTS-OH). Similar to the OMS, the OTS transports the optically multiplexed sections described above. The OTS OH - although not fully defined - is used for maintenance and operational functions. The OTS layer allows the network operator to perform monitoring and maintenance tasks between the NEs which include; OADMs, multiplexers, demultiplexers and optical switches.

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Basic OTN Transport StructureBasic OTN Transport Structure

Client Signal

OH Client Signal

OH Optical Payload Unit

OH Optical Data Unit FEC

Optical Channel

Optical Multiplex Section

Optical Transmission Section

Non

-ass

ocia

ted

Ove

rhea

d

OPU

ODU

OTU

OCh

OMS

OTS

OH is added to the client signal toform the Optical channel Payload Unit (OPU)OH is then added to the OPU thusforming the Optical channel Data Unit (ODU)Additional OH plus FEC are added to form the Optical channel Transport Unit (OTU)Adding further OH creates anOptical Channel (Och)which is carried by one color

Additional OH may be added to the OCh to enable the managementof multiple colors in the OTN

The Optical Multiplex Section(OMS) and theOptical Transmission Section(OTS) are then constructed

Fig. 11 Basic OTN Transport Structure

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2.1.4 Optical Transport Hierarchy OTH Optical Transport Module (OTM) Like SDH, OTN defines a network hierarchy called the Optical Transport Hierarchy (OTH). Though SDH's STM-1s are concatenated to reach higher speeds, OTH's base unit, the Optical Transport Module (OTM), reaches higher speeds by bundling wavelengths. TIP The facts in the following section are presented in a very simplified form. The only intention is to give a principle overview. OTMs can span multiple wavelengths of different carrier capacities. To indicate this difference, OTMs carry two suffixes: • “n” - refers to the maximum number of wavelengths supported at the lowest bit

rate on the wavelength; • “m” indicates the bit rate supported on the interface; These suffixes are written as OTM-n.m.

Three throughput rates are supported: • 2.5Gbits/sec, indicated by a 1; • 10Gbits/sec, indicated by a 2; • 40Gbits/sec, indicated by a 3; An interface might support some combination of these, namely a 2.5Gbit/sec and a 10Gbit/sec combo (1 and 2); a 10Gbit/sec and a 40Gbit/sec combo (2 and 3), or a combination of all three speeds (1, 2, and 3). Thus, an OTM-3.2 indicates an OTM that spans three wavelengths, each operating at least at 10Gbits/sec. Similarly, an OTM-5.12 indicates a channel that spans five wavelengths and can operate at either 2.5Gbits/sec or 10Gbits/sec.

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OTH Multiplexing and Mapping StructureOTH Multiplexing and Mapping Structure

ODU1 OPU1

ODU2 OPU2x 1

ODU3

x 1

OPU3x 1

OTU1[V]

OTU2[V]

OTU3[V] x 1

x 1

x 1

OTM-n.m

x k

x j

x i

OCG-n.m

OCC

OCC

OCCx 1

x 1

x 1OCh

OCh

OCh x 1

x 1

x 1

OTM-nr.m

x k

x jx i

OCG-nr.m

OCCr

OCCr

OCCrx 1

x 1

x 1OChr

OChr

OChr x 1

x 1

x 1

OSCx 1

OOS OTS, OMS, OCh, COMMS OH

x 1

x 1

x 1

OTM-0.m

1 wi + j + k wn

1 wi + j + k wn

Mapping

Multiplexing

Clientsignal

Clientsignal

Clientsignal

Fig. 12

TIP ITU-T Recommendation G.709 "Interfaces for the optical transport network (OTN) Amendment 1" published at 11/2001 adds the following extension to the base document:

• Support of Virtual Concatenation of OPUk signals; • Support of Link Capacity Adjustment Scheme (LCAS) for Virtual Concatenation; • Mapping ODUk Signals into the ODTUjk signal. TIP The index "r", if present, is used to indicate a reduced functionality OTM, OCG, OCC and OCh (non-associated overhead is not supported). Note that for n = 0 the index r is not required as it implies always reduced functionality.

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2.1.5 OTN layers According to ITU-T recommendations G.709, G.805 and G.872, the equipments used to build an optical transport network must implement a layered structure of communications. With this structure, each layer becomes responsible for processing the functions required in the signal to be transported. Beyond the specifications for OCh including its underlying digital OTN layers, the ITU-T G.709 outlines also ONNIs (Optical Network Node Interfaces). The ITU-T G.709 identifies ONNI interconnection points as being inter-domain interfaces (IrDI). IrDIs are the boundaries between different administrative domains within an OTN. The administrative domains may be that of multiple network providers and/or equipment vendors. Intra-domain interfaces (IaDI) are interconnections within a given administrative domain. Examples of IaDIs could be any interface within a single vendor sub-network which may or may not extend to include the entire OTN. TIP Please notice the following important difference as described and recommended by ITU-T G.709: while for the IaDI (intra-domain interface) a proprietary FEC such as enhanced FEC code can be implemented within the OTU frame format, for the IrDI (inter-domain interface), the strict FEC code as defined in G.709 has to be implemented within the OTU frame format so that it can be handled by different equipment vendors at the interconnection point of different domains. OPU is created at the very edge of the network, when a tributary signal enters the optical network and remains unchanged throughout the whole net. Its main purpose consists of adapting the different tributary signals to the fixed transmission bit rates. The ODU is generated next and also remains intact until the signal leaves the network. Therefore, there are only the ODU structures ODU1, ODU2 and ODU3 defined for all interfaces within the network, independent of IrDI or IaDI. Nevertheless, at all 3R regeneration points, the ODU overhead is evaluated and serves for monitoring purposes. The next step is the creation of the OTU. The OTU is created and evaluated in each 3R regeneration point anew. This means that theoretically different OTU types might be used for different 3R regeneration sections. Within a domain, i.e. for IaDIs, the OCh overhead is added at the same time as the OTU is created and therefore is valid for exactly one 3R section.

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ODUOTU OTUOCh OCh

OMS OMS

OTU

OChrOPS

OTS OTS OTSOMS OMS

OTS OTS OTS OTS

OTUOCh

OTN

Optical line amplifier (OTS termination)

Optical cross connect/add-drop/terminal mux(OMS termination)

3-R regeneration (OCh, OTU termination)

Client access (ODU termination)

optical

sub-network

optical sub-networkoptical

sub-network

domain domain

IaDIsIrDI

IaDIs

OTN Overview

Fig. 13 OTN Overview

The OMS overhead connects two points where optical multiplexing or demultiplexing is taking place. This usually also means 3R regeneration, but this is not a must. For example in front of an optical cross connect the signals are demultiplexed but not necessarily regenerated. Finally, the OTS overhead only accompanies the optical signal on a section from one amplifier or regenerator to the next, allowing for the supervision of a single transmission span. The final structure is termed OTM-n.m, with n the number of channels and m the bit rate indicator. As mentioned before, we will find the same structure for a single channel.

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2.2 Enhanced forward error correction Signal rates continue to increase in today's optical networks – from 10 Gbit/s (OC-192 / STM-64) to 40 Gbit/s (OC-768 / STM-256) and beyond. Unfortunately, distance-limiting phenomena such as fiber non-linearities, chromatic dispersion, and PMD have a more pronounced effect at higher transmission rates. With existing technology, additional amplifier and regenerator sites at closer spacing would be needed for these networks, resulting in unacceptable expense to customers. However, techniques such as EFEC can be used, which enable networks to contain spans longer than otherwise possible. In its simplest sense, EFEC is a coding algorithm that enables bit errors to be detected and corrected without the need for re-transmission. The ability to correct bit errors translates directly into the ability to make spans longer. Distances that, without EFEC, would suffer an unacceptable receive-end OSNR can be successfully spanned when EFEC is used. In other words, EFEC provides networks with additional OSNR margin (typically about 8.5 dB extra margin). EFEC is a "forward" scheme because everything is always moving forward – data is encoded at the transmit end and sent to the receive end where it is terminated (decoded). The receiver needs only the information it receives to detect and correct bit errors – it never requests a re-transmission. TIP The type of EFEC implements is known as "Out-of-Band" EFEC (OOB EFEC). In this method, the transmission rate is increased to accommodate the basic SONET/SDH payload plus the added EFEC and management overhead. Recently ratified, the ITU-T G.709, Network Node Interface for the Optical Transport Network provides the standards specification for OTN. By complying with ITU-T G.709, network nodes from various suppliers are ensured of interoperability. The OTN architecture is additionally defined under ITU-T G.872, Architecture of Optical Transport Networks.

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

OTN Frame XX

EFECEncoderEFEC

Encoder

DelayDelay

EFEC

EFECOTN Frame

Transmitter Side

OTN FrameXX

EFECDecoderEFEC

Decoder

DelayDelay

EFEC

EFECOTN Frame

Receiver Side

Error Correction

Fig. 14 Enhanced forward error correction principles

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2.3 Client Signals Mapping of CBR2G5, CBR10G and CBR40G Signals (e.g. STM-16/64/256) into OPUk: Mapping of a CBR2G5, CBR10G or CBR40G signal (with up to ±20 ppm bit-rate tolerance) into an OPUk (k = 1, 2, 3) may be performed according to two different modes based on one generic OPUk frame structure. • Asynchronous mapping: The OPUk signal for the asynchronous mapping is

created from a locally generated clock, which is independent of the CBR2G5, CBR10G or CBR40G client signal.

• Bit synchronous mapping: The OPUk clock for the bit synchronous mapping is derived from the CBR2G5, CBR10G or CBR40G client signal.

Mapping of ATM Cell Stream into OPUk: A constant bit rate ATM cell stream with a capacity that is identical to the OPUk payload area is created by multiplexing the ATM cells of a set of ATM VP signals. Rate adaptation is performed as part of this cell stream creation process. The ATM cell stream is mapped into the OPUk payload area with the ATM cell byte structure aligned to the ODUk payload byte structure. A cell may cross an OPUk frame boundary.

Mapping of GFP Frames into OPUk: The mapping of generic framing procedure (GFP) frames is performed by aligning the byte structure of every GFP frame with the byte structure of the OPUk payload. Since the GFP frames are of variable length (the mapping does not impose any restrictions on the maximum frame length), a frame may cross the OPUk frame boundary. GFP frames arrive as a continuous bit stream with a capacity that is identical to the OPUk payload area, due to the insertion of Idle frames at the GFP encapsulation stage.

Mapping of Test Signals into OPUk: • Mapping of a NULL client into OPUk: An OPUk payload signal with an all-0s

pattern is defined for test purposes. This is referred to as the NULL client. • Mapping of PRBS test signal into OPUk: For test purposes a 2 147 483 647-bit

pseudo-random test sequence (231 - 1) can be mapped into the OPUk payload.

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Mapping of Client Signals into OPUkMapping of Client Signals into OPUk

CBR2G5, CBR10Gand CBR40G signals

Optical Channel Transport Unit OTU

ATM Cell Stream

Test Signals

GFP Frames

Non-specificClient Bit Streams

Fig. 15 Mapping of Client Signals into OPUk

Mapping of a Non-specific Client Bit Stream into OPUk: In addition to the mappings of specific client signals a non-specific client mapping into OPUk is specified. Any (set of) client signal(s), which after encapsulation into a continuous bit stream with a bit rate of the OPUk payload, can be mapped into the OPUk payload. The bit stream must be synchronous with the OPUk signal. Any justification must be included in the continuous bit stream creation process. The continuous bit stream must be scrambled before mapping into the OPUk payload.

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2.4 Applicable ITU recommendations The applicable recommendation is: • ITU-T Recommendation G.709/Y.1331

"Interfaces for the Optical Transport Network (OTN)"

2.4.1 ITU-T Recommendation G.709/Y.1331 This Recommendation defines the interfaces of the optical transport network to be used within and between sub-networks of the optical network. This Recommendation defines the requirements for the optical transport module of order n (OTM-n) signals of the optical transport network, in terms of: • Optical transport hierarchy (OTH); • Functionality of the overhead in support of multi-wavelength optical networks; • Frame structures; • Bit rates; • Formats for mapping client signals.

ITU-T Recommendation G.709/Y.1331

This Recommendation defines the requirements for the optical transport module of order n (OTM-n) signals of the optical transport network, in terms of:

• Optical transport hierarchy (OTH);• Functionality of the overhead in support of multi-wavelength optical networks;• Frame structures;• Bit rates;• Formats for mapping client signals.

Fig. 16 ITU-T Recommandation G.709/Y.1331