Optical Network Curriculum

Development Section

ISSUE

WDM Network Technology

1.0

Objectives

Understand the basic concept of WDM

network ,

Grasp the budgeting method in WDM

network design ,

Know channel coding technology in WDM

network ,

On completion of this course ,you will be able to :

Contents

Chapter 1 WDM network description

Chapter 2 Factors to be considered in

WDM networking budget

Chapter 3 Coding Technology

Chapter 1 WDM network description

1.1 WDM NE Types

1.2 WDM Network Topology

O T M

O T M

1.OTM

OADM 2.OADM

3.OLA

4.REG R E G

R E G

WDM NE Types

OTM

OTU DeMUX

OTU MUX

Optical Amplifier

Optical Amplifier

OSC

OLA

Optical amplifier

OSC

Optical amplifier

OADM

OTU

OADM

OSC

OA

OA

TMUX

OA

OA

REG

DeMUX MUX

Optical amplifier

OSC

O

T

U

DeMUX MUX

Optical amplifier

O

T

U

Chapter 1 WDM network description

1.1 WDM NE Types

1.2 WDM network topology

Point to Point Network

SDH OTM OLA OLA OTM SDH

It consists of OTM and OLA ,and OADM is

not included .

Chain network

STM-16

16个 OTM

STM-16

16个

STM-16

8个 OTM OADM

Ring Network

OADM

OADM OADM

OADM

Note: OADM is including static OADM and back-to-back

OTM (OTM-OTM)

Chapter 1 WDM network description

1.1 WDM NE Types

1.2 WDM network Topology

Contents

Chapter 1 WDM network description

Chapter 2 Factors to be considered in

WDM networking budget

Chapter 3 Coding Technology

Chapter 2 Factors to be considered

in WDM networking budget

2.1 Attenuation

2.2 Dispersion

2.3 OSNR

2.4 Nonlinearity

Attenuation

Under worst condition :

L=(Ps-Pr)/a

Ps :Launched optical power (with single channel) at transmitter side

Pr :Receiving optical power (with single channel) at receiver side .

a :Attenuation coefficient . it is recommended as 0.275dB/km (for G.652)

from ITU-T when taking optical splicing and redundancy into consideration ,

A B S R

L Ps Pr

Attenuation

Example：

Ps=5dBm, Pr= -17dBm, L=

Ps=5dBm, Pr= -28dBm, L=

80km

120km

Chapter 2 Factors to be considered

in WDM networking budget

2.1 Attenuation

2.2 Dispersion

2.3 OSNR

2.4 Nonlinearity

Dispersion

Dispersion limited distance=

(Dispersion tolerance/dispersion coefficient)- (10～30)

(Normally , Redundancy is considered as 10~30 km )

Dispersion tolerance ： it depends on the spectrum

performance of light source .

Dispersion performance with STM-16 level

Dispersion tolerance of light source is 7200ps/nm ,

12800ps/nm etc,.

Dispersion coefficient is about 17ps/nm.km in G.652

fiber (20ps/nm is used in project when redundancy is

considered),

Typically ,dispersion coefficient is about 1~6ps/nm.km

in G.655 fiber .

Dispersion performance with STM-64 level

Dispersion tolerance of light source is 700ps/nm ,

1200ps/nm etc,.

Dispersion coefficient is the same as above .

Max. transmission distance without compensation :

In G.652 L=700/17-(10～30)

In G.655 L=700/6-(10～30)

Distributing compensation is recommended to obtain good effect .

Dispersion compensation

It is necessary to make dispersion

compensation in STM-64 system .

Chapter 2 Factors to be considered

in WDM networking budget

2.1 Attenuation

2.2 Dispersion

2.3 OSNR

2.4 Nonlinearity

Noise（ASE）

Signal

ASE Noise

OSNR

Span

OSNR is used to measure accumulation degree of

ASE noise .defined as :

The ratio of signal power to the noise power inside a specific

band nearby signal:

OSNR

=

ASE

s

P

P OSNR

Adopt Optical Amplifiers with low

noise :

•Application of Preamplifiers with low

noise

•Raman technology

How To Reduce Noise Accumulation

System Design With Raman Amplifiers

Chapter 2 Factors to be considered

in WDM networking budget

2.1 Attenuation

2.2 Dispersion

2.3 OSNR

2.4 Nonlinearity

Nonlinearity Limitation

Nonlinearity Limitation :

Limit optical power of single channel

Limit optical power coupled into fiber

Refer to G.663

Take FWM into account ,channel spacing is

strictly required in G.653 .

Refer to G.692

Nonlinearity :

SBS, SRS, FWM, SPM, XPM etc,.

Contents

Chapter 1 WDM network description

Chapter 2 Factors to be considered in

WDM networking budget

Chapter 3 Coding Technology

How to increase OSNR tolerance in WDM system?

Adopt Forward Error Correction Technology ：

FEC/EFEC

Adopt special coding technology : RZ format etc,.

Chapter 3 Coding Technology

Chapter 3 Coding Technology

3.1 FEC coding

3.2 RZ coding

FEC Technology

FEC（Forward Error correction）

At transmitter end, some additional overheads bits or

bytes are inserted into the transmitting data, at receiver

end, wrong data (error) will be corrected through

additional bits or bytes.

With FEC function, It can improve BER performance

without increasing OSNR requirement.

FEC Technology

•Digital encapsulation

•RS Coding（255，239）

FEC is used to correct code, it doesn’t increase optical

power or improve OSNR, but data error ratio is reduced

under the same condition. In case of BER<10-12

Without FEC :OSNR is required to be more than 20dB

With FEC, OSNR is required to be more than 16dB

In practical application, when FEC function is available,

it means long distance transmission with multiple sections

can be achieved.

OSNR and FEC

Chapter 3 Coding Technology

3.1 FEC coding

3.2 RZ coding

R Z

RZ－Return to Zero Code

NRZ

RZ

1 1 1 1 1 1 1 0 0

CLOCK

T

T

T

RZ Coding

I1-I0

I1-I0

• Small duty cycle and power density；

• Higher ability to resist dispersion and nonlinearity,

• Higher ability to resist eye pattern degradation resulted

by PMD.

Super WDM