# WDM Network Design

• Sample Chapter is provided courtesy of Cisco Press.
• Date: Feb 7, 2003.

### Chapter Description

This sample chapter explores some of the design constraints involved in the WDM network design.

\$55.00

## Examples

The following case studies reinforce the principles discussed so far in this chapter.

### Case 1

Design a 4 ×25 span WDM link with an optical amplifier gain of 22 dB and NF equal to 5 dB.

Calculate the final OSNR if the input power is 0 dB. Calculate the signal power at the receiver.

Will this system work if the receiver sensitivity is a minimum of –25 dB?

Will the system work if the input power is 10 dB?

The answer is shown in Figure 4-16.

Figure 4-16 Answer to the Problem

You can use one of the two methods to determine the final OSNR.

#### Method 2

OSNR stage by stage analysis using the formula:

OSNRstagei = 1/(1/OSNRstage0 + NF.h.v.Δf /Pin) (1/OSNRstage0 = 0)

OSNR Stage 1

NF = 5 dB converting to linear = 3.166 (10NF dB/10)

h = Plank's constant = 6.6260E – 34

v = frequency of light 1.9350E + 14

Δf = bandwidth (measuring the NF) = 12.5 KHz (.1 nm)

Pin = input power at the amplifier

0 – 25 dB = –25 dB

OSNRstage1 = 28 dB

Output from the amplifier = –25 + 22 = –3 dB

OSNR Stage 2

OSNRstage2 = 1/(1/OSNRstage1 + NF.h.v.Δf /Pin)

Input at the second amplifier

= –3 – 25 = –28 dB

OSNRstage2 = 23 dB

Output from the amplifier = –28 + 22 = –6

OSNR Stage 3

OSNRstage3 =1/(1/OSNRstage3 + NF.h.v.Δf /Pin)

Input at the third amplifier = –6 – 25 = –31 dB

OSNRstage3 = 20 dB

Output from the amplifier = –31 + 22 = –9 dB

= –9 – 25 = –34 dB

If the receiver sensitivity is –25, the system will not work.

The solution is to (a) increase the gain of the amplifier (b) increase the input power of the transmitter.

The same solution for 10dB input power is shown in Figure 4-16.

Using Method 1: OSNRfinal = 10 + 58 – 25 – 6 – 5 = 32

Method 2: gives OSNRfinal = 29 dB

The difference in value is due to the approximation made in the parameters.

= 10(Tr) – 25 (loss1) + 22 (gain1) – 25 (loss2) + 22 (gain 2) – 25 (loss 3) + 22 (gain 3) – 25 (loss) = –24 dB

The receiver sensitivity is given as –25 dB, so the system will work.

### Case 2

Calculate the number of spans in this link, given Pin = 0 dB; OSNRfinal = 20 dB total length = 300 km Bit rate = 5 Gbps; NF = 5 dB. (Assume fiber type is SMF ∝ = .2 dB/km)

Total loss over entire length = 300 × 0.2 = 60 dB

Attenuation per span = 60/N (assume N number of spans

OSNRFinal = P0 + 58 – –10logN – NF

20 = 0 + 58 – 60/N –10logN – 5

-33 = –60/N – 10logN

Rearranging, you get this:

60/N – 10logN = 33

Solving for N (for N = 2)

60/2 + 10log2 = 33

Therefore, the number of spans = 2.

### Case 3

OSNR = 20 dB, dispersion of the fiber is 17 ps/nm-km, span loss = 22 dB.

What is the total length of the system? (NF of the amplifier = 4 dB, and dispersion tolerance is given as 1600 ps/nm, Pin = 10)

OSNRFinal = P0 + 58 – –10logN – NF

20 = 10 + 58 – 22 – 4 – 10logN

–22 = –10 logN

N = 158 spans

Therefore, total length = 158 * 22/0.2 = 17,280 km (theoretical limit)

But due to dispersion, max length = 1600 ps.nm/17 ps /nm.km = 94 km.

### Case 4

Customer A wants to build a 200 km OC48 link to transport traffic. Design the link with the following parameters. (Assume SMF-28 fiber with α = .25 dB/km/ and 18 ps/nm/km as the dispersion characteristic.)

Transmit power: +7 to +9 dBm

Dispersion tolerance: 1500 ps/nm

Dispersion penalty: 1.5 dB @ 1500 ps/nm

OSNR tolerance: 20 dB @ Resolution Bandwidth 0.1 nm

EDFA

Input power range: +3 to –25 dBm; Gain: 20 dB to 14 dB

Maximum output power: +17 dBm; Noise Figure: 5 dB

DCU

Loss 5 dB, dispersion compensation –1100 ps

The answer requires several steps. Refer to Figure 4-17.

 Step 1 The total distance is 200 km; therefore, total loss is 200 * .25 = 50 dB. You need amplifiers to reach that distances. Step 2 Total dispersion is 200 km * 18 ps.nm / km = 3600 ps.nm

You need to use dispersion compensators because the given dispersion tolerance is 1500 ps.nm. The maximum distance for this system without DCU is 1500/18 = 83.33 km given DCU = –1100 ps. If you strategically place 3 DCU, you get (3600 – 3 * 1100) = 300 ps, which is well within the limit. The DCU has a 5 dB passthrough loss, so it is best to place it before preamplifiers.

Analysis of the problem

Transmit power = + 7 dB

Stage 1:

loss = –10 (link loss) – 6 (DCU loss) – 1.5 (margin) = –17.5 dB

Pin = Power at the end of stage 1 = 7 – 17.5 = –10.5 dB

Stage 1 amplifier gain: 20

Power output from the amplifier: (power input + gain) – 10.5 dB + 20 = 9.5 dB

OSNR calculation

OSNRstagei =1/(1/OSNR0 + NF.h.v.Δf /Pin) (for stage 1 1/OSNR0 = 0)

OSNR1 = Pin/NF.h.v. Δf

Substituting the values Pin = –10.5 dB = 8.9125E – 05 W (convert to watts)

NF = 5 dB converting to linear = 3.166 (10NF dB/10)

h = planks constant = 6.6260E – 34

v = frequency of light 1.9350E + 14

Δf = bandwidth (measuring the NF) = 12.5 KHz (.1 nm)

Substituting, you get OSNR = 42 dB

Stage 2:

Loss = –20 (link loss – 6 (DCU loss) – 1.5 (margin) = –27.5 dB

Power at the end of stage 2

Pin = 9.5 – 20 – 6 – 1.5 (margin) = –18 dB

Stage 2 amplifier gain: 20

Power output from amplifier: –18 + 20 = 2 dB

OSNRstage2 = 1/(1/OSNR1 + NF.h.v.Δf /Pin)

OSNR1 is the OSNR of stage 1 = 42 dB and Pin = –18 dB

OSNRstage 2 = 34 dB

Stage 3 loss:

–20(link loss) – 6 (DCU loss) – 1.5 (margin) = –27.5 dB (loss)

Power at the end of stage 3

Pin= 2 – 27.5 = –25.5 dB

Stage 3 amplifier gain 20 dB

Power output from the amplifier: = –25.5 + 20 = –5.5 dB

OSNR

OSNRstage3 = 1/(1/OSNRstage2 + NF.h.v.Df /Pin)

OSNR = 26

### Case 5

Calculate the composite power at the output of a DWDM 8 channel multiplexor (shown in Figure 4-18) if the input power is 0 dB (insertion loss = 5 dB).

Figure 4-18 Calculate the Composite Power

Pcomposite = Pchannel + 10logN – Insertion loss (where N is the number of channels)

= 0 + 10log8 – 5 dB = 0 + 9 – 5 dB

= 4 dB

### Exercises

The following exercises are left for you to solve. Use previous examples as guidelines.

1. Calculate the dispersion and the receiver power of an optical link that is 90 km in length. The transmit power = +8 dB, the receiver sensitivity = –18 dB, and the dispersion tolerance = 1500 ps/nm (SMF ∝ = .25 dB/km 18 ps/nm-km). Is it dispersion limited? Design the link for proper network operation if you need to use EDFA 15 dB (gain) and DCU –1100 ps and 5 dB loss).

2. Design a 4 × 25 span WDM link with optical amplifier gain = 18 dB and NF = 6 dB.

3. Calculate final OSNR if the input power is 0 dB. Calculate signal power at the receiver.

(The receiver sensitivity is –25 dB at BER 10-15). Does the system work?

Now calculate the final OSNR by replacing the following:

— The transmitter with input power to +7 dB

— The amplifier gain to 22 dB

4. Calculate power in milliwatts if the input power were 0 dBm.

5. Calculate the power in dBm if the input power were 12 mW.

Hint: XmW power is 10 * log10(x) in dBm

Y dBm power is 10Y/10 in mW

6. Calculate the composite power at the output of a DWDM 16-channel mux if the input power is 4 dB. If the input is plugged into an amplifier, how much attenuation do you need (given NF = 5 dB, input range = 0–25 dB, gain = 22)

### Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.

## Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

## Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

### Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

### Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

### Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

### Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

### Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

### Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

## Other Collection and Use of Information

### Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

### Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

### Do Not Track

This site currently does not respond to Do Not Track signals.

## Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

## Children

This site is not directed to children under the age of 13.

## Marketing

Pearson may send or direct marketing communications to users, provided that

• Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
• Such marketing is consistent with applicable law and Pearson's legal obligations.
• Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
• Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

## Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

## Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

## Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

## Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

## Sharing and Disclosure

Pearson may disclose personal information, as follows:

• As required by law.
• With the consent of the individual (or their parent, if the individual is a minor)
• In response to a subpoena, court order or legal process, to the extent permitted or required by law
• To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
• In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
• To investigate or address actual or suspected fraud or other illegal activities
• To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
• To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
• To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.