Incorporating Passive CWDM Technology vs.

Deploying Additional Optical Fiber The recent advancement in telecommunication applications for voice, video and data is placing additional demands on fiber optic networks.. WDM was deve

Incorporating Passive

CWDM Technology vs

Deploying Additional

Optical Fiber

Passive CWDM Technology

vs Deploying Additional Optical Fiber

The recent advancement in telecommunication applications for voice, video and data is placing additional demands on fiber optic networks Adding additional fiber to existing networks can be very costly to service providers In most cases, a far better – and less costly – option is found

in coarse wavelength division multiplexing (CWDM) technology

This paper will explain CWDM technology and its ability to add greater fiber bandwidth while increasing the flexibility, accessibility, adaptability, manageability and protection of the network for applications up to 60 km

What is CWDM?

CWDM can be viewed as a “third generation” of WDM technology WDM was developed as a fiber exhaust solution and traditionally employed the 1310 nm and 1550 nm wavelength signals In most WDM scenarios, providers with a fixed number of fibers had run short of bandwidth due

to rapid growth and/or unforeseen demand By multiplexing a signal on top of the existing 1310 nm wavelength, they could create additional channels through a single fiber to increase the network’s capacity

However, demand continued to increase dramatically with new innovations and applications such as the internet, text messaging and other high-bandwidth requirements This created the need for very fine channel spacing to add even more wavelengths or channels to each fiber Dense WDM (DWDM) was a major breakthrough as equipment providers pushed

to offer new DWDM equipment, promising nearly unlimited bandwidth potential However, while DWDM was quickly adopted for long-haul and trans-oceanic optical networking, its use in regional, metropolitan, and campus environments was, in most cases, cost prohibitive

A more targeted and cost-effective solution followed with CWDM, a more recent standard of channel spacing developed by the International Telecommunication Union (ITU) organization in 2002 This standard calls for a 20 nm channel spacing grid using wavelengths between

1270 nm and 1610 nm (see Figure 1) The cost of deploying CWDM architectures today is significantly lower than its DWDM predecessors Prior to ITU standardization, CWDM was fairly generic and meant a number

of things For instance, the fact that the choice of channel spacing and

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frequency stability was such that erbium-doped fiber

amplifiers (EDFAs) could not be used was a common

thread One typical definition for CWDM was two or

more signals multiplexed onto a single fiber, one in the

1550 nm band and the other in the 1310 nm band

– basically, the original definition for early WDM

New developments

Even though the ITU’s 20 nm channel spacing offers 20

wavelengths for CWDM, the reality is that wavelengths

below 1470 nm are considered “unusable” on older

G.625 spec fibers due to the increased attenuation

in the 1310-1470 nm bands However, new fibers

that conform to the G.652.C and G.652.D standards,

such as Corning SMF-28e and Samsung Widepass,

nearly eliminate the “water peak” attenuation

peak to allow for full operation of all ITU CWDM

channels in metropolitan and regional networks

This enables a CWDM system to operate effectively

at the low end of the ITU grid where attenuation

was problematic for earlier fibers For example,

an Ethernet LX-4 physical layer uses a CWDM

consisting of four wavelengths near the 1310 nm

wavelength, each carrying a 3.125 Gbits/sec data

stream Together, the four wavelengths can carry 10

Gbits/sec of aggregate data across a single fiber

As mentioned earlier, a major characteristic of the

recent ITU CWDM standard is that the signals are not

spaced appropriately for amplification by EDFAs This

limits the total CWDM optical span to somewhere near

60 km of reach for a 2.5 Gbits/sec signal However,

this distance is suitable for use in metropolitan

applications The relaxed optical frequency stabilization

requirements also allow the associated costs of CWDM

to approach those of non-WDM optical components

Basic implementation

As stated earlier, CWDM’s appeal is firmly rooted

in meeting the additional demands being placed

on fiber networks by a steady stream of new, bandwidth-hungry applications Adding more fiber is one solution, but there are many possible obstacles that will likely make this solution cost prohibitive Although every situation is different and brings unique considerations to the table, nearly any fiber deployment includes rights-of-way, trenching costs, additional equipment, manpower and considerable time

Market studies have indicated accrued costs between

$10,000 and $70,000 per mile to deploy new fiber cable The large disparity is due to different situations – for example, it costs far more to tear up a city street than to simply trench fiber in a rural setting But the key issue is that network architects can incorporate

a CWDM system for much less cost and still achieve the bandwidth increases necessary to meet demand today and well into the foreseeable future

Basically, a CWDM implementation involves placing passive devices, transmitters and receivers, at each end of the network segment CWDM performs two functions First, they filter the light to ensure only the desired combination of wavelengths is used The second function involves multiplexing and demultiplexing the signal across a single fiber link In the multiplex operation, the multiple wavelength bands are combined onto a single fiber for transport In the demultiplex operation, the multiple wavelength bands are separated from the single fiber to multiple outputs (See figures 2 and 3) ADC’s passive network solution adds value by using the value-added module (VAM) platform to multiplex and demultiplex These VAMs can easily be incorporated into central office (CO), multiple service operator (MSO), and mobile switching center (MSC) environments for leveraging the benefits of CWDM The MSC uses CWDM

to multiplex the different hosts on a wireless coverage system to multiple remotes using minimal fiber strands Even a single fiber can service 4, 6, or 8 different remote units From there, an antenna is attached to each device to enable indoor wireless coverage

Designated, dedicated wavelengths

CWDM also offers the benefit of individual wavelengths for allocating specific functions and applications

Out-of-band testing capability is achieved by simply dedicating a separate wavelength or channel for non-intrusive testing and monitoring In fact, any number of different applications can be applied

to specific wavelengths For example, a particular wavelength might be allocated specifically for running overhead or management software systems

This is a common practice in using CWDM for cable television networks, where different wavelengths are dedicated for downstream and upstream signals

Incorporating Passive CWDM Technology vs Deploying Additional Optical Fiber

O-band

1260-1360 1360-1460 E-band

Wavelength (nm)

S-band 1460-1530 1530-1565 C-band 1565-1625 L-band

2

1.5

1

0.5

0 ITU-T G.652 fiber

Water peak

1430 1450 1470 1490 1510 1530 1550 1570 1590 1610

Figure 1: CWDM wavelength grid as specified by ITU-T G.694.2–

Today’s standardized CWDM is better defined as a cost-effective solution

for building a metropolitan access network that promises all the key

characteristics of a network architecture service providers dream about

– offering transparency, scalability, and low cost.

Website: www.adc.com

From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 • For a listing of ADC’s global sales office locations, please refer to our website ADC Telecommunications, Inc., P.O Box 1101, Minneapolis, Minnesota USA 55440-1101

Specifications published here are current as of the date of publication of this document Because we are continuously improving our products, ADC reserves the right to change specifications without prior notice At any time, you may verify product specifications by contacting our headquarters office in Minneapolis ADC Telecommunications, Inc views its patent portfolio as an important corporate asset and vigorously enforces its patents Products or features contained herein may be covered by one or more U.S or foreign patents An Equal Opportunity Employer

104632AE 6/07 Original © 2007 ADC Telecommunications, Inc All Rights Reserved

It should be noted that the downstream

and upstream wavelengths are usually

widely separated For instance, the

downstream signal might be at 1310 nm

while the upstream signal is at 1550 nm

Another recent development in CWDM is the

creation of small form factor pluggable (SFP)

transceivers that use standardized CWDM

wavelengths These devices enable a nearly

seamless upgrade in even legacy systems that

support SFP interfaces, making the migration

to CWDM more cost effective than ever

before A legacy system is easily converted

to allow wavelength multiplexed transport

over one fiber by simply choosing specific

transceiver wavelengths, combined with an

inexpensive passive optical multiplexing device

Conclusion

ADC views the emergence of CWDM as the most cost effective means of moving ever-increasing amounts of information across metropolitan access networks For most providers, deploying new fiber as a means of combating fiber exhaust is not a viable option There are too many high costs involved with trenching the fiber cable, and obtaining rights-of-way can be an intensely complex issue CWDM simply makes sense, particularly with the technological advancements in today’s fiber and transceiver options, including VAM systems CWDM achieves the critical goals

of transparency, scalability, and low cost that providers seek in today’s highly competitive industry – an industry where new applications and increasing demand dictate the pace for modern telecommunication networks

Metro Transport Ring Using CWDM

HEADEND

RESIDENTIAL

Optical Node

Hub

Hub

Hub

Hub

Optical Node

Optical Node

Optical Node

INDUSTRIAL

WIRELESS HANDOFF

HIGH-RISE MDU/ BUSINESS

Satellite Uplink

Fib er L ine

ve le

s

Figure 2: CWDMs in use – For example, MSOs can install a band system at the headend that will drop one wavelength to each node along a particular ring configuration This ring can be utilized as a single fiber Each CWDM device is packaged into the VAM platform – connectorized and labeled – for integration into the actual

fiber panel or cross connect to save floor space and eliminate extra patch cords.