WHITE PAPER Incorporating Passive CWDM Technology vs. Deploying Additional Optical Fiber 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. 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 Page 3 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 1200 1300 O-band 1260-1360 E-band 1360-1460 Wavelength (nm) Fiber attenuation (dB/km) S-band 1460-1530 C-band 1530-1565 L-band 1565-1625 1400 1500 1600 2 1.5 1 0.5 0 ITU-T G.652 fiber Water peak 1270 1290 1310 1330 1350 1370 1390 1410 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. WHITE PAPER 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 F i b e r L i n e W a v e l e n g t h 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.