Anthony Abate was facing a challenge. As Director of Network Engineering at Line Systems, Inc., Anthony is responsible for network infrastructure, and was tasked with adding a new Gigabit Ethernet backbone to an existing SONET OC-12 network. The goal was to expand the capacity of the SONET fiber infrastructure to meet the increasing demand for next-generation Carrier Ethernet Business Services. In addition, he was to achieve this in the most cost-effective means possible, quickly implement the Ethernet services without interruption to the existing services, and provide future growth potential.

Line Systems positions itself as a full-service communications consulting firm, serving business users of communication products and services. Line Systems offers a comprehensive portfolio, supporting a full range of voice, Voice over IP, data and Internet services, including wholesaling to Regional Bell Operating Companies.

The challenge of adding new customers, new services, and bandwidth to existing fiber networks, while controlling capital expenditure costs, is not unique to Line Systems. ILECs, CLECs, Cable Operators, ISPs and even operators of large enterprise networks face similar circumstances of needing to add capacity to their fiber networks while facing pressures to lower costs.

Line Systems operates a network in the Philadelphia Tri-State area that extends through Philadelphia and the surrounding suburbs and Southern New Jersey out to Atlantic City. The network consists of three SONET OC-12 rings with a central hub providing Carrier Class TDM voice and data to thousands of customers. (See Figure 1.) 

Figure 1. Map of the Line Systems’ SONET ring networks.

 Abate considered three options to increase the capacity of Infinite’s fiber optic network: install new fiber, upgrade the SONET equipment, or utilize Wavelength Division Multiplexing. The options he reviewed included:

Option 1. Install New Fiber
Anthony recognized fiber is a very cost-effective cabling medium. The installation is where it gets expensive. Estimates run from $50,000 to $250,000 per mile* depending on whether the installation is in rural or urban areas, and on the method of installation (trenching, blown fiber, or aerial). In addition, fiber installs can take many months of zoning, project planning, construction work, and disruption.

Option 2. Upgrade SONET Equipment
Anthony also knew that TDM networking equipment, particularly SONET multiplexers and routers, can be expensive, and so can the cost of upgrading SONET equipment to higher data rates and running Ethernet over SONET. In addition, Anthony was well aware of the complexities of TDM circuit emulation equipment that carries Ethernet over SONET increases training and support costs.

Option 3. Wavelength Division Multiplexing
In a typical fiber optic network, data signals are transmitted via 1310nm or 1550nm wavelengths. Anthony understood that Wavelength Division Multiplexing (WDM) enables multiple wavelengths, or colors of light, to be transmitted over a fiber optic link, creating independent and simultaneous data streams of different data rates and network protocols. These wavelengths are combined (multiplexed) at the source end, then separated (de-multiplexed) with color filters at the destination end.

WDM wavelengths are also referred to as channels because data from virtually any type of communications protocol and data rates up to 10 Gigabit can be carried over a WDM network, including Ethernet ATM, TDM, SONET, Fiber Channel, and Serial. The primary benefit is that the fiber bandwidth capacity is significantly increased, enabling more information to be sent over the existing fiber media.

After careful evaluation, Anthony selected CWDM to increase the capacity of the SONET network for two reasons. First, Anthony was well aware that dark fiber is an expensive resource. He explained, “We had already made a significant investment in the fiber network to deploy SONET OC-12. When we wanted to add new Ethernet services, CWDM gave us the ability to leverage the existing fiber without making an additional costly physical build. Secondly, we considered upgrading the OC-12 to a higher bandwidth SONET such as OC-48 or OC-192 and running Ethernet over SONET. However, the associated optics costs coupled with the additional complexities of the conversion between Ethernet and SONET made this option much less desirable than CWDM.”

To tackle his task, Anthony built a single CWDM Gigabit Ethernet redundant ring around all three SONET rings using the 1470nm wavelength, providing two independent paths running Spanning Tree Protocol (STP). He selected CWDM MUXes that support the 1470nm, 1490nm, 1590nm, and 1610nm wavelengths. (See Figure 2).This configuration offered him the flexibility to use either the 1310 pass band port or the 1550 expansion (1550 pass band) port, because another challenge he faced was mixed wavelengths in the legacy network.

Figure 2. Anthony selected the 1470, 1490, 1590, and 1610 CWDM wavelengths because they enabled him to reserve areas of the spectrum for the legacy 1310 and 1550 wavelengths.

When the network was originally built, the 1310nm OC-12 optics could not reach the distance required on three segments extending from the hub site. Anthony took care of that by purchasing Omnitron’s fiber-to-fiber media converters that provided 1310nm to 1550nm conversion with higher powered optics to span the longer segments.

It’s important to remember a few things when overlaying a CWDM network onto a SONET ring network. To begin, each SONET node is physically bypassed because the CWDM wavelengths cannot pass through the SONET devices. This is accomplished by installing CWDM MUXes on each side of the SONET nodes, and connecting the 1310 or 1550 native to the pass band ports on the CWDM MUXes. The CWDM channel (the 1470nm Gigabit Ethernet) and the legacy OC-12 network (1310nm or 1550nm) are multiplexed across common links between the CWDM MUXes.

In this particular application, Gigabit Ethernet Switches were installed in locations that required Ethernet access. Anthony bypassed the SONET nodes by connecting the switches to the channel ports on the CWDM MUXes with fiber patch cables (See Figure 3.)

Figure 3. Area of detail from Figure 2 showing how CWMD MUXes are used to bypass SONET Nodes.

Anthony connected the Ethernet switches to the CWDM network with SFP transceivers that support CWDM wavelengths. An SFP that supports the 1470nm CWDM wavelength is installed in the switch and connected to 1470nm channel port on the CWDM MUX at each location.

Since Anthony knew that the selection of the appropriate SFP transceivers depended on the distances between each location, he was also aware of the fiber loss and loss inserted by each CWDM multiplexer. To be sure of his next move, Anthony consulted the design team to ensure the transceiver used at each location met the optical loss budget of each network segment.

His plan was confirmed to be the best approach. At locations where Ethernet access was not yet required, a single patch cable was used to bypass the SONET node. At the central hub site, the SONET hub was bypassed by connecting the 1470nm channel ports on the CWDM MUXes with fiber patch cables.

Anthony's Happy Ending
The CWDM network was installed in just a few days, and Anthony was quite satisfied. (See Figure 3.) "The results were excellent. CWDM effectively provided us with a clean slate of dark fiber with zero impact on the existing OC-12 service. We had no issues during the installation and a seamless turn-up process. Our new Gigabit Ethernet backbone is working flawlessly."

With that, Anthony learned that the beauty of SONET networks is the resiliency to re-route network links. SONET rings operate in both directions, so when the CWDM MUXes were installed, there was no down time on the SONET network.

In summary, Anthony gave accolades to the solution. "CWDM enabled Infinite Telecom to generate new revenues by providing Carrier Ethernet and Layer 2 Bridging Services to multisite customers. "We can now offer a full complement of Metro Ethernet Services such as E-Line, E-Lan, and E-Tree. Furthermore, having a dedicated Layer-2 backbone enables Infinite Telecom to be one of the few MEF Certified Carriers offering Layer-2 services to the SMB marketplace."

Endnotes
* The author researched several sources and found numbers ranging from $5,000 to $1 million. An average of several sources was used for the estimate:
1. Optical Communications Rules of Thumb by John Lester Miller and Ed Friedman. ($35,000 to $100,000),
2. Congressional Budget Office. http://www.cbo.gov/ftpdocs/75xx/doc7564/09-15-GIG-BE.pdf ($150,000 to $300,000),
3. CIO magazine article, Fiber Optics: The Fiber Glut Myth by Bud Bates. October 15, 2002. http://www.cio.com/article/31460/Fiber_Optics_The_Fiber_Glut_Myth ($250,000 to $1 million)

Get on the Same Wavelength
Anthony’s Tutorial on CWDM
The wavelengths used with CWDM implementations are defined by the International Telecommunications Union; reference ITU-T G.694.2, listing 18 wavelengths from 1270nm to 1610nm with 20nm wavelength spacing. The actual center wavelength in ITU-T G.694.2 CWDM wavelengths are 1271, 1291, etc., but are typically referred to as 1270, 1290, etc. These are actually the same wavelengths, the only difference is semantics. CWDM wavelengths can be used for a wide variety of functions and applications. For example, wavelengths can be dedicated to different customers’ traffic, different speeds and services, or used for non-intrusive testing, monitoring, and management.

To connect a communication device into a CWDM network, the device must transmit an optical signal using one of the 18 specific CWDM wavelengths and be multiplexed into the network’s common link, which is a fiber cable that carries all the CWDM wavelengths. Source and destination devices that communicate across a CWDM common link must use the same wavelength (e.g., both devices use 1490nm). New wavelengths can be added to the common link to connect devices, as long as each wavelength is unique.

The heart of a CWDM network is a device called the CWDM multiplexer (MUX) that multiplexes, or combines, unique wavelengths from different communications sources onto a fiber optic cable. This fiber is referred to as the common link. At the other end of the common link, another MUX device is used to demultiplex, or filter the individual wavelengths and deliver them to their destinations. Each CWDM channel is connected to the CWDM MUX via channel ports.

Note that standard (or native) 1310nm and 1550nm wavelengths are not the same as CWDM 1310nm and CWDM 1550nm wavelengths. The center wavelength tolerances for legacy 1310nm and 1550nm are much wider than the CWDM equivalents, and therefore not precise enough to run through CWDM filters. (See Figure 4.)

Figure 4. The CWDM Spectrum of 18 wavelengths with the wider-tolerance legacy 1310 and 1550 shown in the areas of gray shadow. 

When implementing a CWDM network, a standard wavelength can be converted to a CWDM wavelength, or a CWDM MUX with a pass band port can overlay the standard wavelength onto the CWDM common link. A pass band port is an additional channel port on a CWDM MUX that allows a legacy 1310nm or 1550nm signal to pass through the network within a reserved band. The legacy device is connected directly to the pass band port via fiber cabling. Standard wavelengths can be converted to CWDM wavelengths using CWDM Small Form Pluggable (SFP) transceivers, transponders, and media converters that support SFPs.

Another port available on a CWDM MUX is called the expansion port. This port enables the cascading of several CWDM MUX devices, allowing a network designer to expand the channel capacity of a CWDM network. Two 4-channel CWDM/X devices, for example, can be cascaded (daisy chained) to create an 8-channel CWDM network with this feature. Expansion ports typically utilize the 1510nm to 1570nm region of the CWDM spectrum, and can also function as pass band ports for legacy 1550 networks.

About the Author
Ty Estes is the Director of Marketing Communications for Omnitron Systems. He has more than 15 years experience in marketing network technology. For more information, please email: tyestes@omnitron-systems.com or visit: www.omnitron-systems.com

Anthony Abate is the Director of Switch Engineering for Line Systems. He has more than 23 years experience in Electrical and Telecommunications Engineering. For more information, email aabate@linesystems.com or visit www.linesystems.com.

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