The contemporary carrier business climate is marked with vexing challenges:  Fierce price competition, a decline in consumer buying power, higher downstream/upstream bandwidth demands, and increased energy costs.

In response, carriers are seeking to consolidate their metro networks and rethink their access strategies. Long-term profitable service delivery demands converging access and backhaul for residential and business services, reducing the number of active components in the access network, eliminating local exchanges, and reducing power and space requirements.

These are among the leading and varied drivers behind the recent emergence of the Wavelength Division Multiplexing Passive Optical Network (WDM-PON). Carriers see true benefit in having WDM in both the access and backhaul segments of the network, and WDM-PON enables them to do so.

What ramifications does the innovation of the WDM-PON network have in the real-world operational infrastructure of the outside plant (OSP)? The concepts undergirding the emergent technology are actually already well established, but there are considerations to be addressed in order to realize WDM-PON’s unique benefits.

WDM Breaks Out of the Box
Carriers see a terrific, less-competitive business opportunity in Fiber-to-the-x (FTTx), and the emergent WDM-PON is an uncommonly valuable tool for realizing its potential. WDM-PON solutions stand to enable service providers of all kinds to extend the benefits of WDM into the access network, thereby eliminating active sites and reducing overall network costs.

The PON is firmly established as a valuable network architecture for carriers. (See Figure 1.) Other types of PON solutions, based on Time Division Multiplexing (TDM), are already successfully serving as access solutions for residential Triple Play services integrating voice, TV and Internet. The PON is point-to-multipoint; passive optical splitters divide signals among multiple end users, with different wavelengths assigned to downstream and upstream traffic. Sometimes, a separate wavelength is implemented for downstream video.

One of the great benefits of the PON is that it simplifies a carrier’s approach to the OSP. Different speeds and services can be implemented without changing optical components en route between central office and customer premises.

TDM-based PONs (e.g., Ethernet, Broadband and Gigabit PON [EPON, BPON and GPON] solutions) employ protocol-based traffic separation, in which the specific end users’ traffic is carried on different time slots on the optical signal.

While effective for residential services, this approach creates security concerns for high-end users. Many enterprise customers demand connections that are dedicated and non-shared, in order to optimize security and scalability. In traditional PON approaches, bandwidth scalability is limited and affected by the demands of neighboring customers. The loss-intensive splitting concept constrains the reach of such solutions to roughly 20 km. Consequently, TDM-based PONs are not often used to support business services.

The WDM-PON avoids these pitfalls by utilizing hardware-based traffic separation enabled by wavelength-selective filters. Wavelengths can be dedicated per individual end users. As a result, WDM-PON delivers the terrific level of traffic security that high-end business customers require. Plus, bandwidth per cabinet, building, or end user can be scaled nearly indefinitely. Bandwidth can be upgraded per wavelength from 1 Gbit/s to 2.5 Gbit/s to, potentially, 10 Gbit/s speeds, and the rest of the traffic on the WDM-PON network is not impacted. The architecture is also well-suited to second-mile broadband networks where scalable, cost-effective backhaul of traffic from Digital Subscriber Line Access Multiplexer (DSLAM) locations is required.

These are the benefits that have made the WDM-PON an architecture of such promising value to carriers. The WDM-PON resolves the issues that thus far have prevented PON deployments in support of business services, as well as provides the necessary attributes
for second-mile backhaul of increasingly powerful residential services. The WDM-PON figures to coexist with two other PON architectures: a GPON and a next-generation TDM-based PON, “NGPON”- to support various services. GPON and NGPON will support a carrier’s residential services, while WDM-PON will shoulder commercial access and backhaul for cell towers and Very-High-speed Digital Subscriber Line (VDSL) services. (See Figure 2.)

No Boxing In the OSP
What practical concerns for the carrier OSP does the WDM-PON introduce?

The installation process for a WDM-PON splitter is the same as with an EPON, BPON, or GPON splitter, so no additional training for field technicians is required. But a WDM-PON approach does introduce a flexible remote node performing new functionality in the OSP. On one side, this WDM splitter accepts a multiplex of service traffic, separates wavelengths by color and, on the other, intelligently delivers up to 40 channels to individual users.

From a topology perspective, it’s not a concept dissimilar to point-to-point fiber cables fanning out from the OSP. The WDM-PON simply takes this tree-type architecture another step by splitting out wavelengths from one fiber. With the WDM aggregation achieved on each fiber strand, the need for new cable deployments for new customers is, thus, averted.

The stability of this wavelength-selection process is an important question for the OSP, as attenuation and loss are sensitive to both temperature and moisture.

Coarse WDM (CWDM) splitters, often already deployed for point-to-point networks, absolve the issue. CWDM’s coarse grid, with its large degree of separation among wavelengths (20 nanometers), is more forgiving of the wavelength shifts that occur with temperature changes. Therefore, CWDM splitters are a solution for environments where temperature and humidity are of concern, such as the cable duct or street-side cabinet that is not environmentally controlled.

Dense WDM (DWDM) splitters slice the wavelength into more channels, separated by only 0.8 nanometers. The process is more precise and, in turn, can be sensitive to environmental variations; a 0.1 nanometer wave-length shift could result in variations in optical loss and throughput.

Fortunately, the latest athermal arrayed waveguide gratings (AWGs) can alleviate the environmental concerns and deliver stable traffic separation regardless of temperature variations. As the athermal AWG is a recent innovation, prices are higher. But, for the carrier with the oppor-tunity to serve many customers from the same access ring, a WDM-PON approach leveraging DWDM splitters might generate enough revenues and efficiencies to offset the initial capital investment in athermal AWGs.

Three additional things impact the use of WDM-PON in the OSP. They all positively impact the carrier business model for service delivery.

Impact #1: Because the insertion loss of WDM-PON splitters is intrinsically lower than traditional EPON and GPON splitters and because optical amplifiers can be employed, network spans of much longer reach can be deployed—of 100 kilometers or more. This allows a carrier to remove active equipment and sites end-to-end across the network, significantly reducing operational costs driven by powering, heating and cooling.

Impact #2: Because WDM-PON splitters are passive equipment, the carrier gains a greater degree of flexibility in where the devices can be deployed—perhaps, even in decommissioned/non-powered environments.

Impact #3: Because WDM-PON splitters serve more individual users from every fiber within a given cable, less cable must be deployed in the OSP as a carrier's traffic demands increase.

Closing In on the Bottom Line
The WDM-PON's combination of secure traffic separation, scalable bandwidth, extended reach, and unique practical benefits for the OSP enables carriers to serve a wider base of more lucrative customers from fewer active sites and to streamline network operation with less operational staff.

The introduction of WDM into the FTTx environment is a forward-looking and innovative concept. Without introducing significant new burdens for the OSP, the quickly maturing WDM-PON sets up a carrier for long-term profitable delivery of high-demand services.

Dr. Paul R. Morkel is a senior director responsible for applications and solutions development for ADVA Optical Networking products in North America. For more information about ADVA Optical Networking, visit www.advaoptical.com. Paul can be reached at PMorkel@advaoptical.com.

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