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Connectors in the field make plug and play WDMs a snap

Thu, 06/06/2013 - 12:07am
Tom Warren

When FTTH (fiber-to- the-home) technology was in its infancy, the prevailing method of deploying optical splitters in the OSP was to hard splice them in splice cases. In addition to hard splicing the splitters they would also typically hard splice drop cables at pedestals or vaults in the access portion of the network.

As the years went by and connectors and the termination process behind them became more robust, communication providers began to deploy splitter cabinets with terminated splitters and patch panels.

Connectorized drop solutions also have become very popular as well. The reasons for this migration to connectorization were; flexibility, testing access, ease of optical component replacement, and reduced labor and installation times.

As MSOs continue to install wavelength division multiplexing (WDM) solutions, it may behoove them to take a page from the FTTH playbook.

Most MSO fiber engineers that I speak with have similar concerns with their networks. How do we reduce install times, increase flexibility and replace/ upgrade optical components in an organized fashion? Some of the struggles that have been expressed include: disorganized splice cases, long turn-up times and splicing costs.

Alternatives for WDM deployments

With some forethought and up front engineering, the connectorization of WDM components may relieve much of the pain associated with WDM deployments. Let’s take a quick look at how most MSOs are currently installing WDM components and propose some alternatives that could reduce the restoration time of field outages to hours instead of days.

Two popular applications for WDMs are business class services and cell site backhaul.

They have become a priority for MSOs. They are usually longterm, high-revenue opportunities in a very competitive marketplace.

In most cases there is going to be fiber in the general vicinity of the new customer.

Over the years MSOs have stretched the capacity of their existing fibers so many of these fibers will already be in use for HFC. To better utilize the fiber resources for business class and cell backhaul, multiple channels will be combined at the head end and certain channels dropped periodically along the existing fiber routes as needed.

Let’s take a quick look at how MSOs are currently providing service to these types of customers using WDMs.

A customer is identified along a route with existing fiber(s). These fibers are designated as the common fibers that will handle multiple wavelengths.

A multi-channel MUX WDM device is placed at the headend and attached to the appropr iat e common fibers.

The various wavelengths are combined in the MUX and sent to the OSP. Near the customer l o c a t i on a splice case or pedestal will be used as the location for the “drop” filters for the correct wavelength(s). The fiber is midspanned and then the splicer will splice on to the incoming common fiber, the drop leg and the express fiber. The drop fibers will then be spliced in the splice case on to the drop fibers and spliced inside the customer premise as well. In this application we are looking at eight splices for a two-way communication path.

As an alternative, consider a solution that uses a small sealed closure near areas of potential business, for example a business park, strip mall or cell site. In the closure we pre-splice a terminated pigtail on to the common (incoming) fiber. We also presplice a pigtail onto the outgoing (express) fiber. We will then use a simple coupler to mate the incoming fiber to the express fiber.

Now when a customer in the proximity of that closure requires service, a low level technician can be sent out to the closure with a small cassette style WDM drop with female adapters on all ports (let’s say 1470nm and 1490nm in a CWDM system). The common pigtail, express pigtail and connectorized LC DUPLEX drop cables will be plugged into the adapter ports on the cassette. The double-ended drop cable can be pulled/pushed through microduct into the customer premise and plugged into the electronics or tie panel. This solution will require a total of four up-front splices in this solution.

Now if a second customer signs up for service at that same location, a tech can bring out a different plug and play WDM cassette with more drop wavelengths (1510nm and 1530nm) on it to be deployed in the same manner. The original two channel CWDM is unplugged and the new four-channel (1470, 1490, 1510 and 1530) device is plugged in. No need to break into fibers and do additional splicing later on. This solution dramatically reduces install times. It also cuts down on confusion in the splice case and possible outages due to technician errors. Since the drop cables are pre-connectorized on both ends, a splicer is only needed when the closures are initially deployed and not at each turn up.

All about microduct

We should spend a little more time talking about microduct and its ability to reduce installation costs and times.

With its smaller overall footprint, it can be fed into existing congested conduit or placed in a micro trench. Microduct is available in several configurations including OSP Tonable and Non-Tonable Direct Bury, Aerial and Plenum. By using a coupler we are able to mate the OSP microduct to the Indoor Plenum rated duct and utilize a continuous piece of pushable fiber cable from the splice point to the electronics in the premise.

This eliminates the need for a splice point at the building entry to transition from OSP to indoor rated fiber.

The LC Duplex Pushable Assembly is another key component to a plug and play WDM solution. The ability to install an LC Duplex Jumper into an optical component is an important innovation. Most Ethernet deployments require two fibers; one fiber for transmit and one for receive. In the past, terminated push-able fiber assemblies were only available in single fiber SC connector solutions. Now that we can push two pre-polished LC ferrules through a single piece of microduct this creates the twoway communication path needed for business class and cell back haul. Once the two ferrules have been pushed or pulled through the microduct pathway it is a simple procedure to affix the connector bodies to the ferrules.

No splicing is needed for the drop portion of the project.

Node segmentation is another popular application for WDMs. As bandwidth needs increase in HFC networks, the need to split nodes becomes mandatory. Before Internet, VOD and VoIP, nodes were able to serve larger areas and more subscribers. Now that the general population has developed a thirst for highbandwidth applications, these single nodes are no longer able to properly handle demand.

Additional nodes need to be deployed and customers split between the nodes. If additional fibers are not available in that region, WDM technology will need to be used to service multiple nodes on the existing fibers. Each node will now be assigned its own C or DWDM channels and C/DWDM filters will be used to combine them on to single fiber(s). The prevailing method for handling high channel count optical components in this application is to place the WDM filters in a splice case and hard splice the device on to the OSP fibers that will service the nodes.

As a suggested improvement, a small cabinet or splitter pedestal can be placed in the OSP and terminated WDM can be deployed in the cabinet.

In the same cabinet patch panels would be installed that would allow all OSP fibers to be terminated for testing and access. This is especially handy when dealing with highcount DWDM applications.

In the example shown above, the current situation is a sixfiber cable that is feeding three nodes. The entire fiber sheath is consumed and no more capacity is available for additional node splits. To create additional capacity, two 16- channel DWDM modules are installed in the head end and will be fed with SFP’s tuned to the appropriate channels.

The cabinet or PON pedestal mentioned earlier will contain the demux devices and will serve as the separation point for all the various wavelengths.

The two common fibers will now serve the original three nodes plus an additional 13.

It also allows multiple services to be provided in an organized fashion. PON splitters can be installed side by side with WDM devices and techs will easily be able to transition customers to different services as bandwidth needs increase.

Instead of having to enter a splice case, break fibers and re-splice fibers a jumper can be used to re-route fibers to the correct devices.

WDMs make sense for fiber exhaust issues and fiber conservation. They are becoming standard equipment for business class services, node segmentation and cell backhaul applications. Organizing and managing these devices is a challenge using existing solutions.

Using connectors in the field to provide a plug and play solution makes sense for many styles of projects.

Tom Warren is in Applications Engineering with Clearfield. 

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