The splicing of outside plant (OSP) fibers to connectorized pigtails, to allow termination panel
access to the OSP fiber, can be done in two basic configurations: on-frame and off-frame.
On-frame splicing (See figure 1) is performed within the confines of the optical distribution frame (ODF), whereas off-frame splicing is done away from the ODF, generally in or near the OSP cable vault.Original fiber networks incorporated on-frame splicing, since the fiber counts were very small.
Even today, on-frame splicing can be a cost-effective solution for small and medium fiber count (less than 432 fibers) networks where future growth is limited. There are some drawbacks to this method, however.
For one thing, the number of terminations in a single rack is reduced by the presence of the splice panels, so generally there are fewer than 432 terminations in a single frame. One other drawback to on-frame splicing is the access to the ODF.
Different organizational groups are usually responsible for splicing functions and cable installation. Having splicing on the fiber frame limits the functions that can be performed on the fiber network at the same time.
For example, if the splicers are in the office splicing the OSP fibers to the pigtails, they will not want the operations group working on the frame at the same time trying to route patch cords.
This conflict can result in delays in service turn-up as well as possible scheduling conflicts over accessing the ODF, resulting in an increase in the installation costs and an increase in the probability of failure in the network. When OSP fiber counts become larger and floor space is at a premium, off-frame splicing can provide many advantages over on-frame splicing.
Off-frame splicing (see Figure 2) involves splicing the OSP fibers to pigtails in a location away from the ODF, such as the cable vault. The splicing is done in a large-capacity splice frame or wall mount cabinet. Splice cabinets able to handle 864 splices are common. The link between the splice closure and the ODF is made via an intrafacility cable (IFC) that is connectorized on one end.
The connectorized end is loaded into a termination panel. The loading of the connectorized IFC into the termination panel can be done at the factory or in the field. However, experience has shown that factory loading reduces the overall cost of installation (including training costs) and the amount of time required for installation. Factory loading also increases network reliability.
Termination panels with IFC assemblies generally are generally configured in 72- or 96-fiber counts. In large fiber count applications, with more than 432 incoming OSP fibers, splicing in a remote location can increase the termination density with the ODF to the point of reducing the number of racks required.
This allows the floor space within the office to be utilized more cost-efficiently and provides room for future network growth.Off-frame splicing can also improve flexibility in handling incoming OSP cables.
For example, a service provider may have only 48-fiber OSP cables being routed through the network and may be using the most common rack mount splice panels, which come in multiples of 48-splice capacity (up to 192 splices per panel). These panels work well if the incoming OSP cables remain consistent in size through the life of the network.
However, problems can arise when a variety of fiber cable sizes are deployed, with a mix of 24-, 72-, 96- and 144-fiber cables entering an office. In order to match these cables to a 48-splice capacity panel, the cable sub-units must be split between splice panels. The splitting of the sub-units between panels requires additional protection to shield the sub-units from damage.
(Figure 2) Off-frame Splicing
A dedicated splice facility, such as a wall mount splice enclosure accommodating 864 splices with any combination of OSP fiber counts, allows flexibility in the selection and routing of OSP cables.
Another advantage of off-frame splicing is that routing OSP cables through an office can be more difficult than routing IFC cables. OSP cables have a thicker, more rigid jacketing than IFC cables. OSP cables may also have metallic strength members that require special grounding not normally used on ODFs.
In any case, the OSP cable’s stiffness can make it very difficult to route through a central office or headend. IFC cable’s jacketing, on the other hand, is more flexible, but still rugged enough to be routed through an office without any additional protection.
There is a perception that off-frame splicing is more expensive than on-frame splicing, as it requires additional costs for equipment and IFC cable. In actuality, when looking at a system with more than 432 fibers in a cross-connect architecture, the price of a full ODF system with off-frame splicing will be equal to or slightly less than that of a full system with on-frame splicing.
There are two reasons for this cost difference: the elimination of the splice panels from the ODF; and the reduction in the number of racks required. Reducing the number of racks increases the amount of equipment that can be incorporated into the installation, increasing the overall flexibility, and profitability, of the network. Whatever splicing system is chosen, the decision needs to be based on long-term network requirements.
A network in which on-frame splicing works well initially may require off-frame splicing in the future. The ODF system should have the flexibility to easily incorporate both on-frame and off-frame splicing. The operational impact of using the wrong splicing system can include running out of floor space, increasing network installation time and cost, and reducing long-term reliability.
For more information around fusion splicing for fiber optics see our SP4420 Fiber Optic Infrastructure Specialist Course.