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Calculating Fiber Optic Cable Link Budget

Fiber to the X- Fundamentals

Connections and losses

The strength of an optical signal is always higher at its point of origin than at the
receiving end of a line. Light always suffers some degradation over the length
of the network connection. To minimize this loss, fiber cables need to connect
seamlessly to other devices or cables in a network.

Graph Fibre Link Loss

In most cases, fibers are ‘fusion spliced’ - connected directly together by melting the
glass – which will also introduce some optical loss. These fusion splice connections
are permanent. Wherever connections may need to be changed in the future, or
wherever fiber has to link to a device on the network, connectors are used instead of
fusing the cable together. 

The fiber cable is fitted with a connector plug, which goes into an adaptor or a socket on an active device, and a connection is established. The core of an optical fiber is far smaller than a speck of dust - so making a good connection requires enormous precision in the alignment of the fibers.

In all optical systems, transported optical information can ‘leak’ if light is lost as it
transfers from one fiber to the other. The more precisely the cores are aligned, the
less light is lost, and the better the signal at the receiver.

The Math of loss measurement

When measuring the attenuation or insertion loss a ‘zero dB’ reference point is made on selected reference cables using an LSPM (light source and power meter) or OLTS (optical loss test set). A light source (LS) is connected to one end of the cable and the power meter (PM) is connected to the other. Then the connection between the reference cables is separated, and both ends are connected to the line that we want to measure. The measured value is the loss of the total line.

In this case the loss at the connections is measured and the losses in the rest of the cable, for example where the cable has been spliced.

Optical link budget

During cabling design, the link budget is used to predict the amount of light needed to guarantee an uninterrupted communications link. Link loss budget could be described as a ‘worst case scenario’ for a data transmission path. This takes into account elements that can introduce loss, such as splice, coupling or connector losses and fiber attenuation. 

Before designing or installing a fiber optic cabling system, it is important to determine the loss budget to ensure the system will work. Both passive and active circuit components need to be included in the loss budget calculation.

Both before and after installation, the link loss budget is essential. ‘Real life’ test results are
compared to previously calculated loss values to make sure a link will operate as intended.

Calculating the link budget

+ Transceiver power budget

– Losses from multiplexing and demultiplexing

– Fiber losses

– Splice losses

– Patch panel and connector losses


This article has been extracted from "Fiber to the X- Fundamentals - A complete overview of principles, technologies, architectures and business models for future networks." by COMMSCOPE

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