Why does the ALCMS run on glass fibre?
Control and status data travel as pulses of light through fibres laid round the airfield, immune to electrical noise and lightning-induced interference, usually in a ring so that one cut does not isolate a substation.
A fibre-optic network carries ALCMS data as pulses of light along thin glass fibres instead of electrical signals along copper. At each end of a link, a switch or media converter changes the electrical data into light and back again. The light stays inside the core because it reflects off the boundary with the surrounding cladding.
An airfield is hard on electrical signals: long cable runs, lightning, high-voltage series circuits and CCR switching. Glass carries no current, so fibre is immune to electrical noise and does not carry surges or earth potential differences from one building to another, provided the cable is all-dielectric or any metal armour or strength member is bonded as the project design specifies. It also reaches long distances with little loss. The fibres are usually laid as a ring through the substations, so if one section is cut, data simply goes the other way round and every substation stays connected.
Fibre is tested at commissioning for total loss and for the position of splices, connectors and breaks. In service, fibre ends and connectors must be kept clean, bends must respect the minimum bend radius, and nobody should look into a fibre end or port. The fibre type, ring layout and link limits vary with the manufacturer and the project, so the project design documents are the reference for any actual site.
Frequently Asked Questions
Q1What is a fibre-optic network in an ALCMS?
It is the glass-fibre cabling and network equipment that carry ALCMS control commands and status data between the control room and the substations around the airfield. Data travels as pulses of light, and switches or media converters at each end change it to and from electrical signals.
Q2Why use fibre instead of copper cable?
Fibre carries no electrical current, so lightning, high-voltage cables and switching noise do not corrupt the data, and it does not carry surges or earth potential differences from one building to another. It also covers long airfield distances and has spare capacity for future needs. Copper can still be used for short links inside a building.
Q3What is the difference between single-mode and multimode fibre?
Single-mode fibre has a very small core, about 9 µm, and suits long runs between buildings. Multimode fibre has a larger core, commonly 50 µm, and suits shorter links such as those inside a building. The fibre type must match the equipment at both ends of the link.
Q4Why is the fibre usually laid as a ring?
In a ring, every substation can be reached in either direction round the airfield. If one section is cut, data is sent the other way round, so no substation is lost. How quickly and how automatically the switchover happens depends on the network equipment and the project design.
Q5How is a fibre link tested?
At commissioning, the total loss of each link is measured with a light source and power meter and compared with the design limit. An OTDR (optical time-domain reflectometer) shows the fibre as a trace that locates connectors, splices, bends and breaks. The results are recorded so that later faults can be compared with the original.
Q6Is it safe to look into a fibre end?
No. The light is usually invisible and can be strong enough to harm the eye, so nobody should look into a fibre end, connector or port, even if it seems dark. Fibres are disconnected and handled only by trained staff following the site procedure, and unused ports and connectors are capped.
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