What keeps the ALCMS network running after a failure?
Duplicated switches, links and paths mean that one fault moves the traffic to a second path, and an alarm tells the engineers to repair it before the spare is needed.
Network redundancy means there is always a second way for ALCMS data to travel. The usual building blocks are two independent networks (A and B) or a fibre ring, duplicated switches and dual-port field controllers that connect to both paths. If one link or switch fails, the traffic moves to the other path and control carries on.
Redundancy only protects when the paths are truly independent: separate switches, separate cable routes so that one damaged duct does not cut both, and separate, UPS-backed power supplies. It also hides the first failure, so the system must raise an alarm and the fault must be repaired quickly, because the remaining path is then a single point of failure. If both paths are lost, each CCR still has local control as the last fallback.
How fast the switchover happens, and whether it is automatic, depends on the equipment and its settings, so a failover test is part of commissioning and is repeated after changes. Ring and dual-network designs vary with the manufacturer and the project, so the project design documents are the reference for any actual site.
Frequently Asked Questions
Q1What does network redundancy mean in an ALCMS?
It means the control network has a second, independent way to carry data, so one failed cable, switch or port does not remove control or monitoring. It is usually built from duplicated switches, separate network paths or a fibre ring, and field controllers that connect to both paths.
Q2What is the difference between a ring and a dual network?
A ring is one loop of cable in which data can travel either way round, so one cut is bypassed. A dual network has two separate networks, A and B, each with its own switches and routes, and each field controller connects to both. A dual network survives more kinds of failure but needs more equipment. The choice depends on the design.
Q3Why must the two paths be independent?
If both paths share a cable route, a switch or a power supply, one fault can take out both and the redundancy is lost. Separate routes, separate switches and separate power supplies mean that a single event, such as a damaged duct or a failed supply, affects only one path.
Q4If redundancy works, why is an alarm still needed?
Because the system keeps running, nobody would notice that one path is already lost. The alarm shows that the system is now running with no spare, so the fault can be repaired before a second failure causes a loss of control.
Q5How is redundancy tested?
At commissioning, and again after changes, a path is deliberately disconnected or a switch is powered down, under the airport's change control, to check that traffic moves to the other path, that control and monitoring continue and that the correct alarm appears. The switchover time and the results are recorded.
Q6What happens if both paths fail?
Remote control and monitoring are lost. The CCRs normally stay in their last commanded state and can be switched to local control at the CCR or substation, but the exact behaviour depends on the design and the airport procedures.
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