Nowadays the smooth flow of road traffic is significantly dependent on suitable data communication. Optical bypass technology, which functions similar to a railway turnout, protects networks against multiple points of failure. This means if a certain part of the network fails, it will not affect the function of the remaining network.
By Thorsten Ebach, EKS Engel Download PDF version of this article
There is a requirement for uninterrupted operation for networks in traffic control, as prescribed e.g. in the road tunnel equipment and operation guideline (RABT). But redundancy methods are not suited to meet this requirement, despite switching in only a few milliseconds. The reason is that if multiple failures occur simultaneously an interrupt of the data communication is inevitable. To prevent this, EKS Engel has implemented optical bypass technology, up to now mainly used in traditional IT, into the industrially integrated optical bypass x-light which withstands the harsh environments in traffic control systems. The x-light works in a protocol transparent way and therefore supports Ethernet as well as all field busses and even numerous interfaces like RS-485. The optical bypass also works seamlessly with third-party network devices and can be integrated either in ring or line topologies.
Various kinds of traffic-control equipment, e.g. intelligent traffic guidance systems, warning signals for wrong-way drivers, or roadside emergency telephones, constantly monitor the flow of traffic to ensure the highest degree of security. Tunnels are additionally equipped with fire alarm systems and smoke extraction systems. Nowadays nearly all these traffic management systems are linked via fiber optic cables, mainly with single-mode transmission systems featuring attenuation of only 0.3dB per kilometer up to 100km. With transmission-distances up to 5km maximum, mainly multi-mode fiber cables are in use with attenuation 1dB per kilometer. Even in terms of reliability, fiber optics is the measure of all things because it is not sensitive to electromagnetic fields. Fiber optics consists of insulating material, which means data is transmitted via an electric insulator. Therefore, there are no potential equalization currents which are much dreaded in larger extended plants. There is also no risk of damaging connected devices even in the event of direct lightning strikes.
Active network components like Ethernet switches or field-bus converters are put through their paces by their manufacturers prior to delivery. But despite this, failures in operation cannot be completely avoided. The main reasons are, apart from incorrect use, problems in hardware and software and power failure.
To avoid problems within the network several protection methods are in use. Power failure can be compensated at least for a certain time with an uninterruptible power supply (UPS). Software is evaluated or rather validated as well as tested with structure, function, and so-called black-box tests. The magic word in hardware is redundancy, which means that devices, plug connectors, and wiring are present twice. Also, the network has a redundant layout which results in a kind of hot standby effect: if a device or the network itself fails, the system switches automatically over to a redundant one.
Figure 1. The x-light bypass is parallel connected to the subscriber via two optical duplex ports. If the subscriber connection fails, it will be physically bridged.
But this is often a complex and costly strategy. To solve this problem the ring topology was developed. In a ring topology, the data communication has one direction, but in case of an interruption it works the other direction. This maintains the function of the network. The ring topology, also often used in traffic management systems, builds in redundancy which ensures fast switching, achieving the premier class of safety against failure. In contrast to line topology it can cope with a single point of failure, i.e. the failure of one participant. But if there is a failure of another subscriber or more (multiple points of failure), even ring topology is overstrained. To avoid situations like this, networks are monitored with systems which display the state of the active components, like FiberView, and the condition of the individual fiber transmission lines, to detect excessive attenuation.
Optical bypass technology takes one step further because it starts at the network participant level. This could include all devices featuring an optical port. If one of them fails the bypass takes over and maintains the data communication between adjacent participants. Therefore, the only application affected is the one controlled by the failed subscriber. The remaining network continues to be kept in function physically, and therefore works even with multiple points of failure. This is also valid even if the bypass, which is powered either by the protected participant or a separate power supply, no longer receives any power at all.
So, how does this work? With optical bypass technology fiber transmission lines are switched over physically immediately after a failure occurs at the respective participant. The principle is similar to railway turnouts working with strong electromagnets: if no current flows the turnout automatically switches and this is why the bypass technology works even without any voltage which is the trick. Since the bypass technology works at the physical level of the network it is protocol transparent and vendor independent.
Figure 2. In normal operation, the optical bypass sends the data to the parallel connected subscriber (on top). In the case of failure, data is not transmitted to the output but to the second input (below).
X-light is parallel connected to the respective participant via two optical input and output duplex ports. In case of disturbance its differential relay sends a signal to the electrical port of x-light and thereby activates the bypass function. After that the data is not transmitted any longer to the faulty participant but to the next participant in the network, in other words, the participant is bridged physically. To reconnect it safely later on to the network, e.g. after booting, there is an individual adjustable turn-on delay.
With optical bypass technology, networks of traffic management systems are not only protected against the effects of hardware and software errors, breakdown of power supply, shortfall of defined values, and so on, but parts of the network can also be specifically disconnected, e.g. for maintenance work. For this purpose, a voltage is applied to the electrical port of the respective bypass and therefore to the linked participant. This disconnects the connected participant physically from the network without the need to disconnect fiber connectors. Disconnecting connectors could eventually cause problems because of dirt particles which could get into the link.
An optical bypass is not only of worth in traffic management systems but also in all cases where power failure could cause serious problems. This includes production lines in automotive industry, plants in the petrochemical sector, and wind farms. Since the x-light has been developed as stand-alone device, retrofitting is no problem and neither transmitting protocols nor manufacturer-specific standards play a role. Integrating the bypass function into an Ethernet switch or field bus converter would lose this flexibility despite the fact that it would be possible.