THE USE OF FIBER OPTICS IN A NETWORKED FIRE ALARM SYSTEM
BY JOE TOBIN
LaGuardia Airport, located in the New York City borough of Queens, is a 650-acre campus-type facility that presents unique fire detection and fire control problems. As in any airport of this size, one of the key components is accurate information to the Central Fire Command Station during local fire alarm system monitoring.
Interconnecting all of the local fire alarm control panels to the Central Fire Command Station via traditional copper wiring presents challenges in a facility of this size and use. The extraordinary distances can result in degraded signal and data loss. Furthermore, the widespread use of radar and other specialized, high-power transmitting equipment can inject noise into the network, a condition that could adversely affect the transmission of fire alarm data. One way to reduce the problems inherent in a network of this type is to use fiber optic cable in-stead of the traditional copper wiring.
When originally installed at this facility in 1994, the system represented the first use of fiber optics for signal transmission in New York City. Los Angeles International Airport and Prudential Insurance in Roseland, New Jersey, also use this technology for fire alarm procedures.
Fiber optics are used as a mode of signal transmission to link local nodes, which, in the case of LaGuardia Airport, are the fire alarm systems. In this mode, light is used to transmit data instead of radiowaves, because light has a frequency of 100 Thz (100 trillion hertz), which is much higher than methods such as line-of-sight microwave and satellite link. Therefore, more data channels can be transmitted simultaneously across a single fiber optic link, enabling a quicker response to and from the Central Fire Command Station.
Signal transmission using fiber optic technology begins with electrical signals, which are then converted into pulses of light using small semiconductor lasers. These pulses are sent through the network by way of minute glass fibers. At the end of the loop, photo detectors convert these incoming signals back to electrical signals for easier translation at the destination.
At LaGuardia Airport, this technology is used for data transmission of fire alarm signals. When it was installed, the goal of this system was to provide the facility with a closed-loop proprietary signaling system with individually addressable points. Each device on the network is assigned a specific loop, address, and message. This information is coordinated so that no two addresses or points are the same. The specific infrastructure of this network comprises 15 local multiplexers, called FDINs (Field Drop and Insert Nodes), which are at various locations. There is one main multiplexer, called a CDIN (Central Drop and Insert Node), which is at the Central Fire Command Station.
At the FDIN units (field multiplexers), a single pair of twisted shielded #16 wires connects the local fire alarm control panels to the multiplexer panels. Each fire alarm panel requires specific software for its devices. This software must be programmed to accept the signals coming from the field devices and then transmit them through the fiber optic network. To maintain the integrity of the system across the network, only one manufacturer`s model should be used for all local fire alarm control panels directly connected to the central system. The multiplexers loop back to the Central Fire Command Station CDIN, which then distributes the information to four remote video display terminals and three remote printers.
This network is a point-to-multipoint dual fiber ring with a primary loop and a secondary loop. Because there are two loops (each traversing in a different direction), each FDIN must have four port connections: for the primary transmit, the primary receive, the secondary transmit, and the secondary receive. This configuration allows for a “Style 7 system,” under NFPA 72, in which a single fault will not interrupt the flow of fire alarm data to the Central Fire Command Station, thereby allowing for appropriate redundancy measures within the system.
The CDIN polls the FDIN units in a counterclockwise rotation every three seconds. Each multiplexer can operate and communicate properly under three conditions: repeater, secondary to primary, and primary to secondary. In the normal operating condition, the CDIN will read “primary to secondary,” and the FDIN units will read “repeater,” usually over the primary loop.
At the Central Fire Command Station, the supervisory computer monitors the status of all nodes on the network. In addition, during this monitoring, the fiber connections between the nodes can be diagnosed. These nodes have color-coded status blocks that indicate re-peater, primary to secondary, secondary to primary, polling, unknown, and rebuilding.
A log of activities for each of these nodes is available for review on a daily basis through the supervisory computer. Coincidentally, this feature was useful after the installation. One week after the coordinated network test was completed and accepted, one of the field multiplexers had a problem with the primary transmit connection, and the power went down at that unit an hour later. The supervisory log stored all of this information, which made it easy to determine that there had been system tampering. The proper authorities were summoned. To prevent further tampering, numbered seals were placed on all node cabinets. Since this incident, there have been few problems with the coordinated network.
To ensure that the system is performing as required over a longer period of time, certain tests are conducted–for example, a coordinated network test of the system (as mentioned above). During this test, a simulated fail condition of all possible scenarios is performed to determine if the network will reconfigure itself to ensure proper data flow through the system. Specifically, for each node, a simulated primary link fail, a secondary link fail, and a dual link fail test are performed. To simulate a line break between the nodes, the primary and secondary links are removed. The resultant condition (assuming the test was passed) is that the loop reconfigures itself but will poll in the shape of a horseshoe instead of a continuous ring.
Although the use of fiber optics is still in its infancy, it is becoming increasingly evident that it exhibits superior reliability and, as used in this configuration, has redundancy features traditional copper wiring does not offer–that is, if the primary loop breaks, the secondary loop takes over. If both the primary and secondary loops break, the multiplexers can still communicate via a different configuration. Additionally, if a node fails completely, all other nodes can rebuild around it.
As previously noted, this system was installed in 1994. It comprises components from different manufacturers to achieve a common goal: a reliable, durable, and trouble-free networked fire alarm system. Since that time, this technology has been embraced more readily by various industries; subsequently, numerous fire alarm system manufacturers have developed their own fiber optic components that are mated to their networked systems. It is probable that the use of matched components by one manufacturer would enhance the reliability and ease of future system expansion. The use of fiber optics for fire alarm systems will increase as time goes by and as more airports and public buildings realize the benefits of this type of life safety mechanism. n
(Special thanks to RSM/MD.)
n JOE TOBIN is a project engineer with the Port Authority of New York and New Jersey at LaGuardia Airport. He previously worked as a supervising inspector in the Suppression Unit of the Fire Department of New York Bureau of Fire Prevention. He has a B.A. from John Jay College and is a firefighter with the Hillsdale (NJ) Fire Department.