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Presently, there are three basic network topologies under serious consideration for use in aircraft data distribution systems. These are star networks, active rings, and linear busses. The greatest factor involved in the selection of a topology for a typical application has generally been the number of nodes which may be accessed by that topology. In that respect, active rings are able to utilize the greatest number of nodes, followed by star networks and finally linear topologies. Active rings have the advantage of requiring the minimal performance from the optical components (transmitters, receivers, couplers), but have two major drawbacks. First, each node functions as a repeater leading to degradation in the system bit error rate. Second, in order to eliminate single point failures complicated systems for bypassing nodes and counter rotation schemes must be used. Star networks require little dynamic range and offer potentially large numbers of nodes, but are also susceptible to single point failures. In addition, a large amount of cable is required in a star network, especially for redundant systems. Linear topologies offer a minimum of cabling for a passive system resulting in ease of installation and maintenance, but until recently have been ruled out for more than about ten nodes due to the additive nature of the power losses and the relatively high insertion loss of the optical taps. The development of low loss couplers with non-reciprocal coupling of optical power between tap fibers and bus fibers has allowed the consideration of linear topologies for much larger numbers of nodes. "Asymmetric" tap couplers provide the maximum amount of optical power to be launched onto the bus fiber while maintaining the low insertion loss and tapoff ratio necessary for accessing many nodes. These couplers essentially launch more power onto the bus than they tap out of the bus, while preserving their inherently low excess loss. In this paper, a description of the operation of the asymmetric coupler is given, along with test data indicating the important performance parameters. Additional test data is provided for a linear data bus configured from such couplers. The impact of linear topologies and various data transmission schemes on transmitter and receiver design will be addressed. Further discussions are presented regarding the environmental conditions which must be addressed for aircraft applications, and additional data are presented indicating actual performance of the linear data bus and its various components under those environmental conditions.
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