APPLICATION OF FIBER OPTICS TO COMPUTER SYSTEMS

APPLICATION OF FIBER OPTICSTO COMPUTER SYSTEMSDaniel J. StiglianiIBM Corp.P. O. Box 390Poughkeepsie, N. Y. 12602AbstractThe use of fiber optic communications in a modern computing system is investigated.The scope of this paper is limited to intra -unit interconnection and the in -plantnetwork. The inherent properties (e.g., electromagnetic compatibility, ground level de-pendence, volume reduction, data rate, delay, etc.) are discussed in relation to the areaof usage within a computing system. The evaluation of these properties indicates that theinitial usage of fiber optics will probably occur for inter -unit in -plant network in-terconnection. It is shown that the associated signal delay of a fiber optic link as wellas the dissimilar materials technologies will preclude the initial use of fiber optics forintra- processor interconnection. A representative local in -plant data network is analyzedwith respect to data requirements and equipment complement. This configuration is used asa basis for implementation of a fiber optic data communication network. Various fiber op-tic interconnection configurations (e.g., point -to- point, star, multidrop and bus) areinvestigated. The advantages /disadvantages of these configurations are analyzed and com-parisons made with respect to total cable length and optical loss. This analysisindicates that point -to -point Space Division Multiplexed and Star data configurations areviable in fiber optics interconnection techniques.IntroductionThe development of fiber optic communications promises to have a major impact towardsimproving the present communications facilities. The introduction of low loss (less than20dB /Km) and wide bandwidth (greater than 1 ghz /Km) optical fibers has stimulated theimagination of many communication engineers. Fiber optics has enabled the design engineerto uncouple cable attenuation and signal bandwidth in order to achieve wide bandwidths andlong transmission lengths simultaneously. Moreover, it is anticipated that these resultscan be achieved at a cost which is competitive with present narrow bandwidth wire communi-cation channels.Interest in optical communications has also developed for application in data pro-cessing systems. Present computer systems are interconnected via coaxial cables or stand-ard telephone lines (remote units). Some of these interconnections are parallel (byte wide)while others are serial. The trend has been toward wider bandwidth interfaces which willallow more effective use of modern computing equipment. In the electrical domain there aretradeoffs (e.g., data rate,cable length, ground level shift, etc.) that must be made indesigning a computer interconnection technique and in many instances the results are notoptimum for a data processing system. The use of fiber optics will alleviate some of theseproblems and allow a more optimum data processing system design.Properties of Fiber Optic CommunicationThe use of fiber optics as a communication medium offers some significant advantagesas compared to wire. The advantages which are most pertinent to data communication willbe discussed in the following paragraphs as well as some disadvantages of the technology.AdvantagesElectromagnetic Compatibility (EMC). The use of the optical domain for transmissionof data allows the use of a dielectric waveguide to convey the optical energy. Consequent-ly, the optical cable is not susceptible to and does not radiate radio frequency (RF).Optical communications has inherent Electromagnetic Compatibility.The susceptibility of electrical lines to electromagnetic interference causes random"burst" errors to appear on a line. In order to insure data integrity, error correctingtechniques and /or error detection and retransmission techniques are employed. These ap-proaches add hardware cost and decrease the effective data rate of the transmission inter-