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Show moreTechnical field: This invention relates to the art of distributed data communication. The invention finds particular application in bus communication systems which transmit data packets between a plurality of stations and will be described with particular reference thereto. The invention is also applicable to other communication systems.Prior articles: Heretofore, communication in distributed multistation systems was commonly implemented with packet broadcasting. In packet broadcasting, each message or transmission unit was segmented into transmission segments or packets. The message commonly included an address transmission field to indicate a specific receiving station or that the transmission was intended for reception by all stations. To implement the distributed multistation packet broadcasting system, a high bandwidth common channel was shared among many contending stations. The station which momentarily had control of the common channel transmitted its message including its data packet. This mode of operation was advantageous in that it combined the advantages of both packet switching and broadcast communication. Packet switching offered efficient sharing of communication resources, whereas broadcast communication eliminated complex topology and routing problems. To eliminate contention among the many stations, a means of access control, i.e. a multi-access protocol, was established. There were two common multi-access protocols: time-division multiplexing (TDM) and frequency-division multiplexing (FDM). In frequency-division multiplexing, an available range of frequencies was divided or allocated among the stations. Frequency-division multiplexing was a relatively expensive access protocol. In time-division multiplexing, the stations share a single "baseband" frequency which was implemented with less expense. Time-division multiplexing was commonly either synchronous (STDM) or asynchronous (ATDM). In synchronous time-division multiplexing, a centralized controller cyclically assigned a time slice to each station without regard to whether or not the station needed access to the common channel. The synchronous time-division multiplexing tended to be time wasteful, particularly when the data traffic was highly bursty. Asynchronous time-division multiplexing provided common channel access only to those stations which requested access, i.e. a demand access scheme. Asynchronous time-division multiplexing was commonly either random or controlled. The most common random form of asynchronous time-division multiplexing was carrier-sensed multiple access (CSMA) which was an indeterministic, distributed protocol. The carrier-sensed multiple access protocols utilized either a collision detection technique (CSMA/CD) or a collision avoidance technique (CSMA/CA) in controlling access to the common channel.
http://www.google.com/patents?vid=USPAT4569044
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