By Claude Baggett, Paul Workman, and Michael Ellis, Cox Cable Communications, Inc.
As an industry, we have spent many years perfecting the art of building analog RF wideband communications systems. Although the meaning of the term "wideband" takes on a new meaning every few years, and the technology of RF circuit design may still be more black art than science, still we have become fairly comfortable with this discipline. Now, however, we have witnessed the marriage or perhaps elopement of digital data processing with our broadband RF analog cable systems. What will be the ultimate issue of this union cannot be fully determined at this time, however, it can be said that it is here to stay. Cox Cable has responded to this stimulus by the development of its Tandem T-16 based INDAX system. INDAX, which is an acronym for INteractive DAta eXchange system, is a versatile, modularly expandable interactive data system for subscriber services and is based upon the multi-processor, fully loadsharing Tandem T-16 system. Communications between this central processor and the user terminals occurs over dedicated bandwidth on the subscriber cable. The basic data transmission rate for all INDAX communications is 28 KBS. This occurs in two different modes. A number of 28 KBS channels are set aside for what is essentially one-way common user communications. In this mode, data items of common interest, such as sport briefs, stock market reports, news and the like are constantly cycled. A subscriber may tap into one of these 28 KBS channels at any time without requiring a unique response from the INDAX processor. The second mode of operation is a two-way interactive process, which once again is implemented over dedicated bandwidth on the cable. This service also operates at 28 KBS, but in a different manner. After extensive investigation Cox has determined that the most efficient implementation for this kind of service is a packetized system utilizing carrier sense multiple access 105 with collision detection, commonly called CSMA-CD protocol. With this technique, all communications occur in standard digital packets in a listen-before-transmit mode. This is the CSMA portion of the protocol. The CD, or collision detection, means that the transmitter listens to see if another transmitter came up at exactly the same time. If so, a random time is delayed before attempting to retransmit. Utilizing this process, the INDAX two-way interactive system achieves a 70% efficiency.