Telebit Corp of Mountain View, California has demonstrated the ability to send error-free data at up to 28K-bits-per-second across ordinary dial-up phone lines: the laboratory results were part of the company’s recent presentation to the CCITT. The agency is considering standards for high-speed modems that will be used on impaired phone lines (those designed for voice rather than data), and Telebit’s multichannel system is competing with a single-channel system proposed by Illinois based US Robotics. Standard modem systems use a single frequency to send or receive data – half duplex, or at most a pair of frequencies to send and receive simultaneously – full duplex, notes Ted Brown, Telebit’s manager of systems engineering. Any line noise in that frequency range will create data errors; in addition, telephone company multiplexing systems designed for voice lines can further scramble the data. The Telebit system, dubbed PEP, for Packetised Ensemble Protocol, divides the available bandwidth into 512 frequencies; 112 are used for housekeeping and error detection, and the rest are available for data, with up to six data bits per channel. The system constantly monitors the reliability of the data channels, Telebit says; error-detection is performed within the modem, and erroneous packets are automatically retransmitted. Consequently, modems using the PEP can send and receive at high speeds, even on noisy lines designed for voice only, with no error-detection required by sending or receiving computers, claims the company. PEP was developed by Telebit founder Paul Baran, and in the company’s current line of Trailblazer modems, it’s implemented in the form of a Motorola 68000 microprocessor, a Texas Instruments TMS32010 digital signal processor, and software in EPROM. But it’s not cheap: the Trailblazer Plus, rated for 18,000 bps, is $1,345 as a stand alone unit or $1,195 as a full-sized board for a standard MS-DOS micro – and you need another one at the other end.
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