mo@LBL-UNIX@sri-unix (09/18/82)
From: mo at LBL-UNIX (Mike O'Dell [system]) Date: 14 Sep 1982 21:07:03-PDT No, the digital circuit they are talking about is NOT connected to the BPSS (which hasn't been aproved by the FCC yet). It is "Digital Dataphone Service". This is a scheme whereby your bit stream is sent as binary digital data. With analog circuits, the data is converted to genuine analog signals via combinations of phase and amplitude modulation. The resulting signal is a very complex analog beast. The DDS line uses weird current-mode line drivers and the data goes as real bits. Where available, the ARPAnet inter-IMP trunks are DDS because they have much, much better error performance than the analog circuits. Makes sense, though. If you are using an analog circuit, the phone company digitizes the waveform and sends it over their digital (but NOT packet-switched) backbone where possible (you may hear a "T1 carrier" referred to). By going DDS, your bits stay bits and just get multiplexed as one more data stream, only instead of being a PCM version of a conversation with Aunt Tillie, it is a data-stream conversation between your ECU's. By all means, use the DDS service!! -Mike