In what looks like an attempt to pressure the ATM Forum into adopting the Carrierless Amplitude Modulation, Phase Modulation or CAP proposal for a physical layer specification for the planned Asynchronous Transfer Mode standard, several industry heavyweights have publicly endorsed it. The group comprises AT&T Co and its NCR Corp subsidiary, Hewlett-Packard Corp, Newbridge Networks Corp’s Vivid unit, and Standard Microsystems Corp. Carrierless Amplitude-Phase is currently under consideration by the ATM Forum as one of several proposals for a modulation scheme necessary for moving Asynchronous Transfer frames on and off the cable. It provides a single Asynchronous Mode physical layer specification for different grades of twisted pair cable at different data rates – which is, say the companies, the technology’s main advantage, since the same adaptors and hubs can be used for the different media and speeds. The alternative proposals made to the ATM Forum call for multiple specifications according to the type of cabling and speed of transmission. As it stands, however, the Carrierless Amplitude Modulation, Phase Modulation technology does have its limitations, notably the distances over which it can operate. The group is playing these down, since they say the specification is needed primarily for local transmissions and that it can handle great enough distances for most requirements. Over Category 3 unshielded twisted pair cabling it runs at 51.84Mbps for distances up to 600 feet, or at half this speed for greater distances, dropping down to 12.96Mbps for distances over about 1,200 feet. While its ability to be used over Category 5 unshielded twisted pair or shielded twisted pair cabling has yet to be tested, the companies say that it is very feasible for it to run at 155.52Mbps at distances of 300 feet. To add weight to their proposal, NCR and Standard Microsystems were demonstrating Asynchronous Transfer Mode operating over Category 3 unshielded twisted pair cabling at 51.84Mbps, at their respective booths at the InterOp show in Paris last week.
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