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Aucbvax.6485 fa.works utcsrgv!utzoo!decvax!ucbvax!works Mon Mar 15 03:54:40 1982 Mike O'Dell and Ethernet >From Odonnell@Yale Wed Mar 10 13:48:16 1982 Several points should be made:

First, about broadband. Current RF modems have problems with a cumulative-noise phenomenon; 40 dB down is not necessarily good enough if you plan hundreds of modems. Also, as I understand it a broadband cable is almost, but not quite, many logically independent networks. Problems arise on collisions: transmitters beat against each other, spraying noise across all frequencies on the cable.

Second, cable bandwidth. Ethernet provides one channel, with well analyzed and understood behavior under wide-ranging loads. The CATV systems partition cable bandwidth in ways that may not correspond to the load; also, many of them use questionable FDM and packet-allocation practices that look nice in theory, but may match real loads quite poorly. Comparing 'raw' bandwidths is quite misleading.

Third, DC coupling. Properly designed transcievers provide ground isolation. Lighting has struck near PARC several times without problems (problems DID occur once when cable ground was not properly isolated from building ground; this is equally dangerous and/or likely with broadband systems). No argument here between Ether/Broadband makes sense, unless the 'broadband' medium is fiber optic.

Our view is that indeed, some mix of ether and broadband makes a great deal of sense in planning for a large campus: in-building Ether distribution linked via broadband gateways (the Ungermann-Bass systems look attractive for this). -------

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