smb@ulysses.UUCP (Steven Bellovin) (01/28/85)
We're about to wire our building with Ethernet coax. Initially, about half a dozen 3B2s and maybe ATT 6300s will be connected, but we expect substantial growth in the future. The problem I'm worrying about now is how to plan for easy additions to the net, specifically adding transceivers. One solution, of course, is to use vampire taps. But the Ethernet package for a 3B2 includes a 3Com transceiver, which uses barrel connectors. So we're contemplating putting connectors every 12.5 meters or thereabouts, allowing for easy access. (Total cable length will be about 300 meters, comfortably within the spec.) Is that likely to add too much noise? All of the wiring will be in the ceiling, which is moderately accessible. If we need to add a second transceiver in the same splice as an existing one, we'll just add a 2.5 meter jumper cable between the two transceivers.
rpw3@redwood.UUCP (Rob Warnock) (02/02/85)
+--------------- | We're about to wire our building with Ethernet coax... | One solution, of course, is to use vampire taps. But the Ethernet package | for a 3B2 includes a 3Com transceiver, which uses barrel connectors. | So we're contemplating putting connectors every 12.5 meters or thereabouts, | allowing for easy access... Is that likely to add too much noise? All | of the wiring will be in the ceiling, which is moderately accessible... +--------------- Two cautionary suggestions: First, make very sure that unused connectors (male-barrel-male), in fact ALL connectors and terminators, are insulated from any metal contact, such as ground, cable trays, light fixtures, etc. (The coax should be grounded to a solid earth ground in EXACTLY one place, maybe in your "main" machine room.) Ground loops (multiple ground connections) can be a cause of damage, both due to A/C currents (most buildings are NOT at the same potential at both ends!) and due to lightning (even nearby strikes induce large voltages). Use rubber "boots" rather than tape, as tape gets "sticky/gunky" after a while. (See the Ethernet 2.0 spec, section 7.6.3, p.82-83) Second, it would be best if your coax were a single length of cable that was cut AFTER it was installed. Since that is hard (putting connectors on in the ceiling is rough!), you can equivalently specify that the sections be cut from a single piece of cable, or failing that, from the same manu- facturer and lot. This will lower the impedance variations in the cable. (See the Ethernet 2.0 spec, section 7.6.1, p.81-82) Otherwise, sounds fine. (If you begin also using vampire taps, remember to keep them on 2.5m spacings from your connectors.) Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404