pag@hao.UUCP (Peter Gross) (01/04/84)
The following are the mail responses I received concerning the use of Ethernet repeaters. --peter gross ****************************************************Response 1**** There are two kinds of repeaters: 1) a simple state machine connected (via 50 meter tranceiver cables) to a pair of tranceivers on two coax segments. Xerox for sure makes these, costs several hundreds of dollars (plus tranceivers) 2) a repeater with 1km of cable in it's middle. I've never seen this as a distinct product. I suspect you really use two repeaters and 1km of coax, getting the extra distance out of only two taps, not 100 so few reflections and low load. Codenol makes a fiber-optic cable tranceiver which will drive 1km of untapped cable, but this may not be cheaper. The official rule is that any path between two nodes on a single node may not pass thru more than 2 repeaters, nor more than 2.5km (longer cable would compromise collision detection). You can cut the cost of coax by using RG-8 type cable instead of "real" Ether- net cable. Belden 8214 coax is both 50 ohm and 78% light-speed propagation, as real cable. The main differences are : 1) non-teflon so you can't use in plenums (but you can also buy non-teflon ethernet cable at 20% cost) 2) only a single shield so more signal leakage both in and out, so probably not advisable for large nets or long cable runs. (XSIS, who markets Dolphins actually told me about RG-8) ****************************************************Response 2**** I think repeaters are just a way of isolating coax segments and (hopefully) isolating some of the cumulative phase problems. DEC lists a repeater as a product. It connects via standard modems so the modem-repeater is the standard length. In addition, you can have two repeaters in any path so they have an option to put up to 1000 meter fiber optic link between two repeaters so as to connect two different coax segments. This is really no different, with respect to the spec, than having the two repeaters connected by a piece of ethernet coax. I have a glossy from DEC on this, call me if you like. Another cute item that DEC has for their own Ethernet interfaces is a local network interconnect that lets you hook up as many as 8 of their interface boards together without any ethernet coax or transcievers. The Local Network Interconnect box CAN be connected to a chunk of coax with a transciever in which case the interfaces plugged into it will all share use of that transciever. This ONLY works with their interfaces -- it looks like a bunch of the brains in their system is out in the transciever. Doesn't Interlan make a repeater? ****************************************************Response 3**** As I understand it, repeaters are indeed connected by transcievers to their coaxes. The only purpose is to extend the electrical length of the cable by regenerating the signals at each end. I would expect all the usual rules about transceiver cables to apply, and remember that the end-to-end distance for the total system still has to be within the system limit because of propagation delay limits. ****************************************************Response 4**** Two segments of Ethernet coax can be connected by a repeater. The Ethernet segment-to-repeater cable is transceiver cable (50m max.). This type of repeater is called a local repeater. A remote repeater is similar to a local repeater cut in half, with each half connected to its coax segment over transceiver cable (as before). However, the remote repeater halves may have up to 1Km of coax between them. So the max length of an Ethernet system (according to v2.0 specs and IEEE 802) would be: 50m X 2 transceiver cable for the transceivers 50m X 2 transceiver cable for the remote repeater 1Km coax cable between the halves of the remote repeater 50m X 2 transceiver cable for another local repeater (specs allow for a max of 2 repeaters between any 2 stations, worst case) 500m X 3 coax cable for the 3 segments that the signal will have to travel, worst case. ---------------- 2.8 Km max. In the systems we design, we usually use the Ungermann-Bass repeater. The 2.8 Km limit will allow a fairly large LAN to be constructed if you use a physical star topology.