From: utzoo!decvax!harpo!eagle!allegra!jdd Newsgroups: net.lan Title: Re: U-B survey, second installment... Article-I.D.: allegra.910 Posted: Wed Feb 16 13:15:11 1983 Received: Mon Feb 21 07:02:11 1983 From unm-ivax!dd Tue Feb 15 09:26:28 1983 In-Real-Life: DD Subject: U-B survey, second installment... (unm-ivax.136) Newsgroups: net.lan ---------------- Rice University --------------------------------- Keep in mind that all U-B equipment claiming to run on ethernet is INCOMPATIBLE with Xerox Ethernet above the signal voltage level. Two competing protocols can SHARE the cable but users of U-B stuff can't access, for instance, a line printer using Xerox protocol/hardware! P.S.: Most people don't believe this when they first hear it. Call U-B and they will finally admit it after a few comments about protocol levels ... The Ethernet spec covers a great deal more than signal levels (for example, bit-rate, bit-encoding, bit-synchronization, bit-ordering, byte-ordering, basic packet format, addressing, minimum and maximum packet sizes, packet spacing, CRC, error detection and handling, and so forth and so on). I doubt that U-B equipment is incompatible with these. However, there are also higher-level protocols where systems can diverge. For example, a VAX talking only TCP-IP cannot communicate with a Xerox Print Server talking only PUP (say), even if each uses the same Ethernet for transport. The trick here is the word "only". It makes some sense for something as dumb as a print server to have a single higher-level protocol (handling such things as virtual circuits, flow control, error control, as well as features specific to printing) built into it. These would not be changable as a user option, but you could twiddle your other machines to talk the right way to funny peripherals. On the other hand, the VAX can be expected to be more flexible. I agree that it seems inexcusable for an Ethernet controller for a general-purpose machine, such as a VAX, not to allow access to the raw net (possibly in addition to providing certain higher-level protocols for efficiency). Cheers, John DeTreville Bell Labs, Murray Hill