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