Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!husc6!ogccse!blake!uw-beaver!cornell!batcomputer!itsgw!steinmetz!uunet!portal!cup.portal.com!ts
From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.protocols.appletalk
Subject: Re: What is EtherTalk speed?
Message-ID: <12735@cup.portal.com>
Date: 18 Dec 88 09:34:26 GMT
References: <1731@cayman.COM> <12454674092.150.A.ALDERSON@MACBETH.STANFORD.EDU>
Organization: The Portal System (TM)
Lines: 24

Good heavens, don't we at least get to assume some sort of Ethernet chip
in these calculations?  These should "watch the wire" for us and only
cause an interrupt when a packet for us arrives.

Anyway, existing EtherTalk cards use on board buffering.  Apple's EtherTalk
card has either 32k or 64k ( I forget which ).  Dove's FastNet III has
64k.  FastNet II has 256k of RAM on board.  FastNet SCSI contains 512K.
FastNet SE has 64k.  I don't know what Kinetics uses ( I don't have
access to any of their products, but I have EtherTalk and all the
FastNet stuff ).

Some of the Ethernet chips are pretty nice about buffering.  For
example, the AMD LANCE, which Dove uses, lets you give it a ring
of pointers to buffers.  The chip fills the buffers and then sets
a bit that says that the CPU owns the buffer.  When the CPU is done
with the buffer, it resets the bit so the chip knows that it can
fill the buffer again ( actually, it is the other way around:
a set bit means that the chip has the buffer and a clear bit
means that the CPU owns the buffer ).  The same thing is done
for transmitting.  FastNet III, for instance, uses 32 1.5k buffers,
so a burst of up to 32 packets can be received before the CPU
has to do something about them.

						Tim Smith