From: andrew@megadata.mega.oz.au (Andrew McRae)
Subject: Re: Arcnet for an Embedded system
Originator: andrew@noah
Organization: Megadata P/L, North Ryde, Sydney, Aust.
Date: Fri, 4 Sep 1992 01:34:25 GMT

From article <4403@equinox.unr.edu>, by essary@pyramid.unr.edu (Bill Essary):
> We have just settled on Arcnet for our system (386, 8050 and Siemens 166
> processor mix.  Has anyone seen an OSI type (well defined physical, network
> and transport layer) protocol stack written to a small realtime kernel
> interface?  We have an in-house kernel and would love to port rather
> than write the network.  Actually, writing the network would be fun, 
> but... time.
> 
> Bill Essary

We currently have TCP/IP running over 802.4 Token Bus, with the
physical media being in one case 5 Mbit carrier band, and in another
case 2 Mbit RS-485 Twisted Pair (I'm not sure if the TP 802.4 has made it
all the way to a final standard yet). The TP is dual media (very nice
implementation). We fit this into a 68000 box where the entire
application code plus kernel, networking, drivers etc. fits in under
128Kb ROM. A future plan is to run the BSD 4.4 OSI stack over this
media and run MAP, though we would estimate that we would need our
bigger 68020 processor with 1 Mbyte RAM.

If you write your kernel interface to the BSD networking code, then
the BSD 4.4 OSI code should drop in easily. I took the BSD 4.3 Tahoe
code and interfaced it to our embedded kernel. One of the design goals
was to keep the BSD code unchanged so that I could later drop the
4.4 code in without changes; as it is, some change would be needed
as the socket address structure changed slightly and the mbuf stuff
has changed. But the changes would be easily incorporated.

This has been an interesting thread about what networking to
use in embedded systems; it is indicative of the state of the
industry that no one standard (de facto or de jure) has emerged as the
prime candidate, not like ethernet in the office environment. When
we went through the exercise of choosing a network (media and protocol)
we had a set of design criteria, and no one network met
all our requirements. A major problem with a lack of standards is that
you do not get the chip makers designing and releasing integrated
controllers that do most of the hard work in running a medium speed
network. Some people have mentioned Arcnet controllers, and I
wish I had known about them (Down Under here we unfortunately do not
get informed about the entire range of available products from vendors).

The Token Bus stuff seems very good, albeit a little more expensive,
probably because it runs faster than most other PLC RS-485 based
networks. What swayed us in the end is that we cannot afford not
to go with a standard, even though that standard seems to be poorly
supported. Catch-22, really.

For more info about the porting of the BSD code, see the paper
"Porting TCP/IP to 802.4 Token Bus" in the Australian Unix Users
Group winter 1991 conference proceedings (I can mail the paper to
anyone who is interested).

Andrew McRae			inet:	andrew@megadata.mega.oz.au
Megadata Pty Ltd,		uucp:	..!uunet!megadata.mega.oz.au!andrew
North Ryde  2113		Phone:	+61 2 805 0899
NSW    AUSTRALIA		Fax:	+61 2 887 4847
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